OperatingSystemsandNetworks

NetworkLecture4:LinkLayer(2)

AdrianPerrigNetworkSecurityGroupETHZürich

2

PendingIssues• Howtoreadthecoursetextbook?• Howtopreparefortheexamgiventhatthereisahugeamountofmaterial?

3

WhereweareintheCourse• FinishingofftheLinkLayer!

– Buildsonthephysicallayertotransfer framesoverconnectedlinks

PhysicalLink

NetworkTransportApplication

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Topics1. Framing

– Delimitingstart/endofframes

2. Errordetection/correction– Handlingerrors

Done

DSL

5

Topics(2)3. Retransmissions

– Handlingloss

4. MultipleAccess– ClassicEthernet,802.11

5. Switching– ModernEthernet

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Retransmissions(ARQ)(§3.3)• Twostrategiestohandleerrors:1. Detecterrorsandretransmitframe(AutomaticRepeatreQuest,

ARQ)

2. Correcterrorswithanerrorcorrectingcode

Donethis

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ContextonReliability• Whereinthestackshouldweplacereliabilityfunctions?

PhysicalLink

NetworkTransportApplication

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ContextonReliability(2)• Everywhere!Itisakeyissue

– Different layerscontributedifferently

PhysicalLink

NetworkTransportApplication

Recoveractions(correctness)

Maskerrors(performanceoptimization)

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ARQ(AutomaticRepeatreQuest)• ARQoftenusedwhenerrorsarecommonormustbecorrected– E.g.,WiFi,andTCP(later)

• Rulesatsenderandreceiver:– Receiverautomaticallyacknowledges correct frameswithanACK

– Senderautomaticallyresendsafteratimeout,untilanACKisreceived

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ARQ(2)• Normaloperation(noloss,noerror)

Frame

ACKTimeout Time

Sender Receiver

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ARQ(3)• Lossandretransmission

Frame

Timeout Time

Sender Receiver

Frame

ACK

X

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SoWhat’sTrickyAboutARQ?• Twonon-trivialissues:

– Howlongtosetthetimeout?– Howtoavoidacceptingduplicate framesasnewframes

• Wantperformanceinthecommoncaseandcorrectnessalways

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Timeouts• Timeoutshouldbe:

– Nottoobig(linkgoesidle)– Nottoosmall(spuriousresend)

• FairlyeasyonaLAN– Clearworstcase,littlevariation

• FairlydifficultovertheInternet– Muchvariation,noobviousbound– We’llrevisitthiswithTCP(later)

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Duplicates• WhathappensifanACKislost?

Frame

ACK

X

Frame

ACKTimeout

Sender Receiver

NewFrame??

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Duplicates(2)• Orthetimeoutisearly?

Frame

ACK

Frame

ACK

Timeout

Sender Receiver

NewFrame??

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SequenceNumbers• FramesandACKsmustbothcarrysequencenumbersforcorrectness

• Todistinguishthecurrentframefromthenextone,asinglebit(twonumbers)issufficient– CalledStop-and-Wait

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Stop-and-Wait• Inthenormalcase:

Time

Sender Receiver

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Stop-and-Wait(2)• Inthenormalcase:

Frame0

ACK0Timeout Time

Sender Receiver

Frame1

ACK1

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Stop-and-Wait(3)• WithACKloss:

Frame0

ACK0

X

Frame0

ACK0Timeout

Sender Receiver

It’saResend!

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Stop-and-Wait(4)• Withearlytimeout:

Frame0

ACK0

Frame0

ACK0

Timeout

Sender Receiver

It’saResend

OK…

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LimitationofStop-and-Wait• Itallowsonlyasingleframetobeoutstandingfromthesender:– GoodforLAN,inefficient forhighBD(bandwidth-delay product)

• Ex:R=1Mbps,D=50ms– Howmanyframes/sec? IfR=10Mbps?

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SlidingWindow• Generalizationofstop-and-wait

– AllowsWframestobeoutstanding– CansendWframesperRTT (=2D)

– Variousoptionsfornumbering frames/ACKsandhandlingloss• WilllookatalongwithTCP(later)

Multiplexing(§2.5.3,2.5.4)• Multiplexingisthenetworkwordforthesharingofaresource

• Classicscenarioissharingalinkamongdifferentusers– TimeDivisionMultiplexing(TDM)– FrequencyDivisionMultiplexing(FDM)

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TimeDivisionMultiplexing(TDM)

• Userstaketurnsonafixedschedule

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2 2 2 2

FrequencyDivisionMultiplexing (FDM)• Putdifferentusersondifferentfrequencybands

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Overall FDMchannel

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TDMversusFDM• InTDMausersendsatahighrateafractionofthetime;inFDM,ausersendsatalowrateallthetime

Rate

TimeFDM

TDM

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TDM/FDMUsage• Staticallydividearesource

– Suitedforcontinuous traffic,fixednumberofusers

• Widelyusedintelecommunications– TVandradiostations(FDM)– GSM(2Gcellular)allocatescallsusingTDMwithinFDM

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MultiplexingNetworkTraffic• Networktrafficisbursty

– ON/OFF sources– Loadvariesgreatlyovertime

Rate

TimeRate

Time

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MultiplexingNetworkTraffic(2)• Networktrafficisbursty

– Inefficient toalwaysallocateusertheirON needswithTDM/FDM

Rate

TimeRate

Time

R

R

MultiplexingNetworkTraffic(3)• Multipleaccessschemesmultiplexusersaccordingtotheirdemands– forgainsofstatisticalmultiplexing

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Rate

TimeRate

Time

Rate

Time

R

R

R’<2R

Twousers,eachneedR TogethertheyneedR’<2R

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MultipleAccess• Wewilllookattwokindsofmultipleaccessprotocols1. Randomized.Nodesrandomize their resourceaccessattempts

– Goodforlowloadsituations

2. Contention-free. Nodesordertheirresourceaccessattempts– Goodforhighloadorguaranteedqualityofservicesituations

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RandomizedMultipleAccess(§4.2.1-4.2.2,4.3.1-4.3.3)

• Howdonodesshareasinglelink?Whosendswhen,e.g.,inWiFI?– Explorewithasimplemodel

• Assumeno-oneisincharge;thisisadistributedsystem

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RandomizedMultipleAccess(2)• Wewillexplorerandommultipleaccesscontrol ormediumaccesscontrol (MAC)protocols– ThisisthebasisforclassicEthernet– Remember:datatraffic isbursty

Zzzz..Busy! Hohum

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ALOHANetwork• SeminalcomputernetworkconnectingtheHawaiianislandsinthelate1960s– Whenshouldnodessend?– A newprotocolwasdevisedbyNormAbramson…

Hawaii

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ALOHAProtocol• Simpleidea:

– Nodejustsendswhenithastraffic.– Iftherewasacollision(noACKreceived) thenwaitarandomtimeandresend

• That’sit!

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ALOHAProtocol(2)• Someframeswillbelost,butmanymaygetthrough…

• Goodidea?

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ALOHAProtocol(3)• Simple,decentralizedprotocolthatworkswellunderlowload!

• Notefficientunderhighload– Analysisshowsatmost18%efficiency– Improvement:divide timeintoslotsandefficiency goesupto36%

• We’lllookatotherimprovements

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ClassicEthernet• ALOHAinspiredBobMetcalfetoinventEthernetforLANsin1973– Nodesshare10Mbpscoaxialcable– Hugelypopular in1980s,1990s

:©2009IEEE

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CSMA(CarrierSenseMultipleAccess)

• ImproveALOHAbylisteningforactivitybeforewesend(Doh!)– Candoeasilywithwires,notwireless

• Sodoesthiseliminatecollisions?– Whyorwhynot?

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CSMA(2)• Stillpossibletolistenandhearnothingwhenanothernodeissendingbecauseofdelay

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CSMA/CD(withCollisionDetection)• Canreducethecostofcollisionsbydetectingthemandaborting(Jam)therestoftheframetime– Again,wecandothiswithwires

XX X X X X X XJam! Jam!

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CSMA/CDComplications• Wanteveryonewhocollidestoknowthatithappened

– Timewindow inwhichanodemayhearofacollisionis2Dseconds

X

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CSMA/CDComplications(2)• Imposeaminimumframesizethatlastsfor2D seconds

– Sonodecan’tfinishbeforecollision– Ethernetminimumframeis64bytes

X

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CSMA“Persistence”• Whatshouldanodedoifanothernodeissending?

• Idea:Waituntilitisdone,andsend

Whatnow?

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CSMA“Persistence”(2)• Problemisthatmultiplewaitingnodeswillqueueupthencollide– Moreload,moreofaproblem

Now! Now!Uhoh

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CSMA“Persistence”(3)• Intuitionforabettersolution

– IfthereareNqueuedsenders,wewanteachtosendnextwithprobability1/N

Sendp=½WhewSendp=½

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BinaryExponentialBackoff(BEB)• Cleverlyestimatestheprobability

– 1stcollision,wait0or1frametimes– 2ndcollision,waitfrom0to3times– 3rdcollision,waitfrom0to7times…

• BEBdoublesintervalforeachsuccessivecollision– Quicklygetslargeenoughtowork– Veryefficient inpractice

ClassicEthernet,orIEEE802.3• MostpopularLANofthe1980s,1990s

– 10Mbpsoversharedcoaxialcable,withbasebandsignals– Multipleaccesswith“1-persistentCSMA/CDwithBEB”– Withreasonableparameters,85%efficiency (Book4.3.3)

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EthernetFrameFormat• Hasaddressestoidentifythesenderandreceiver• CRC-32forerrordetection;noACKsorretransmission• Startofframeidentifiedwithphysicallayerpreamble

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PacketfromNetworklayer(IP)

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ModernEthernet• Basedonswitches,notmultipleaccess,butstillcalledEthernet– We’llgettoitlater

Switch

TwistedpairSwitchports

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WirelessMultipleAccess(§4.2.5,4.4)• Howdowirelessnodesshareasinglelink?(Yes,thisisWiFi!)– Buildonoursimple,wiredmodel

Send? Send?

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WirelessComplications• Wirelessismorecomplicatedthanthewiredcase(Surprise!)1. Nodesmayhavedifferentareasofcoverage– doesn’tfit

CarrierSense2. Nodescan’thearwhilesending– can’tCollisionDetect

≠CSMA/CD

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DifferentCoverageAreas• Wirelesssignalisbroadcastandreceivednearby,wherethereissufficientSNR

HiddenTerminals• NodesAandCarehiddenterminals whensendingtoB

– Can’theareachother(tocoordinate)yetcollideatB– Wewanttoavoidtheinefficiency ofcollisions

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ExposedTerminals• BandCareexposedterminals whensendingtoAandD

– Canheareachotheryetdon’tcollideatreceiversAandD– Wewanttosendconcurrently toincreaseperformance

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NodesCan’tHearWhileSending• Withwires,detectingcollisions(andaborting)lowerstheircost

• Morewastedtimewithwireless

Time XXXXXXXXX

XXXXXXXXX

WirelessCollision

ResendX

X

WiredCollision

Resend

PossibleSolution:MACA• MACAusesashorthandshakeinsteadofCSMA(Karn,1990)

– 802.11usesarefinementofMACA(later)

• Protocolrules:1. AsendernodetransmitsaRTS(Request-To-Send,withframelength)2. ThereceiverreplieswithaCTS(Clear-To-Send,withframelength)3. SendertransmitstheframewhilenodeshearingtheCTSstaysilent– CollisionsontheRTS/CTSarestillpossible,butlesslikely

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MACA– HiddenTerminals• AàBwithhiddenterminalC

1. AsendsRTS,toB

DCBA

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MACA– HiddenTerminals(2)• AàBwithhiddenterminalC

2. BsendsCTS,toA,andCtoo

DCBARTS

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MACA– HiddenTerminals(3)• AàBwithhiddenterminalC

2. BsendsCTS,toA,andCtoo

DCBARTS

CTSCTS

Alert!

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MACA– HiddenTerminals(4)• AàBwithhiddenterminalC

3. AsendsframewhileCdefers

FrameQuiet...

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MACA– ExposedTerminals• BàA,CàDasexposedterminals– BandCsendRTStoAandD

DCBA

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MACA– ExposedTerminals(2)• BàA,CàDasexposedterminals– AandDsendCTStoBandC

DCBARTSRTS

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MACA– ExposedTerminals(3)• BàA,CàDasexposedterminals– AandDsendCTStoBandC

DCBARTSRTS

CTSCTS

AllOKAllOK

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MACA– ExposedTerminals(4)• BàA,CàDasexposedterminals– AandDsendCTStoBandC

DCBAFrameFrame

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802.11,orWiFi• VerypopularwirelessLAN

startedinthe1990s• Clientsgetconnectivity froma

(wired)AP(AccessPoint)• It’samulti-accessproblemJ• Variousflavorshavebeen

developed overtime– Faster,morefeatures

AccessPoint

Client

ToNetwork

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802.11PhysicalLayer• Uses20/40MHzchannelsonISMbands

– 802.11b/g/non2.4GHz– 802.11a/non5GHz

• OFDMmodulation (except legacy802.11b)– Differentamplitudes/phasesforvaryingSNRs– Ratesfrom6to54Mbpspluserrorcorrection– 802.11nusesmultipleantennas;see“802.11withMultipleAntennasfor

Dummies”

802.11LinkLayer• MultipleaccessusesCSMA/CA(next);RTS/CTSoptional• FramesareACKed andretransmittedwithARQ• Funkyaddressing(threeaddresses!)duetoAP• Errorsaredetectedwitha32-bitCRC• Many,manyfeatures(e.g.,encryption,powersave)

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PacketfromNetworklayer(IP)

802.11CSMA/CAforMultipleAccess• Senderavoidscollisionsbyinsertingsmallrandomgaps

– E.g.,whenbothBandCsend,Cpicksasmallergap,goesfirst

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Time

Send?

Send?

TheFutureof802.11(Guess)• LikelyubiquitousforInternetconnectivity– Greaterdiversity,fromlow- tohigh-enddevices

• Innovationinphysical layerdrivesspeed– Andpower-efficientoperationtoo

• Moreseamlessintegrationofconnectivity– Toomanualnow,andlimited(e.g.,device-to-device)

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Contention-FreeMultipleAccess(§4.2.3)• Anotherapproachtomultipleaccess

– Basedonturns,notrandomization

1

32

4

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IssueswithRandomMultipleAccess• CSMAisgoodunderlowload:– Grantsimmediateaccess– Littleoverhead(fewcollisions)

• Butnotsogoodunderhighload:– Highoverhead(expectcollisions)– Accesstimevaries(lucky/unlucky)

• Wewanttodobetterunderload!

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Turn-TakingMultipleAccessProtocols

• Theydefineanorderinwhichnodesgetachancetosend– Orpass,ifnotrafficatpresent

• Wejustneedsomeordering…– E.g.,TokenRing– E.g.,nodeaddresses

TokenRing• Arrangenodesinaring;tokenrotates“permissiontosend”toeachnodeinturn

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Node

Directionoftransmission

Token

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Turn-TakingAdvantages• Fixedoverheadwithnocollisions–Moreefficientunderload

• Regularchancetosendwithnounluckynodes– Predictableservice,easilyextendedtoguaranteedqualityofservice

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Turn-TakingDisadvantages• Complexity–Morethingsthatcangowrongthanrandomaccessprotocols!• E.g.,whatifthetokenislost?• Electaleaderwhomanagesthetoken,whattodoifleadercrashes?

– Higheroverheadatlowload

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Turn-TakinginPractice• Regularlytriedasanimprovementofferingbetterservice– E.g.,qualitiesofservice

• Butrandommultipleaccessishardtobeat– Simple,andusuallygoodenough– Scalesfromfewtomanynodes

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LANSwitches(§4.3.4,4.8.1-4.8.2,4.8.4)• Howdoweconnectnodeswithaswitch insteadofmultipleaccess– Usesmultiplelinks/wires– Basisofmodern(switched)Ethernet

Switch

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SwitchedEthernet• HostsarewiredtoEthernetswitcheswithtwistedpair

– Switchservestoconnect thehosts– Wiresusuallyruntoacloset

Switch

TwistedpairSwitchports

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What’sinthebox?• Rememberfromprotocollayers:

Network

LinkNetwork

Link

Link Link

Physical PhysicalHub,orrepeater

Switch

Router

Alllooklikethis:

InsideaHub• Allportsarewiredtogether;moreconvenientandreliablethanasinglesharedwire

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↔

InsideaSwitch• Usesframeaddressestoconnectinputporttotherightoutputport;multipleframesmaybeswitchedinparallel

82

...

Fabric

InsideaSwitch(2)• Portmaybeusedforbothinputandoutput(full-duplex)

– Justsend,nomultipleaccessprotocol

83

...

123

4

1à 4and2à 3

InsideaSwitch(3)• Needbuffersformultipleinputstosendtooneoutput

84

...

...

... ...

InputBuffer OutputBufferFabric

Input Output

InsideaSwitch(4)• Sustainedoverloadwillfillbufferandleadtoframeloss

85

...

...

... ...

InputBuffer OutputBufferFabric

Input Output

XXX

Loss!

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AdvantagesofSwitches• Switchesandhubshavereplacedthesharedcableofclassic

Ethernet– Convenienttorunwirestoonelocation– Morereliable;wirecutisnotasinglepointoffailurethatishardto

find

• Switchesofferscalableperformance– E.g.,100Mbpsperportinsteadof100Mbpsforallnodesofshared

cable/hub

SwitchForwarding• SwitchneedstofindtherightoutputportforthedestinationaddressintheEthernetframe.How?– Wanttolethostsbemovedaroundreadily;don’tlookatIP

87

...

...

... ...

Source

Destination

EthernetFrame

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BackwardLearning• Switchforwardsframeswithaport/addresstableasfollows:1. Tofillthetable,itlooksatthesourceaddressofinput

frames2. Toforward,itsendstotheport,orelsebroadcaststo

allports

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BackwardLearning(2)• 1:AsendstoD

Switch

D

Address PortABCD

90

BackwardLearning(3)• 2:DsendstoA

Switch

D

Address PortA 1BCD

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BackwardLearning(4)• 3:AsendstoD

Switch

D

Address PortA 1BCD 4

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BackwardLearning(5)• 3:AsendstoD

Switch

D

Address PortA 1BCD 4

LearningwithMultipleSwitches• Justworkswithmultipleswitchesandamixofhubsassumingnoloops,e.g.,AsendstoDthenDsendstoA

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Switch

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SwitchSpanningTree(§4.8.3)• Howcanweconnectswitchesinanytopologysotheyjustwork?

Loops– yikes!

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Problem– ForwardingLoops• Mayhavealoopinthetopology

– Redundancy incaseoffailures– Orasimplemistake

• WantLANswitchesto“justwork”– Plug-and-play,nochangestohosts– Butloopscauseaproblem…

RedundantLinks

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ForwardingLoops(2)• SupposethenetworkisstartedandAsendstoF.Whathappens?

Left/Right

A B

C

D

E F

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ForwardingLoops(3)• SupposethenetworkisstartedandAsendstoF.Whathappens?– Aà Cà B,D-left,D-right– D-leftà C-right,E,F– D-rightà C-left,E,F– C-rightà D-left,A,B– C-leftà D-right,A,B– D-leftà…– D-rightà …

Left/Right

A B

C

D

E F

98

SpanningTreeSolution• Switchescollectivelyfindaspanningtreeforthetopology– Asubsetoflinksthatisatree(noloops)andreachesallswitches

– Switchesforwardasnormalbutonlyonspanningtree– Broadcastswillgouptotherootofthetreeanddownallthebranches

SpanningTree(2)

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Topology OneST AnotherST

Root

100

Radia Perlman(1951–)• Keyearlyworkonroutingprotocols– RoutingintheARPANET– SpanningTreeforswitches(next)– Link-staterouting(later)

• Nowfocusedonnetworksecurity

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SpanningTreeAlgorithm• Rulesofthedistributedgame:

– Allswitchesrunthesamealgorithm– Theystartwithnoinformation– Operate inparallel andsendmessages– Alwayssearchforthebestsolution

• Ensuresahighlyrobustsolution– Anytopology,withnoconfiguration– Adaptstolink/switch failures,…

102

SpanningTreeAlgorithm(2)• Outline:

1. Electarootnodeofthetree(switchwiththelowestaddress)

2. Growtreeasshortestdistancesfromtheroot(usinglowestaddresstobreakdistanceties)

3. Turnoffportsforforwardingiftheyarenotonthespanningtree

SpanningTreeAlgorithm(3)• Details:

– Eachswitchinitiallybelievesitistherootofthetree– Eachswitchsendsperiodic updatestoneighborswith:

• Itsaddress,addressoftheroot,anddistance(inhops)toroot– Switchesfavorsportswithshorterdistancestolowestroot

• Useslowestaddressasatiefordistances

103

C

Hi,I’mC,therootisA,it’s2 hopsaway or(C,A,2)

104

SpanningTreeExample• 1st round,sending:

– Asends(A,A,0)tosayitisroot– B,C,D,E,andFdolikewise

• 1st round,receiving:– Astillthinksisit(A,A,0)– Bstillthinks(B,B,0)– Cupdatesto(C,A,1)– Dupdatesto(D,C,1)– Eupdatesto(E,A,1)– Fupdatesto(F,B,1)

A,A,0 B,B,0

C,C,0

D,D,0

E,E,0 F,F,0

105

SpanningTreeExample(2)• 2nd round,sending

– Nodessendtheirupdatedstate• 2nd roundreceiving:

– Aremains(A,A,0)– Bupdatesto(B,A,2)viaC– Cremains(C,A,1)– Dupdatesto(D,A,2)viaC– Eremains(E,A,1)– Fremains(F,B,1)

A,A,0 B,B,0

C,A,1

D,C,1

E,A,1 F,B,1

106

SpanningTreeExample(3)• 3rd round,sending

– Nodessendtheirupdatedstate• 3rd roundreceiving:

– Aremains(A,A,0)– Bremains(B,A,2)viaC– Cremains(C,A,1)– Dremains(D,A,2)viaC-left– Eremains(E,A,1)– Fupdatesto(F,A,3)viaB

A,A,0 B,A,2

C,A,1

D,A,2

E,A,1 F,B,1

107

SpanningTreeExample(4)• 4th round

– Steady-statehasbeenreached– Nodesturnoffforwarding thatisnotonthespanningtree

• Algorithmcontinuestorun– Adaptsbytimingoutinformation– E.g.,ifAfails,othernodesforgetit,andBwillbecomethenewroot

A,A,0 B,A,2

C,A,1

D,A,2

E,A,1 F,A,3

108

SpanningTreeExample(5)• ForwardingproceedsasusualontheST• InitiallyDsendstoF:

• AndFsendsbacktoD:

A,A,0 B,A,2

C,A,1

D,A,2

E,A,1 F,A,3

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SpanningTreeExample(6)• ForwardingproceedsasusualontheST• InitiallyDsendstoF:

– Dà C-left– Cà A,B– Aà E– Bà F

• AndFsendsbacktoD:– Fà B– Bà C– Cà D(hm,notsuchagreatroute)

A,A,0 B,A,2

C,A,1

D,A,2

E,A,1 F,A,3