計算機網(wǎng)絡(luò)(自頂向下方法)配套課件Chapter3

Chapter3

TransportLayerComputerNetworking:ATopDownApproach

4thedition.

JimKurose,KeithRoss

Addison-Wesley,July2007.

Anoteontheuseofthesepptslides:We’remakingtheseslidesfreelyavailabletoall(faculty,students,readers).They’reinPowerPointformsoyoucanadd,modify,anddeleteslides(includingthisone)andslidecontenttosuityourneeds.Theyobviouslyrepresentalotofworkonourpart.Inreturnforuse,weonlyaskthefollowing:Ifyouusetheseslides(e.g.,inaclass)insubstantiallyunalteredform,thatyoumentiontheirsource(afterall,we’dlikepeopletouseourbook!)Ifyoupostanyslidesinsubstantiallyunalteredformonawwwsite,thatyounotethattheyareadaptedfrom(orperhapsidenticalto)ourslides,andnoteourcopyrightofthismaterial.Thanksandenjoy!JFK/KWRAllmaterialcopyright1996-2007J.FKuroseandK.W.Ross,AllRightsReserved1TransportLayerChapter3:TransportLayerOurgoals:

understandprinciplesbehindtransportlayerservices:multiplexing/demultiplexingreliabledatatransferflowcontrolcongestioncontrollearnabouttransportlayerprotocolsintheInternet:UDP:connectionlesstransportTCP:connection-orientedtransportTCPcongestioncontrol2TransportLayerVocabularyandTermTransport(傳輸/運輸)Multiplex/De-multiplex(多路復(fù)用/多路分流)UDP(UserDatagramProtocol)

(用戶數(shù)據(jù)報協(xié)議)TCP(TransmissionControlProtocol)

(傳輸控制協(xié)議)Reliability(Error/Loss)

Control(可靠性（差錯/丟失）控制)QoS(QualityofService)(服務(wù)質(zhì)量)Segments(報文段)datagram(數(shù)據(jù)報/報文段)Packet(包/分組)3TransportLayerChapter3outline3.1Transport-layerservices3.2Multiplexinganddemultiplexing3.3Connectionlesstransport:UDP3.4Principlesofreliabledatatransfer3.5Connection-orientedtransport:TCPsegmentstructurereliabledatatransferflowcontrolconnectionmanagement3.6Principlesofcongestioncontrol3.7TCPcongestioncontrol4TransportLayerTcessesrunningondifferenthoststransportprotocolsruninendsystems

sendside:breaksappl.messagesintosegments,passestonetworklayerreceiverside:re-assemblessegmentsintomessages,passestoapplicationlayermorethanonetransportprotocolavailabletoapplicationsInternet:TCPandUDPapplicationtransportnetworkdatalinkphysicalapplicationtransportnetworkdatalinkphysicallogicalend-endtransport5TransportLayerTworklayertransportlayer:

logicalcommunicationbetweenprocesses

relieson,enhances,networklayerservices

networklayer:

logicalcommunicationbetweenhosts6TransportLayerTworklayerHouseholdanalogy:12kidssendinglettersto12kidsprocesses=kidsapplicationmessages=lettersinenvelopeshosts=housestransportprotocol=AnnandBillnetwork-layerprotocol=postalservice7TransportLayerInternettransport-layerprotocolsreliable,in-orderdelivery:TCPcongestioncontrolflowcontrolconnectionsetupunreliable,unordereddelivery:UDPno-frillsextensionof“best-effort”IPservicesnotavailable:delayguaranteesbandwidthguaranteesapplicationtransportnetworkdatalinkphysicalnetworkdatalinkphysicalnetworkdatalinkphysicalnetworkdatalinkphysicalnetworkdatalinkphysicalnetworkdatalinkphysicalnetworkdatalinkphysicalapplicationtransportnetworkdatalinkphysicallogicalend-endtransport8TransportLayerChapter3outline3.1Transport-layerservices3.2Multiplexinganddemultiplexing3.3Connectionlesstransport:UDP3.4Principlesofreliabledatatransfer3.5Connection-orientedtransport:TCPsegmentstructurereliabledatatransferflowcontrolconnectionmanagement3.6Principlesofcongestioncontrol3.7TCPcongestioncontrol9TransportLayerMultiplexingAppl.1Port1UDPAppl.2Port2Appl.NPortNIPUDPdatagramtransmit10TransportLayerDe-multiplexingUDPIPUDPdatagramarrivesAppl.1Appl.2Appl.NPort1Port2PortN11TransportLayerMultiplexing/demultiplexingapplicationtransportnetworklinkphysicalP1applicationtransportnetworklinkphysicalapplicationtransportnetworklinkphysicalP2P3P4P1host1host2host3=process=socketdeliveringreceivedsegmentstocorrectsocketDemultiplexingatrcvhost:gatheringdatafrommultiplesockets,envelopingdatawithheader(fordemultiplexing)Multiplexingatsendhost:12TransportLayerHowde-multiplexingworkshostreceivesIPdatagramseachdatagramhassourceIPaddress,destinationIPaddresseachdatagramcarries1transport-layersegmenteachsegmenthassource,destinationportnumberhostusesIPaddresses&portnumberstodirectsegmenttoappropriatesocketsourceport#destport#32bitsapplicationdata(message)otherheaderfieldsUDPdatagram/segmentformat13TransportLayerConnectionlessdemultiplexingCreatesocketswithportnumbers:DatagramSocketmySocket1=newDatagramSocket(12534);DatagramSocketmySocket2=newDatagramSocket(12535);UDPsocketidentifiedbytwo-tuple:(IPaddress,portnumber)WhenhostreceivesUDPsegment:checksdestinationportnumberinsegmentdirectsUDPsegmenttosocketwiththatportnumberIPdatagramswithdifferentsourceIPaddressesand/orsourceportnumbersdirectedtosamesocket14TransportLayerConnectionlessdemuxltiplexingDatagramSocketserverSocket=newDatagramSocket(6428);ClientIP:BP2clientIP:AP1P1P3serverIP:CSP:6428DP:9157SP:9157DP:6428SP:6428DP:5775SP:5775DP:6428SPprovides“returnaddress”15TransportLayerConnection-orienteddemuxltiplexingTCPsocketidentifiedby4-tuple:sourceIPaddresssourceportnumberdestinationIPaddressdestinationportnumberreceivehostusesallfourvaluestodirectsegmenttoappropriatesocketSocketClientSocket=newSocket("hostname",6789)SocketconnectionSocket=welcomeSocket.accept()16TransportLayerConnection-orienteddemuxltiplexingServerhostmaysupportmanysimultaneousTCPsockets:eachsocketidentifiedbyitsown4-tupleWebservershavedifferentsocketsforeachconnectingclientnon-persistentHTTPwillhavedifferentsocketforeachrequest17TransportLayerConnection-orienteddemux(cont.)ClientIP:BP1clientIP:AP1P2P4serverIP:CSP:9157DP:80SP:9157DP:80P5P6P3D-IP:CS-IP:AD-IP:CS-IP:BSP:5775DP:80D-IP:CS-IP:B18TransportLayerConnection-orienteddemux:ThreadedWebServerClientIP:BP1clientIP:AP1P2serverIP:CSP:9157DP:80SP:9157DP:80P4P3D-IP:CS-IP:AD-IP:CS-IP:BSP:5775DP:80D-IP:CS-IP:B19TransportLayerChapter3outline3.1Transport-layerservices3.2Multiplexinganddemultiplexing3.3Connectionlesstransport:UDP3.4Principlesofreliabledatatransfer3.5Connection-orientedtransport:TCPsegmentstructurereliabledatatransferflowcontrolconnectionmanagement3.6Principlesofcongestioncontrol3.7TCPcongestioncontrol20TransportLayerWhyisthereaUDP?Simplenoconnectionstateatsender,receiverEfficientsmallsegmentheaderQuick（啟動快）noconnectionestablishment(whichcanadddelay)nocongestioncontrol（傳輸快）UDPcanblastawayasfastasdesiredApplicationshaveBettercontroloverwhatandwhentosent21TransportLayerUDP:UserDatagramProtocol[RFC768]“nofrills,”“barebones”Internettransportprotocol“besteffort”service,UDPsegmentsmaybe:lostdeliveredoutofordertoappconnectionless:nohandshakingbetweenUDPsender,receivereachUDPsegmenthandledindependentlyofothersWhyisthereaUDP?noconnectionestablishment(whichcanadddelay)simple:noconnectionstateatsender,receiversmallsegmentheadernocongestioncontrol:UDPcanblastawayasfastasdesired22TransportLayerUDP:moreoftenusedforstreamingmultimediaappslosstolerantratesensitiveotherUDPusesDNSSNMPreliabletransferoverUDP:addreliabilityatapplicationlayerapplication-specificerrorrecovery!sourceport#destport#32bitsApplicationdata(message)UDPsegmentformatlengthchecksumLength,inbytesofUDPsegment,includingheader23TransportLayerMultiplexingMultiplexing（多路復(fù)用）MultipleapplicationsusingonetransportprotocolIdentifyapplicationusingportnumber(ApplicationAddressing)

UDP

TCP

111

161

69

25

21

23

RPC

SNMP

TFTP

SMTP

FTP

TELNET

IP

24TransportLayerPortnumberandProtocolnumberDeterminethehigherlayerprotocoltransportnetworkTCPUDPProtocol

NumbersIP17625TransportLayerPortswell-knownPorts(眾所周知端口，公共端口)0~1023ReservedforstandardpublicserviceAlreadyallocated：1~1000RegisteredPorts（注冊端口）1024~49151Registered：1025~48619Dynamic/PrivatePorts（私有端口）Notforpublicuse49152~6553526TransportLayerPortsAllocationAllocationAuthority/assignments/port-numbers(lastupdated26March2008)27TransportLayerUDPchecksumSender:treatsegmentcontentsassequenceof16-bitintegerschecksum:addition(1’scomplementsum)ofsegmentcontentssenderputschecksumvalueintoUDPchecksumfieldReceiver:computechecksumofreceivedsegmentcheckifcomputedchecksumequalschecksumfieldvalue:NO-errordetectedYES-noerrordetected.Butmaybeerrorsnonetheless?Morelater….Goal:detect“errors”(e.g.,flippedbits)intransmittedsegment28TransportLayerInternetChecksumExampleNoteWhenaddingnumbers,acarryoutfromthemostsignificantbitneedstobeaddedtotheresultExample:addtwo16-bitintegers1111001100110011011101010101010101110111011101110111101110111011110010100010001000011wraparoundsumchecksum29TransportLayerHomework&ExtensiveReadingReviews：4Problems：130TransportLayerChapter3Outline3.1Transport-layerservices3.2Multiplexinganddemultiplexing3.3Connectionlesstransport:UDP3.4Principlesofreliabledatatransfer3.5Connection-orientedtransport:TCPsegmentstructurereliabledatatransferflowcontrolconnectionmanagement3.6Principlesofcongestioncontrol3.7TCPcongestioncontrol31TransportLayerTCP:Overview

point-to-pointonesender,onereceiversend&receivebuffers

reliable,in-orderbytestreamno“messageboundaries”pipelinedTCPcongestionandflowcontrolsetwindowsizesocketdoorTCPsendbufferTCPreceivebuffersocketdoorsegmentapplicationwritesdataapplicationreadsdata32TransportLayerTCPfullduplex（全雙工）databi-directionaldataflowinsameconnectionconnection-orientedhandshaking(exchangeofcontrolmsgs)initiatessender,receiverstatebeforedataexchangeflowcontrolledsenderwillnotoverwhelm（淹沒）

receiversocketdoorTCPsendbufferTCPreceivebuffersocketdoorsegmentapplicationwritesdataapplicationreadsdata33TransportLayerRFC：793,1122,1323,2001,2018,2581

RF793TransmissionControlProtocolJ.Postel.September1981.RFC2001TCPSlowStart,CongestionAvoidance,FastRetransmit,andFast

RecoveryAlgorithms.

W.Stevens.January1997RFC2018TCPSelectiveAcknowledgmentOptionsM.Mathis,J.Mahdavi,S.Floyd,A.Romanow,October1996.RFC2581TCPCongestionControlM.Allman,V.Paxson,W.Stevens.April1999.34TransportLayerTCPScientistTCP/IPInventor1970：Cerf,V.,Carr,C.,Crocker,S.,Kahn,R.-NetworkControlProtocol,"Host-hostCommunicationProtocolintheARPANetwork“,AFIPSProceedingsofSJCC.IEEETrans.onCommunicationTechnology,197435TransportLayer36TransportLayerTCPScientistActiveQueueManagementTCP-FriendlycongestioncontrolHigh-PerformanceTCPSallyFloydLixiaZhangTCPTimers37TransportLayerChapter3outline3.1Transport-layerservices3.2Multiplexinganddemultiplexing3.3Connectionlesstransport:UDP3.4Principlesofreliabledatatransfer3.5Connection-orientedtransport:TCPsegmentstructurereliabledatatransferflowcontrolconnectionmanagement3.6Principlesofcongestioncontrol3.7TCPcongestioncontrol38TransportLayerVocabularyandTermReliability（Error/Loss)

Control可靠性（差錯/丟失）控制FlowControl（流量控制）CongestionControl（擁塞控制）Acknowledge（ACK）確認Pipeline（流水線）RoundTripTime(RTT)往返時間Expire失效SlidingWindow

滑動窗口39TransportLayerTCPsegmentstructuresourceport#destport#32bitsapplicationdata(variablelength)sequencenumberacknowledgementnumberReceivewindowUrgdatapointerchecksumFSRPAUheadlennotusedOptions(variablelength)StartbyteinbytestreamsNextBytethereceiverexpectsForvariablelengthwithoptionsIt’sanACK.ACK#isValidSynchronize:initiateconnectionUrgentData:transmitimmediatelyPush(ImportantData):delivertoapp.quicklyDataFinished:teardowntheconnectionFatalError:Resettheconnection#bytesreceiver

iswilling

toaccept.UsedforflowcontrolInternet

checksum

(asinUDP)locationofthelastbyteof

urgentdataLikeMSS(MaximumSegmentSize)40TransportLayerTCPSequenceNo.’sandACKsSeq.#’s:bytestream“number”offirstbyteinsegment’sdataACKs:Seq#ofnextbyteexpectedfromothersideCumulative(累計)

ACKQ:howreceiverhandlesout-of-ordersegmentsA:TCPspecificationdoesn’tsay,-uptoimplementor(實現(xiàn)者)

HostAHostBSeq=42,ACK=79,data=‘C’Seq=79,ACK=43,data=‘C’Seq=43,ACK=80Usertypes‘C’hostACKsreceiptofechoed‘C’hostACKsreceiptof‘C’,echoesback‘C’timesimpletelnetscenario41TransportLayerWindowsize=1

Sender

ReceiverTCPSimple-ACKTCPSimple-ACKWindowsize=1

Sender

ReceiverSend1Receive1TCPSimple-ACKWindowsize=1

Sender

ReceiverSend1Receive1ReceiveACK2SendACK2TCPSimple-ACKWindowsize=1

Sender

ReceiverSend1Receive1ReceiveACK2SendACK2Send2Receive2TCPSimple-ACKWindowsize=1

Sender

ReceiverSend1Receive1ReceiveACK2SendACK2Send2Receive2ReceiveACK3SendACK3TCPSimple-ACKWindowsize=1

Sender

ReceiverSend1Receive1ReceiveACK2SendACK2Send2Receive2ReceiveACK3SendACK3Send3Receive3Windowsize=1

Sender

ReceiverSend1Receive1ReceiveACK2SendACK2Send2Receive2ReceiveACK3SendACK3Send3Receive3ReceiveACK4SendACK4TCPSimple-ACKPipelinedprotocolsPipelining:senderallowsmultiple,“in-flight”,yet-to-be-acknowledgedbytes/packetsrangeofsequencenumbersmustbeincreasedbufferingatsenderandreceiver49TransportLayerPipelining:increasedutilizationfirstpacketbittransmitted,t=0senderreceiverRTTlastbittransmitted,t=L/Rfirstpacketbitarriveslastpacketbitarrives,sendACKACKarrives,sendnextpacket,t=RTT+L/Rlastbitof2ndpacketarrives,sendACKlastbitof3rdpacketarrives,sendACKIncreaseutilizationbyafactorof3!50TransportLayerSourcePortDest.Port…Sequence#Acknowledgement#SourceDest.Seq.Ack.102823101TCP:seq.#’sandACKsData:40Bsent#10-#49.TCP:seq.#’sandACKsgot

#10-#49,nowIneed#50.SourcePortDest.Port…Sequence#Acknowledgement#102823SourceDest.10Seq.1Ack.102823SourceDest.50Seq.1Ack.sent#10-#49.Data:40BSourcePortDest.Port…Sequence#Acknowledgement#102823SourceDest.50Seq.2Ack.102823SourceDest.10Seq.1Ack.102823SourceDest.50Seq.1Ack.TCP:seq.#’sandACKsData:70BNowsent

#50-#119.SourcePortDest.Port…Sequence#Acknowledgement#102823SourceDest.50Seq.2Ack.102823SourceDest.10Seq.1Ack.102823SourceDest.50Seq.1Ack.102823SourceDest.120Seq.2Ack.TCP:seq.#’sandACKsData:70Bgot

#10-#119,nowIneed

#120.sent#50-#119.TCPRoundTripTimeandTimeoutQ:howtosetTCPtimeoutvalue?longerthanRTTbutRTTvariestooshort:prematuretimeoutunnecessaryretransmissionstoolong:slowreactiontosegmentloss55TransportLayerTCPRoundTripTimeandTimeoutQ:howtoestimateRTT?SampleRTT:measuredtimefromsegmenttransmissionuntilACKreceiptignoreretransmissionsSampleRTTwillvary,wantestimatedRTT“smoother”averageseveralrecentmeasurements,notjustcurrentSampleRTT56TransportLayerTCPRoundTripTimeandTimeoutEstimatedRTT=(1-)*EstimatedRTT+*SampleRTTExponentialWeightedMovingAverage(EWMA)（指數(shù)加權(quán)移動平均）influenceofpastsampledecreasesexponentiallyfasttypicalvalue:=0.12557TransportLayerExampleRTTestimation:58TransportLayerTCPRoundTripTimeandTimeoutSettingthetimeoutEstimtedRTTplus“safetymargin”largevariationinEstimatedRTT->largersafetymarginfirstestimateofhowmuchSampleRTTdeviatesfromEstimatedRTT:TimeoutInterval=EstimatedRTT+4*DevRTTDevRTT=(1-)*DevRTT+

*|SampleRTT-EstimatedRTT|(typically,=0.25)

Thensettimeoutinterval:59TransportLayerTCPRoundTripTimeandTimeoutTimeoutInterval=EstimatedRTT+4*DevRTTDevRTT=(1-)*DevRTT+*|SampleRTT-EstimatedRTT|

Thensettimeout(expiration)interval:EstimatedRTT=(1-)*EstimatedRTT+*SampleRTT60TransportLayerChapter3Outline3.1Transport-layerservices3.2Multiplexinganddemultiplexing3.3Connectionlesstransport:UDP3.4

Principlesofreliabledatatransfer3.5Connection-orientedtransport:TCPsegmentstructurereliabledatatransferflowcontrolconnectionmanagement3.6Principlesofcongestioncontrol3.7TCPcongestioncontrol61TransportLayer

TCP

Transmission

State62TransportLayerTCPreliabledatatransferTCPcreatesrdtserviceontopofIP’sunreliableservicePipelinedsegmentsCumulativeAcksTCPusessingleretransmissiontimerRetransmissionsaretriggeredby:timeouteventsduplicateAcksInitiallyconsidersimplifiedTCPsender:ignoreduplicateAcksignoreflowcontrol,congestioncontrol63TransportLayerTCPsenderevents:Datareceivedfromapplication:Createsegmentwithseq#seq#isbyte-streamnumberoffirstdatabyteinsegment（報文段）starttimerifnotalreadyrunning(thinkoftimerasforoldestunackedsegment)expirationinterval:TimeOutInterval（超時間隔）64TransportLayerTCPsenderevents:timeout:retransmitsegmentthatcausedtimeoutrestarttimerAckreceived:Ifacknowledgespreviouslyunackedsegmentsupdatewhatisknowntobeackedstarttimerifthereareoutstandingsegments65TransportLayerTCP

sender

(simplified)

NextSeqNum=InitialSeqNumSendBase=InitialSeqNum

loop(forever){

switch(event)

event:datareceivedfromapplicationabovecreateTCPsegmentwithsequencenumberNextSeqNumif(timercurrentlynotrunning)starttimerpasssegmenttoIPNextSeqNum=NextSeqNum+length(data)

event:timertimeoutretransmitnot-yet-acknowledgedsegmentwithsmallestsequencenumberstarttimer

event:ACKreceived,withACKfieldvalueofyif(y>SendBase){SendBase=yif(therearecurrentlynot-yet-acknowledgedsegments)starttimer}

}/*endofloopforever*/

Comment:SendBase-1:lastcumulatively

ack’edbyteExample:SendBase-1=71;

y=73,sothercvr

wants73+;

y>SendBase,so

thatnewdatais

acked66TransportLayerTCP:retransmissionscenariosHostASeq=92,8bytesdataACK=100losstimeoutlostACKscenarioHostBXSeq=92,8bytesdataACK=100timeSendBase=10067TransportLayerTCP:retransmissionscenariosHostASeq=100,20bytesdataACK=100timeprematuretimeout（過早超時）HostBSeq=92,8bytesdataACK=120Seq=92,8bytesdataSeq=92timeoutACK=120Seq=92timeoutSendBase=120SendBase=120Sendbase=10068TransportLayerTCPretransmissionscenarios(more)HostASeq=92,8bytesdataACK=100losstimeoutCumulativeACKscenarioHostBXSeq=100,20bytesdataACK=120timeSendBase=12069TransportLayerTCPAckgeneration

[RFC1122,RFC2581]EventatReceiverArrivalofin-ordersegmentwithexpectedseq#.Alldatauptoexpectedseq#alreadyACKedArrivalofin-ordersegmentwithexpectedseq#.OneothersegmenthasACKpendingArrivalofout-of-ordersegmenthigher-than-expectseq.#.GapdetectedArrivalofsegmentthatpartiallyorcompletelyfillsgapTCPReceiveractionDelayedACK.Waitupto500msfornextsegment.Ifnonextsegment,sendACKImmediatelysendsinglecumulativeACK,ACKingbothin-ordersegmentsImmediatelysendduplicateACK,indicatingseq.#ofnextexpectedbyteImmediatesendACK,providedthatsegmentstartsatlowerendofgap70TransportLayerFastRetransmitTime-outperiodoftenrelativelylong:longdelaybeforeresendinglostpacketDetectlostsegmentsviaduplicate(冗余)

Acks.Senderoftensendsmanysegmentsback-to-backIfsegmentislost,therewilllikelybemanyduplicateACKs.71TransportLayerFastRetransmitIfsenderreceives3Acksforthesamedata,itsupposesthatsegmentafterAckeddatawaslost:fastretransmit:

resendsegmentbeforetimerexpires72TransportLayerFastRetransmit73TransportLayer

event:ACKreceived,withACKfieldvalueofyif(y>SendBase){SendBase=yif(therearecurrentlynot-yet-acknowledgedsegments)starttimer}else{incrementcountofdupACKsreceivedforyif(countofdupACKsreceivedfory=3){resendsegmentwithsequencenumbery}

Fastretransmitalgorithm:aduplicateACKforalreadyACKedsegmentfastretransmit74TransportLayerGo-Back-NSender:k-bitseq#inpktheader“window”ofuptoN,consecutiveunack’edpktsallowedACK(n):ACKsallpktsupto,includingseq#n-“cumulativeACK”mayreceiveduplicateACKs(seereceiver)timerforeachin-flightpkttimeout(n):retransmitpktnandallhigherseq#pktsinwindow75TransportLayerGBNin

action76TransportLayerGo-Back-N：strength“window”ofuptoN,consecutiveunack’edpktsallowednextseqnum:smallestunusedsequencenumberACK(n):ACKsallpktsupto,

includingseq#n

-

“cumulativeACK”ReduceACKmessages,goodthroughput！77TransportLayerGo-Back-N：weakpointsnextseqnum:smallestunusedsequencenumberACK(n):ACKsallpktsupto,includingseq#n-“cumulativeACK”timerforeachin-flightpkttimeout(n):retransmitpktnandallhigherseq#pktsinwindowLOSTRe-transmit78TransportLayerSelectiveRepeatreceiverindividuallyacknowledgesallcorrectlyreceivedpktsbufferspkts,asneeded,foreventualin-orderdeliverytoupperlayersenderonlyresendspktsforwhichACKnotreceivedsendertimerforeachunACKedpktsenderwindowNconsecutiveseq#’sagainlimitsseq#sofsent,unACKedpkts79TransportLayerSelectiverepeat:sender,receiverwindows80TransportLayerSelectiverepeatdatafromabove:ifnextavailableseq#inwindow,sendpkttimeout(n):resendpktn,restarttimerACK(n)in[sendbase,sendbase+N]:markpktnasreceivedifnsmallestunACKedpkt,advancewindowbasetonextunACKedseq#senderpktnin[rcvbase,rcvbase+N-1]sendACK(n)out-of-order:bufferin-order:deliver(alsodeliverbuffered,in-orderpkts),advancewindowtonextnot-yet-receivedpktpktnin[rcvbase-N,rcvbase-1]ACK(n)otherwise:

ignorereceiver81TransportLayerSelectiverepeatinaction82TransportLayerHomework&ExtensiveReadingReviews：5,6Problems：2283TransportLayerChapter3Outline3.1Transport-layerservices3.2Multiplexinganddemultiplexing3.3Connectionlesstransport:UDP3.4Principlesofreliabledatatransfer3.5Connection-orientedtransport:TCPsegmentstructurereliabledatatransferflowcontrolconnectionmanagement3.6Principlesofcongestioncontrol3.7TCPcongestioncontrol84TransportLayerTCPFlowControlreceivesideofTCPconnectionhasareceivebuffer:applicationprocessmaybeslowatreadingfrombuffer85TransportLayerTCPFlowControlspeed-matchingservice:matchingthesendratetothereceivingapplication’sdrain(排干)

rate!senderwon’toverflowreceiver’sbufferbytransmittingtoomuch,toofastflowcontrol86TransportLayerTCPFlowcontrol:howitworks(SupposeTCPreceiverdiscardsout-of-ordersegments)spareroominbuffer=RcvWindow=RcvBuffer-[LastByteRcvd-LastByteRead]ReceiveradvertisesspareroombyincludingvalueofRcvWindowinsegmentsSenderlimitsunACKeddatatoRcvWindowguaranteesreceivebufferdoesn’toverflow87TransportLayerChapter3outline3.1Transport-layerservices3.2Multiplexinganddemultiplexing3.3Connectionlesstransport:UDP3.4Principlesofreliabledatatransfer3.5Connection-orientedtransport:TCPsegmentstructurereliabledatatransferflowcontrolconnectionmanagement3.6Principlesofcongestioncontrol3.7TCPcongestioncontrol88TransportLayerTCPConnectionManagementRecall:TCPsender,receiverestablish“connection”beforeexchangingdatasegmentsinitializeTCPvariables:seq.#s【ISN,InitialSequenceNumber】buffers,flowcontrolinfo(e.g.RcvWindow)client:connectioninitiator

SocketclientSocket=newSocket("hostname","portnumber");

connect（server，destPort）

server:contactedbyclient

SocketconnectionSocket=welcomeSocket.accept();89TransportLayerTCPConnectionManagementSYN,SEQ=xSYN,SEQ=y,ACK=x+1SYN,SEQ=x+1,ACK=y+1Threewayhandshake:Step1:

clienthostsendsTCPSYNsegmenttoserverspecifiesinitialseq#nodataStep2:

serverhostreceivesSYN,replieswithSYNACKsegmentserverallocatesbuffersspecifiesserverinitialseq.#Step3:clientreceivesSYNACK,replieswithACKsegment,whichmaycontaindata90TransportLayerTCPConnectionManagement(cont.)Closingaconnection:clientclosessocket:

clientSocket.close();

Step1:

clientendsystemsendsTCPFINcontrolsegmenttoserver

Step2:

serverreceivesFIN,replieswithACK.Closesconnection,sendsFIN.clientFINserverACKACKFINclosecloseclosedtimedwait91TransportLayerTCPConnectionManagement(cont.)S