note Chapter5
LinkLayerandLANs
A 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 obviously
represent a lot of work on our part. In return for use, we only ask the
following:
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, and
note our copyright of this material.
ComputerNetworking:ATop
DownApproach
5thedition.
JimKurose,KeithRoss
AddisonWesley,April2009.
Thanks and enjoy! JFK/KWR
All material copyright 1996-2009
J.F Kurose and K.W. Ross, All Rights Reserved
5: DataLink Layer
5-1
Chapter5:TheDataLinkLayer
Ourgoals:
understandprinciplesbehinddatalinklayerservices:
r errordetection,correction
r sharingabroadcastchannel:multipleaccess
r linklayeraddressing
r reliabledatatransfer,flowcontrol:done!
instantiationandimplementationofvariouslinklayer
technologies
5: DataLink Layer
5-2
LinkLayer
5.1Introductionandservices
5.6Linklayerswitches
correction
5.3Multipleaccessprotocols
5.4LinklayerAddressing
5.5Ethernet
5.8Linkvirtualization:MPLS
5.2Errordetectionand
5.7PPP
5.9Adayinthelifeofaweb
request
5: DataLink Layer
5-3
LinkLayer:Introduction
Someterminology:
hostsandroutersarenodes
c o m m unic a tio nc h a nne ls th a t
c o nne c ta d ja c e ntno d e s a lo ng
c o m m unic a tio np a th a re links
r
r
r
wire d links
wire le s s links
LANs
la y e r2 p a c ke tis a frame,
e nc a p s ula te s d a ta g ra m
data-link layerhasresponsibilityof
transferringdatagramfromonenode
toadjacentnodeoveralink
5: DataLink Layer
5-4
Linklayer:context
datagramtransferredbydifferent
linkprotocolsoverdifferent
links:
r
e.g.,Ethernetonfirstlink,frame
relayonintermediatelinks,802.11
onlastlink
eachlinkprotocolprovides
differentservices
r
e.g.,mayormaynotproviderdt
overlink
transportationanalogy
tripfromPrincetontoLausanne
r
r
r
limo:PrincetontoJFK
plane:JFKtoGeneva
train:GenevatoLausanne
tourist=datagram
transportsegment=
communicationlink
transportationmode=linklayer
protocol
travelagent=routingalgorithm
5: DataLink Layer
5-5
LinkLayerServices
framing,linkaccess:
r
r
r
encapsulatedatagramintoframe,addingheader,trailer
channelaccessifsharedmedium
MACaddressesusedinframeheaderstoidentifysource,dest
differentfromIPaddress!
reliabledeliverybetweenadjacentnodes
r welearnedhowtodothisalready(chapter3)!
r seldomusedonlowbiterrorlink(fiber,sometwistedpair)
r wirelesslinks:higherrorrates
Q:whybothlinklevelandendendreliability?
5: DataLink Layer
5-6
LinkLayerServices(more)
flowcontrol:
r
pacingbetweenadjacentsendingandreceivingnodes
errordetection:
r errorscausedbysignalattenuation,noise.
r receiverdetectspresenceoferrors:
signalssenderforretransmissionordropsframe
errorcorrection:
r receiveridentifiesandcorrectsbiterror(s)withoutresortingto
retransmission
halfduplexandfullduplex
r withhalfduplex,nodesatbothendsoflinkcantransmit,butnotatsame
time
5: DataLink Layer
5-7
Whereisthelinklayerimplemented?
ineachandeveryhost
linklayerimplementedin
adaptor(akanetwork
interfacecardNIC)
r
r
Ethernetcard,PCMCIcard,
802.11card
implementslink,physicallayer
attachesintohostssystembuses
combinationofhardware,
software,firmware
host schematic
application
transport
network
link
cpu
memory
host
bus
(e.g., PCI)
controller
link
physical
physical
transmission
network adapter
card
5: DataLink Layer
5-8
AdaptorsCommunicating
datagram
datagram
controller
controller
receiving host
sending host
datagram
frame
sendingside:
r encapsulatesdatagraminframe
r addserrorcheckingbits,rdt,flow
control,etc.
receivingside
r looksforerrors,rdt,flowcontrol,
etc
r extractsdatagram,passestoupper
layeratreceivingside
5: DataLink Layer
5-9
LinkLayer
5.1Introductionandservices
5.6Linklayerswitches
correction
5.3Multipleaccessprotocols
5.4LinklayerAddressing
5.5Ethernet
5.8Linkvirtualization:MPLS
5.2Errordetectionand
5.7PPP
5.9Adayinthelifeofaweb
request
5: DataLink Layer
5-
ErrorDetection
EDC=ErrorDetectionandCorrectionbits(redundancy)
D=Dataprotectedbyerrorchecking,mayincludeheaderfields
Errordetectionnot100%reliable!
protocolmaymisssomeerrors,butrarely
largerEDCfieldyieldsbetterdetectionandcorrection
otherwise
5: DataLink Layer
5-
ParityChecking
SingleBitParity:
Detect single bit errors
TwoDimensionalBitParity:
Detect and correct single bit errors
0
0
5: DataLink Layer
5-
Internetchecksum(review)
Goal:detecterrors(e.g.,flippedbits)intransmittedpacket(note:used
attransportlayeronly)
Sender:
treatsegmentcontentsassequence
of16bitintegers
checksum:addition(1s
complementsum)ofsegment
contents
senderputschecksumvalueinto
UDPchecksumfield
Receiver:
computechecksumofreceivedsegment
checkifcomputedchecksumequals
checksumfieldvalue:
r NOerrordetected
r YESnoerrordetected.But
maybeerrorsnonetheless?
5: DataLink Layer
5-
Checksumming:CyclicRedundancyCheck
viewdatabits,D,asabinarynumber
chooser+1bitpattern(generator),G
goal:chooserCRCbits,R,suchthat
r
r
r
<D,R>exactlydivisiblebyG(modulo2)
receiverknowsG,divides<D,R>byG.Ifnonzeroremainder:errordetected!
candetectallbursterrorslessthanr+1bits
widelyusedinpractice(Ethernet,802.11WiFi,ATM)
5: DataLink Layer
5-
CRCExample
Want:
D.2rXORR=nG
equivalently:
D.2r=nGXORR
equivalently:
ifwedivideD.2rbyG,
wantremainderR
D.2r
R=remainder[]
G
5: DataLink Layer
5-
LinkLayer
5.1Introductionandservices
5.6Linklayerswitches
correction
5.3Multipleaccessprotocols
5.4LinklayerAddressing
5.5Ethernet
5.8Linkvirtualization:MPLS
5.2Errordetectionand
5.7PPP
5.9Adayinthelifeofaweb
request
5: DataLink Layer
5-
MultipleAccessLinksandProtocols
Twotypesoflinks:
pointtopoint
r PPPfordialupaccess
r pointtopointlinkbetweenEthernetswitchandhost
broadcast(sharedwireormedium)
r oldfashionedEthernet
r upstreamHFC
r 802.11wirelessLAN
sharedwire(e.g.,
cabledEthernet)
sharedRF
(e.g.,802.11WiFi)
sharedRF
(satellite)
humansata
cocktailparty
(sharedair,acoustical)
5: DataLink Layer
5-
MultipleAccessprotocols
singlesharedbroadcastchannel
twoormoresimultaneoustransmissionsbynodes:interference
r collisionifnodereceivestwoormoresignalsatthesametime
multipleaccessprotocol
distributedalgorithmthatdetermineshownodessharechannel,i.e.,
determinewhennodecantransmit
communicationaboutchannelsharingmustusechannelitself!
r
nooutofbandchannelforcoordination
5: DataLink Layer
5-
IdealMultipleAccessProtocol
BroadcastchannelofrateRbps
1.whenonenodewantstotransmit,itcansendatrateR.
2.whenMnodeswanttotransmit,eachcansendataveragerate
R/M
3.fullydecentralized:
r
r
nospecialnodetocoordinatetransmissions
nosynchronizationofclocks,slots
4.simple
5: DataLink Layer
5-
MACProtocols:ataxonomy
Threebroadclasses:
ChannelPartitioning
r
r
dividechannelintosmallerpieces(timeslots,frequency,code)
allocatepiecetonodeforexclusiveuse
RandomAccess
r channelnotdivided,allowcollisions
r recoverfromcollisions
Takingturns
r nodestaketurns,butnodeswithmoretosendcantakelongerturns
5: DataLink Layer
5-
ChannelPartitioningMACprotocols:TDMA
TDMA:timedivisionmultipleaccess
accesstochannelin"rounds"
eachstationgetsfixedlengthslot(length=pkttranstime)ineach
round
unusedslotsgoidle
example:6stationLAN,1,3,4havepkt,slots2,5,6idle
6slot
frame
1
3
4
1
3
4
5: DataLink Layer
5-
ChannelPartitioningMACprotocols:FDMA
FDMA:frequencydivisionmultipleaccess
channelspectrumdividedintofrequencybands
eachstationassignedfixedfrequencyband
unusedtransmissiontimeinfrequencybandsgoidle
example:6stationLAN,1,3,4havepkt,frequencybands2,5,6idle
FDMcable
frequencybands
time
5: DataLink Layer
5-
RandomAccessProtocols
Whennodehaspackettosend
r transmitatfullchanneldatarateR.
r noaprioricoordinationamongnodes
twoormoretransmittingnodescollision,
randomaccessMACprotocolspecifies:
r howtodetectcollisions
r howtorecoverfromcollisions(e.g.,viadelayedretransmissions)
ExamplesofrandomaccessMACprotocols:
r slottedALOHA
r ALOHA
r CSMA,CSMA/CD,CSMA/CA
5: DataLink Layer
5-
SlottedALOHA
Assumptions:
allframessamesize
timedividedintoequalsize
slots(timetotransmit1frame)
nodesstarttotransmitonlyslot
beginning
nodesaresynchronized
if2ormorenodestransmitin
slot,allnodesdetectcollision
Operation:
whennodeobtainsfreshframe,
transmitsinnextslot
r ifnocollision:nodecansend
newframeinnextslot
r ifcollision:noderetransmits
frameineachsubsequentslot
withprob.puntilsuccess
5: DataLink Layer
5-
SlottedALOHA
Pros
singleactivenodecan
continuouslytransmitatfull
rateofchannel
highlydecentralized:only
slotsinnodesneedtobein
sync
simple
Cons
collisions,wastingslots
idleslots
nodesmaybeabletodetect
collisioninlessthantimeto
transmitpacket
clocksynchronization
5: DataLink Layer
5-
SlottedAlohaefficiency
Efficiency:longrun
fractionofsuccessfulslots
(manynodes,allwithmanyframes
tosend)
suppose:Nnodeswithmany
framestosend,eachtransmits
inslotwithprobabilityp
probthatgivennodehas
successinaslot=p(1p)N-1
probthatanynodehasa
success=Np(1p)N-1
maxefficiency:findp*that
maximizes
Np(1p)N-1
formanynodes,takelimitof
Np*(1p*)N-1 asNgoesto
infinity,gives:
Maxefficiency=1/e=.37
Atbest:channel
usedforuseful
transmissions37%
oftime!
5: DataLink Layer
!
5-
Pure(unslotted)ALOHA
unslottedAloha:simpler,nosynchronization
whenframefirstarrives
r transmitimmediately
collisionprobabilityincreases:
r framesentatt0collideswithotherframessentin[t01,t0+1]
5: DataLink Layer
5-
PureAlohaefficiency
P(successbygivennode)=P(nodetransmits).
P(noothernodetransmitsin[p01,p0].
P(noothernodetransmitsin[p01,p0]
=p.(1p)N-1.(1p)N-1
= p.(1p)2(N-1)
choosingoptimumpandthenlettingn>infty...
=1/(2e)=.18
evenworsethanslottedAloha!
5: DataLink Layer
5-
CSMA(CarrierSenseMultipleAccess)
CSMA:listenbeforetransmit:
Ifchannelsensedidle:transmitentireframe
Ifchannelsensedbusy,defertransmission
humananalogy:dontinterruptothers!
5: DataLink Layer
5-
CSMAcollisions
spatiallayoutofnodes
collisionscanstilloccur:
propagationdelaymeans
twonodesmaynothear
eachotherstransmission
collision:
entirepackettransmission
timewasted
note:
roleofdistance&propagationdelayin
determiningcollisionprobability
5: DataLink Layer
5-
CSMA/CD(CollisionDetection)
CSMA/CD:carriersensing,deferralasinCSMA
collisionsdetectedwithinshorttime
r collidingtransmissionsaborted,reducingchannelwastage
r
collisiondetection:
r easyinwiredLANs:measuresignalstrengths,comparetransmitted,
receivedsignals
r difficultinwirelessLANs:receivedsignalstrengthoverwhelmedby
localtransmissionstrength
humananalogy:thepoliteconversationalist
5: DataLink Layer
5-
CSMA/CDcollisiondetection
5: DataLink Layer
5-
TakingTurnsMACprotocols
channelpartitioningMACprotocols:
r sharechannelefficientlyandfairlyathighload
r inefficientatlowload:delayinchannelaccess,1/Nbandwidth
allocatedevenifonly1activenode!
RandomaccessMACprotocols
r efficientatlowload:singlenodecanfullyutilizechannel
r highload:collisionoverhead
takingturnsprotocols
lookforbestofbothworlds!
5: DataLink Layer
5-
TakingTurnsMACprotocols
Polling:
masternodeinvitesslave
nodestotransmitinturn
typicallyusedwithdumb
slavedevices
concerns:
r
r
r
pollingoverhead
latency
singlepointoffailure
(master)
data
poll
master
data
slaves
5: DataLink Layer
5-
TakingTurnsMACprotocols
Tokenpassing:
controltoken passedfrom
onenodetonextsequentially.
tokenmessage
concerns:
r
r
r
tokenoverhead
latency
singlepointoffailure(token)
T
(nothing
tosend)
T
data
5: DataLink Layer
5-
SummaryofMACprotocols
channelpartitioning,bytime,frequencyorcode
r TimeDivision,FrequencyDivision
randomaccess(dynamic),
r ALOHA,SALOHA,CSMA,CSMA/CD
r carriersensing:easyinsometechnologies(wire),hardinothers(wireless)
r CSMA/CDusedinEthernet
r CSMA/CAusedin802.11
takingturns
r pollingfromcentralsite,tokenpassing
r Bluetooth,FDDI,IBMTokenRing
5: DataLink Layer
5-
LinkLayer
5.1Introductionandservices
5.6Linklayerswitches
correction
5.3Multipleaccessprotocols
5.4LinkLayerAddressing
5.5Ethernet
5.8Linkvirtualization:MPLS
5.2Errordetectionand
5.7PPP
5.9Adayinthelifeofaweb
request
5: DataLink Layer
5-
MACAddressesandARP
32bitIPaddress:
networklayeraddress
r usedtogetdatagramtodestinationIPsubnet
r
MAC(orLANorphysicalorEthernet)address:
function:getframefromoneinterfacetoanotherphysically
connectedinterface(samenetwork)
r 48bitMACaddress(formostLANs)
r
burnedinNICROM,alsosometimessoftwaresettable
5: DataLink Layer
5-
LANAddressesandARP
EachadapteronLANhasuniqueLANaddress
Broadcastaddress=
FFFFFFFFFFFF
1A2FBB7609AD
7165F72B0853
LAN
(wiredor
wireless)
=adapter
5823D7FA20B0
0CC4116FE398
5: DataLink Layer
5-
LANAddress(more)
MACaddressallocationadministeredbyIEEE
manufacturerbuysportionofMACaddressspace(toassure
uniqueness)
analogy:
(a)MACaddress:likeSocialSecurityNumber
(b)IPaddress:likepostaladdress
MACflataddressportability
r
canmoveLANcardfromoneLANtoanother
IPhierarchicaladdressNOTportable
r addressdependsonIPsubnettowhichnodeisattached
5: DataLink Layer
5-
ARP:AddressResolutionProtocol
Question:howtodetermine
MACaddressofB
knowingBsIPaddress?
137.196.7.78
1A2FBB7609AD
137.196.7.23
137.196.7.14
EachIPnode(host,router)on
LANhasARPtable
ARPtable:IP/MACaddress
mappingsforsomeLANnodes
<IPaddress;MACaddress;TTL>
r
TTL(TimeToLive):timeafter
whichaddressmappingwillbe
forgotten(typically20min)
LAN
7165F72B0853
137.196.7.88
5823D7FA20B0
0CC4116FE398
5: DataLink Layer
5-
ARPprotocol:SameLAN(network)
AwantstosenddatagramtoB,and
BsMACaddressnotinAsARP
table.
AbroadcastsARPquerypacket,
containingB'sIPaddress
r destMACaddress=FFFF
FFFFFFFF
r allmachinesonLANreceive
ARPquery
BreceivesARPpacket,repliestoA
withits(B's)MACaddress
r
framesenttoAsMACaddress
(unicast)
Acaches(saves)IPtoMACaddress
pairinitsARPtableuntil
informationbecomesold(timesout)
r softstate:informationthat
timesout(goesaway)unless
refreshed
ARPisplugandplay:
r nodescreatetheirARPtables
withoutinterventionfromnet
administrator
5: DataLink Layer
5-
Addressing:routingtoanotherLAN
walkthrough:senddatagramfromAtoBviaR
assumeAknowsBsIPaddress
88-B2-2F-54-1A-0F
74-29-9C-E8-FF-55
A
111.111.111.111
E6-E9-00-17-BB-4B
1A-23-F9-CD-06-9B
222.222.222.220
111.111.111.110
111.111.111.112
R
222.222.222.221
222.222.222.222
B
49-BD-D2-C7-56-2A
CC-49-DE-D0-AB-7D
twoARPtablesinrouterR,oneforeachIPnetwork(LAN)
5: DataLink Layer
5-
AcreatesIPdatagramwithsourceA,destinationB
AusesARPtogetRsMACaddressfor111.111.111.110
AcreateslinklayerframewithR'sMACaddressasdest,framecontainsAtoB
IPdatagram
Thisisareallyimportant
AsNICsendsframe
examplemakesureyou
understand!
RsNICreceivesframe
RremovesIPdatagramfromEthernetframe,seesitsdestinedtoB
RusesARPtogetBsMACaddress
RcreatesframecontainingAtoBIPdatagramsendstoB
88-B2-2F-54-1A-0F
74-29-9C-E8-FF-55
A
E6-E9-00-17-BB-4B
111.111.111.111
1A-23-F9-CD-06-9B
222.222.222.220
111.111.111.110
111.111.111.112
R
222.222.222.221
222.222.222.222
B
49-BD-D2-C7-56-2A
CC-49-DE-D0-AB-7D
5: DataLink Layer
5-
LinkLayer
5.1Introductionandservices
5.6Linklayerswitches
correction
5.3Multipleaccessprotocols
5.4LinkLayerAddressing
5.5Ethernet
5.8Linkvirtualization:MPLS
5.2Errordetectionand
5.7PPP
5.9Adayinthelifeofaweb
request
5: DataLink Layer
5-
Ethernet
dominantwiredLANtechnology:
cheap$20forNIC
firstwidelyusedLANtechnology
simpler,cheaperthantokenLANsandATM
keptupwithspeedrace:10Mbps10Gbps
MetcalfesEthernet
sketch
5: DataLink Layer
5-
Startopology
bustopologypopularthroughmid90s
r allnodesinsamecollisiondomain(cancollidewitheachother)
today:startopologyprevails
r activeswitchincenter
r eachspokerunsa(separate)Ethernetprotocol(nodesdonotcollidewith
eachother)
switch
bus:coaxialcable
star
5: DataLink Layer
5-
EthernetFrameStructure
SendingadapterencapsulatesIPdatagram(orothernetworklayer
protocolpacket)inEthernetframe
Preamble:
7byteswithpattern10101010followedbyonebytewithpattern
10101011
usedtosynchronizereceiver,senderclockrates
5: DataLink Layer
5-
EthernetFrameStructure(more)
Addresses:6bytes
r ifadapterreceivesframewithmatchingdestinationaddress,orwith
broadcastaddress(egARPpacket),itpassesdatainframetonetworklayer
protocol
r otherwise,adapterdiscardsframe
Type:indicateshigherlayerprotocol(mostlyIPbutotherspossible,
e.g.,NovellIPX,AppleTalk)
CRC:checkedatreceiver,iferrorisdetected,frameisdropped
5: DataLink Layer
5-
Ethernet:Unreliable,connectionless
connectionless:NohandshakingbetweensendingandreceivingNICs
unreliable:receivingNICdoesntsendacksornackstosendingNIC
r streamofdatagramspassedtonetworklayercanhavegaps(missingdatagrams)
r gapswillbefilledifappisusingTCP
r otherwise,appwillseegaps
EthernetsMACprotocol:unslottedCSMA/CD
5: DataLink Layer
5-
EthernetCSMA/CDalgorithm
1.NICreceivesdatagramfromnetwork 4.IfNICdetectsanothertransmission
layer,createsframe
whiletransmitting,abortsand
sendsjamsignal
2.IfNICsenseschannelidle,starts
frametransmissionIfNICsenses 5.Afteraborting,NICenters
channelbusy,waitsuntilchannel
exponential DataLinkframe!
5: backoff:after
idle,thentransmits
mthcollision,NICchoosesKat
randomfrom
3.IfNICtransmitsentireframe
{0,1,2,,2m1}.NICwaitsK512bit
withoutdetectinganother
times,returnstoStep2
transmission,NICisdonewith
Layer
5-
EthernetsCSMA/CD(more)
JamSignal:makesureallother
transmittersareawareofcollision;
48bits
Bittime:.1microsecfor10Mbps
Ethernet;
forK=1023,waittimeisabout50
msec
See/interactwithJava
appletonAWLWebsite:
highlyrecommended!
ExponentialBackoff:
Goal:adaptretransmissionattemptsto
estimatedcurrentload
r heavyload:randomwaitwillbe
longer
firstcollision:chooseKfrom{0,1};
delayisK512bittransmissiontimes
aftersecondcollision:chooseKfrom
{0,1,2,3}
aftertencollisions,chooseKfrom
{0,1,2,3,4,,1023}
5: DataLink Layer
5-
CSMA/CDefficiency
Tprop=maxpropdelaybetween2nodesinLAN
ttrans=timetotransmitmaxsizeframe
efficiency =
1
1 + 5t prop /t trans
efficiencygoesto1
r astpropgoesto0
r
asttransgoestoinfinity
betterperformancethanALOHA:andsimple,cheap,decentralized !
5: DataLink Layer
5-
802.3EthernetStandards:Link&PhysicalLayers
manydifferentEthernetstandards
r commonMACprotocolandframeformat
r differentspeeds:2Mbps,10Mbps,100Mbps,1Gbps,10Gbps
r differentphysicallayermedia:fiber,cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T2
100BASE-FX
100BASE-T4
100BASE-SX
100BASE-BX
copper(twister
pair)physicallayer
fiberphysicallayer
5: DataLink Layer
5-
Manchesterencoding
usedin10BaseT
eachbithasatransition
allowsclocksinsendingandreceivingnodestosynchronizetoeach
other
r
noneedforacentralized,globalclockamongnodes!
Hey,thisisphysicallayerstuff!
5: DataLink Layer
5-
LinkLayer
5.1Introductionandservices
5.2Errordetectionand
correction
5.3Multipleaccessprotocols
5.4LinklayerAddressing
5.5Ethernet
5.6Linklayerswitches,LANs,
VLANs
5.7PPP
5.8Linkvirtualization:MPLS
5.9Adayinthelifeofaweb
request
5: DataLink Layer
5-
Hubs
physicallayer(dumb)repeaters:
r bitscominginonelinkgooutallotherlinksatsamerate
r allnodesconnectedtohubcancollidewithoneanother
r noframebuffering
r noCSMA/CDathub:hostNICsdetectcollisions
twistedpair
hub
5: DataLink Layer
5-
Switch
linklayerdevice:smarterthanhubs,takeactiverole
r store,forwardEthernetframes
r examineincomingframesMACaddress,selectivelyforward
frametooneormoreoutgoinglinkswhenframeistobe
forwardedonsegment,usesCSMA/CDtoaccesssegment
transparent
r hostsareunawareofpresenceofswitches
plugandplay,selflearning
r switchesdonotneedtobeconfigured
5: DataLink Layer
5-
Switch:allowsmultiplesimultaneoustransmissions
A
hostshavededicated,direct
connectiontoswitch
switchesbufferpackets
Ethernetprotocolusedoneach
incominglink,butnocollisions;full
duplex
r
B
1
6
5
2
3
4
C
eachlinkisitsowncollisiondomain
switching:AtoAandBtoB
simultaneously,withoutcollisions
r
C
notpossiblewithdumbhub
B
A
switchwithsixinterfaces
(1,2,3,4,5,6)
5: DataLink Layer
5-
SwitchTable
A
Q:howdoesswitchknowthatA
reachableviainterface4,Breachable C
viainterface5?
A:eachswitchhasaswitchtable,each
entry:
r
Q:howareentriescreated,maintained
inswitchtable?
somethinglikearoutingprotocol?
1
6
5
(MACaddressofhost,interfacetoreach
host,timestamp)
lookslikearoutingtable!
r
B
2
3
4
C
B
A
switchwithsixinterfaces
(1,2,3,4,5,6)
5: DataLink Layer
5-
Switch:selflearning
switchlearnswhichhostscanbe
reachedthroughwhichinterfaces
r
r
Source:A
Dest:A
AA
A
C
B
whenframereceived,switch
learnslocationofsender:
incomingLANsegment
recordssender/locationpairin
switchtable
1
6
5
2
3
4
C
B
A
MACaddrinterfaceTTL
A
1
60
Switchtable
(initiallyempty)
5: DataLink Layer
5-
Switch:framefiltering/forwarding
Whenframereceived:
1.recordlinkassociatedwithsendinghost
2.indexswitchtableusingMACdestaddress
3. if entryfoundfordestination
then {
if destonsegmentfromwhichframearrived
thendroptheframe
elseforwardtheframeoninterfaceindicated
}
elseflood
forwardonallbuttheinterface
onwhichtheframearrived
5: DataLink Layer
5-
Selflearning,
forwarding:
example
Source:A
Dest:A
AA
A
C
B
framedestination
unknown: flood
6
AA
1
5
destinationAlocation
known:
selectivesend
2
3
4
C
AA
B
A
MACaddrinterfaceTTL
A
A
1
4
60
60
Switchtable
(initiallyempty)
5: DataLink Layer
5-
Interconnectingswitches
switchescanbeconnectedtogether
S4
S1
A
B
S3
S2
C
F
D
E
I
G
H
Q:sendingfromAtoGhowdoesS1knowtoforwardframe
destinedtoFviaS4andS3?
A:selflearning!(worksexactlythesameasinsingleswitchcase!)
5: DataLink Layer
5-
Selflearningmultiswitchexample
SupposeCsendsframetoI,IrespondstoC
S4
1
S1
S2
A
B
2
C
S3
F
D
E
I
G
H
Q:showswitchtablesandpacketforwardinginS1,S2,S3,S4
5: DataLink Layer
5-
Institutionalnetwork
toexternal
network
mailserver
router
webserver
IPsubnet
5: DataLink Layer
5-
Switchesvs.Routers
bothstoreandforwarddevices
r routers:networklayerdevices(examinenetworklayerheaders)
r switchesarelinklayerdevices
routersmaintainroutingtables,implementroutingalgorithms
switchesmaintainswitchtables,implementfiltering,learning
algorithms
5: DataLink Layer
5-
VLANs:motivation
Whatswrongwiththispicture?
Whathappensif:
CSusermovesofficetoEE,but
wantsconnecttoCSswitch?
singlebroadcastdomain:
r
Computer
Science
alllayer2broadcasttraffic(ARP,
DHCP)crossesentireLAN
(security/privacy,efficiencyissues)
eachlowestlevelswitchhasonlyfew
Electrical
Engineering
Computer
Engineering
portsinuse
5: DataLink Layer
5-
VLANs
PortbasedVLAN:switchportsgrouped(byswitch
managementsoftware)sothatsinglephysical
switch
1
Switch(es)supportingVLAN
capabilitiescanbeconfigured
todefinemultiplevirtual
LANSoversinglephysical
LANinfrastructure.
9
15
2
Virtual Local
Area Network
7
8
10
16
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
operatesasmultiplevirtualswitches
1
7
9
15
2
8
10
16
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-16)
5: DataLink Layer
5-
PortbasedVLAN
router
trafficisolation:framesto/from
ports18canonlyreachports18
r
canalsodefineVLANbasedonMAC
addressesofendpoints,ratherthan
switchport
1
bedynamicallyassignedamong
VLANs
9
15
2
dynamicmembership:portscan
7
8
10
16
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
forwardingbetweenVLANS:donevia
routing(justaswithseparateswitches)
r
inpracticevendorssellcombinedswitches
plusrouters
5: DataLink Layer
5-
VLANSspanningmultipleswitches
1
7
9
15
1
3
5
7
2
8
10
16
2
4
6
8
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
Ports 2,3,5 belong to EE VLAN
Ports 4,6,7,8 belong to CS VLAN
trunkport:carriesframesbetweenVLANSdefinedovermultiple
physicalswitches
r
r
framesforwardedwithinVLANbetweenswitchescantbevanilla802.1frames
(mustcarryVLANIDinfo)
802.1qprotocoladds/removedadditionalheaderfieldsforframesforwarded
betweentrunkports
5: DataLink Layer
5-
802.1QVLANframeformat
Type
802.1frame
802.1Qframe
2-byte Tag Protocol Identifier
(value: 81-00)
Recomputed
CRC
Tag Control Information (12 bit VLAN ID field,
3 bit priority field like IP TOS)
5: DataLink Layer
5-
LinkLayer
5.1Introductionandservices
5.6Linklayerswitches
correction
5.3Multipleaccessprotocols
5.4LinkLayerAddressing
5.5Ethernet
5.8Linkvirtualization:MPLS
5.2Errordetectionand
5.7PPP
5.9Adayinthelifeofaweb
request
5: DataLink Layer
5-
PointtoPointDataLinkControl
onesender,onereceiver,onelink:easierthanbroadcastlink:
noMediaAccessControl
r noneedforexplicitMACaddressing
r e.g.,dialuplink,ISDNline
popularpointtopointDLCprotocols:
r PPP(pointtopointprotocol)
r HDLC:Highleveldatalinkcontrol(Datalinkusedtobe
consideredhighlayerinprotocolstack!
r
5: DataLink Layer
5-
PPPDesignRequirements[RFC1557]
packetframing:encapsulationofnetworklayerdatagramindata
linkframe
r carrynetworklayerdataofanynetworklayerprotocol(notjust
IP)atsametime
r abilitytodemultiplexupwards
bittransparency:mustcarryanybitpatterninthedatafield
errordetection(nocorrection)
connectionliveness:detect,signallinkfailuretonetworklayer
networklayeraddressnegotiation:endpointcanlearn/configure
eachothersnetworkaddress
5: DataLink Layer
5-
PPPnonrequirements
noerrorcorrection/recovery
noflowcontrol
outoforderdeliveryOK
noneedtosupportmultipointlinks(e.g.,polling)
Errorrecovery,flowcontrol,datareordering
allrelegatedtohigherlayers!
5: DataLink Layer
5-
PPPDataFrame
Flag:delimiter(framing)
Address:doesnothing(onlyoneoption)
Control:doesnothing;inthefuturepossiblemultiplecontrolfields
Protocol:upperlayerprotocoltowhichframedelivered(eg,PPP
LCP,IP,IPCP,etc)
5: DataLink Layer
5-
PPPDataFrame
info:upperlayerdatabeingcarried
check:cyclicredundancycheckforerrordetection
5: DataLink Layer
5-
ByteStuffing
datatransparencyrequirement:datafieldmustbeallowedto
includeflagpattern<01111110>
r Q:isreceived<01111110>dataorflag?
Sender:adds(stuffs)extra<01111110>byteaftereach<
01111110>databyte
Receiver:
r two01111110bytesinarow:discardfirstbyte,continuedata
reception
r single01111110:flagbyte
5: DataLink Layer
5-
ByteStuffing
flagbyte
pattern
indata
tosend
flagbytepatternplus
stuffedbyteintransmitteddata
5: DataLink Layer
5-
PPPDataControlProtocol
Beforeexchangingnetworklayerdata,
datalinkpeersmust
configurePPPlink(max.frame
length,authentication)
learn/configurenetwork
layerinformation
r forIP:carryIPControlProtocol
(IPCP)msgs(protocolfield:8021)
toconfigure/learnIPaddress
5: DataLink Layer
5-
LinkLayer
5.1Introductionandservices
5.6Linklayerswitches
correction
5.3Multipleaccessprotocols
5.4LinkLayerAddressing
5.5Ethernet
5.8Linkvirtualization:MPLS
5.2Errordetectionand
5.7PPP
5.9Adayinthelifeofaweb
request
5: DataLink Layer
5-
Virtualizationofnetworks
Virtualizationofresources:powerfulabstractioninsystems
engineering:
computingexamples:virtualmemory,virtualdevices
r Virtualmachines:e.g.,java
r IBMVMosfrom1960s/70s
layeringofabstractions:dontsweatthedetailsofthelowerlayer,
onlydealwithlowerlayersabstractly
5: DataLink Layer
5-
TheInternet:virtualizingnetworks
1974:multipleunconnectednets
ARPAnet
r dataovercablenetworks
r packetsatellitenetwork(Aloha)
r packetradionetwork
r
ARPAnet
"A Protocol for Packet Network Intercommunication",
V. Cerf, R. Kahn, IEEE Transactions on Communications,
May, 1974, pp. 637-648.
differingin:
addressingconventions
r packetformats
r errorrecovery
r routing
r
satellitenet
5: DataLink Layer
5-
TheInternet:virtualizingnetworks
Gateway:
Internetworklayer(IP):
embedinternetworkpacketsinlocal
addressing:internetworkappearsas
packetformatorextractthem
single,uniformentity,despiteunderlying
route(atinternetworklevel)tonext
localnetworkheterogeneity
gateway
networkofnetworks
gateway
ARPAnet
satellitenet
5: DataLink Layer
5-
Cerf&KahnsInternetworkArchitecture
Whatisvirtualized?
twolayersofaddressing:internetworkandlocalnetwork
newlayer(IP)makeseverythinghomogeneousatinternetworklayer
underlyinglocalnetworktechnology
cable
r satellite
r 56Ktelephonemodem
r today:ATM,MPLS
invisibleatinternetworklayer.Lookslikealinklayer
technologytoIP!
r
5: DataLink Layer
5-
ATMandMPLS
ATM,MPLSseparatenetworksintheirownright
r differentservicemodels,addressing,routingfromInternet
viewedbyInternetaslogicallinkconnectingIProuters
r justlikedialuplinkisreallypartofseparatenetwork(telephone
network)
ATM,MPLS:oftechnicalinterestintheirownright
5: DataLink Layer
5-
AsynchronousTransferMode:ATM
1990s/00 standard for high-speed (155Mbpsto622
Mbpsandhigher)BroadbandIntegratedServiceDigitalNetwork
architecture
Goal:integrated,endendtransportofcarryvoice,video,data
r meetingtiming/QoSrequirementsofvoice,video(versusInternet
besteffortmodel)
r nextgenerationtelephony:technicalrootsintelephoneworld
r packetswitching(fixedlengthpackets,calledcells)using
virtualcircuits
5: DataLink Layer
5-
Multiprotocollabelswitching(MPLS)
initialgoal:speedupIPforwardingbyusingfixedlengthlabel
(insteadofIPaddress)todoforwarding
r
r
borrowingideasfromVirtualCircuit(VC)approach
butIPdatagramstillkeepsIPaddress!
PPP or Ethernet
header
MPLS header
label
20
IP header
remainder of link-layer frame
Exp S TTL
3
1
5
5: DataLink Layer
5-
MPLScapablerouters
a.k.a.labelswitchedrouter
forwardspacketstooutgoinginterfacebasedonlyonlabel
value(dontinspectIPaddress)
r
MPLSforwardingtabledistinctfromIPforwardingtables
signalingprotocolneededtosetupforwarding
r RSVPTE
r forwardingpossiblealongpathsthatIPalonewouldnotallow(e.g.,
sourcespecificrouting)!!
r useMPLSfortrafficengineering
mustcoexistwithIPonlyrouters
5: DataLink Layer
5-
MPLSforwardingtables
in
out
label label dest
interface
10
12
8
out
A
D
A
0
0
1
in
out
label label dest
interface
out
0
R4
R5
6
A
1
12
R6
10
9
D
0
0
1
R3
D
1
0
0
R2
in
out
label label dest
interface
8
6
A
out
0
in
outR1
label label dest
interface
6
-
A
A
out
0
5: DataLink Layer
5-
LinkLayer
5.1Introductionandservices
5.6Linklayerswitches
correction
5.3Multipleaccessprotocols
5.4LinkLayerAddressing
5.5Ethernet
5.8Linkvirtualization:MPLS
5.2Errordetectionand
5.7PPP
5.9Adayinthelifeofaweb
request
5: DataLink Layer
5-
Synthesis:adayinthelifeofawebrequest
journeydownprotocolstackcomplete!
r application,transport,network,link
puttingitalltogether:synthesis!
r goal:identify,review,understandprotocols(atalllayers)
involvedinseeminglysimplescenario:requestingwwwpage
r scenario:studentattacheslaptoptocampusnetwork,
requests/receiveswww.google.com
5: DataLink Layer
5-
Adayinthelife:scenario
DNS server
browser
Comcast network
68.80.0.0/13
school network
68.80.2.0/24
web page
web server
64.233.169.105
Googles network
64.233.160.0/19
5: DataLink Layer
5-
AdayinthelifeconnectingtotheInternet
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
connectinglaptopneedstogetits
ownIPaddress,addroffirsthop
router,addrofDNSserver:use
DHCP
DHCP
DHCPrequestencapsulatedin
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
UDP,encapsulatedinIP,
encapsulatedin802.1Ethernet
router
(runsDHCP)
Ethernetframebroadcast(dest:
FFFFFFFFFFFF)onLAN,receivedat
routerrunningDHCPserver
EthernetdemuxedtoIP
demuxed,UDPdemuxedtoDHCP
5: DataLink Layer
5-
AdayinthelifeconnectingtotheInternet
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCPserverformulatesDHCP
ACKc o nta ining c lie nts IP
a d d re s s ,IP a d d re s s o ffirs t
h o p ro u te rfo rc lie nt,na m e &
IP a d d re s s o fDNS s e rve r
encapsulationatDHCPserver,
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
frameforwarded(switch
learning) th ro ug h LAN,
d e m u ltip le xing a tc lie nt
router
(runsDHCP)
DHCPclientreceivesDHCPACK
reply
ClientnowhasIPaddress,knowsname&addrofDNS
server,IPaddressofitsfirsthoprouter
5: DataLink Layer
5-
AdayinthelifeARP(beforeDNS,beforeHTTP)
DNS
DNS
DNS
ARP query
beforesendingHTTP request,needIP
DNS
UDP
IP
ARP
Eth
Phy
addressofwww.google.com:DNS
DNSquerycreated,encapsulatedinUDP,
ARP
ARP reply
Eth
Phy
encapsulatedinIP,encasulatedinEth.In
ordertosendframetorouter,needMAC
addressofrouterinterface:ARP
ARP querybroadcast,receivedby
router,whichreplieswithARP reply
givingMACaddressofrouterinterface
clientnowknowsMACaddressoffirst
hoprouter,socannowsendframe
containingDNSquery
5: DataLink Layer
5-
AdayinthelifeusingDNS
DNS
DNS
DNS
DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
DNS
UDP
IP
Eth
Phy
DNS server
Comcast network
68.80.0.0/13
IPdatagramforwardedfromcampus
IPdatagramcontainingDNSquery
forwardedviaLANswitchfrom
clientto1sthoprouter
networkintocomcastnetwork,routed
(tablescreatedbyRIP, OSPF, IS-IS
and/orBGProutingprotocols)toDNS
server
demuxedtoDNSserver
DNSserverrepliestoclientwithIP
addressofwww.google.com
5: DataLink Layer
5-
AdayinthelifeTCPconnectioncarryingHTTP
HTTP
HTTP
TCP
IP
Eth
Phy
SYNACK
SYN
SYNACK
SYN
SYNACK
SYN
tosendHTTPrequest,client
SYNACK
SYN
SYNACK
SYN
SYN
SYNACK
TCP
IP
Eth
Phy
web server
64.233.169.105
firstopensTCP socketto
webserver
TCPSYN segment(step1in3
wayhandshake)inter-domain
routedtowebserver
webserverrespondswithTCP
SYNACK(step2in3way
handshake)
TCPconnection established!
5: DataLink Layer
5-
AdayinthelifeHTTPrequest/reply
HTTP
HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
webpagefinally (!!!)displayed
HTTP requestsentinto
TCPsocket
HTTP
HTTP
HTTP
HTTP
HTTP
TCP
IP
Eth
Phy
web server
64.233.169.105
IPdatagramcontainingHTTP
requestroutedtowww.google.com
webserverrespondswithHTTP
reply(containingwebpage)
IPdatgramcontainingHTTPreply
routedbacktoclient
5: DataLink Layer
5-
Chapter5:Summary
principlesbehinddatalinklayerservices:
r errordetection,correction
r sharingabroadcastchannel:multipleaccess
r linklayeraddressing
instantiationandimplementationofvariouslinklayertechnologies
Ethernet
r switchedLANS,VLANs
r PPP
r virtualizednetworksasalinklayer:MPLS
synthesis:adayinthelifeofawebrequest
r
5: DataLink Layer
5-
Chapter5:letstakeabreath
journeydownprotocolstackcomplete(exceptPHY)
solidunderstandingofnetworkingprinciples,practice
..couldstophere.butlotsofinterestingtopics!
r wireless
r multimedia
r security
r networkmanagement
5: DataLink Layer
5-