Data link layer -- June 20041 Data link layer Computer Networks.
5: DataLink Layer5-1 Link Layer 5.1 Introduction and services 5.2 Error detection and correction ...
-
Upload
alysa-hancox -
Category
Documents
-
view
232 -
download
0
Transcript of 5: DataLink Layer5-1 Link Layer 5.1 Introduction and services 5.2 Error detection and correction ...
5 DataLink Layer 5-1
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-2
Link Layer IntroductionSome terminology hosts and routers are nodes communication channels
that connect adjacent nodes along communication path are linkso wired links wireless linkso Point-to-point links multi-
access (or broadcast) links layer-2 packet is a frame
encapsulates datagram
data-link layer has responsibility of transferring datagram from one node to adjacent node over a link
5 DataLink Layer 5-3
Link layer context
datagram transferred by different link protocols over different linkso eg Ethernet on first link frame relay on
intermediate links 80211 on last link
each link protocol provides different serviceso eg may or may not provide rdt over link
Introduction 1-4
source
application
transportnetwork
linkphysical
HtHn M
segment Ht
datagram
destination
application
transportnetwork
linkphysical
HtHnHl M
HtHn M
Ht M
M
networklink
physical
linkphysical
HtHnHl M
HtHn M
HtHn M
HtHnHl M
router
switch
Encapsulationmessage M
Ht M
Hn
frame
5 DataLink Layer 5-5
Link Layer Services framing
encapsulate datagram into frame adding header trailer
link access o channel access if shared mediumo ldquoMACrdquo addresses used in frame headers to identify
source dest bull different from IP address
reliable delivery between adjacent nodeso we learned how to do this already (chapter 3)o seldom used on low bit-error link (fiber some twisted
pair)o wireless links high error ratesbull Q why both link-level and end-end reliability
5 DataLink Layer 5-6
Link Layer Services (more)
flow control o pacing between adjacent sending and receiving
nodes
error detection o errors caused by signal attenuation noise o receiver detects presence of errors
bull signals sender for retransmission or drops frame
error correction o receiver identifies and corrects bit error(s) without
resorting to retransmission
5 DataLink Layer 5-7
Where is the link layer implemented
in each and every host link layer implemented in
ldquoadaptorrdquo (aka network interface card NIC)o Ethernet card PCMCI card
80211 cardo implements link physical
layer
attaches into hostrsquos system buses
combination of hardware software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
5 DataLink Layer 5-8
Adaptors Communicating
sending sideo encapsulates datagram
in frameo adds error checking bits
rdt flow control etc
receiving sideo looks for errors rdt flow
control etco extracts datagram passes
to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
5 DataLink Layer 5-9
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-10
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-11
Internet checksum (review)
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field valueo NO - error detectedo YES - no error detected
But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
5 DataLink Layer 5-12
Checksumming Cyclic Redundancy Check view data bits D as a binary number (actually a
polynomial with binary coefficients) choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
o ltDRgt exactly divisible by G (modulo 2) o receiver knows G divides ltDRgt by G If non-zero
remainder error detectedo can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Cyclic Redundancy Check Modulo 2 arithmetic
o addition = subtraction = XOR Each bit string represents a polynomial Example 10011011 corresponds to
A polynomial G(x) of degree r is known to both sender and receiver
Sender appends r bits (called CRC code) to the message so that the resulting polynomial can be divided evenly by G(x)
Receiver checks if the received frame (message together with CRC) is still divisible by G(x)
If not there are transmission errors in the frame
7 4 3( ) 1D x x x x x
Common polynomials for G(x)
CRC
CRC-8
CRC-10
CRC-12
CRC-16
CRC-CCITT
CRC-32
C(x)
x8+x2+x1+1
x10+x9+x5+x4+x1+1
x12+x11+x3+x2+x1+1
x16+x15+x2+1
x16+x12+x5+1
x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1
5 DataLink Layer 5-15
CRC Example
31001 represents 1x
5 DataLink Layer 5-16
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-17
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
o PPP for dial-up accesso point-to-point link between Ethernet switch and host
broadcast (shared wire or medium)o old-fashioned Etherneto upstream HFC (cable network)o 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Introduction 1-18
Cable Network Architecture Overview
home
cable headend
cable distributionnetwork (simplified)
Typically 500 to 5000 homes
5 DataLink Layer 5-19
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference o collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself o no out-of-band channel for coordination
5 DataLink Layer 5-20
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at the full rate say R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
o no special node to coordinate transmissionso no synchronization of clocks slots
4 simple
5 DataLink Layer 5-21
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
o divide channel into smaller ldquopiecesrdquo (time slots frequency code)
o allocate piece to node for exclusive use
Random Accesso channel not divided allow collisionso ldquorecoverrdquo from collisions
ldquoTaking turnsrdquoo nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-22
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-23
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-24
Random Access Protocols
When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA
5 DataLink Layer 5-25
ALOHA
When a node has a frame to send send immediately
Set a timer for a random amount of time If an ACK arrives before the timer expires
fine otherwise resend the frame
(Works like stop-and-wait with random timeout interval)
5 DataLink Layer 5-26
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-27
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
5 DataLink Layer 5-28
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
o collisions detected within short timeo colliding transmissions aborted reducing
channel wastage collision detection
o easy in wired LANs measure signal strengths compare transmitted amp received signals
o difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-29
CSMACD collision detection
5 DataLink Layer 5-30
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
o share channel efficiently and fairly at high load
o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolso efficient at low load single node can fully
utilize channelo high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-31
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concernso polling overhead o latencyo single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
o token overhead o latencyo single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-33
Summary of MAC protocols
channel partitioning by time frequency or codeo Time Division Frequency Division
random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)
hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211
taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-34
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-35
MAC Addresses and ARP
32-bit IP address o network-layer addresso used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address o function get frame from one interface to
another physically-connected interface (in same network)
o 48 bit MAC addressbull burned in NIC ROM
5 DataLink Layer 5-36
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-37
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
o can move LAN card from one LAN to another
IP hierarchical address NOT portableo address depends on IP subnet to which node is
attached
5 DataLink Layer 5-38
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgto TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Q given a nodersquos IP address how to determine its MAC address
1A-2F-BB-76-09-AD
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-39
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-
FF-FF-FF-FF-FFo all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ARP is ldquoplug-and-playrdquoo nodes create their
ARP tables without intervention from net administrator
5 DataLink Layer 5-40
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
LAN LAN
5 DataLink Layer 5-41
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-42
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-2
Link Layer IntroductionSome terminology hosts and routers are nodes communication channels
that connect adjacent nodes along communication path are linkso wired links wireless linkso Point-to-point links multi-
access (or broadcast) links layer-2 packet is a frame
encapsulates datagram
data-link layer has responsibility of transferring datagram from one node to adjacent node over a link
5 DataLink Layer 5-3
Link layer context
datagram transferred by different link protocols over different linkso eg Ethernet on first link frame relay on
intermediate links 80211 on last link
each link protocol provides different serviceso eg may or may not provide rdt over link
Introduction 1-4
source
application
transportnetwork
linkphysical
HtHn M
segment Ht
datagram
destination
application
transportnetwork
linkphysical
HtHnHl M
HtHn M
Ht M
M
networklink
physical
linkphysical
HtHnHl M
HtHn M
HtHn M
HtHnHl M
router
switch
Encapsulationmessage M
Ht M
Hn
frame
5 DataLink Layer 5-5
Link Layer Services framing
encapsulate datagram into frame adding header trailer
link access o channel access if shared mediumo ldquoMACrdquo addresses used in frame headers to identify
source dest bull different from IP address
reliable delivery between adjacent nodeso we learned how to do this already (chapter 3)o seldom used on low bit-error link (fiber some twisted
pair)o wireless links high error ratesbull Q why both link-level and end-end reliability
5 DataLink Layer 5-6
Link Layer Services (more)
flow control o pacing between adjacent sending and receiving
nodes
error detection o errors caused by signal attenuation noise o receiver detects presence of errors
bull signals sender for retransmission or drops frame
error correction o receiver identifies and corrects bit error(s) without
resorting to retransmission
5 DataLink Layer 5-7
Where is the link layer implemented
in each and every host link layer implemented in
ldquoadaptorrdquo (aka network interface card NIC)o Ethernet card PCMCI card
80211 cardo implements link physical
layer
attaches into hostrsquos system buses
combination of hardware software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
5 DataLink Layer 5-8
Adaptors Communicating
sending sideo encapsulates datagram
in frameo adds error checking bits
rdt flow control etc
receiving sideo looks for errors rdt flow
control etco extracts datagram passes
to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
5 DataLink Layer 5-9
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-10
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-11
Internet checksum (review)
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field valueo NO - error detectedo YES - no error detected
But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
5 DataLink Layer 5-12
Checksumming Cyclic Redundancy Check view data bits D as a binary number (actually a
polynomial with binary coefficients) choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
o ltDRgt exactly divisible by G (modulo 2) o receiver knows G divides ltDRgt by G If non-zero
remainder error detectedo can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Cyclic Redundancy Check Modulo 2 arithmetic
o addition = subtraction = XOR Each bit string represents a polynomial Example 10011011 corresponds to
A polynomial G(x) of degree r is known to both sender and receiver
Sender appends r bits (called CRC code) to the message so that the resulting polynomial can be divided evenly by G(x)
Receiver checks if the received frame (message together with CRC) is still divisible by G(x)
If not there are transmission errors in the frame
7 4 3( ) 1D x x x x x
Common polynomials for G(x)
CRC
CRC-8
CRC-10
CRC-12
CRC-16
CRC-CCITT
CRC-32
C(x)
x8+x2+x1+1
x10+x9+x5+x4+x1+1
x12+x11+x3+x2+x1+1
x16+x15+x2+1
x16+x12+x5+1
x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1
5 DataLink Layer 5-15
CRC Example
31001 represents 1x
5 DataLink Layer 5-16
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-17
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
o PPP for dial-up accesso point-to-point link between Ethernet switch and host
broadcast (shared wire or medium)o old-fashioned Etherneto upstream HFC (cable network)o 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Introduction 1-18
Cable Network Architecture Overview
home
cable headend
cable distributionnetwork (simplified)
Typically 500 to 5000 homes
5 DataLink Layer 5-19
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference o collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself o no out-of-band channel for coordination
5 DataLink Layer 5-20
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at the full rate say R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
o no special node to coordinate transmissionso no synchronization of clocks slots
4 simple
5 DataLink Layer 5-21
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
o divide channel into smaller ldquopiecesrdquo (time slots frequency code)
o allocate piece to node for exclusive use
Random Accesso channel not divided allow collisionso ldquorecoverrdquo from collisions
ldquoTaking turnsrdquoo nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-22
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-23
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-24
Random Access Protocols
When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA
5 DataLink Layer 5-25
ALOHA
When a node has a frame to send send immediately
Set a timer for a random amount of time If an ACK arrives before the timer expires
fine otherwise resend the frame
(Works like stop-and-wait with random timeout interval)
5 DataLink Layer 5-26
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-27
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
5 DataLink Layer 5-28
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
o collisions detected within short timeo colliding transmissions aborted reducing
channel wastage collision detection
o easy in wired LANs measure signal strengths compare transmitted amp received signals
o difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-29
CSMACD collision detection
5 DataLink Layer 5-30
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
o share channel efficiently and fairly at high load
o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolso efficient at low load single node can fully
utilize channelo high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-31
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concernso polling overhead o latencyo single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
o token overhead o latencyo single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-33
Summary of MAC protocols
channel partitioning by time frequency or codeo Time Division Frequency Division
random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)
hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211
taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-34
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-35
MAC Addresses and ARP
32-bit IP address o network-layer addresso used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address o function get frame from one interface to
another physically-connected interface (in same network)
o 48 bit MAC addressbull burned in NIC ROM
5 DataLink Layer 5-36
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-37
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
o can move LAN card from one LAN to another
IP hierarchical address NOT portableo address depends on IP subnet to which node is
attached
5 DataLink Layer 5-38
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgto TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Q given a nodersquos IP address how to determine its MAC address
1A-2F-BB-76-09-AD
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-39
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-
FF-FF-FF-FF-FFo all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ARP is ldquoplug-and-playrdquoo nodes create their
ARP tables without intervention from net administrator
5 DataLink Layer 5-40
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
LAN LAN
5 DataLink Layer 5-41
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-42
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-3
Link layer context
datagram transferred by different link protocols over different linkso eg Ethernet on first link frame relay on
intermediate links 80211 on last link
each link protocol provides different serviceso eg may or may not provide rdt over link
Introduction 1-4
source
application
transportnetwork
linkphysical
HtHn M
segment Ht
datagram
destination
application
transportnetwork
linkphysical
HtHnHl M
HtHn M
Ht M
M
networklink
physical
linkphysical
HtHnHl M
HtHn M
HtHn M
HtHnHl M
router
switch
Encapsulationmessage M
Ht M
Hn
frame
5 DataLink Layer 5-5
Link Layer Services framing
encapsulate datagram into frame adding header trailer
link access o channel access if shared mediumo ldquoMACrdquo addresses used in frame headers to identify
source dest bull different from IP address
reliable delivery between adjacent nodeso we learned how to do this already (chapter 3)o seldom used on low bit-error link (fiber some twisted
pair)o wireless links high error ratesbull Q why both link-level and end-end reliability
5 DataLink Layer 5-6
Link Layer Services (more)
flow control o pacing between adjacent sending and receiving
nodes
error detection o errors caused by signal attenuation noise o receiver detects presence of errors
bull signals sender for retransmission or drops frame
error correction o receiver identifies and corrects bit error(s) without
resorting to retransmission
5 DataLink Layer 5-7
Where is the link layer implemented
in each and every host link layer implemented in
ldquoadaptorrdquo (aka network interface card NIC)o Ethernet card PCMCI card
80211 cardo implements link physical
layer
attaches into hostrsquos system buses
combination of hardware software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
5 DataLink Layer 5-8
Adaptors Communicating
sending sideo encapsulates datagram
in frameo adds error checking bits
rdt flow control etc
receiving sideo looks for errors rdt flow
control etco extracts datagram passes
to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
5 DataLink Layer 5-9
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-10
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-11
Internet checksum (review)
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field valueo NO - error detectedo YES - no error detected
But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
5 DataLink Layer 5-12
Checksumming Cyclic Redundancy Check view data bits D as a binary number (actually a
polynomial with binary coefficients) choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
o ltDRgt exactly divisible by G (modulo 2) o receiver knows G divides ltDRgt by G If non-zero
remainder error detectedo can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Cyclic Redundancy Check Modulo 2 arithmetic
o addition = subtraction = XOR Each bit string represents a polynomial Example 10011011 corresponds to
A polynomial G(x) of degree r is known to both sender and receiver
Sender appends r bits (called CRC code) to the message so that the resulting polynomial can be divided evenly by G(x)
Receiver checks if the received frame (message together with CRC) is still divisible by G(x)
If not there are transmission errors in the frame
7 4 3( ) 1D x x x x x
Common polynomials for G(x)
CRC
CRC-8
CRC-10
CRC-12
CRC-16
CRC-CCITT
CRC-32
C(x)
x8+x2+x1+1
x10+x9+x5+x4+x1+1
x12+x11+x3+x2+x1+1
x16+x15+x2+1
x16+x12+x5+1
x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1
5 DataLink Layer 5-15
CRC Example
31001 represents 1x
5 DataLink Layer 5-16
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-17
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
o PPP for dial-up accesso point-to-point link between Ethernet switch and host
broadcast (shared wire or medium)o old-fashioned Etherneto upstream HFC (cable network)o 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Introduction 1-18
Cable Network Architecture Overview
home
cable headend
cable distributionnetwork (simplified)
Typically 500 to 5000 homes
5 DataLink Layer 5-19
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference o collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself o no out-of-band channel for coordination
5 DataLink Layer 5-20
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at the full rate say R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
o no special node to coordinate transmissionso no synchronization of clocks slots
4 simple
5 DataLink Layer 5-21
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
o divide channel into smaller ldquopiecesrdquo (time slots frequency code)
o allocate piece to node for exclusive use
Random Accesso channel not divided allow collisionso ldquorecoverrdquo from collisions
ldquoTaking turnsrdquoo nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-22
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-23
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-24
Random Access Protocols
When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA
5 DataLink Layer 5-25
ALOHA
When a node has a frame to send send immediately
Set a timer for a random amount of time If an ACK arrives before the timer expires
fine otherwise resend the frame
(Works like stop-and-wait with random timeout interval)
5 DataLink Layer 5-26
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-27
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
5 DataLink Layer 5-28
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
o collisions detected within short timeo colliding transmissions aborted reducing
channel wastage collision detection
o easy in wired LANs measure signal strengths compare transmitted amp received signals
o difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-29
CSMACD collision detection
5 DataLink Layer 5-30
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
o share channel efficiently and fairly at high load
o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolso efficient at low load single node can fully
utilize channelo high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-31
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concernso polling overhead o latencyo single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
o token overhead o latencyo single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-33
Summary of MAC protocols
channel partitioning by time frequency or codeo Time Division Frequency Division
random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)
hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211
taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-34
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-35
MAC Addresses and ARP
32-bit IP address o network-layer addresso used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address o function get frame from one interface to
another physically-connected interface (in same network)
o 48 bit MAC addressbull burned in NIC ROM
5 DataLink Layer 5-36
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-37
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
o can move LAN card from one LAN to another
IP hierarchical address NOT portableo address depends on IP subnet to which node is
attached
5 DataLink Layer 5-38
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgto TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Q given a nodersquos IP address how to determine its MAC address
1A-2F-BB-76-09-AD
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-39
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-
FF-FF-FF-FF-FFo all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ARP is ldquoplug-and-playrdquoo nodes create their
ARP tables without intervention from net administrator
5 DataLink Layer 5-40
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
LAN LAN
5 DataLink Layer 5-41
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-42
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
Introduction 1-4
source
application
transportnetwork
linkphysical
HtHn M
segment Ht
datagram
destination
application
transportnetwork
linkphysical
HtHnHl M
HtHn M
Ht M
M
networklink
physical
linkphysical
HtHnHl M
HtHn M
HtHn M
HtHnHl M
router
switch
Encapsulationmessage M
Ht M
Hn
frame
5 DataLink Layer 5-5
Link Layer Services framing
encapsulate datagram into frame adding header trailer
link access o channel access if shared mediumo ldquoMACrdquo addresses used in frame headers to identify
source dest bull different from IP address
reliable delivery between adjacent nodeso we learned how to do this already (chapter 3)o seldom used on low bit-error link (fiber some twisted
pair)o wireless links high error ratesbull Q why both link-level and end-end reliability
5 DataLink Layer 5-6
Link Layer Services (more)
flow control o pacing between adjacent sending and receiving
nodes
error detection o errors caused by signal attenuation noise o receiver detects presence of errors
bull signals sender for retransmission or drops frame
error correction o receiver identifies and corrects bit error(s) without
resorting to retransmission
5 DataLink Layer 5-7
Where is the link layer implemented
in each and every host link layer implemented in
ldquoadaptorrdquo (aka network interface card NIC)o Ethernet card PCMCI card
80211 cardo implements link physical
layer
attaches into hostrsquos system buses
combination of hardware software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
5 DataLink Layer 5-8
Adaptors Communicating
sending sideo encapsulates datagram
in frameo adds error checking bits
rdt flow control etc
receiving sideo looks for errors rdt flow
control etco extracts datagram passes
to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
5 DataLink Layer 5-9
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-10
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-11
Internet checksum (review)
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field valueo NO - error detectedo YES - no error detected
But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
5 DataLink Layer 5-12
Checksumming Cyclic Redundancy Check view data bits D as a binary number (actually a
polynomial with binary coefficients) choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
o ltDRgt exactly divisible by G (modulo 2) o receiver knows G divides ltDRgt by G If non-zero
remainder error detectedo can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Cyclic Redundancy Check Modulo 2 arithmetic
o addition = subtraction = XOR Each bit string represents a polynomial Example 10011011 corresponds to
A polynomial G(x) of degree r is known to both sender and receiver
Sender appends r bits (called CRC code) to the message so that the resulting polynomial can be divided evenly by G(x)
Receiver checks if the received frame (message together with CRC) is still divisible by G(x)
If not there are transmission errors in the frame
7 4 3( ) 1D x x x x x
Common polynomials for G(x)
CRC
CRC-8
CRC-10
CRC-12
CRC-16
CRC-CCITT
CRC-32
C(x)
x8+x2+x1+1
x10+x9+x5+x4+x1+1
x12+x11+x3+x2+x1+1
x16+x15+x2+1
x16+x12+x5+1
x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1
5 DataLink Layer 5-15
CRC Example
31001 represents 1x
5 DataLink Layer 5-16
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-17
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
o PPP for dial-up accesso point-to-point link between Ethernet switch and host
broadcast (shared wire or medium)o old-fashioned Etherneto upstream HFC (cable network)o 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Introduction 1-18
Cable Network Architecture Overview
home
cable headend
cable distributionnetwork (simplified)
Typically 500 to 5000 homes
5 DataLink Layer 5-19
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference o collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself o no out-of-band channel for coordination
5 DataLink Layer 5-20
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at the full rate say R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
o no special node to coordinate transmissionso no synchronization of clocks slots
4 simple
5 DataLink Layer 5-21
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
o divide channel into smaller ldquopiecesrdquo (time slots frequency code)
o allocate piece to node for exclusive use
Random Accesso channel not divided allow collisionso ldquorecoverrdquo from collisions
ldquoTaking turnsrdquoo nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-22
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-23
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-24
Random Access Protocols
When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA
5 DataLink Layer 5-25
ALOHA
When a node has a frame to send send immediately
Set a timer for a random amount of time If an ACK arrives before the timer expires
fine otherwise resend the frame
(Works like stop-and-wait with random timeout interval)
5 DataLink Layer 5-26
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-27
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
5 DataLink Layer 5-28
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
o collisions detected within short timeo colliding transmissions aborted reducing
channel wastage collision detection
o easy in wired LANs measure signal strengths compare transmitted amp received signals
o difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-29
CSMACD collision detection
5 DataLink Layer 5-30
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
o share channel efficiently and fairly at high load
o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolso efficient at low load single node can fully
utilize channelo high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-31
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concernso polling overhead o latencyo single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
o token overhead o latencyo single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-33
Summary of MAC protocols
channel partitioning by time frequency or codeo Time Division Frequency Division
random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)
hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211
taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-34
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-35
MAC Addresses and ARP
32-bit IP address o network-layer addresso used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address o function get frame from one interface to
another physically-connected interface (in same network)
o 48 bit MAC addressbull burned in NIC ROM
5 DataLink Layer 5-36
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-37
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
o can move LAN card from one LAN to another
IP hierarchical address NOT portableo address depends on IP subnet to which node is
attached
5 DataLink Layer 5-38
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgto TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Q given a nodersquos IP address how to determine its MAC address
1A-2F-BB-76-09-AD
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-39
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-
FF-FF-FF-FF-FFo all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ARP is ldquoplug-and-playrdquoo nodes create their
ARP tables without intervention from net administrator
5 DataLink Layer 5-40
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
LAN LAN
5 DataLink Layer 5-41
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-42
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-5
Link Layer Services framing
encapsulate datagram into frame adding header trailer
link access o channel access if shared mediumo ldquoMACrdquo addresses used in frame headers to identify
source dest bull different from IP address
reliable delivery between adjacent nodeso we learned how to do this already (chapter 3)o seldom used on low bit-error link (fiber some twisted
pair)o wireless links high error ratesbull Q why both link-level and end-end reliability
5 DataLink Layer 5-6
Link Layer Services (more)
flow control o pacing between adjacent sending and receiving
nodes
error detection o errors caused by signal attenuation noise o receiver detects presence of errors
bull signals sender for retransmission or drops frame
error correction o receiver identifies and corrects bit error(s) without
resorting to retransmission
5 DataLink Layer 5-7
Where is the link layer implemented
in each and every host link layer implemented in
ldquoadaptorrdquo (aka network interface card NIC)o Ethernet card PCMCI card
80211 cardo implements link physical
layer
attaches into hostrsquos system buses
combination of hardware software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
5 DataLink Layer 5-8
Adaptors Communicating
sending sideo encapsulates datagram
in frameo adds error checking bits
rdt flow control etc
receiving sideo looks for errors rdt flow
control etco extracts datagram passes
to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
5 DataLink Layer 5-9
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-10
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-11
Internet checksum (review)
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field valueo NO - error detectedo YES - no error detected
But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
5 DataLink Layer 5-12
Checksumming Cyclic Redundancy Check view data bits D as a binary number (actually a
polynomial with binary coefficients) choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
o ltDRgt exactly divisible by G (modulo 2) o receiver knows G divides ltDRgt by G If non-zero
remainder error detectedo can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Cyclic Redundancy Check Modulo 2 arithmetic
o addition = subtraction = XOR Each bit string represents a polynomial Example 10011011 corresponds to
A polynomial G(x) of degree r is known to both sender and receiver
Sender appends r bits (called CRC code) to the message so that the resulting polynomial can be divided evenly by G(x)
Receiver checks if the received frame (message together with CRC) is still divisible by G(x)
If not there are transmission errors in the frame
7 4 3( ) 1D x x x x x
Common polynomials for G(x)
CRC
CRC-8
CRC-10
CRC-12
CRC-16
CRC-CCITT
CRC-32
C(x)
x8+x2+x1+1
x10+x9+x5+x4+x1+1
x12+x11+x3+x2+x1+1
x16+x15+x2+1
x16+x12+x5+1
x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1
5 DataLink Layer 5-15
CRC Example
31001 represents 1x
5 DataLink Layer 5-16
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-17
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
o PPP for dial-up accesso point-to-point link between Ethernet switch and host
broadcast (shared wire or medium)o old-fashioned Etherneto upstream HFC (cable network)o 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Introduction 1-18
Cable Network Architecture Overview
home
cable headend
cable distributionnetwork (simplified)
Typically 500 to 5000 homes
5 DataLink Layer 5-19
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference o collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself o no out-of-band channel for coordination
5 DataLink Layer 5-20
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at the full rate say R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
o no special node to coordinate transmissionso no synchronization of clocks slots
4 simple
5 DataLink Layer 5-21
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
o divide channel into smaller ldquopiecesrdquo (time slots frequency code)
o allocate piece to node for exclusive use
Random Accesso channel not divided allow collisionso ldquorecoverrdquo from collisions
ldquoTaking turnsrdquoo nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-22
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-23
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-24
Random Access Protocols
When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA
5 DataLink Layer 5-25
ALOHA
When a node has a frame to send send immediately
Set a timer for a random amount of time If an ACK arrives before the timer expires
fine otherwise resend the frame
(Works like stop-and-wait with random timeout interval)
5 DataLink Layer 5-26
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-27
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
5 DataLink Layer 5-28
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
o collisions detected within short timeo colliding transmissions aborted reducing
channel wastage collision detection
o easy in wired LANs measure signal strengths compare transmitted amp received signals
o difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-29
CSMACD collision detection
5 DataLink Layer 5-30
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
o share channel efficiently and fairly at high load
o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolso efficient at low load single node can fully
utilize channelo high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-31
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concernso polling overhead o latencyo single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
o token overhead o latencyo single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-33
Summary of MAC protocols
channel partitioning by time frequency or codeo Time Division Frequency Division
random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)
hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211
taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-34
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-35
MAC Addresses and ARP
32-bit IP address o network-layer addresso used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address o function get frame from one interface to
another physically-connected interface (in same network)
o 48 bit MAC addressbull burned in NIC ROM
5 DataLink Layer 5-36
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-37
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
o can move LAN card from one LAN to another
IP hierarchical address NOT portableo address depends on IP subnet to which node is
attached
5 DataLink Layer 5-38
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgto TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Q given a nodersquos IP address how to determine its MAC address
1A-2F-BB-76-09-AD
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-39
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-
FF-FF-FF-FF-FFo all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ARP is ldquoplug-and-playrdquoo nodes create their
ARP tables without intervention from net administrator
5 DataLink Layer 5-40
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
LAN LAN
5 DataLink Layer 5-41
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-42
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-6
Link Layer Services (more)
flow control o pacing between adjacent sending and receiving
nodes
error detection o errors caused by signal attenuation noise o receiver detects presence of errors
bull signals sender for retransmission or drops frame
error correction o receiver identifies and corrects bit error(s) without
resorting to retransmission
5 DataLink Layer 5-7
Where is the link layer implemented
in each and every host link layer implemented in
ldquoadaptorrdquo (aka network interface card NIC)o Ethernet card PCMCI card
80211 cardo implements link physical
layer
attaches into hostrsquos system buses
combination of hardware software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
5 DataLink Layer 5-8
Adaptors Communicating
sending sideo encapsulates datagram
in frameo adds error checking bits
rdt flow control etc
receiving sideo looks for errors rdt flow
control etco extracts datagram passes
to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
5 DataLink Layer 5-9
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-10
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-11
Internet checksum (review)
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field valueo NO - error detectedo YES - no error detected
But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
5 DataLink Layer 5-12
Checksumming Cyclic Redundancy Check view data bits D as a binary number (actually a
polynomial with binary coefficients) choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
o ltDRgt exactly divisible by G (modulo 2) o receiver knows G divides ltDRgt by G If non-zero
remainder error detectedo can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Cyclic Redundancy Check Modulo 2 arithmetic
o addition = subtraction = XOR Each bit string represents a polynomial Example 10011011 corresponds to
A polynomial G(x) of degree r is known to both sender and receiver
Sender appends r bits (called CRC code) to the message so that the resulting polynomial can be divided evenly by G(x)
Receiver checks if the received frame (message together with CRC) is still divisible by G(x)
If not there are transmission errors in the frame
7 4 3( ) 1D x x x x x
Common polynomials for G(x)
CRC
CRC-8
CRC-10
CRC-12
CRC-16
CRC-CCITT
CRC-32
C(x)
x8+x2+x1+1
x10+x9+x5+x4+x1+1
x12+x11+x3+x2+x1+1
x16+x15+x2+1
x16+x12+x5+1
x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1
5 DataLink Layer 5-15
CRC Example
31001 represents 1x
5 DataLink Layer 5-16
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-17
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
o PPP for dial-up accesso point-to-point link between Ethernet switch and host
broadcast (shared wire or medium)o old-fashioned Etherneto upstream HFC (cable network)o 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Introduction 1-18
Cable Network Architecture Overview
home
cable headend
cable distributionnetwork (simplified)
Typically 500 to 5000 homes
5 DataLink Layer 5-19
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference o collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself o no out-of-band channel for coordination
5 DataLink Layer 5-20
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at the full rate say R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
o no special node to coordinate transmissionso no synchronization of clocks slots
4 simple
5 DataLink Layer 5-21
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
o divide channel into smaller ldquopiecesrdquo (time slots frequency code)
o allocate piece to node for exclusive use
Random Accesso channel not divided allow collisionso ldquorecoverrdquo from collisions
ldquoTaking turnsrdquoo nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-22
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-23
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-24
Random Access Protocols
When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA
5 DataLink Layer 5-25
ALOHA
When a node has a frame to send send immediately
Set a timer for a random amount of time If an ACK arrives before the timer expires
fine otherwise resend the frame
(Works like stop-and-wait with random timeout interval)
5 DataLink Layer 5-26
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-27
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
5 DataLink Layer 5-28
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
o collisions detected within short timeo colliding transmissions aborted reducing
channel wastage collision detection
o easy in wired LANs measure signal strengths compare transmitted amp received signals
o difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-29
CSMACD collision detection
5 DataLink Layer 5-30
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
o share channel efficiently and fairly at high load
o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolso efficient at low load single node can fully
utilize channelo high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-31
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concernso polling overhead o latencyo single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
o token overhead o latencyo single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-33
Summary of MAC protocols
channel partitioning by time frequency or codeo Time Division Frequency Division
random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)
hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211
taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-34
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-35
MAC Addresses and ARP
32-bit IP address o network-layer addresso used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address o function get frame from one interface to
another physically-connected interface (in same network)
o 48 bit MAC addressbull burned in NIC ROM
5 DataLink Layer 5-36
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-37
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
o can move LAN card from one LAN to another
IP hierarchical address NOT portableo address depends on IP subnet to which node is
attached
5 DataLink Layer 5-38
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgto TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Q given a nodersquos IP address how to determine its MAC address
1A-2F-BB-76-09-AD
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-39
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-
FF-FF-FF-FF-FFo all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ARP is ldquoplug-and-playrdquoo nodes create their
ARP tables without intervention from net administrator
5 DataLink Layer 5-40
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
LAN LAN
5 DataLink Layer 5-41
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-42
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-7
Where is the link layer implemented
in each and every host link layer implemented in
ldquoadaptorrdquo (aka network interface card NIC)o Ethernet card PCMCI card
80211 cardo implements link physical
layer
attaches into hostrsquos system buses
combination of hardware software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
5 DataLink Layer 5-8
Adaptors Communicating
sending sideo encapsulates datagram
in frameo adds error checking bits
rdt flow control etc
receiving sideo looks for errors rdt flow
control etco extracts datagram passes
to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
5 DataLink Layer 5-9
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-10
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-11
Internet checksum (review)
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field valueo NO - error detectedo YES - no error detected
But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
5 DataLink Layer 5-12
Checksumming Cyclic Redundancy Check view data bits D as a binary number (actually a
polynomial with binary coefficients) choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
o ltDRgt exactly divisible by G (modulo 2) o receiver knows G divides ltDRgt by G If non-zero
remainder error detectedo can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Cyclic Redundancy Check Modulo 2 arithmetic
o addition = subtraction = XOR Each bit string represents a polynomial Example 10011011 corresponds to
A polynomial G(x) of degree r is known to both sender and receiver
Sender appends r bits (called CRC code) to the message so that the resulting polynomial can be divided evenly by G(x)
Receiver checks if the received frame (message together with CRC) is still divisible by G(x)
If not there are transmission errors in the frame
7 4 3( ) 1D x x x x x
Common polynomials for G(x)
CRC
CRC-8
CRC-10
CRC-12
CRC-16
CRC-CCITT
CRC-32
C(x)
x8+x2+x1+1
x10+x9+x5+x4+x1+1
x12+x11+x3+x2+x1+1
x16+x15+x2+1
x16+x12+x5+1
x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1
5 DataLink Layer 5-15
CRC Example
31001 represents 1x
5 DataLink Layer 5-16
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-17
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
o PPP for dial-up accesso point-to-point link between Ethernet switch and host
broadcast (shared wire or medium)o old-fashioned Etherneto upstream HFC (cable network)o 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Introduction 1-18
Cable Network Architecture Overview
home
cable headend
cable distributionnetwork (simplified)
Typically 500 to 5000 homes
5 DataLink Layer 5-19
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference o collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself o no out-of-band channel for coordination
5 DataLink Layer 5-20
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at the full rate say R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
o no special node to coordinate transmissionso no synchronization of clocks slots
4 simple
5 DataLink Layer 5-21
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
o divide channel into smaller ldquopiecesrdquo (time slots frequency code)
o allocate piece to node for exclusive use
Random Accesso channel not divided allow collisionso ldquorecoverrdquo from collisions
ldquoTaking turnsrdquoo nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-22
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-23
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-24
Random Access Protocols
When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA
5 DataLink Layer 5-25
ALOHA
When a node has a frame to send send immediately
Set a timer for a random amount of time If an ACK arrives before the timer expires
fine otherwise resend the frame
(Works like stop-and-wait with random timeout interval)
5 DataLink Layer 5-26
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-27
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
5 DataLink Layer 5-28
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
o collisions detected within short timeo colliding transmissions aborted reducing
channel wastage collision detection
o easy in wired LANs measure signal strengths compare transmitted amp received signals
o difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-29
CSMACD collision detection
5 DataLink Layer 5-30
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
o share channel efficiently and fairly at high load
o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolso efficient at low load single node can fully
utilize channelo high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-31
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concernso polling overhead o latencyo single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
o token overhead o latencyo single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-33
Summary of MAC protocols
channel partitioning by time frequency or codeo Time Division Frequency Division
random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)
hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211
taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-34
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-35
MAC Addresses and ARP
32-bit IP address o network-layer addresso used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address o function get frame from one interface to
another physically-connected interface (in same network)
o 48 bit MAC addressbull burned in NIC ROM
5 DataLink Layer 5-36
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-37
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
o can move LAN card from one LAN to another
IP hierarchical address NOT portableo address depends on IP subnet to which node is
attached
5 DataLink Layer 5-38
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgto TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Q given a nodersquos IP address how to determine its MAC address
1A-2F-BB-76-09-AD
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-39
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-
FF-FF-FF-FF-FFo all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ARP is ldquoplug-and-playrdquoo nodes create their
ARP tables without intervention from net administrator
5 DataLink Layer 5-40
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
LAN LAN
5 DataLink Layer 5-41
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-42
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-8
Adaptors Communicating
sending sideo encapsulates datagram
in frameo adds error checking bits
rdt flow control etc
receiving sideo looks for errors rdt flow
control etco extracts datagram passes
to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
5 DataLink Layer 5-9
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-10
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-11
Internet checksum (review)
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field valueo NO - error detectedo YES - no error detected
But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
5 DataLink Layer 5-12
Checksumming Cyclic Redundancy Check view data bits D as a binary number (actually a
polynomial with binary coefficients) choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
o ltDRgt exactly divisible by G (modulo 2) o receiver knows G divides ltDRgt by G If non-zero
remainder error detectedo can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Cyclic Redundancy Check Modulo 2 arithmetic
o addition = subtraction = XOR Each bit string represents a polynomial Example 10011011 corresponds to
A polynomial G(x) of degree r is known to both sender and receiver
Sender appends r bits (called CRC code) to the message so that the resulting polynomial can be divided evenly by G(x)
Receiver checks if the received frame (message together with CRC) is still divisible by G(x)
If not there are transmission errors in the frame
7 4 3( ) 1D x x x x x
Common polynomials for G(x)
CRC
CRC-8
CRC-10
CRC-12
CRC-16
CRC-CCITT
CRC-32
C(x)
x8+x2+x1+1
x10+x9+x5+x4+x1+1
x12+x11+x3+x2+x1+1
x16+x15+x2+1
x16+x12+x5+1
x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1
5 DataLink Layer 5-15
CRC Example
31001 represents 1x
5 DataLink Layer 5-16
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-17
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
o PPP for dial-up accesso point-to-point link between Ethernet switch and host
broadcast (shared wire or medium)o old-fashioned Etherneto upstream HFC (cable network)o 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Introduction 1-18
Cable Network Architecture Overview
home
cable headend
cable distributionnetwork (simplified)
Typically 500 to 5000 homes
5 DataLink Layer 5-19
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference o collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself o no out-of-band channel for coordination
5 DataLink Layer 5-20
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at the full rate say R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
o no special node to coordinate transmissionso no synchronization of clocks slots
4 simple
5 DataLink Layer 5-21
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
o divide channel into smaller ldquopiecesrdquo (time slots frequency code)
o allocate piece to node for exclusive use
Random Accesso channel not divided allow collisionso ldquorecoverrdquo from collisions
ldquoTaking turnsrdquoo nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-22
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-23
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-24
Random Access Protocols
When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA
5 DataLink Layer 5-25
ALOHA
When a node has a frame to send send immediately
Set a timer for a random amount of time If an ACK arrives before the timer expires
fine otherwise resend the frame
(Works like stop-and-wait with random timeout interval)
5 DataLink Layer 5-26
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-27
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
5 DataLink Layer 5-28
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
o collisions detected within short timeo colliding transmissions aborted reducing
channel wastage collision detection
o easy in wired LANs measure signal strengths compare transmitted amp received signals
o difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-29
CSMACD collision detection
5 DataLink Layer 5-30
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
o share channel efficiently and fairly at high load
o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolso efficient at low load single node can fully
utilize channelo high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-31
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concernso polling overhead o latencyo single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
o token overhead o latencyo single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-33
Summary of MAC protocols
channel partitioning by time frequency or codeo Time Division Frequency Division
random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)
hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211
taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-34
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-35
MAC Addresses and ARP
32-bit IP address o network-layer addresso used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address o function get frame from one interface to
another physically-connected interface (in same network)
o 48 bit MAC addressbull burned in NIC ROM
5 DataLink Layer 5-36
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-37
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
o can move LAN card from one LAN to another
IP hierarchical address NOT portableo address depends on IP subnet to which node is
attached
5 DataLink Layer 5-38
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgto TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Q given a nodersquos IP address how to determine its MAC address
1A-2F-BB-76-09-AD
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-39
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-
FF-FF-FF-FF-FFo all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ARP is ldquoplug-and-playrdquoo nodes create their
ARP tables without intervention from net administrator
5 DataLink Layer 5-40
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
LAN LAN
5 DataLink Layer 5-41
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-42
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-9
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-10
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-11
Internet checksum (review)
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field valueo NO - error detectedo YES - no error detected
But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
5 DataLink Layer 5-12
Checksumming Cyclic Redundancy Check view data bits D as a binary number (actually a
polynomial with binary coefficients) choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
o ltDRgt exactly divisible by G (modulo 2) o receiver knows G divides ltDRgt by G If non-zero
remainder error detectedo can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Cyclic Redundancy Check Modulo 2 arithmetic
o addition = subtraction = XOR Each bit string represents a polynomial Example 10011011 corresponds to
A polynomial G(x) of degree r is known to both sender and receiver
Sender appends r bits (called CRC code) to the message so that the resulting polynomial can be divided evenly by G(x)
Receiver checks if the received frame (message together with CRC) is still divisible by G(x)
If not there are transmission errors in the frame
7 4 3( ) 1D x x x x x
Common polynomials for G(x)
CRC
CRC-8
CRC-10
CRC-12
CRC-16
CRC-CCITT
CRC-32
C(x)
x8+x2+x1+1
x10+x9+x5+x4+x1+1
x12+x11+x3+x2+x1+1
x16+x15+x2+1
x16+x12+x5+1
x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1
5 DataLink Layer 5-15
CRC Example
31001 represents 1x
5 DataLink Layer 5-16
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-17
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
o PPP for dial-up accesso point-to-point link between Ethernet switch and host
broadcast (shared wire or medium)o old-fashioned Etherneto upstream HFC (cable network)o 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Introduction 1-18
Cable Network Architecture Overview
home
cable headend
cable distributionnetwork (simplified)
Typically 500 to 5000 homes
5 DataLink Layer 5-19
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference o collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself o no out-of-band channel for coordination
5 DataLink Layer 5-20
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at the full rate say R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
o no special node to coordinate transmissionso no synchronization of clocks slots
4 simple
5 DataLink Layer 5-21
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
o divide channel into smaller ldquopiecesrdquo (time slots frequency code)
o allocate piece to node for exclusive use
Random Accesso channel not divided allow collisionso ldquorecoverrdquo from collisions
ldquoTaking turnsrdquoo nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-22
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-23
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-24
Random Access Protocols
When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA
5 DataLink Layer 5-25
ALOHA
When a node has a frame to send send immediately
Set a timer for a random amount of time If an ACK arrives before the timer expires
fine otherwise resend the frame
(Works like stop-and-wait with random timeout interval)
5 DataLink Layer 5-26
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-27
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
5 DataLink Layer 5-28
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
o collisions detected within short timeo colliding transmissions aborted reducing
channel wastage collision detection
o easy in wired LANs measure signal strengths compare transmitted amp received signals
o difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-29
CSMACD collision detection
5 DataLink Layer 5-30
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
o share channel efficiently and fairly at high load
o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolso efficient at low load single node can fully
utilize channelo high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-31
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concernso polling overhead o latencyo single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
o token overhead o latencyo single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-33
Summary of MAC protocols
channel partitioning by time frequency or codeo Time Division Frequency Division
random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)
hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211
taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-34
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-35
MAC Addresses and ARP
32-bit IP address o network-layer addresso used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address o function get frame from one interface to
another physically-connected interface (in same network)
o 48 bit MAC addressbull burned in NIC ROM
5 DataLink Layer 5-36
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-37
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
o can move LAN card from one LAN to another
IP hierarchical address NOT portableo address depends on IP subnet to which node is
attached
5 DataLink Layer 5-38
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgto TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Q given a nodersquos IP address how to determine its MAC address
1A-2F-BB-76-09-AD
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-39
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-
FF-FF-FF-FF-FFo all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ARP is ldquoplug-and-playrdquoo nodes create their
ARP tables without intervention from net administrator
5 DataLink Layer 5-40
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
LAN LAN
5 DataLink Layer 5-41
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-42
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-10
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-11
Internet checksum (review)
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field valueo NO - error detectedo YES - no error detected
But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
5 DataLink Layer 5-12
Checksumming Cyclic Redundancy Check view data bits D as a binary number (actually a
polynomial with binary coefficients) choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
o ltDRgt exactly divisible by G (modulo 2) o receiver knows G divides ltDRgt by G If non-zero
remainder error detectedo can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Cyclic Redundancy Check Modulo 2 arithmetic
o addition = subtraction = XOR Each bit string represents a polynomial Example 10011011 corresponds to
A polynomial G(x) of degree r is known to both sender and receiver
Sender appends r bits (called CRC code) to the message so that the resulting polynomial can be divided evenly by G(x)
Receiver checks if the received frame (message together with CRC) is still divisible by G(x)
If not there are transmission errors in the frame
7 4 3( ) 1D x x x x x
Common polynomials for G(x)
CRC
CRC-8
CRC-10
CRC-12
CRC-16
CRC-CCITT
CRC-32
C(x)
x8+x2+x1+1
x10+x9+x5+x4+x1+1
x12+x11+x3+x2+x1+1
x16+x15+x2+1
x16+x12+x5+1
x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1
5 DataLink Layer 5-15
CRC Example
31001 represents 1x
5 DataLink Layer 5-16
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-17
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
o PPP for dial-up accesso point-to-point link between Ethernet switch and host
broadcast (shared wire or medium)o old-fashioned Etherneto upstream HFC (cable network)o 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Introduction 1-18
Cable Network Architecture Overview
home
cable headend
cable distributionnetwork (simplified)
Typically 500 to 5000 homes
5 DataLink Layer 5-19
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference o collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself o no out-of-band channel for coordination
5 DataLink Layer 5-20
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at the full rate say R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
o no special node to coordinate transmissionso no synchronization of clocks slots
4 simple
5 DataLink Layer 5-21
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
o divide channel into smaller ldquopiecesrdquo (time slots frequency code)
o allocate piece to node for exclusive use
Random Accesso channel not divided allow collisionso ldquorecoverrdquo from collisions
ldquoTaking turnsrdquoo nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-22
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-23
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-24
Random Access Protocols
When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA
5 DataLink Layer 5-25
ALOHA
When a node has a frame to send send immediately
Set a timer for a random amount of time If an ACK arrives before the timer expires
fine otherwise resend the frame
(Works like stop-and-wait with random timeout interval)
5 DataLink Layer 5-26
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-27
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
5 DataLink Layer 5-28
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
o collisions detected within short timeo colliding transmissions aborted reducing
channel wastage collision detection
o easy in wired LANs measure signal strengths compare transmitted amp received signals
o difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-29
CSMACD collision detection
5 DataLink Layer 5-30
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
o share channel efficiently and fairly at high load
o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolso efficient at low load single node can fully
utilize channelo high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-31
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concernso polling overhead o latencyo single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
o token overhead o latencyo single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-33
Summary of MAC protocols
channel partitioning by time frequency or codeo Time Division Frequency Division
random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)
hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211
taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-34
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-35
MAC Addresses and ARP
32-bit IP address o network-layer addresso used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address o function get frame from one interface to
another physically-connected interface (in same network)
o 48 bit MAC addressbull burned in NIC ROM
5 DataLink Layer 5-36
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-37
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
o can move LAN card from one LAN to another
IP hierarchical address NOT portableo address depends on IP subnet to which node is
attached
5 DataLink Layer 5-38
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgto TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Q given a nodersquos IP address how to determine its MAC address
1A-2F-BB-76-09-AD
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-39
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-
FF-FF-FF-FF-FFo all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ARP is ldquoplug-and-playrdquoo nodes create their
ARP tables without intervention from net administrator
5 DataLink Layer 5-40
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
LAN LAN
5 DataLink Layer 5-41
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-42
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-11
Internet checksum (review)
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field valueo NO - error detectedo YES - no error detected
But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
5 DataLink Layer 5-12
Checksumming Cyclic Redundancy Check view data bits D as a binary number (actually a
polynomial with binary coefficients) choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
o ltDRgt exactly divisible by G (modulo 2) o receiver knows G divides ltDRgt by G If non-zero
remainder error detectedo can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Cyclic Redundancy Check Modulo 2 arithmetic
o addition = subtraction = XOR Each bit string represents a polynomial Example 10011011 corresponds to
A polynomial G(x) of degree r is known to both sender and receiver
Sender appends r bits (called CRC code) to the message so that the resulting polynomial can be divided evenly by G(x)
Receiver checks if the received frame (message together with CRC) is still divisible by G(x)
If not there are transmission errors in the frame
7 4 3( ) 1D x x x x x
Common polynomials for G(x)
CRC
CRC-8
CRC-10
CRC-12
CRC-16
CRC-CCITT
CRC-32
C(x)
x8+x2+x1+1
x10+x9+x5+x4+x1+1
x12+x11+x3+x2+x1+1
x16+x15+x2+1
x16+x12+x5+1
x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1
5 DataLink Layer 5-15
CRC Example
31001 represents 1x
5 DataLink Layer 5-16
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-17
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
o PPP for dial-up accesso point-to-point link between Ethernet switch and host
broadcast (shared wire or medium)o old-fashioned Etherneto upstream HFC (cable network)o 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Introduction 1-18
Cable Network Architecture Overview
home
cable headend
cable distributionnetwork (simplified)
Typically 500 to 5000 homes
5 DataLink Layer 5-19
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference o collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself o no out-of-band channel for coordination
5 DataLink Layer 5-20
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at the full rate say R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
o no special node to coordinate transmissionso no synchronization of clocks slots
4 simple
5 DataLink Layer 5-21
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
o divide channel into smaller ldquopiecesrdquo (time slots frequency code)
o allocate piece to node for exclusive use
Random Accesso channel not divided allow collisionso ldquorecoverrdquo from collisions
ldquoTaking turnsrdquoo nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-22
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-23
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-24
Random Access Protocols
When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA
5 DataLink Layer 5-25
ALOHA
When a node has a frame to send send immediately
Set a timer for a random amount of time If an ACK arrives before the timer expires
fine otherwise resend the frame
(Works like stop-and-wait with random timeout interval)
5 DataLink Layer 5-26
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-27
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
5 DataLink Layer 5-28
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
o collisions detected within short timeo colliding transmissions aborted reducing
channel wastage collision detection
o easy in wired LANs measure signal strengths compare transmitted amp received signals
o difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-29
CSMACD collision detection
5 DataLink Layer 5-30
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
o share channel efficiently and fairly at high load
o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolso efficient at low load single node can fully
utilize channelo high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-31
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concernso polling overhead o latencyo single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
o token overhead o latencyo single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-33
Summary of MAC protocols
channel partitioning by time frequency or codeo Time Division Frequency Division
random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)
hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211
taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-34
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-35
MAC Addresses and ARP
32-bit IP address o network-layer addresso used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address o function get frame from one interface to
another physically-connected interface (in same network)
o 48 bit MAC addressbull burned in NIC ROM
5 DataLink Layer 5-36
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-37
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
o can move LAN card from one LAN to another
IP hierarchical address NOT portableo address depends on IP subnet to which node is
attached
5 DataLink Layer 5-38
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgto TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Q given a nodersquos IP address how to determine its MAC address
1A-2F-BB-76-09-AD
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-39
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-
FF-FF-FF-FF-FFo all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ARP is ldquoplug-and-playrdquoo nodes create their
ARP tables without intervention from net administrator
5 DataLink Layer 5-40
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
LAN LAN
5 DataLink Layer 5-41
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-42
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-12
Checksumming Cyclic Redundancy Check view data bits D as a binary number (actually a
polynomial with binary coefficients) choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
o ltDRgt exactly divisible by G (modulo 2) o receiver knows G divides ltDRgt by G If non-zero
remainder error detectedo can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Cyclic Redundancy Check Modulo 2 arithmetic
o addition = subtraction = XOR Each bit string represents a polynomial Example 10011011 corresponds to
A polynomial G(x) of degree r is known to both sender and receiver
Sender appends r bits (called CRC code) to the message so that the resulting polynomial can be divided evenly by G(x)
Receiver checks if the received frame (message together with CRC) is still divisible by G(x)
If not there are transmission errors in the frame
7 4 3( ) 1D x x x x x
Common polynomials for G(x)
CRC
CRC-8
CRC-10
CRC-12
CRC-16
CRC-CCITT
CRC-32
C(x)
x8+x2+x1+1
x10+x9+x5+x4+x1+1
x12+x11+x3+x2+x1+1
x16+x15+x2+1
x16+x12+x5+1
x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1
5 DataLink Layer 5-15
CRC Example
31001 represents 1x
5 DataLink Layer 5-16
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-17
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
o PPP for dial-up accesso point-to-point link between Ethernet switch and host
broadcast (shared wire or medium)o old-fashioned Etherneto upstream HFC (cable network)o 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Introduction 1-18
Cable Network Architecture Overview
home
cable headend
cable distributionnetwork (simplified)
Typically 500 to 5000 homes
5 DataLink Layer 5-19
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference o collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself o no out-of-band channel for coordination
5 DataLink Layer 5-20
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at the full rate say R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
o no special node to coordinate transmissionso no synchronization of clocks slots
4 simple
5 DataLink Layer 5-21
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
o divide channel into smaller ldquopiecesrdquo (time slots frequency code)
o allocate piece to node for exclusive use
Random Accesso channel not divided allow collisionso ldquorecoverrdquo from collisions
ldquoTaking turnsrdquoo nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-22
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-23
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-24
Random Access Protocols
When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA
5 DataLink Layer 5-25
ALOHA
When a node has a frame to send send immediately
Set a timer for a random amount of time If an ACK arrives before the timer expires
fine otherwise resend the frame
(Works like stop-and-wait with random timeout interval)
5 DataLink Layer 5-26
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-27
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
5 DataLink Layer 5-28
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
o collisions detected within short timeo colliding transmissions aborted reducing
channel wastage collision detection
o easy in wired LANs measure signal strengths compare transmitted amp received signals
o difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-29
CSMACD collision detection
5 DataLink Layer 5-30
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
o share channel efficiently and fairly at high load
o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolso efficient at low load single node can fully
utilize channelo high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-31
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concernso polling overhead o latencyo single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
o token overhead o latencyo single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-33
Summary of MAC protocols
channel partitioning by time frequency or codeo Time Division Frequency Division
random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)
hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211
taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-34
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-35
MAC Addresses and ARP
32-bit IP address o network-layer addresso used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address o function get frame from one interface to
another physically-connected interface (in same network)
o 48 bit MAC addressbull burned in NIC ROM
5 DataLink Layer 5-36
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-37
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
o can move LAN card from one LAN to another
IP hierarchical address NOT portableo address depends on IP subnet to which node is
attached
5 DataLink Layer 5-38
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgto TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Q given a nodersquos IP address how to determine its MAC address
1A-2F-BB-76-09-AD
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-39
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-
FF-FF-FF-FF-FFo all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ARP is ldquoplug-and-playrdquoo nodes create their
ARP tables without intervention from net administrator
5 DataLink Layer 5-40
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
LAN LAN
5 DataLink Layer 5-41
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-42
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
Cyclic Redundancy Check Modulo 2 arithmetic
o addition = subtraction = XOR Each bit string represents a polynomial Example 10011011 corresponds to
A polynomial G(x) of degree r is known to both sender and receiver
Sender appends r bits (called CRC code) to the message so that the resulting polynomial can be divided evenly by G(x)
Receiver checks if the received frame (message together with CRC) is still divisible by G(x)
If not there are transmission errors in the frame
7 4 3( ) 1D x x x x x
Common polynomials for G(x)
CRC
CRC-8
CRC-10
CRC-12
CRC-16
CRC-CCITT
CRC-32
C(x)
x8+x2+x1+1
x10+x9+x5+x4+x1+1
x12+x11+x3+x2+x1+1
x16+x15+x2+1
x16+x12+x5+1
x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1
5 DataLink Layer 5-15
CRC Example
31001 represents 1x
5 DataLink Layer 5-16
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-17
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
o PPP for dial-up accesso point-to-point link between Ethernet switch and host
broadcast (shared wire or medium)o old-fashioned Etherneto upstream HFC (cable network)o 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Introduction 1-18
Cable Network Architecture Overview
home
cable headend
cable distributionnetwork (simplified)
Typically 500 to 5000 homes
5 DataLink Layer 5-19
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference o collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself o no out-of-band channel for coordination
5 DataLink Layer 5-20
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at the full rate say R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
o no special node to coordinate transmissionso no synchronization of clocks slots
4 simple
5 DataLink Layer 5-21
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
o divide channel into smaller ldquopiecesrdquo (time slots frequency code)
o allocate piece to node for exclusive use
Random Accesso channel not divided allow collisionso ldquorecoverrdquo from collisions
ldquoTaking turnsrdquoo nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-22
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-23
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-24
Random Access Protocols
When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA
5 DataLink Layer 5-25
ALOHA
When a node has a frame to send send immediately
Set a timer for a random amount of time If an ACK arrives before the timer expires
fine otherwise resend the frame
(Works like stop-and-wait with random timeout interval)
5 DataLink Layer 5-26
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-27
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
5 DataLink Layer 5-28
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
o collisions detected within short timeo colliding transmissions aborted reducing
channel wastage collision detection
o easy in wired LANs measure signal strengths compare transmitted amp received signals
o difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-29
CSMACD collision detection
5 DataLink Layer 5-30
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
o share channel efficiently and fairly at high load
o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolso efficient at low load single node can fully
utilize channelo high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-31
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concernso polling overhead o latencyo single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
o token overhead o latencyo single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-33
Summary of MAC protocols
channel partitioning by time frequency or codeo Time Division Frequency Division
random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)
hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211
taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-34
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-35
MAC Addresses and ARP
32-bit IP address o network-layer addresso used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address o function get frame from one interface to
another physically-connected interface (in same network)
o 48 bit MAC addressbull burned in NIC ROM
5 DataLink Layer 5-36
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-37
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
o can move LAN card from one LAN to another
IP hierarchical address NOT portableo address depends on IP subnet to which node is
attached
5 DataLink Layer 5-38
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgto TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Q given a nodersquos IP address how to determine its MAC address
1A-2F-BB-76-09-AD
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-39
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-
FF-FF-FF-FF-FFo all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ARP is ldquoplug-and-playrdquoo nodes create their
ARP tables without intervention from net administrator
5 DataLink Layer 5-40
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
LAN LAN
5 DataLink Layer 5-41
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-42
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
Common polynomials for G(x)
CRC
CRC-8
CRC-10
CRC-12
CRC-16
CRC-CCITT
CRC-32
C(x)
x8+x2+x1+1
x10+x9+x5+x4+x1+1
x12+x11+x3+x2+x1+1
x16+x15+x2+1
x16+x12+x5+1
x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1
5 DataLink Layer 5-15
CRC Example
31001 represents 1x
5 DataLink Layer 5-16
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-17
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
o PPP for dial-up accesso point-to-point link between Ethernet switch and host
broadcast (shared wire or medium)o old-fashioned Etherneto upstream HFC (cable network)o 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Introduction 1-18
Cable Network Architecture Overview
home
cable headend
cable distributionnetwork (simplified)
Typically 500 to 5000 homes
5 DataLink Layer 5-19
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference o collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself o no out-of-band channel for coordination
5 DataLink Layer 5-20
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at the full rate say R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
o no special node to coordinate transmissionso no synchronization of clocks slots
4 simple
5 DataLink Layer 5-21
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
o divide channel into smaller ldquopiecesrdquo (time slots frequency code)
o allocate piece to node for exclusive use
Random Accesso channel not divided allow collisionso ldquorecoverrdquo from collisions
ldquoTaking turnsrdquoo nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-22
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-23
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-24
Random Access Protocols
When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA
5 DataLink Layer 5-25
ALOHA
When a node has a frame to send send immediately
Set a timer for a random amount of time If an ACK arrives before the timer expires
fine otherwise resend the frame
(Works like stop-and-wait with random timeout interval)
5 DataLink Layer 5-26
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-27
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
5 DataLink Layer 5-28
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
o collisions detected within short timeo colliding transmissions aborted reducing
channel wastage collision detection
o easy in wired LANs measure signal strengths compare transmitted amp received signals
o difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-29
CSMACD collision detection
5 DataLink Layer 5-30
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
o share channel efficiently and fairly at high load
o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolso efficient at low load single node can fully
utilize channelo high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-31
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concernso polling overhead o latencyo single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
o token overhead o latencyo single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-33
Summary of MAC protocols
channel partitioning by time frequency or codeo Time Division Frequency Division
random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)
hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211
taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-34
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-35
MAC Addresses and ARP
32-bit IP address o network-layer addresso used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address o function get frame from one interface to
another physically-connected interface (in same network)
o 48 bit MAC addressbull burned in NIC ROM
5 DataLink Layer 5-36
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-37
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
o can move LAN card from one LAN to another
IP hierarchical address NOT portableo address depends on IP subnet to which node is
attached
5 DataLink Layer 5-38
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgto TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Q given a nodersquos IP address how to determine its MAC address
1A-2F-BB-76-09-AD
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-39
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-
FF-FF-FF-FF-FFo all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ARP is ldquoplug-and-playrdquoo nodes create their
ARP tables without intervention from net administrator
5 DataLink Layer 5-40
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
LAN LAN
5 DataLink Layer 5-41
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-42
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-15
CRC Example
31001 represents 1x
5 DataLink Layer 5-16
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-17
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
o PPP for dial-up accesso point-to-point link between Ethernet switch and host
broadcast (shared wire or medium)o old-fashioned Etherneto upstream HFC (cable network)o 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Introduction 1-18
Cable Network Architecture Overview
home
cable headend
cable distributionnetwork (simplified)
Typically 500 to 5000 homes
5 DataLink Layer 5-19
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference o collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself o no out-of-band channel for coordination
5 DataLink Layer 5-20
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at the full rate say R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
o no special node to coordinate transmissionso no synchronization of clocks slots
4 simple
5 DataLink Layer 5-21
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
o divide channel into smaller ldquopiecesrdquo (time slots frequency code)
o allocate piece to node for exclusive use
Random Accesso channel not divided allow collisionso ldquorecoverrdquo from collisions
ldquoTaking turnsrdquoo nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-22
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-23
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-24
Random Access Protocols
When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA
5 DataLink Layer 5-25
ALOHA
When a node has a frame to send send immediately
Set a timer for a random amount of time If an ACK arrives before the timer expires
fine otherwise resend the frame
(Works like stop-and-wait with random timeout interval)
5 DataLink Layer 5-26
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-27
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
5 DataLink Layer 5-28
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
o collisions detected within short timeo colliding transmissions aborted reducing
channel wastage collision detection
o easy in wired LANs measure signal strengths compare transmitted amp received signals
o difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-29
CSMACD collision detection
5 DataLink Layer 5-30
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
o share channel efficiently and fairly at high load
o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolso efficient at low load single node can fully
utilize channelo high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-31
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concernso polling overhead o latencyo single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
o token overhead o latencyo single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-33
Summary of MAC protocols
channel partitioning by time frequency or codeo Time Division Frequency Division
random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)
hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211
taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-34
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-35
MAC Addresses and ARP
32-bit IP address o network-layer addresso used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address o function get frame from one interface to
another physically-connected interface (in same network)
o 48 bit MAC addressbull burned in NIC ROM
5 DataLink Layer 5-36
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-37
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
o can move LAN card from one LAN to another
IP hierarchical address NOT portableo address depends on IP subnet to which node is
attached
5 DataLink Layer 5-38
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgto TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Q given a nodersquos IP address how to determine its MAC address
1A-2F-BB-76-09-AD
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-39
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-
FF-FF-FF-FF-FFo all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ARP is ldquoplug-and-playrdquoo nodes create their
ARP tables without intervention from net administrator
5 DataLink Layer 5-40
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
LAN LAN
5 DataLink Layer 5-41
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-42
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-16
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-17
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
o PPP for dial-up accesso point-to-point link between Ethernet switch and host
broadcast (shared wire or medium)o old-fashioned Etherneto upstream HFC (cable network)o 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Introduction 1-18
Cable Network Architecture Overview
home
cable headend
cable distributionnetwork (simplified)
Typically 500 to 5000 homes
5 DataLink Layer 5-19
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference o collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself o no out-of-band channel for coordination
5 DataLink Layer 5-20
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at the full rate say R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
o no special node to coordinate transmissionso no synchronization of clocks slots
4 simple
5 DataLink Layer 5-21
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
o divide channel into smaller ldquopiecesrdquo (time slots frequency code)
o allocate piece to node for exclusive use
Random Accesso channel not divided allow collisionso ldquorecoverrdquo from collisions
ldquoTaking turnsrdquoo nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-22
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-23
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-24
Random Access Protocols
When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA
5 DataLink Layer 5-25
ALOHA
When a node has a frame to send send immediately
Set a timer for a random amount of time If an ACK arrives before the timer expires
fine otherwise resend the frame
(Works like stop-and-wait with random timeout interval)
5 DataLink Layer 5-26
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-27
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
5 DataLink Layer 5-28
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
o collisions detected within short timeo colliding transmissions aborted reducing
channel wastage collision detection
o easy in wired LANs measure signal strengths compare transmitted amp received signals
o difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-29
CSMACD collision detection
5 DataLink Layer 5-30
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
o share channel efficiently and fairly at high load
o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolso efficient at low load single node can fully
utilize channelo high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-31
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concernso polling overhead o latencyo single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
o token overhead o latencyo single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-33
Summary of MAC protocols
channel partitioning by time frequency or codeo Time Division Frequency Division
random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)
hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211
taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-34
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-35
MAC Addresses and ARP
32-bit IP address o network-layer addresso used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address o function get frame from one interface to
another physically-connected interface (in same network)
o 48 bit MAC addressbull burned in NIC ROM
5 DataLink Layer 5-36
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-37
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
o can move LAN card from one LAN to another
IP hierarchical address NOT portableo address depends on IP subnet to which node is
attached
5 DataLink Layer 5-38
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgto TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Q given a nodersquos IP address how to determine its MAC address
1A-2F-BB-76-09-AD
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-39
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-
FF-FF-FF-FF-FFo all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ARP is ldquoplug-and-playrdquoo nodes create their
ARP tables without intervention from net administrator
5 DataLink Layer 5-40
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
LAN LAN
5 DataLink Layer 5-41
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-42
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-17
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
o PPP for dial-up accesso point-to-point link between Ethernet switch and host
broadcast (shared wire or medium)o old-fashioned Etherneto upstream HFC (cable network)o 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Introduction 1-18
Cable Network Architecture Overview
home
cable headend
cable distributionnetwork (simplified)
Typically 500 to 5000 homes
5 DataLink Layer 5-19
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference o collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself o no out-of-band channel for coordination
5 DataLink Layer 5-20
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at the full rate say R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
o no special node to coordinate transmissionso no synchronization of clocks slots
4 simple
5 DataLink Layer 5-21
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
o divide channel into smaller ldquopiecesrdquo (time slots frequency code)
o allocate piece to node for exclusive use
Random Accesso channel not divided allow collisionso ldquorecoverrdquo from collisions
ldquoTaking turnsrdquoo nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-22
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-23
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-24
Random Access Protocols
When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA
5 DataLink Layer 5-25
ALOHA
When a node has a frame to send send immediately
Set a timer for a random amount of time If an ACK arrives before the timer expires
fine otherwise resend the frame
(Works like stop-and-wait with random timeout interval)
5 DataLink Layer 5-26
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-27
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
5 DataLink Layer 5-28
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
o collisions detected within short timeo colliding transmissions aborted reducing
channel wastage collision detection
o easy in wired LANs measure signal strengths compare transmitted amp received signals
o difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-29
CSMACD collision detection
5 DataLink Layer 5-30
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
o share channel efficiently and fairly at high load
o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolso efficient at low load single node can fully
utilize channelo high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-31
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concernso polling overhead o latencyo single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
o token overhead o latencyo single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-33
Summary of MAC protocols
channel partitioning by time frequency or codeo Time Division Frequency Division
random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)
hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211
taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-34
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-35
MAC Addresses and ARP
32-bit IP address o network-layer addresso used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address o function get frame from one interface to
another physically-connected interface (in same network)
o 48 bit MAC addressbull burned in NIC ROM
5 DataLink Layer 5-36
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-37
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
o can move LAN card from one LAN to another
IP hierarchical address NOT portableo address depends on IP subnet to which node is
attached
5 DataLink Layer 5-38
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgto TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Q given a nodersquos IP address how to determine its MAC address
1A-2F-BB-76-09-AD
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-39
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-
FF-FF-FF-FF-FFo all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ARP is ldquoplug-and-playrdquoo nodes create their
ARP tables without intervention from net administrator
5 DataLink Layer 5-40
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
LAN LAN
5 DataLink Layer 5-41
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-42
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
Introduction 1-18
Cable Network Architecture Overview
home
cable headend
cable distributionnetwork (simplified)
Typically 500 to 5000 homes
5 DataLink Layer 5-19
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference o collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself o no out-of-band channel for coordination
5 DataLink Layer 5-20
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at the full rate say R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
o no special node to coordinate transmissionso no synchronization of clocks slots
4 simple
5 DataLink Layer 5-21
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
o divide channel into smaller ldquopiecesrdquo (time slots frequency code)
o allocate piece to node for exclusive use
Random Accesso channel not divided allow collisionso ldquorecoverrdquo from collisions
ldquoTaking turnsrdquoo nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-22
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-23
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-24
Random Access Protocols
When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA
5 DataLink Layer 5-25
ALOHA
When a node has a frame to send send immediately
Set a timer for a random amount of time If an ACK arrives before the timer expires
fine otherwise resend the frame
(Works like stop-and-wait with random timeout interval)
5 DataLink Layer 5-26
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-27
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
5 DataLink Layer 5-28
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
o collisions detected within short timeo colliding transmissions aborted reducing
channel wastage collision detection
o easy in wired LANs measure signal strengths compare transmitted amp received signals
o difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-29
CSMACD collision detection
5 DataLink Layer 5-30
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
o share channel efficiently and fairly at high load
o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolso efficient at low load single node can fully
utilize channelo high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-31
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concernso polling overhead o latencyo single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
o token overhead o latencyo single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-33
Summary of MAC protocols
channel partitioning by time frequency or codeo Time Division Frequency Division
random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)
hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211
taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-34
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-35
MAC Addresses and ARP
32-bit IP address o network-layer addresso used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address o function get frame from one interface to
another physically-connected interface (in same network)
o 48 bit MAC addressbull burned in NIC ROM
5 DataLink Layer 5-36
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-37
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
o can move LAN card from one LAN to another
IP hierarchical address NOT portableo address depends on IP subnet to which node is
attached
5 DataLink Layer 5-38
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgto TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Q given a nodersquos IP address how to determine its MAC address
1A-2F-BB-76-09-AD
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-39
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-
FF-FF-FF-FF-FFo all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ARP is ldquoplug-and-playrdquoo nodes create their
ARP tables without intervention from net administrator
5 DataLink Layer 5-40
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
LAN LAN
5 DataLink Layer 5-41
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-42
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-19
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference o collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself o no out-of-band channel for coordination
5 DataLink Layer 5-20
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at the full rate say R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
o no special node to coordinate transmissionso no synchronization of clocks slots
4 simple
5 DataLink Layer 5-21
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
o divide channel into smaller ldquopiecesrdquo (time slots frequency code)
o allocate piece to node for exclusive use
Random Accesso channel not divided allow collisionso ldquorecoverrdquo from collisions
ldquoTaking turnsrdquoo nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-22
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-23
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-24
Random Access Protocols
When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA
5 DataLink Layer 5-25
ALOHA
When a node has a frame to send send immediately
Set a timer for a random amount of time If an ACK arrives before the timer expires
fine otherwise resend the frame
(Works like stop-and-wait with random timeout interval)
5 DataLink Layer 5-26
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-27
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
5 DataLink Layer 5-28
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
o collisions detected within short timeo colliding transmissions aborted reducing
channel wastage collision detection
o easy in wired LANs measure signal strengths compare transmitted amp received signals
o difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-29
CSMACD collision detection
5 DataLink Layer 5-30
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
o share channel efficiently and fairly at high load
o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolso efficient at low load single node can fully
utilize channelo high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-31
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concernso polling overhead o latencyo single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
o token overhead o latencyo single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-33
Summary of MAC protocols
channel partitioning by time frequency or codeo Time Division Frequency Division
random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)
hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211
taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-34
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-35
MAC Addresses and ARP
32-bit IP address o network-layer addresso used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address o function get frame from one interface to
another physically-connected interface (in same network)
o 48 bit MAC addressbull burned in NIC ROM
5 DataLink Layer 5-36
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-37
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
o can move LAN card from one LAN to another
IP hierarchical address NOT portableo address depends on IP subnet to which node is
attached
5 DataLink Layer 5-38
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgto TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Q given a nodersquos IP address how to determine its MAC address
1A-2F-BB-76-09-AD
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-39
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-
FF-FF-FF-FF-FFo all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ARP is ldquoplug-and-playrdquoo nodes create their
ARP tables without intervention from net administrator
5 DataLink Layer 5-40
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
LAN LAN
5 DataLink Layer 5-41
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-42
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-20
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at the full rate say R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
o no special node to coordinate transmissionso no synchronization of clocks slots
4 simple
5 DataLink Layer 5-21
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
o divide channel into smaller ldquopiecesrdquo (time slots frequency code)
o allocate piece to node for exclusive use
Random Accesso channel not divided allow collisionso ldquorecoverrdquo from collisions
ldquoTaking turnsrdquoo nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-22
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-23
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-24
Random Access Protocols
When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA
5 DataLink Layer 5-25
ALOHA
When a node has a frame to send send immediately
Set a timer for a random amount of time If an ACK arrives before the timer expires
fine otherwise resend the frame
(Works like stop-and-wait with random timeout interval)
5 DataLink Layer 5-26
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-27
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
5 DataLink Layer 5-28
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
o collisions detected within short timeo colliding transmissions aborted reducing
channel wastage collision detection
o easy in wired LANs measure signal strengths compare transmitted amp received signals
o difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-29
CSMACD collision detection
5 DataLink Layer 5-30
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
o share channel efficiently and fairly at high load
o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolso efficient at low load single node can fully
utilize channelo high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-31
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concernso polling overhead o latencyo single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
o token overhead o latencyo single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-33
Summary of MAC protocols
channel partitioning by time frequency or codeo Time Division Frequency Division
random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)
hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211
taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-34
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-35
MAC Addresses and ARP
32-bit IP address o network-layer addresso used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address o function get frame from one interface to
another physically-connected interface (in same network)
o 48 bit MAC addressbull burned in NIC ROM
5 DataLink Layer 5-36
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-37
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
o can move LAN card from one LAN to another
IP hierarchical address NOT portableo address depends on IP subnet to which node is
attached
5 DataLink Layer 5-38
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgto TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Q given a nodersquos IP address how to determine its MAC address
1A-2F-BB-76-09-AD
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-39
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-
FF-FF-FF-FF-FFo all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ARP is ldquoplug-and-playrdquoo nodes create their
ARP tables without intervention from net administrator
5 DataLink Layer 5-40
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
LAN LAN
5 DataLink Layer 5-41
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-42
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-21
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
o divide channel into smaller ldquopiecesrdquo (time slots frequency code)
o allocate piece to node for exclusive use
Random Accesso channel not divided allow collisionso ldquorecoverrdquo from collisions
ldquoTaking turnsrdquoo nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-22
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-23
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-24
Random Access Protocols
When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA
5 DataLink Layer 5-25
ALOHA
When a node has a frame to send send immediately
Set a timer for a random amount of time If an ACK arrives before the timer expires
fine otherwise resend the frame
(Works like stop-and-wait with random timeout interval)
5 DataLink Layer 5-26
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-27
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
5 DataLink Layer 5-28
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
o collisions detected within short timeo colliding transmissions aborted reducing
channel wastage collision detection
o easy in wired LANs measure signal strengths compare transmitted amp received signals
o difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-29
CSMACD collision detection
5 DataLink Layer 5-30
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
o share channel efficiently and fairly at high load
o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolso efficient at low load single node can fully
utilize channelo high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-31
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concernso polling overhead o latencyo single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
o token overhead o latencyo single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-33
Summary of MAC protocols
channel partitioning by time frequency or codeo Time Division Frequency Division
random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)
hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211
taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-34
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-35
MAC Addresses and ARP
32-bit IP address o network-layer addresso used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address o function get frame from one interface to
another physically-connected interface (in same network)
o 48 bit MAC addressbull burned in NIC ROM
5 DataLink Layer 5-36
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-37
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
o can move LAN card from one LAN to another
IP hierarchical address NOT portableo address depends on IP subnet to which node is
attached
5 DataLink Layer 5-38
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgto TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Q given a nodersquos IP address how to determine its MAC address
1A-2F-BB-76-09-AD
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-39
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-
FF-FF-FF-FF-FFo all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ARP is ldquoplug-and-playrdquoo nodes create their
ARP tables without intervention from net administrator
5 DataLink Layer 5-40
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
LAN LAN
5 DataLink Layer 5-41
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-42
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-22
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-23
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-24
Random Access Protocols
When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA
5 DataLink Layer 5-25
ALOHA
When a node has a frame to send send immediately
Set a timer for a random amount of time If an ACK arrives before the timer expires
fine otherwise resend the frame
(Works like stop-and-wait with random timeout interval)
5 DataLink Layer 5-26
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-27
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
5 DataLink Layer 5-28
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
o collisions detected within short timeo colliding transmissions aborted reducing
channel wastage collision detection
o easy in wired LANs measure signal strengths compare transmitted amp received signals
o difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-29
CSMACD collision detection
5 DataLink Layer 5-30
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
o share channel efficiently and fairly at high load
o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolso efficient at low load single node can fully
utilize channelo high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-31
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concernso polling overhead o latencyo single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
o token overhead o latencyo single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-33
Summary of MAC protocols
channel partitioning by time frequency or codeo Time Division Frequency Division
random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)
hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211
taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-34
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-35
MAC Addresses and ARP
32-bit IP address o network-layer addresso used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address o function get frame from one interface to
another physically-connected interface (in same network)
o 48 bit MAC addressbull burned in NIC ROM
5 DataLink Layer 5-36
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-37
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
o can move LAN card from one LAN to another
IP hierarchical address NOT portableo address depends on IP subnet to which node is
attached
5 DataLink Layer 5-38
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgto TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Q given a nodersquos IP address how to determine its MAC address
1A-2F-BB-76-09-AD
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-39
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-
FF-FF-FF-FF-FFo all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ARP is ldquoplug-and-playrdquoo nodes create their
ARP tables without intervention from net administrator
5 DataLink Layer 5-40
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
LAN LAN
5 DataLink Layer 5-41
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-42
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-23
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-24
Random Access Protocols
When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA
5 DataLink Layer 5-25
ALOHA
When a node has a frame to send send immediately
Set a timer for a random amount of time If an ACK arrives before the timer expires
fine otherwise resend the frame
(Works like stop-and-wait with random timeout interval)
5 DataLink Layer 5-26
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-27
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
5 DataLink Layer 5-28
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
o collisions detected within short timeo colliding transmissions aborted reducing
channel wastage collision detection
o easy in wired LANs measure signal strengths compare transmitted amp received signals
o difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-29
CSMACD collision detection
5 DataLink Layer 5-30
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
o share channel efficiently and fairly at high load
o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolso efficient at low load single node can fully
utilize channelo high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-31
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concernso polling overhead o latencyo single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
o token overhead o latencyo single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-33
Summary of MAC protocols
channel partitioning by time frequency or codeo Time Division Frequency Division
random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)
hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211
taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-34
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-35
MAC Addresses and ARP
32-bit IP address o network-layer addresso used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address o function get frame from one interface to
another physically-connected interface (in same network)
o 48 bit MAC addressbull burned in NIC ROM
5 DataLink Layer 5-36
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-37
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
o can move LAN card from one LAN to another
IP hierarchical address NOT portableo address depends on IP subnet to which node is
attached
5 DataLink Layer 5-38
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgto TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Q given a nodersquos IP address how to determine its MAC address
1A-2F-BB-76-09-AD
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-39
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-
FF-FF-FF-FF-FFo all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ARP is ldquoplug-and-playrdquoo nodes create their
ARP tables without intervention from net administrator
5 DataLink Layer 5-40
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
LAN LAN
5 DataLink Layer 5-41
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-42
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-24
Random Access Protocols
When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA
5 DataLink Layer 5-25
ALOHA
When a node has a frame to send send immediately
Set a timer for a random amount of time If an ACK arrives before the timer expires
fine otherwise resend the frame
(Works like stop-and-wait with random timeout interval)
5 DataLink Layer 5-26
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-27
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
5 DataLink Layer 5-28
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
o collisions detected within short timeo colliding transmissions aborted reducing
channel wastage collision detection
o easy in wired LANs measure signal strengths compare transmitted amp received signals
o difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-29
CSMACD collision detection
5 DataLink Layer 5-30
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
o share channel efficiently and fairly at high load
o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolso efficient at low load single node can fully
utilize channelo high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-31
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concernso polling overhead o latencyo single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
o token overhead o latencyo single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-33
Summary of MAC protocols
channel partitioning by time frequency or codeo Time Division Frequency Division
random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)
hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211
taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-34
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-35
MAC Addresses and ARP
32-bit IP address o network-layer addresso used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address o function get frame from one interface to
another physically-connected interface (in same network)
o 48 bit MAC addressbull burned in NIC ROM
5 DataLink Layer 5-36
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-37
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
o can move LAN card from one LAN to another
IP hierarchical address NOT portableo address depends on IP subnet to which node is
attached
5 DataLink Layer 5-38
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgto TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Q given a nodersquos IP address how to determine its MAC address
1A-2F-BB-76-09-AD
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-39
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-
FF-FF-FF-FF-FFo all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ARP is ldquoplug-and-playrdquoo nodes create their
ARP tables without intervention from net administrator
5 DataLink Layer 5-40
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
LAN LAN
5 DataLink Layer 5-41
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-42
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-25
ALOHA
When a node has a frame to send send immediately
Set a timer for a random amount of time If an ACK arrives before the timer expires
fine otherwise resend the frame
(Works like stop-and-wait with random timeout interval)
5 DataLink Layer 5-26
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-27
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
5 DataLink Layer 5-28
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
o collisions detected within short timeo colliding transmissions aborted reducing
channel wastage collision detection
o easy in wired LANs measure signal strengths compare transmitted amp received signals
o difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-29
CSMACD collision detection
5 DataLink Layer 5-30
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
o share channel efficiently and fairly at high load
o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolso efficient at low load single node can fully
utilize channelo high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-31
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concernso polling overhead o latencyo single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
o token overhead o latencyo single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-33
Summary of MAC protocols
channel partitioning by time frequency or codeo Time Division Frequency Division
random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)
hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211
taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-34
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-35
MAC Addresses and ARP
32-bit IP address o network-layer addresso used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address o function get frame from one interface to
another physically-connected interface (in same network)
o 48 bit MAC addressbull burned in NIC ROM
5 DataLink Layer 5-36
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-37
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
o can move LAN card from one LAN to another
IP hierarchical address NOT portableo address depends on IP subnet to which node is
attached
5 DataLink Layer 5-38
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgto TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Q given a nodersquos IP address how to determine its MAC address
1A-2F-BB-76-09-AD
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-39
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-
FF-FF-FF-FF-FFo all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ARP is ldquoplug-and-playrdquoo nodes create their
ARP tables without intervention from net administrator
5 DataLink Layer 5-40
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
LAN LAN
5 DataLink Layer 5-41
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-42
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-26
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-27
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
5 DataLink Layer 5-28
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
o collisions detected within short timeo colliding transmissions aborted reducing
channel wastage collision detection
o easy in wired LANs measure signal strengths compare transmitted amp received signals
o difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-29
CSMACD collision detection
5 DataLink Layer 5-30
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
o share channel efficiently and fairly at high load
o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolso efficient at low load single node can fully
utilize channelo high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-31
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concernso polling overhead o latencyo single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
o token overhead o latencyo single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-33
Summary of MAC protocols
channel partitioning by time frequency or codeo Time Division Frequency Division
random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)
hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211
taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-34
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-35
MAC Addresses and ARP
32-bit IP address o network-layer addresso used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address o function get frame from one interface to
another physically-connected interface (in same network)
o 48 bit MAC addressbull burned in NIC ROM
5 DataLink Layer 5-36
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-37
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
o can move LAN card from one LAN to another
IP hierarchical address NOT portableo address depends on IP subnet to which node is
attached
5 DataLink Layer 5-38
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgto TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Q given a nodersquos IP address how to determine its MAC address
1A-2F-BB-76-09-AD
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-39
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-
FF-FF-FF-FF-FFo all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ARP is ldquoplug-and-playrdquoo nodes create their
ARP tables without intervention from net administrator
5 DataLink Layer 5-40
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
LAN LAN
5 DataLink Layer 5-41
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-42
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-27
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
5 DataLink Layer 5-28
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
o collisions detected within short timeo colliding transmissions aborted reducing
channel wastage collision detection
o easy in wired LANs measure signal strengths compare transmitted amp received signals
o difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-29
CSMACD collision detection
5 DataLink Layer 5-30
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
o share channel efficiently and fairly at high load
o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolso efficient at low load single node can fully
utilize channelo high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-31
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concernso polling overhead o latencyo single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
o token overhead o latencyo single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-33
Summary of MAC protocols
channel partitioning by time frequency or codeo Time Division Frequency Division
random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)
hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211
taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-34
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-35
MAC Addresses and ARP
32-bit IP address o network-layer addresso used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address o function get frame from one interface to
another physically-connected interface (in same network)
o 48 bit MAC addressbull burned in NIC ROM
5 DataLink Layer 5-36
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-37
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
o can move LAN card from one LAN to another
IP hierarchical address NOT portableo address depends on IP subnet to which node is
attached
5 DataLink Layer 5-38
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgto TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Q given a nodersquos IP address how to determine its MAC address
1A-2F-BB-76-09-AD
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-39
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-
FF-FF-FF-FF-FFo all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ARP is ldquoplug-and-playrdquoo nodes create their
ARP tables without intervention from net administrator
5 DataLink Layer 5-40
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
LAN LAN
5 DataLink Layer 5-41
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-42
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-28
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
o collisions detected within short timeo colliding transmissions aborted reducing
channel wastage collision detection
o easy in wired LANs measure signal strengths compare transmitted amp received signals
o difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-29
CSMACD collision detection
5 DataLink Layer 5-30
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
o share channel efficiently and fairly at high load
o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolso efficient at low load single node can fully
utilize channelo high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-31
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concernso polling overhead o latencyo single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
o token overhead o latencyo single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-33
Summary of MAC protocols
channel partitioning by time frequency or codeo Time Division Frequency Division
random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)
hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211
taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-34
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-35
MAC Addresses and ARP
32-bit IP address o network-layer addresso used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address o function get frame from one interface to
another physically-connected interface (in same network)
o 48 bit MAC addressbull burned in NIC ROM
5 DataLink Layer 5-36
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-37
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
o can move LAN card from one LAN to another
IP hierarchical address NOT portableo address depends on IP subnet to which node is
attached
5 DataLink Layer 5-38
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgto TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Q given a nodersquos IP address how to determine its MAC address
1A-2F-BB-76-09-AD
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-39
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-
FF-FF-FF-FF-FFo all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ARP is ldquoplug-and-playrdquoo nodes create their
ARP tables without intervention from net administrator
5 DataLink Layer 5-40
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
LAN LAN
5 DataLink Layer 5-41
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-42
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-29
CSMACD collision detection
5 DataLink Layer 5-30
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
o share channel efficiently and fairly at high load
o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolso efficient at low load single node can fully
utilize channelo high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-31
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concernso polling overhead o latencyo single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
o token overhead o latencyo single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-33
Summary of MAC protocols
channel partitioning by time frequency or codeo Time Division Frequency Division
random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)
hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211
taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-34
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-35
MAC Addresses and ARP
32-bit IP address o network-layer addresso used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address o function get frame from one interface to
another physically-connected interface (in same network)
o 48 bit MAC addressbull burned in NIC ROM
5 DataLink Layer 5-36
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-37
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
o can move LAN card from one LAN to another
IP hierarchical address NOT portableo address depends on IP subnet to which node is
attached
5 DataLink Layer 5-38
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgto TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Q given a nodersquos IP address how to determine its MAC address
1A-2F-BB-76-09-AD
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-39
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-
FF-FF-FF-FF-FFo all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ARP is ldquoplug-and-playrdquoo nodes create their
ARP tables without intervention from net administrator
5 DataLink Layer 5-40
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
LAN LAN
5 DataLink Layer 5-41
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-42
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-30
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
o share channel efficiently and fairly at high load
o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolso efficient at low load single node can fully
utilize channelo high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-31
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concernso polling overhead o latencyo single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
o token overhead o latencyo single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-33
Summary of MAC protocols
channel partitioning by time frequency or codeo Time Division Frequency Division
random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)
hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211
taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-34
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-35
MAC Addresses and ARP
32-bit IP address o network-layer addresso used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address o function get frame from one interface to
another physically-connected interface (in same network)
o 48 bit MAC addressbull burned in NIC ROM
5 DataLink Layer 5-36
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-37
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
o can move LAN card from one LAN to another
IP hierarchical address NOT portableo address depends on IP subnet to which node is
attached
5 DataLink Layer 5-38
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgto TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Q given a nodersquos IP address how to determine its MAC address
1A-2F-BB-76-09-AD
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-39
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-
FF-FF-FF-FF-FFo all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ARP is ldquoplug-and-playrdquoo nodes create their
ARP tables without intervention from net administrator
5 DataLink Layer 5-40
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
LAN LAN
5 DataLink Layer 5-41
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-42
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-31
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concernso polling overhead o latencyo single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
o token overhead o latencyo single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-33
Summary of MAC protocols
channel partitioning by time frequency or codeo Time Division Frequency Division
random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)
hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211
taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-34
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-35
MAC Addresses and ARP
32-bit IP address o network-layer addresso used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address o function get frame from one interface to
another physically-connected interface (in same network)
o 48 bit MAC addressbull burned in NIC ROM
5 DataLink Layer 5-36
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-37
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
o can move LAN card from one LAN to another
IP hierarchical address NOT portableo address depends on IP subnet to which node is
attached
5 DataLink Layer 5-38
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgto TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Q given a nodersquos IP address how to determine its MAC address
1A-2F-BB-76-09-AD
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-39
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-
FF-FF-FF-FF-FFo all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ARP is ldquoplug-and-playrdquoo nodes create their
ARP tables without intervention from net administrator
5 DataLink Layer 5-40
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
LAN LAN
5 DataLink Layer 5-41
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-42
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
o token overhead o latencyo single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-33
Summary of MAC protocols
channel partitioning by time frequency or codeo Time Division Frequency Division
random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)
hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211
taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-34
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-35
MAC Addresses and ARP
32-bit IP address o network-layer addresso used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address o function get frame from one interface to
another physically-connected interface (in same network)
o 48 bit MAC addressbull burned in NIC ROM
5 DataLink Layer 5-36
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-37
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
o can move LAN card from one LAN to another
IP hierarchical address NOT portableo address depends on IP subnet to which node is
attached
5 DataLink Layer 5-38
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgto TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Q given a nodersquos IP address how to determine its MAC address
1A-2F-BB-76-09-AD
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-39
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-
FF-FF-FF-FF-FFo all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ARP is ldquoplug-and-playrdquoo nodes create their
ARP tables without intervention from net administrator
5 DataLink Layer 5-40
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
LAN LAN
5 DataLink Layer 5-41
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-42
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-33
Summary of MAC protocols
channel partitioning by time frequency or codeo Time Division Frequency Division
random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)
hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211
taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-34
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-35
MAC Addresses and ARP
32-bit IP address o network-layer addresso used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address o function get frame from one interface to
another physically-connected interface (in same network)
o 48 bit MAC addressbull burned in NIC ROM
5 DataLink Layer 5-36
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-37
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
o can move LAN card from one LAN to another
IP hierarchical address NOT portableo address depends on IP subnet to which node is
attached
5 DataLink Layer 5-38
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgto TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Q given a nodersquos IP address how to determine its MAC address
1A-2F-BB-76-09-AD
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-39
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-
FF-FF-FF-FF-FFo all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ARP is ldquoplug-and-playrdquoo nodes create their
ARP tables without intervention from net administrator
5 DataLink Layer 5-40
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
LAN LAN
5 DataLink Layer 5-41
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-42
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-34
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-35
MAC Addresses and ARP
32-bit IP address o network-layer addresso used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address o function get frame from one interface to
another physically-connected interface (in same network)
o 48 bit MAC addressbull burned in NIC ROM
5 DataLink Layer 5-36
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-37
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
o can move LAN card from one LAN to another
IP hierarchical address NOT portableo address depends on IP subnet to which node is
attached
5 DataLink Layer 5-38
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgto TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Q given a nodersquos IP address how to determine its MAC address
1A-2F-BB-76-09-AD
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-39
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-
FF-FF-FF-FF-FFo all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ARP is ldquoplug-and-playrdquoo nodes create their
ARP tables without intervention from net administrator
5 DataLink Layer 5-40
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
LAN LAN
5 DataLink Layer 5-41
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-42
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-35
MAC Addresses and ARP
32-bit IP address o network-layer addresso used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address o function get frame from one interface to
another physically-connected interface (in same network)
o 48 bit MAC addressbull burned in NIC ROM
5 DataLink Layer 5-36
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-37
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
o can move LAN card from one LAN to another
IP hierarchical address NOT portableo address depends on IP subnet to which node is
attached
5 DataLink Layer 5-38
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgto TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Q given a nodersquos IP address how to determine its MAC address
1A-2F-BB-76-09-AD
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-39
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-
FF-FF-FF-FF-FFo all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ARP is ldquoplug-and-playrdquoo nodes create their
ARP tables without intervention from net administrator
5 DataLink Layer 5-40
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
LAN LAN
5 DataLink Layer 5-41
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-42
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-36
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-37
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
o can move LAN card from one LAN to another
IP hierarchical address NOT portableo address depends on IP subnet to which node is
attached
5 DataLink Layer 5-38
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgto TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Q given a nodersquos IP address how to determine its MAC address
1A-2F-BB-76-09-AD
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-39
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-
FF-FF-FF-FF-FFo all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ARP is ldquoplug-and-playrdquoo nodes create their
ARP tables without intervention from net administrator
5 DataLink Layer 5-40
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
LAN LAN
5 DataLink Layer 5-41
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-42
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-37
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
o can move LAN card from one LAN to another
IP hierarchical address NOT portableo address depends on IP subnet to which node is
attached
5 DataLink Layer 5-38
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgto TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Q given a nodersquos IP address how to determine its MAC address
1A-2F-BB-76-09-AD
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-39
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-
FF-FF-FF-FF-FFo all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ARP is ldquoplug-and-playrdquoo nodes create their
ARP tables without intervention from net administrator
5 DataLink Layer 5-40
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
LAN LAN
5 DataLink Layer 5-41
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-42
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-38
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgto TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Q given a nodersquos IP address how to determine its MAC address
1A-2F-BB-76-09-AD
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-39
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-
FF-FF-FF-FF-FFo all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ARP is ldquoplug-and-playrdquoo nodes create their
ARP tables without intervention from net administrator
5 DataLink Layer 5-40
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
LAN LAN
5 DataLink Layer 5-41
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-42
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-39
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-
FF-FF-FF-FF-FFo all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ARP is ldquoplug-and-playrdquoo nodes create their
ARP tables without intervention from net administrator
5 DataLink Layer 5-40
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
LAN LAN
5 DataLink Layer 5-41
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-42
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-40
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
LAN LAN
5 DataLink Layer 5-41
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-42
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-41
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-42
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-42
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-43
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-44
Star topology bus topology popular through mid 90s
o all nodes in same collision domain (can collide with each other)
today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-45
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver amp sender clock
rates
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-46
Ethernet Frame Structure (more) Addresses 6 bytes
o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
o otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-47
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot
send acks or nacks to sending NICo stream of datagrams passed to network layer can have
gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps
Ethernetrsquos MAC protocol CSMACD
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-48
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects collision while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff
after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1
(1 slot = 512 bit-times)
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-49
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current loado heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bpso different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-51
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each othero no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-52
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs
59 A day in the life of a web request
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-53
Hubsphysical-layer (ldquodumbrdquo) repeaters
o bits coming in one link go out all other links at same rate
o all nodes connected to hub can collide with one another
o no frame bufferingo no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-54
Switch link-layer device smarter than hubs take active
roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparento hosts are unaware of presence of switches
plug-and-play self-learningo switches do not need to be configured
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplexo each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions o not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entryo (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table o something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaceso when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
o records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-61
Self-learning multi-switch exampleSuppose A sends frame to I I responds to A
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2 3
34
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-63
Switches vs Routers both store-and-forward devices
o routers network layer devices (examine network layer headers)o switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Switch
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-64
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 59 A day in the life of
a web request
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-65
Synthesis a day in the life of a web request
journey down protocol stack completeo application transport network link
putting-it-all-together synthesiso goal identify review understand protocols
(at all layers) involved in seemingly simple scenario requesting www page
o scenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-66
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-67
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-68
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-69
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-70
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-71
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-72
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-
5 DataLink Layer 5-73
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topicso Internetworking (CSE 678 TCPIP socket
programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)
- Link Layer
- Link Layer Introduction
- Link layer context
- Encapsulation
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- Cyclic Redundancy Check
- Slide 14
- CRC Example
- Slide 16
- Multiple Access Links and Protocols
- Cable Network Architecture Overview
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 31
- Slide 32
- Summary of MAC protocols
- Slide 34
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Addressing routing to another LAN
- Slide 41
- Slide 42
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Slide 52
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- Slide 64
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 68
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
- Chapter 5 letrsquos take a breath
-