Chap 5 Data Link Layer

8/3/2019 Chap 5 Data Link Layer

1/143

5: DataLink Layer 5a-1

Chapter 5Data Link Layer

Computer Networking: A Top Down Approach Featuring the Internet ,

2nd edition.Jim Kurose, Keith RossAddison-Wesley, July2002.

A note on the use of these ppt slides:Were making these slides freely available to all (faculty, students, readers).Theyre in PowerPoint form so you can add, modify, and delete slides(including this one) and slide content to suit your needs. They obviouslyrepresent a lot of work on our part. In return for use, we only ask thefollowing:

If you use these slides (e.g., in a class) in substantially unaltered form,that you mention their source (after all, wed like people to use our book!) If you post any slides in substantially unaltered form on a www site, that

you note that they are adapted from (or perhaps identical to) our slides, andnote our copyright of this material.

Thanks and enjoy! JFK/KWR

All material copyright 1996-2002

J.F Kurose and K.W. Ross, All Rights Reserved


2/143


Chapter 5: The Data Link LayerOur goals:

understand principles behind data link layerservices:

error detection, correction

sharing a broadcast channel: multiple accesslink layer addressingreliable data transfer, flow control: done!

instantiation and implementation of various link

layer technologies


3/143


Chapter 5 outline

5.1 Introduction andservices 5.2 Error detectionand correction5.3Multiple accessprotocols5.4 LAN addressesand ARP5.5 Ethernet

5.6 Hubs, bridges, andswitches5.7 Wireless links andLANs5.8 PPP5.9 ATM5.10 Frame Relay


4/143


Link Layer: IntroductionSome terminology: hosts and routers are nodes

(bridges and switches too)communication channels thatconnect adjacent nodes alongcommunication path are links

wired linkswireless linksLANs

2-PDU is a frame , encapsulates datagram

link

data-link layer has responsibility oftransferring datagram from one node

to adjacent node over a link


5/143


Link layer: context

Datagram transferred bydifferent link protocolsover different links:

e.g., Ethernet on first link,frame relay onintermediate links, 802.11on last link

Each link protocolprovides different

servicese.g., may or may notprovide rdt over link

transportation analogytrip from Princeton toLausanne

limo: Princeton to JFKplane: JFK to Geneva

train: Geneva to Lausannetourist = datagram transport segment =communication link

transportation mode =link layer protocol travel agent = routingalgorithm


6/143


Link Layer ServicesFraming, link access:

encapsulate datagram into frame, adding header, trailerchannel access if shared mediumphysical addresses used in frame headers to identify

source, dest different from IP address!Reliable delivery between adjacent nodes

we learned how to do this already (chapter 3)!

seldom used on low bit error link (fiber, some twistedpair)wireless links: high error rates

Q: why both link-level and end-end reliability?


7/143


Link Layer Services (more)

Flow Control: pacing between adjacent sending and receiving nodes

Error Detection : errors caused by signal attenuation, noise.receiver detects presence of errors:

signals sender for retransmission or drops frame

Error Correction: receiver identifies and corrects bit error(s) withoutresorting to retransmission

Half-duplex and full-duplex with half duplex, nodes at both ends of link can transmit,but not at same time


8/143


Adaptors Communicating

link layer implemented inadaptor (aka NIC)

Ethernet card, PCMCIcard, 802.11 card

sending side:encapsulates datagram ina frameadds error checking bits,rdt, flow control, etc.

receiving sidelooks for errors, rdt, flowcontrol, etcextracts datagram, passes

to rcving nodeadapter is semi-autonomouslink & physical layers

sendingnode

frame

rcvingnode

datagram

frame adapter adapter

link layer protocol


9/143


Chapter 5 outline

5.1 Introduction andservices5.2 Error detectionand correction 5.3Multiple accessprotocols5.4 LAN addressesand ARP5.5 Ethernet



10/143


Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking, may include header fields

Error detection not 100% reliable! protocol may miss some errors, but rarely larger EDC field yields better detection and correction


11/143


Parity CheckingSingle Bit Parity: Detect single bit errors

Two Dimensional Bit Parity: Detect and correct single bit errors

0 0


12/143


Internet checksum

Sender: treat segment contentsas sequence of 16-bitintegerschecksum: addition (1scomplement sum) ofsegment contentssender puts checksumvalue into UDP checksumfield

Receiver:

compute checksum of receivedsegmentcheck if computed checksumequals checksum field value:

NO - error detectedYES - no error detected. But maybe errors nonetheless? More later .

Goal: detect errors (e.g., flipped bits) in transmittedsegment (note: used at transport layer only )


13/143


Checksumming: Cyclic Redundancy Check

view data bits, D, as a binary numberchoose r+1 bit pattern (generator), G goal: choose r CRC bits,R, such that

exactly divisible by G (modulo 2)receiver knows G, divides by G. If non-zero remainder:error detected!can detect all burst errors less than r+1 bits

widely used in practice (ATM, HDCL)


14/143


CRC Example

Want: D.2r XOR R = nGequivalently:

D.2r = nG XOR Requivalently:

if we divide D.2r byG, want remainder R

R= remainder[ ]D.2rG


15/143


Chapter 5 outline

5.1 Introduction andservices5.2 Error detectionand correction5.3Multiple accessprotocols 5.4 LAN addressesand ARP5.5 Ethernet



16/143


Multiple Access Links and ProtocolsTwo types of links:

point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host

broadcast (shared wire or medium)traditional Ethernetupstream HFC802.11 wireless LAN


17/143


Multiple Access protocolssingle shared broadcast channeltwo or more simultaneous transmissions by nodes:interference

only one node can send successfully at a time

multiple access protocol distributed algorithm that determines how nodesshare channel, i.e., determine when node can transmitcommunication about channel sharing must use channelitself!what to look for in multiple access protocols:


18/143


Ideal Mulitple Access Protocol

Broadcast channel of rate R bps 1. When one node wants to transmit, it can send at

rate R.

2. When M nodes want to transmit, each can send ataverage rate R/M3. Fully decentralized:

no special node to coordinate transmissionsno synchronization of clocks, slots

4. Simple


19/143


MAC Protocols: a taxonomy

Three broad classes:Channel Partitioning divide channel into smaller pieces (time slots,frequency, code)allocate piece to node for exclusive use

Random Access channel not divided, allow collisionsrecover from collisions

Taking turns tightly coordinate shared access to avoid collisions


20/143


Channel Partitioning MAC protocols: TDMA

TDMA: time division multiple access access to channel in "rounds"each station gets fixed length slot (length = pkttrans time) in each roundunused slots go idleexample: 6-station LAN, 1,3,4 have pkt, slots 2,5,6idle


21/143


Channel Partitioning MAC protocols: FDMA

FDMA: frequency division multiple access channel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idleexample: 6-station LAN, 1,3,4 have pkt, frequencybands 2,5,6 idle

f r e q u e n c y

b a n

d s


22/143


Channel Partitioning (CDMA)

CDMA (Code Division Multiple Access) unique code assigned to each user; i.e., code set partitioning used mostly in wireless broadcast channels (cellular, satellite,etc)all users share same frequency, but each user has ownchipping sequence (i.e., code) to encode data encoded signal = (original data) X (chipping sequence)decoding: inner-product of encoded signal and chippingsequenceallows multiple users to coexist and transmit simultaneouslywith minimal interference (if codes are orthogonal)


23/143


CDMA Encode/Decode


24/143


CDMA: two-sender interference


25/143


Random Access Protocols

When node has packet to sendtransmit at full channel data rate R.no a priori coordination among nodes

two or more transmitting nodes - > collision, random access MAC protocol specifies:

how to detect collisionshow to recover from collisions (e.g., via delayedretransmissions)

Examples of random access MAC protocols:slotted ALOHAALOHACSMA, CSMA/CD, CSMA/CA


26/143


Slotted ALOHA

Assumptions all frames same sizetime is divided into

equal size slots, time totransmit 1 framenodes start to transmitframes only atbeginning of slotsnodes are synchronizedif 2 or more nodestransmit in slot, allnodes detect collision

Operation when node obtains freshframe, it transmits in nextslotno collision, node can sendnew frame in next slotif collision, noderetransmits frame in eachsubsequent slot with prob.p until success


27/143


Slotted ALOHA

Pros single active node cancontinuously transmitat full rate of channelhighly decentralized:only slots in nodesneed to be in syncsimple

Cons collisions, wasting slotsidle slots

nodes may be able todetect collision in lessthan time to transmitpacket


28/143


Slotted Aloha efficiency

Suppose N nodes withmany frames to send,each transmits in slotwith probability p

prob that 1st node hassuccess in a slot = p(1-p)N-1

prob that any node hasa success = Np(1-p)N-1

For max efficiencywith N nodes, find p*that maximizesNp(1-p)N-1For many nodes, takelimit of Np*(1-p*)N-1as N goes to infinity,gives 1/e = .37

Efficiency is the long-runfraction of successful slotswhen theres many nodes, eachwith many frames to send

At best: channelused for usefultransmissions 37%of time!


29/143


Pure (unslotted) ALOHAunslotted Aloha: simpler, no synchronizationwhen frame first arrives

transmit immediatelycollision probability increases:

frame sent at t 0 collides with other frames sent in [t 0-1,t0+1]


30/143


Pure Aloha efficiency

P(success by given node) = P(node transmits) . P(no other node transmits in [p 0-1,p0] . P(no other node transmits in [p 0,p0+1]

= p . (1-p)N-1 . (1-p)N-1

= p . (1-p)2(N-1)

choosing optimum p and then letting n -> infty ...

= 1/(2e) = .18Even worse !


31/143


CSMA (Carrier Sense Multiple Access)

CSMA: listen before transmit:If channel sensed idle: transmit entire frameIf channel sensed busy, defer transmission

Human analogy: dont interrupt others!


32/143


CSMA collisionscollisionscan still occur: propagation delay meanstwo nodes may not heareach others transmission

collision: entire packet transmissiontime wasted

spatial layout of nodes

note: role of distance & propagationdelay in determining collisionprobability


33/143


CSMA/CD (Collision Detection)

CSMA/CD:carrier sensing, deferral as in CSMAcollisionsdetected within short timecolliding transmissions aborted, reducing channelwastage

collision detection: easy in wired LANs: measure signal strengths,compare transmitted, received signals

difficult in wireless LANs: receiver shut off whiletransmitting human analogy: the polite conversationalist


34/143


CSMA/CD collision detection

CSMA (C i l i l


35/143


CSMA (Carrier-sense multipleaccess)

If propagation time is much less than transmission time - allstations know that a transmission has started almostimmediatelyFirst listen for clear medium (carrier sense)

If medium idle, transmit

Collision occurs if another user starts transmitting withinthe time it takes for the first bit to reach this user(propagation delay)Collision detected by waiting round trip plus ACK contention

No ACK then retransmit

Max utilization depends on propagation time (medium length)and frame lengthLonger frame and shorter propagation gives better utilization


36/143


CSMA/CDWith CSMA, collision occupies medium forduration of transmission

Even if the station next to transmitting station collided,collision will be detected after >= RTT

Instead CD= collision detect: Stations listen whilst transmittingIf medium idle, transmitIf busy, listen for idle, then transmit (and listen)If collision detected, jam (send noise) then ceasetransmission

After jam, wait random time then start againBinary exponential back off


37/143


Collision Detection

Collision produces much higher signal voltage thansignalCollision detected if cable signal greater than

single station signalSignal attenuated over distanceLimit distance to 500m (10Base5) or 200m(10Base2)

For twisted pair (star-topology) activity on morethan one port is collisionFrames repeated, for CD to work


38/143


Why Jam? Tanenbaum: to make sure the sender does not miss thecollision (48 bits) Halsall: Ensure that the collision is detected by all stationsinvolved Stallings: Assure all staitons know that there has been acollision Keshav: Sequence of 512 bits to ensure that every activestation on the network knows that a collision happened andincrements its backoff counter; to ensure that all collidingstations agree that a collision has happened


39/143


CSMA/CDOperation


40/143


Collision detection

Collision detection can still take as long as


41/143


Collision detection

Transmitting stations may detect collisions almostimmediately, and stop transmission

Saves time and bandwidth

Will improve upon just CSMA only if collision isdetected during frame transmissionThis is possible if frames are long enough (andprop. Delay is short enough) so that collision isdetected while transmission

Guideline used in IEEE 802.3Frame transmission time >= 2*prop + delta


42/143


CSMA/CD efficiency

T prop = max prop between 2 nodes in LANt trans = time to transmit max-size frame

Efficiency goes to 1 as t prop goes to 0Goes to 1 as t

transgoes to infinity

Much better than ALOHA, but still decentralized,simple, and cheap

trans prop t t / 511

efficiency


43/143


Taking Turns MAC protocols

channel partitioning MAC protocols: share channel efficiently and fairly at high loadinefficient at low load: delay in channel access,1/N bandwidth allocated even if only 1 activenode!

Random access MAC protocols efficient at low load: single node can fullyutilize channelhigh load: collision overhead

taking turns protocols look for best of both worlds!


44/143


Taking Turns MAC protocols

Polling: master nodeinvites slave nodesto transmit in turn

concerns:polling overheadlatencysingle point of

failure (master)

Token passing: control token passed fromone node to nextsequentially.

token messageconcerns:token overheadlatency

single point of failure (token)


45/143


Summary of MAC protocols

What do you do with a shared media?Channel Partitioning, by time, frequency or code

Time Division,Code Division, Frequency Division

Random partitioning (dynamic), ALOHA, S-ALOHA, CSMA, CSMA/CD carrier sensing: easy in some technologies (wire), hard

in others (wireless) CSMA/CD used in Ethernet

Taking Turns polling from a central site, token passing


46/143


LAN technologies

Data link layer so far:services, error detection/correction, multipleaccess

Next: LAN technologiesaddressingEthernethubs, bridges, switches

802.11PPPATM


47/143


Ethernet

dominant LAN technology:cheap $20 for 100Mbs!first widely used LAN technologySimpler, cheaper than token LANs and ATM

Kept up with speed race: 10, 100, 1000 Mbps

Metcalfes Ethernet sketch


48/143


Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or othernetwork layer protocol packet) in Ethernet frame

Preamble: 7 bytes with pattern 10101010 followed by onebyte with pattern 10101011

used to synchronize receiver, sender clock rates

E h F S


49/143


Ethernet Frame Structure(more)

Addresses: 6 bytesif adapter receives frame with matching destinationaddress, or with broadcast address, it passes data inframe to net-layer protocol

otherwise, adapter discards frameType: indicates the higher layer protocol, mostlyIP but others may be supported such as NovellIPX and AppleTalk)

CRC:checked at receiver, if error is detected, theframe is simply dropped


50/143


Ethernet min frame lengthMin length needed for CD: for 2500m distancespecification, RT prop delay is determined to be50 sec

Frame transmission time >= 50 secAt 10Mbps, bits transmitted in 50 sec is 500


51/143


Unreliable, connectionless service

Connectionless: No handshaking between sendingand receiving adapter.Unreliable: receiving adapter doesnt send acks ornacks to sending adapter

stream of datagrams passed to network layer can havegapsgaps will be filled if app is using TCPotherwise, app will see the gaps


52/143


Ethernet uses CSMA/CD

No slotsadapter doesnt transmitif it senses that someother adapter istransmitting, that is,carrier sense transmitting adapteraborts when it sensesthat another adapter istransmitting, that is,collision detection

Before attempting aretransmission,adapter waits arandom time, that is,random access

E h CSMA/CD l i h


53/143


Ethernet CSMA/CD algorithm

1. Adaptor gets datagramfrom and creates frame2. If adapter senses channel

idle, it starts to transmitframe. If it senseschannel busy, waits untilchannel idle and thentransmits

3. If adapter transmitsentire frame withoutdetecting anothertransmission, the adapteris done with frame !

4. If adapter detectsanother transmission whiletransmitting, aborts andsends jam signal

5. After aborting, adapterenters exponentialbackoff : after the mthcollision, adapter choosesa K at random from{0,1,2,,2m-1}. Adapterwaits K*512 bit times andreturns to Step 2


54/143


Ethernets CSMA/CD (more)

Jam Signal: make sure allother transmitters areaware of collision; 48 bits;

Bit time: .1 microsec for 10Mbps Ethernet ;

for K=1023, wait time isabout 50 msec

Exponential Backoff: Goal : adapt retransmissionattempts to estimatedcurrent load

heavy load: random waitwill be longer

first collision: choose Kfrom {0,1}; delay is K x 512bit transmission timesafter second collision:

choose K from {0,1,2,3} after ten collisions, chooseK from {0,1,2,3,4,,1023}

See/interact with Javaapplet on AWL Web site:highly recommended !


55/143


Ethernet Technologies: 10Base210: 10Mbps;2: under 200 meters max cable lengththin coaxial cable in a bus topology

repeaters used to connect up to multiple segmentsrepeater repeats bits it hears on one interface toits other interfaces: physical layer device only!has become a legacy technology

10B T d 100B T


56/143


10BaseT and 100BaseT 10/100 Mbps rate; latter called fast ethernet

T stands for Twisted PairNodes connect to a hub: star topology; 100 mmax distance between nodes and hub

Hubs are essentially physical-layer repeaters:bits coming in one link go out all other linksno frame bufferingno CSMA/CD at hub: adapters detect collisionsprovides net management functionality

hub

nodes


57/143


Fast EthernetHigher bit rate media (100 Mbps) is available.Can it be used for Ethernet?Recall minimum frame length?

Set=512 bits by calculating time needed to detect collisions inEthernets of upto 2.5km length, of 10Mbps bit rate

Can higher bit rates be used without changing protocolspecs, and still make it work?Frame transmission time for 512 bit frame @100Mbps ~ 5 sec5 sec >= twice prop. delayShould be ~200m

This is what was Fast Ethernet: transmission media wasavailable, Ethernet wires were anyway not stretching very faraway - > perfect solution say, for e.g. server room LAN


58/143


Gbit Ethernet

use standard Ethernet frame formatallows for point-to-point links and sharedbroadcast channelsin shared mode, CSMA/CD is used; short distancesbetween nodes to be efficientuses hubs, called here Buffered Distributors Full-Duplex at 1 Gbps for point-to-point links

10 Gbps now !


59/143


Gigabit Ethernet1000 Mbps transmission media available.

Cannot continue reducing max lengthTwo enhancements to basic CSMA/CD

Carrier extension: Pad MAC frames to be atleast 4096 bits This means ~4 sec frame transmission time 2*Prop delay < 4 sec : Length restrictions


60/143


LAN Addresses and ARP

32-bit IP address: network-layer addressused to get datagram to destination IP network

(recall IP network definition)

LAN (or MAC or physical or Ethernet) address:used to get datagram from one interface to anotherphysically-connected interface (same network)

48 bit MAC address (for most LANs)burned in the adapter ROM


61/143


LAN Addresses and ARPEach adapter on LAN has unique LAN address


62/143


LAN Address (more)

MAC address allocation administered by IEEEmanufacturer 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 anotherIP hierarchical address NOT portabledepends on IP network to which node is attached


63/143


Recall earlier routing discussion

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2 223.1.3.1

223.1.3.27

A

B

E

Starting at A, given IPdatagram addressed to B:look up net. address of B, find Bon same net. as Alink layer send datagram to Binside link-layer frame

Bs MAC addr

As MAC addr

As IP addr

Bs IP addr IP payload

datagramframe

frame source,dest address

datagram source,dest address


64/143


ARP: Address Resolution Protocol

Each IP node (Host,Router) on LAN hasARPtableARP Table: IP/MAC

address mappings forsome LAN nodes< IP address; MAC address; TTL>

TTL (Time To Live): timeafter which addressmapping will be forgotten(typically 20 min)

Question: how to determineMAC address of Bknowing Bs IP address?


65/143


ARP protocol

A wants to send datagramto B, and A knows Bs IPaddress.Suppose Bs MAC addressis not in As ARP table. A broadcasts ARP querypacket, containing B's IPaddress

all machines on LANreceive ARP query

B receives ARP packet,replies to A with its (B's)MAC address

frame sent to As MACaddress (unicast)

A caches (saves) IP-to-MAC address pair in itsARP table until informationbecomes old (times out)

soft state: informationthat times out (goesaway) unless refreshed

ARP is plug-and-play: nodes create their ARP

tables withoutintervention from netadministrator

Routing to another LAN


66/143


Routing to another LANwalkthrough: send datagram from A to B via R

assume A knows B IP address

Two ARP tables in router R, one for each IPnetwork (LAN)

A

R B


67/143


68/143


Chapter 5 outline

5.1 Introduction andservices5.2 Error detectionand correction5.3Multiple accessprotocols5.4 LAN addressesand ARP5.5 Ethernet

5.6 Hubs, bridges, andswitches 5.7 Wireless links andLANs5.8 PPP5.9 ATM5.10 Frame Relay


69/143


Interconnecting LAN segments

HubsBridgesSwitches

Remark: switches are essentially multi-portbridges.What we say about bridges also holds forswitches!

Interconnecting with hubs


70/143


Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become onelarge collision domain

if a node in CS and a node EE transmit at same time: collision

Cant interconnect 10BaseT & 100BaseT

Bridges


71/143


BridgesLink layer device

stores and forwards Ethernet framesexamines frame header and selectively forwards frame based on MAC dest addresswhen frame is to be forwarded on segment,uses CSMA/CD to access segment

transparenthosts are unaware of presence of bridges

plug-and-play, self-learningbridges do not need to be configured

Bridges: traffic isolation


72/143


Bridges: traffic isolationBridge installation breaks LAN into LAN segmentsbridges filter packets:

same-LAN-segment frames not usuallyforwarded onto other LAN segments

segments become separate collision domains

bridge collisiondomaincollisiondomain

= hub= host

LAN (IP network)

LAN segment LAN segment


73/143


Forwarding

How do determine to which LAN segment toforward frame? Looks like a routing problem...


74/143


Self learning

A bridge has a bridge table entry in bridge table:

(Node LAN Address, Bridge Interface, Time Stamp)stale entries in table dropped (TTL can be 60 min)

bridges learn which hosts can be reached throughwhich interfaces

when frame received, bridge learns location ofsender: incoming LAN segmentrecords sender/location pair in bridge table

Filtering/Forwarding


75/143


Filtering/ForwardingWhen bridge receives a frame:

index bridge table using MAC dest address if entry found for destination

then{if dest on segment from which frame arrived

then drop the frameelse forward the frame on interface indicated

} else flood

forward on all but the interface on which the frame arrived


76/143


Bridge exampleSuppose C sends frame to D and D replies back with

frame to C.

Bridge receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table, bridge sends frame intointerfaces 2 and 3

frame received by D


77/143


Bridge Learning: example

D generates frame for C, sendsbridge receives frame

notes in bridge table that D is on interface 2 bridge knows C is on interface 1, so selectively forwardsframe to interface 1

h b kb


78/143


Interconnection without backbone

Not recommended for two reasons:

- single point of failure at Computer Science hub- all traffic between EE and SE must path overCS segment

B kb fi i


79/143


Backbone configuration

Recommended !

B id S i T


80/143


Bridges Spanning Tree

for increased reliability, desirable to haveredundant, alternative paths from source to destwith multiple paths, cycles result - bridges maymultiply and forward frame forever

solution: organize bridges in a spanning tree bydisabling subset of interfaces

Disabled


81/143


Multiple LANs


82/143


Needed: Routing

Complex large LANs need alternative routesLoad balancingFault tolerance

Bridge must decide whether to forward frameBridge must decide which LAN to forward frame onRouting selected for each source-destination pairof LANs

Done in configurationUsually least hop routeOnly changed when topology changes


83/143


Spanning TreeBridge automatically develops routing tableAutomatically update in response tochanges

Frame forwardingAddress learningLoop resolution


84/143


Frame forwardingMaintain forwarding database for each port

List station addresses reached through each portFor a frame arriving on port X:

Search forwarding database to see if MAC address is

listed for any port except XIf address not found, forward to all ports except XIf address listed for port Y, check port Y for blocking orforwarding state

Blocking prevents port from receiving or transmitting

If not blocked, transmit frame through port Y


85/143


Address Learning

When frame arrives at port X, it has come formthe LAN attached to port XUse the source address to update forwarding

database for port X to include that addressTimer on each entry in database (reset wheneverframe received)Each time frame arrives, source address checkedagainst forwarding database


86/143


Loop of Bridges


87/143


Spanning Tree AlgorithmCreates a logical, or active topology thatbehaves like a spanning tree

Makes alternate bridges redundant

Is run periodically, so will discover failures anduse alternate bridges if necessary

Reference: Fred Halsall: Data Communications, Computer Networks and Open Systems, 4 th Edition.


88/143


Spanning Tree AlgorithmVariables:

1. Each bridge has a Priority Value and a unique Identifier2. Each LAN segment has a Designated Cost (DC) inversely

proportional to the bit rate3. Each port of a bridge has a Path Cost (PC) = DC of the LAN

segment to which it is attached


89/143


Spanning Tree AlgorithmWorking: Bridges regularly exchange frames known as Bridge ProtocolData Units (BPDUs). This exchange does the following:

1. Bridge with highest priority and smallest ID is selected as root bridge.2. Each bridge determines for each port, the least cost path from root bridge

to this port. This is the Root Path Cost (RPC) for this port.a) Select the port which has the least RPC and designate it as the Root Port (RP).

This is the port which will be used for communicating with the root.3. Once root port is determined, one bridge port is selected for each LAN

segment as the designated bridge port (DP) to which frames will be sent forthat LAN segment.

a) This is a port (which is NOT a root port) which has the least path cost to the rootb) The ports of the root bridge are always DPs for the LAN segments connected to

the root bridge

4. The state of the bridge ports can be set either to forwarding or blocking.a) All ports that are either RPs or DPs are forwarding, the rest are blocking.


90/143


Topology InitializationBPDUs are sent to a broadcast MAC address of all bridges on the LANAll bridges initially assume they are the root bridgeEach BPDU contains (self ID, root ID, transmitting port ID, RPC of thisport)If necessary,

Update root ID based on received BPDUsAdd path cost of the port on which frame was received to the RPC in theframeSends out this new info on all other ports with all updated IdsProcedure repeated by all bridges

Will determine RPCs of each port Will select Root Ports based on this

Two or more bridges on the same segment will exchange BPDUs so thatdesignated bridge-port can be seleted


91/143


Topology ChangeRoot bridge regularly transmits BPDUs, forwarded by allbridges on all portsBridges will keep timers associated with each of itsforwarding portsWhen timers expire, procedure similar to topologyinitialization is done

Details

Some bridge features


92/143


Some bridge featuresIsolates collision domains resulting in higher totalmax throughputlimitless number of nodes and geographicalcoverageCan connect different Ethernet typesTransparent (plug -and-play): no configurationnecessary

Bridges vs Routers


93/143


Bridges vs. Routersboth store-and-forward devices

routers: network layer devices (examine network layerheaders)bridges are link layer devices

routers maintain routing tables, implement routing

algorithmsbridges maintain bridge tables, implement filtering,learning and spanning tree algorithms

Routers vs Bridges


94/143


Routers vs. Bridges

Bridges + and - + Bridge operation is simpler requiring less packet

processing+ Bridge tables are self learning- All traffic confined to spanning tree, even when

alternative bandwidth is available- Bridges do not offer protection from broadcast

storms

Routers vs Bridges


95/143


Routers vs. Bridges

Routers + and - + arbitrary topologies can be supported, cycling is

limited by TTL counters (and good routing protocols)

+ provide protection against broadcast storms- require IP address configuration (not plug and play)- require higher packet processing

bridges do well in small (few hundred hosts) whilerouters used in large networks (thousands of hosts)

Ethernet Switches


96/143


t e et Sw tc esEssentially a multi-

interface bridgelayer 2 (frame) forwarding,filtering using LANaddresses

Switching: A-to-A and B-to- B simultaneously, nocollisionslarge number of interfaces

often: individual hosts,star-connected into switchEthernet, but nocollisions!

Ethernet Switches


97/143


Ethernet Switches

cut-through switching: frame forwardedfrom input to output port without awaitingfor assembly of entire frame

slight reduction in latencycombinations of shared/dedicated,10/100/1000 Mbps interfaces

Not an atypical LAN (IP network)


98/143


Not an atypical LAN (IP network)

Dedicated

Shared

Summary comparison


99/143


Summary comparison

hubs bridges routers switches

traffic

isolation

no yes yes yes

plug & play yes yes no yes

optimalrouting

no no yes no

cutthrough

yes no no yes

Chapter 5 outline


100/143


Chapter 5 outline

5.1 Introduction andservices5.2 Error detectionand correction

5.3Multiple accessprotocols5.4 LAN addressesand ARP5.5 Ethernet

5.6 Hubs, bridges, andswitches5.7 Wireless links andLANs

5.8 PPP5.9 ATM5.10 Frame Relay

IEEE 802 11 Wireless LAN


101/143


IEEE 802.11 Wireless LAN

802.11b 2.4-5 GHz unlicensedradio spectrumup to 11 Mbps

direct sequence spreadspectrum (DSSS) inphysical layer

all hosts use samechipping code

widely deployed, usingbase stations

802.11a 5-6 GHz rangeup to 54 Mbps

802.11g 2.4-5 GHz rangeup to 54 Mbps

All use CSMA/CA formultiple access

All have base-stationand ad-hoc networkversions

Base station approach


102/143


ppWireless host communicates with a base station

base station = access point (AP)Basic Service Set (BSS) (a.k.a. cell) contains:

wireless hostsaccess point (AP): base station

BSSs combined to form distribution system (DS)

Ad Hoc Network approach


103/143


ppNo AP (i.e., base station)wireless hosts communicate with each other

to get packet from wireless host A to B mayneed to route through wireless hosts X,Y,Z

Applications:laptop meeting in conference room, car interconnection of personal devices battlefield

IETF MANET (Mobile Ad hoc Networks)working group

IEEE 802.11: multiple access


104/143


pCollision if 2 or more nodes transmit at same time

CSMA makes sense:get all the bandwidth if youre the only one transmitting shouldnt cause a collision if you sense another transmission

Collision detection doesnt work: hidden terminal

problem

IEEE 802 11 MAC Protocol: CSMA/CA


105/143


IEEE 802.11 MAC Protocol: CSMA/CA802.11 CSMA: sender- if sense channel idle for

DISF sec.then transmit entire frame(no collision detection)

-if sense channel busythen binary backoff

802.11 CSMA receiver

- if received OK return ACK after SIFS(ACK is needed due tohidden terminal problem)

Collision avoidance mechanisms


106/143


Problem: two nodes, hidden from each other, transmit completeframes to base stationwasted bandwidth for long duration !

Solution:

small reservation packetsnodes track reservation interval with internalnetwork allocation vector (NAV)

Collision Avoidance: RTS-CTS


107/143


exchangesender transmits shortRTS (request to send)packet: indicatesduration of transmission

receiver replies withshort CTS (clear to send)packet

notifying (possibly hidden)nodes

hidden nodes will nottransmit for specifiedduration: NAV

Collision Avoidance: RTS-CTS


108/143


exchangeRTS and CTS short:collisions less likely, of

shorter durationend result similar to

collision detectionIEEE 802.11 allows:CSMACSMA/CA: reservationspolling from AP

A word about Bluetooth


109/143


A word about Bluetooth

Low-power, small radius,wireless networkingtechnology

10-100 meters

omnidirectionalnot line-of-sight infraredInterconnects gadgets2.4-2.5 GHz unlicensed

radio bandup to 721 kbps

Interference fromwireless LANs, digitalcordless phones,microwave ovens:

frequency hopping helpsMAC protocol supports:error correctionARQ

Each node has a 12-bitaddress

Chapter 5 outline


110/143


Chapter 5 outline


5.3Multiple accessprotocols5.4 LAN addressesand ARP

5.5 Ethernet


5.8 PPP 5.9 ATM5.10 Frame Relay

Point to Point Data Link Control


111/143


Point to Point Data Link Control

one sender, one receiver, one link: easier thanbroadcast link:no Media Access Controlno need for explicit MAC addressing

e.g., dialup link, ISDN linepopular point-to-point DLC protocols:PPP (point-to-point protocol)HDLC: High level data link control (Data linkused to be considered high layer in protocolstack!

PPP Design Requirements [RFC 1557]


112/143


packet framing: encapsulation of network-layerdatagram in data link frame

carry network layer data of any network layerprotocol (not just IP) at same time ability to demultiplex upwards

bit transparency: must carry any bit pattern in thedata fielderror detection (no correction)

connection liveness: detect, signal link failure tonetwork layernetwork layer address negotiation: endpoint canlearn/configure each others network address

PPP non-requirements


113/143


PPP non requirements

no error correction/recoveryno flow controlout of order delivery OKno need to support multipoint links (e.g., polling)

Error recovery, flow control, data re-orderingall relegated to higher layers!

PPP Data Frame


114/143


PPP Data Frame

Flag: delimiter (framing)Address: does nothing (only one option)Control: does nothing; in the future possiblemultiple control fieldsProtocol: upper layer protocol to which framedelivered (eg, PPP-LCP, IP, IPCP, etc)

PPP Data Frame


115/143


PPP Data Frame

info: upper layer data being carriedcheck: cyclic redundancy check for errordetection

Byte Stuffing


116/143


Byte Stuffing data transparency requirement: data field mustbe allowed to include flag pattern

Q: is received data or flag?

Sender: adds (stuffs) extra < 01111110> byteafter each < 01111110>data byteReceiver:

two 01111110 bytes in a row: discard first byte,continue data receptionsingle 01111110: flag byte

Byte Stuffing


117/143


Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte intransmitted data

PPP Data Control Protocol


118/143


PPP Data Control ProtocolBefore exchanging network-

layer data, data link peersmustconfigure PPP link(max.frame length,authentication)learn/configure network layer information

for IP: carry IP ControlProtocol (IPCP) msgs(protocol field: 8021) toconfigure/learn IPaddress

Chapter 5 outline


119/143


Chapter 5 outline



5.5 Ethernet


5.8 PPP5.9 ATM 5.10 Frame Relay

Asynchronous Transfer Mode: ATM


120/143

5: DataLink Layer 5a-

120

y1990s/00 standard for high -speed (155Mbps to622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal: integrated, end-end transport of carry voice,video, data

meeting timing/QoS requirements of voice, video(versus Internet best-effort model)next generation telephony: technical roots intelephone worldpacket-switching (fixed length packets, calledcells) using virtual circuits

ATM architecture


121/143


121

adaptation layer: only at edge of ATM network data segmentation/reassembly

roughly analogous to Internet transport layerATM layer: network layer cell switching, routing

physical layer

ATM: network or link layer?


122/143


122

: etwo o aye ?Vision:end-to-end

transport: ATM fromdesktop to desktop ATM is a networktechnology

Reality: used to connectIP backbone routersIP over ATM ATM as switched

link layer,connecting IProuters

ATM Adaptation Layer (AAL)


123/143


123

p y ( )

ATM Adaptation Layer (AAL): adapts upperlayers (IP or native ATM applications) to ATMlayer belowAAL present only in end systems , not in switches

AAL layer segment (header/trailer fields, data)fragmented across multiple ATM cellsanalogy: TCP segment in many IP packets

ATM Adaptation Layer (AAL) [more]


124/143


Different versions of AAL layers, depending on ATMservice class:AAL1:for CBR (Constant Bit Rate) services, e.g. circuit emulationAAL2:for VBR (Variable Bit Rate) services, e.g., MPEG videoAAL5:for data (e.g., IP datagrams)

AAL PDU

ATM cell

User data

AAL5 - Simple And EfficientAL (SEAL)


125/143


AL (SEAL)AAL5: low overhead AAL used to carry IPdatagrams

4 byte cyclic redundancy checkPAD ensures payload multiple of 48byteslarge AAL5 data unit to be fragmented into 48-byte ATM cells

ATM Layer


126/143


Service: transport cells across ATM network

analogous to IP network layervery different services than IP network layer

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteedminimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss) nocongestionnocongestionyes

no

Guarantees ?

ATM Layer: Virtual Circuits


127/143


yVC transport: cells carried on VC from source to dest

call setup, teardown for each call before data can floweach packet carries VC identifier (not destination ID)every switch on source- dest path maintain state for eachpassing connection

link,switch resources (bandwidth, buffers) may be allocated toVC: to get circuit-like perf.Permanent VCs (PVCs)

long lasting connections

typically: permanent route between to IP routers Switched VCs (SVC): dynamically set up on per-call basis

ATM VCs


128/143


Advantages of ATM VC approach: QoS performance guarantee for connectionmapped to VC (bandwidth, delay, delay jitter)

Drawbacks of ATM VC approach:

Inefficient support of datagram trafficone PVC between each source/dest pair) doesnot scale (N*2 connections needed)SVC introduces call setup latency, processingoverhead for short lived connections

ATM Layer: ATM cell


129/143


y5-byte ATM cell header48-byte payload

Why?: small payload -> short cell-creation delayfor digitized voicehalfway between 32 and 64 (compromise!)

Cell header

Cell format

ATM cell header


130/143


VCI: virtual channel IDwillchange from link to link thru netPT: Payload type (e.g. RM cell versus data cell)CLP: Cell Loss Priority bit

CLP = 1 implies low priority cell, can bediscarded if congestion

HEC: Header Error Checksumcyclic redundancy check

ATM Physical Layer (more)


131/143


y y ( )

Two pieces (sublayers) of physical layer:Transmission Convergence Sublayer (TCS): adaptsATM layer above to PMD sublayer belowPhysical Medium Dependent:depends on physicalmedium being used

TCS Functions: Header checksum generation: 8 bits CRCCelldelineationWith unstructured PMD sublayer, transmissionof idle cells when no data cells to send

ATM Physical Layer


132/143


y y

Physical Medium Dependent (PMD) sublayer SONET/SDH : transmission frame structure (like acontainer carrying bits);

bit synchronization;bandwidth partitions (TDM);several speeds: OC3 = 155.52 Mbps; OC12 = 622.08Mbps; OC48 = 2.45 Gbps, OC192 = 9.6 Gbps

TI/T3 : transmission frame structure (oldtelephone hierarchy): 1.5 Mbps/ 45 Mbpsunstructured : just cells (busy/idle)

IP-Over-ATMIP over ATM


133/143


Classic IP only

3 networks (e.g.,LAN segments)MAC (802.3) and IPaddresses

replace network

(e.g., LAN segment)with ATM networkATM addresses, IPaddresses

ATMnetwork

EthernetLANs

EthernetLANs

IP-Over-ATM


134/143


Issues: IP datagrams intoATM AAL5 PDUsfrom IP addressesto ATM addresses

just like IPaddresses to802.3 MACaddresses!

ATMnetwork

EthernetLANs

Datagram Journey in IP-over-ATM Network


135/143


at Source Host: IP layer maps between IP, ATM dest address (using ARP)passes datagram to AAL5AAL5 encapsulates data, segments cells, passes to ATM layer

ATM network: moves cell along VC to destination at Destination Host:

AAL5 reassembles cells into original datagramif CRC OK, datagram is passed to IP

Chapter 5 outline


136/143




5.5 Ethernet


5.8 PPP5.9 ATM5.10 Frame Relay

Frame Relay


137/143


Like ATM: wide area network technologiesVirtual-circuit oriented

origins in telephony worldcan be used to carry IP datagrams

can thus be viewed as link layers by IP

protocol

Frame Relay


138/143


Designed in late 80s, widely deployed in the 90s Frame relay service:

no error controlend-to-end congestion control

Frame Relay (more)


139/143


Designed to interconnect corporate customer LANstypically permanent VCs: pipe carrying aggregatetraffic between two routersswitched VCs: as in ATM

corporate customer leases FR service from publicFrame Relay network (e.g., Sprint, ATT)

Frame Relay (more)


140/143


Flag bits, 01111110, delimit frameaddress:

10 bit VC ID field3 congestion control bits FECN: forward explicit congestion

notification (frame experienced congestion

on path) BECN: congestion on reverse path DE: discard eligibility

addressflags data CRC flags

Frame Relay -VC Rate Control


141/143


Committed Information Rate (CIR) defined, guaranteed for each VC negotiated at VC set up timecustomer pays based on CIR

DE bit: Discard Eligibility bit Edge FR switch measures traffic rate for each VC;marks DE bit

DE = 0: high priority, rate compliant frame; deliverat all costs DE = 1: low priority, eligible for congestion discard

Frame Relay - CIR & Frame Marking


142/143


Access Rate : rate R of the access link betweensource router (customer) and edge FR switch (provider); 64Kbps


143/143

principles behind data link layer services:error detection, correctionsharing a broadcast channel: multiple accesslink layer addressing, ARP

link layer technologies: Ethernet, hubs,bridges, switches,IEEE 802.11 LANs, PPP,ATM, Frame Relay

journey down the protocol stack now OVER! next stops: multimedia, security, networkmanagement

Chap 5 Data Link Layer

Documents

Transcript of Chap 5 Data Link Layer