计算机网络英文版课件：slide 11 The Transport Layer – Part 2

TheTransportLayer–Part2ContentsTCP(TransmissionControlProtocol)(Section6.5)IntroductiontoTCPTheTCPServiceModelTheTCPSegmentHeaderTCPConnectionEstablishmentTCPConnectionReleaseTCPConnectionManagementModelingTCPTransmissionPolicyTCPCongestionControlTCPTimerManagementIntroductiontoTCPTCP(TransmissionControlProtocol)wasspecificallydesignedtoprovideareliable

end-to-end

bytestreamoveranunreliableinternetwork.connection-oriented:aTCPconnectionisestablishedbetweentwoapplicationsbeforetheycanexchangedataPoint-to-point:broadcastingandmulticastingarenotapplicabletoTCPATCPconnectionisabytestream,notamessagestreameverybyteonaTCPconnectionhasitsown32-bitsequencenumber.TCPsegmentTCP’sadaptivetimeoutandretransmissionTCPalsoprovidesflowcontrol:AreceivingTCPonlyallowstheotherendtosendasmuchdataasthereceiverhasbuffersfor.TCPdoesnotinterpretthecontentsofthebytesatallTCPprovidesafull-duplexservicetotheapplicationlayerRFCs:793,1122,1323,2018,2581TheTCPServiceModelSomeassignedports.PortProtocolUse21FTPFiletransfer23TelnetRemotelogin25SMTPE-mail69TFTPTrivialFileTransferProtocol79FingerLookupinfoaboutauser80HTTPWorldWideWeb110POP-3Remotee-mailaccess119NNTPUSENETnewsTCPPort:istheTCPnameforaTSAP.TheTCPServiceModel(2)(a)Four512-bytesegmentssentasseparateIPdatagrams.(b)The2048bytesofdatadeliveredtotheapplicationinasingleREADCALL.TheTCPSegmentHeaderTCPHeader.TCPConnectionEstablishment(a)TCPconnectionestablishmentinthenormalcase.(b)Callcollision.6-31ClientServerSYNJSYNk,ack

J+1ack

k+1FINMack

M+1ack

N+1FINN(activeopen)SYN_SENTLISTEN(passiveopen)SYN_RCVDESTABLISHEDESTABLISHED(activeclose)FIN_WAIT_1CLOSE_WAIT(passiveclose)FIN_WAIT_2TIME_WAITLAST_ACKCLOSEDFigure18.13TCPstatescorrespondingtonormalconnectionestablishmentandterminationTCPConnectionManagementModelingThestatesusedintheTCPconnectionmanagementfinitestatemachine.TCPConnectionManagementModeling(2)Theheavysolidlineisthenormalpathforaclient.Theheavydashedlineisthenormalpathforaserver.Thelightlinesareunusualevents.Eachtransitionislabeledbytheeventcausingitandtheactionresultingfromit,separatedbyaslash.TCPconnectionmanagementfinitestatemachine.TCPTransmissionPolicyWindowmanagementinTCP.TCPTransmissionPolicy(2)Sillywindowsyndrome.Clark'ssolutionistopreventthereceiverfromsendingawindowupdatefor1byte.Insteaditisforcedtowaituntilithasadecentamountofspaceavailableandadvertisethatinstead.TCPCongestionControlAlltheInternetTCPalgorithmsassumethattimeoutsarecausedbycongestionandmonitortimeoutsforsignsoftrouble.TCPendermaintainstwowindows:thewindowthereceiverhasgrantedandasecondwindow,thecongestionwindow.Eachreflectsthenumberofbytesthesendermaytransmit.Thenumberofbytesthatmaybesentistheminimumofthetwowindows.TCPCongestionControl(a)Afastnetworkfeedingalowcapacityreceiver.(b)Aslownetworkfeedingahigh-capacityreceiver.TCPCongestionControl(2)AnexampleoftheInternetcongestionalgorithm.TCPTimerManagementTCPusesmultipletimers(atleastconceptually)retransmissiontimer:isusedwhenexpectinganacknowledgmentfromtheotherendpersistencetimer:Whenthepersistencetimergoesoff,thesendertransmitsaprobetothereceiver.Theresponsetotheprobegivesthewindowsize.keepalivetimer:Whenaconnectionhasbeenidleforalongtime,thekeepalivetimermaygoofftocauseonesidetocheckwhethertheothersideisstillthere.TCPTimerManagement(a)ProbabilitydensityofACKarrivaltimesinthedatalinklayer.(b)ProbabilitydensityofACKarrivaltimesforTCP.Round-TripTimeMeasurement(1)SmoothedRTTestimator(calledR):R←αR+(1-α)Mwhereα

isasmoothingfactorwitharecommendedvalueof0.9Retransmissiontimeoutvalue(RTO):RTO=Rβ

whereβisadelayvariancefactorwitharecommendedvalueof2Round-TripTimeMeasurement(2)TokeeptrackofthevarianceintheRTTmeasurements,inadditiontothesmoothedRTTestimator

Err=M–AA←A+g

ErrD←D+h(∣Err∣-D)RTO=A+4DWhere:

AisthesmoothedRTT(anestimatoroftheaverage)Disthesmoothedmeandeviation.

ErristhedifferencebetweenthemeasuredvaluejustobtainedandthecurrentRTTestimatorThegaingisfortheaverageandissetto1/8(0.125)Thegainforthedeviationishandissetto0.25.ThelargergainforthedeviationmakestheRTOgoupfasterwhentheRTTchanges.1:257(256)ack1ack257,win7936slip.1024vangogh.discard12257:513(256)ack1513:769(256)ack134ack749ack5135769:1025(256)ack11025:1281(256)ack11281:1537(256)ack1ack1025ack12811537:1793(256)ack11793:2049(256)ack167891012111513RTT#3（1.015ms）RTT#2（0.808ms）RTT#1（1.061ms）Figure21.2PacketexchangeandRTTmeasurementExponentialRTOBackoffBackoffprocess:aTCPsourceincreaseitsRTOeachtimethesamesegmentisretransmitted.Thismaygivetheinternettimetoclearthecurrentcongestion.AsimpletechniqueforimplementingRTObackofistomultiplytheRTOforasegmentbyaconstantvalueforeachretransmission:RTO=q

*RTOThemostcommonlyusedvalueofqis2.Karn’sAlgorithmretransmissionambiguityprob