Roadmap 1.1 What is the Internet? 1.2 Network edge 1.3 Network core 1.4 Network access and physical...
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Roadmap
1.1 What is the Internet?
1.2 Network edge
1.3 Network core
1.4 Network access and physical media
1.5 Internet structure and ISPs
1.6 Delay & loss in packet-switched networks
1.7 Protocol layers, service models
1.8 History
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Protocol “Layers”Networks are complex! • many “pieces”:
– hosts– routers– links of various
media– applications– protocols– hardware,
software
Question: Is there any hope of organizing structure of
network?
Or at least our discussion of networks?
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Organization of air travel
• a series of steps
ticket (purchase)
baggage (check)
gates (load)
runway takeoff
airplane routing
ticket (complain)
baggage (claim)
gates (unload)
runway landing
airplane routing
airplane routing
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ticket (purchase)
baggage (check)
gates (load)
runway (takeoff)
airplane routing
departureairport
arrivalairport
intermediate air-trafficcontrol centers
airplane routing airplane routing
ticket (complain)
baggage (claim
gates (unload)
runway (land)
airplane routing
ticket
baggage
gate
takeoff/landing
airplane routing
Layering of airline functionality
Layers: each layer implements a service– via its own internal-layer actions– relying on services provided by layer below
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Why layering?Dealing with complex systems:• explicit structure allows identification, relationship of
complex system’s pieces– layered reference model for discussion
• modularization eases maintenance, updating of system– change of implementation of layer’s service
transparent to rest of system– e.g., change in gate procedure doesn’t affect rest
of system• layering considered harmful?
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2. TCP/IP Reference Model (Layers)
IP, ICMP and IGMP
IEEE 802 Ethernet X.25 V.24 V.28 EIA-232 ISDN etc.
Gateway protocols
TCP UDP
FTP TELNET MAIL …….. (4)
(3)
(2)
(1)
(1) Data link and physical layer: The protocols at this layer needed to manage a specific physical medium, such as Ethernet or a point to point line
(2) Network layer: IP, which provides the basic service of getting datagrams to their destination
(3) Transport layer: A protocol such as TCP that provides services need by many applications
(4) Application layer: An application protocol such as mail
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Internet protocol stack• application: supporting network
applications– FTP, SMTP, HTTP
• transport: process-process data transfer– TCP, UDP
• network: routing of datagrams from source to destination– IP, routing protocols,
• link: data transfer between neighboring network elements– PPP, Ethernet
• physical: bits “on the wire”
application
transport
network
link
physical
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sourceapplicatio
ntransportnetwork
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
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Figure 3-1
OSI Model
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Figure 3-2 OSI Layers
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Figure 3-3
WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998
An Exchange Using the OSI Model
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Figure 3-4
Physical Layer
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Figure 3-14
Summary of Layer Functions
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Introduction: Summary
Covered a “ton” of material!• Internet overview• what’s a protocol?• network edge, core,
access network– packet-switching versus
circuit-switching• Internet/ISP structure• performance: loss, delay• layering and service
models• history
You now have: • context, overview,
“feel” of networking• more depth, detail
to follow!
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MAC Addresses and ARP
• 32-bit IP address: – network-layer address– used to get datagram to destination IP subnet
• MAC (or LAN or physical or Ethernet) address: – used to get frame from one interface to another
physically-connected interface (same network)– 48 bit MAC address (for most LANs)
burned in the adapter ROM
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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)
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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
– can move LAN card from one LAN to another
• IP hierarchical address NOT portable– depends on IP subnet to which node is attached
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ARP: Address Resolution Protocol
• Each IP node (Host, Router) on LAN has ARP table
• ARP Table: IP/MAC address mappings for some LAN nodes
< IP address; MAC address; TTL>
– TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min)
Question: how to determineMAC address of Bknowing B’s IP address?
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
137.196.7.23
137.196.7.78
137.196.7.14
137.196.7.88
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ARP protocol: Same LAN (network)
• A wants to send datagram to B, and B’s MAC address not in A’s ARP table.
• A broadcasts ARP query packet, containing B's IP address – Dest MAC address = FF-
FF-FF-FF-FF-FF– all machines on LAN
receive ARP query • B receives ARP packet,
replies to A with its (B's) MAC address– frame sent to A’s MAC
address (unicast)
• A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) – soft state: information that
times out (goes away) unless refreshed
• ARP is “plug-and-play”:– nodes create their ARP
tables without intervention from net administrator
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Routing to another LANwalkthrough: send datagram from A to B via R
assume A know’s B IP address
• Two ARP tables in router R, one for each IP network (LAN)
• In routing table at source Host, find router 111.111.111.110• In ARP table at source, find MAC address E6-E9-00-17-BB-4B, etc
A
RB
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• A creates datagram with source A, destination B
• A uses ARP to get R’s MAC address for 111.111.111.110
• A creates link-layer frame with R's MAC address as dest, frame contains A-to-B IP datagram
• A’s adapter sends frame
• R’s adapter receives frame
• R removes IP datagram from Ethernet frame, sees its destined to B
• R uses ARP to get B’s MAC address
• R creates frame containing A-to-B IP datagram sends to B
A
RB
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Link Layer
• 5.1 Introduction and services
• 5.2 Error detection and correction
• 5.3Multiple access protocols
• 5.4 Link-Layer Addressing
• 5.5 Ethernet
• 5.6 Hubs and switches
• 5.7 PPP• 5.8 Link Virtualization:
ATM
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Ethernet“dominant” wired LAN technology: • cheap $20 for 100Mbs!• first widely used LAN technology• Simpler, cheaper than token LANs and ATM• Kept up with speed race: 10 Mbps – 10 Gbps
Metcalfe’s Ethernetsketch
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Star topology
• Bus topology popular through mid 90s• Now star topology prevails• Connection choices: hub or switch (more later)
hub orswitch
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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, sender clock rates
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Ethernet Frame Structure (more)
• Addresses: 6 bytes– if adapter receives frame with matching destination
address, or with broadcast address (eg ARP packet), it passes data in frame to net-layer protocol
– otherwise, adapter discards frame
• Type: indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
• CRC: checked at receiver, if error is detected, the frame is simply dropped
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Unreliable, connectionless service
• Connectionless: No handshaking between sending and receiving adapter.
• Unreliable: receiving adapter doesn’t send acks or nacks to sending adapter– stream of datagrams passed to network layer can
have gaps– gaps will be filled if app is using TCP– otherwise, app will see the gaps
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10BaseT and 100BaseT• 10/100 Mbps rate; latter called “fast ethernet”• T stands for Twisted Pair• Nodes connect to a hub: “star topology”; 100
m max distance between nodes and hub
twisted pair
hub
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HubsHubs are essentially physical-layer repeaters:
– bits coming from one link go out all other links– at the same rate– no frame buffering– no CSMA/CD at hub: adapters detect collisions– provides net management functionality
twisted pair
hub
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Manchester encoding
• Used in 10BaseT• Each bit has a transition• Allows clocks in sending and receiving nodes to
synchronize to each other– no need for a centralized, global clock among
nodes!• Hey, this is physical-layer stuff!
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Gbit Ethernet
• uses standard Ethernet frame format• allows for point-to-point links and shared
broadcast channels• in shared mode, CSMA/CD is used; short
distances between nodes required for efficiency• uses hubs, called here “Buffered Distributors”• Full-Duplex at 1 Gbps for point-to-point links• 10 Gbps now !