计算机通信与网络_Chapter5_Peer-to-Peer_Protocols_and_Data_Link_Layer

ModifiedbyLangFei2010,Chapter5Peer-to-PeerProtocolsandDataLinkLayer,PARTI:Peer-to-PeerProtocolsPeer-to-PeerProtocolsandServiceModelsARQProtocolsandReliableDataTransferFlowControlTimingRecoveryTCPReliableStreamServiceandAllocateresources,e.g.buffersMessagetransferphaseExchangeofSDUsDisconnectphaseExample:TCP,PPP,Connection-OrientedTransferService,pipePDU,ModifiedbyLangFei2010,NoConnectionsetup,simplysendSDUEachmessagesendindependentlyMustprovidealladdressinformationpermessageSimpleblockrequestsfromhigherlayerIftimeoutexpiresretransmitframeandresettimerIfACKreceived:Ifsequencenumberisincorrectoriferrorsdetected:ignoreACKIfsequencenumberiscorrect(Rnext=(Slast+1)mod2):acceptframe,gotoReadystate,ModifiedbyLangFei2010,ReceiverAlwaysinReadyStateWaitforarrivalofnewframeWhenframearrives,checkforerrorsIfnoerrorsdetectedandsequencenumberiscorrect(Slast=Rnext),thenacceptframe,updateRnext=(Rnext+1)mod2Send-backACKframewithupdatedRnext,Deliver-forwardpackettohigherlayerIfnoerrorsdetectedandwrongsequencenumberdiscardframesendACKframewithnon-updatedRnextIferrorsdetecteddiscardframeandnofurtheractiontaken,ModifiedbyLangFei2010,ApplicationsofStop-and-WaitARQp297,IBMBinarySynchronousCommunicationsprotocol(Bisync):character-orienteddatalinkcontrolXmodem:modemfiletransferprotocolTrivialFileTransferProtocol(RFC1350):simpleprotocolforfiletransferoverUDP,ModifiedbyLangFei2010,PerformanceIssuesStop-and-WaitEfficiency,10000bitframe1Mbpstakes10mstotransmitIfwaitforACK=1ms,thenefficiency=10/11=91%IfwaitforACK=20ms,thenefficiency=10/30=33%,P297exampleprefered,ModifiedbyLangFei2010,Delay-BandwidthProductImpacttransmissionEfficiency,FromFig.5.13,SsendwindowemptyAllarriveatreceiverAllACKslostTransmitterresendsframe0,Receiverwindowstartsat0,WrWindowslidesforwardtoWs,Ws+Wr-1Receiverrejectsframe0becauseitisoutsidereceivewindow,ModifiedbyLangFei2010,ApplicationsofSelectiveRepeatARQ,TCP(TransmissionControlProtocol):transportlayerprotocolusesvariationofselectiverepeattoprovidereliablestreamserviceServiceSpecificConnectionOrientedProtocol:errorcontrolforsignalingmessagesinATMnetworks,ModifiedbyLangFei2010,EfficiencyofSelectiveRepeat,AssumePfframelossprobability,thennumberoftransmissionsrequiredtodeliveraframeis:tf/(1-Pf),ModifiedbyLangFei2010,Example:ImpactBitErrorRateonSelectiveRepeat,nf=1250bytes=10000bits,na=no=25bytes=200bitsCompareSClient-to-Serverconnectionestablished,ACKSeq.#=Init.Seq.#+1,ModifiedbyLangFei2010,2ndHandshake:ServertoClientConnectionRequest,SYNbitsetindicatesrequesttoestablishconnectionfromservertoclient,InitialSeq.#fromservertoclient,ModifiedbyLangFei2010,3rdHandshake:ACKfromClient,ACKbitsetacknowledgesconnectionrequest;Connectionsinbothdirectionsestablished,ACKSeq.#=Init.Seq.#+1,ModifiedbyLangFei2010,WhyThree-wayhandshakes?p608,HostAcanalwaysusethesameinitialsequencenumberintwodifferentconnections.HostBcannotdistinguishadelayedsegmentfromthenewone.Weshouldverifythatiftheinitialsequencenumberisalwaysunique,thedelayedsegmentisveryunlikelytopossessalegalsequencenumberandthuscanbedetectedanddiscarded.,ModifiedbyLangFei2010,TCPDataExchange,ApplicationLayerswritebytesintobuffersTCPsenderformssegmentsWhenbytesexceedthresholdortimerexpiresUponPUSHcommandfromapplicationsConsecutivebytesfrombufferinsertedinpayloadSequence#client-to-serverconnectionclosed,ACKSeq.#=PreviousSeq.#+1,ModifiedbyLangFei2010,FlowControl,TCPreceivercontrolsrateatwhichsendertransmitstopreventbufferoverflowTCPreceiveradvertisesawindowsizespecifyingnumberofbytesthatcanbeaccommodatedbyreceiverWA=WR(RnewRlast)WA:advertisedwindowsTCPsenderobligedtokeep#outstandingbytesbelowWA(Srecent-Slast)WA,ModifiedbyLangFei2010,TCPwindowflowcontrol,ModifiedbyLangFei2010,TCPRetransmissionTimeout,TCPretransmitsasegmentaftertimeoutperiodTimeouttooshort:excessivenumberofretransmissionsTimeouttoolong:recoverytooslowTimeoutdependsonRTT:timefromwhensegmentissenttowhenACKisreceivedRoundtriptime(RTT)inInternetishighlyvariableRoutesvaryandcanchangeinmid-connectionTrafficfluctuatesTCPusesadaptiveestimationofRTTMeasureRTTeachtimeACKreceived:tntRTT(new)=atRTT(old)+(1a)tna=7/8typical,ModifiedbyLangFei2010,RTTVariability,Estimatevariances2ofRTTvariationEstimatefortimeout:tout=tRTT+ksRTTIfRTThighlyvariable,timeoutincreaseaccordinglyIfRTTnearlyconstant,timeoutclosetoRTTestimateApproximateestimationofdeviationdRTT(new)=bdRTT(old)+(1-b)|tn-tRTT|tout=tRTT+4dRTT,ModifiedbyLangFei2010,Chapter5Peer-to-PeerProtocolsandDataLinkLayer,PARTII:DataLinkControlsFramingPoint-to-PointProtocolHigh-LevelDataLinkControlLinkSharingUsingStatisticalMultiplexing,ModifiedbyLangFei2010,DataLinkProtocols,Directlyconnected,wire-likeLossesConnectionsacrossWANs,DataLinksServicesFramingErrorcontrolFlowcontrolMultiplexingLinkMaintenanceSecurity:AuthenticationETX(endoftext)=0 x03;Byteusedtocarrynon-printablecharactersinframeDLE(datalinkescape)=0 x10DLESTX(DLEETX)usedtoindicatebeginning(end)offrameInsertextraDLEinfrontofoccurrenceofDLESTX(DLEETX)inframeAllDLEsoccurinpairsexceptatframeboundaries,ModifiedbyLangFei2010,2.Flag-BasedFrameSynchronizationFlag,BitStuffing,andByteStuffing,FramedelineatedbyflagcharacterHDLCusesbitstuffingtopreventoccurrenceofflag01111110insidetheframeTransmitterinsertsextra0aftereachconsecutivefive1sinsidetheframeReceiverchecksforfiveconsecutive1sifnextbit=0,itisremovedifnexttwobitsare10,thenflagisdetected6个1Ifnexttwobitsare11,thenframehaserrors7个1,ModifiedbyLangFei2010,Example:Bitstuffingdial-uptorouter1.ProvidesFramingandErrorDetectionCharacter-orientedHDLC-likeframestructure2.LinkControlProtocolBringingup,testing,bringingdownlines;negotiatingoptionsAuthentication:keycapabilityinISPaccess3.AfamilyofNetworkControlProtocolsspecifictodifferentnetworklayerprotocolsIP,OSInetworklayer,IPX(Novell),Appletalk,ModifiedbyLangFei2010,PPPApplications,PPPusedinmanypoint-to-pointapplicationsTelephoneModemLinks30kbpsPacketoverSONET600Mbpsto10GbpsIPPPPSONETPPPisalsousedoversharedlinkssuchasEthernettoprovideLCP,NCP,andauthenticationfeaturesPPPoverEthernet(RFC2516)UsedoverDSL,ModifiedbyLangFei2010,PPPcansupportmultiplenetworkprotocolssimultaneouslySpecifieswhatkindofpacketiscontainedinthepayloade.g.LCP,NCP,IP,OSICLNP,IPX.,PPPFrameFormat,ModifiedbyLangFei2010,PPPExample,ModifiedbyLangFei2010,PPPPhases,HomePCtoInternetServiceProvider1.PCcallsrouterviamodem2.PCandrouterexchangeLCPpacketstonegotiatePPPparameters3.Checkonidentities4.NCPpacketsexchangedtoconfigurethenetworklayer,e.g.TCP/IP(requiresIPaddressassignment)5.Datatransport,e.g.send/receiveIPpackets6.NCPusedtoteardownthenetworklayerconnection(freeupIPaddress);LCPusedtoshutdowndatalinklayerconnection7.Modemhangsup,ModifiedbyLangFei2010,PPPAuthentication,PasswordAuthenticationProtocolInitiatormustsendIDSecondarysetsF=1inlastI-frameinresponsePrimariesandsecondariesalwaysinteractviapairedP/Fbits,ModifiedbyLangFei2010,Frameslostduetoloss-of-synchorreceiverbufferoverflowFramesmayundergoerrorsintransmissionCRCsdetecterrorsandsuchframesaretreatedaslostRecoverythroughACKs,timeouts&retransmissionSequencenumberingtoidentifyout-of-sequence&duplicateframesHDLCprovidesforoptionsthatimplementseveralARQmethods,ErrorDetection&LossRecovery,ModifiedbyLangFei2010,Supervisoryframes,Usedforerror(ACK,NAK)andflowcontrol(DontSend):ReceiveReady(RR),SS=00ACKsframesuptoN(R)-1whenpiggybacknotavailableREJECT(REJ),SS=01NegativeACKindicatingN(R)isfirstframenotreceivedcorrectly.TransmittermustresendN(R)andlaterframesReceiveNotReady(RNR),SS=10ACKsframeN(R)-1&requeststhatnomoreI-framesbesentSelectiveREJECT(SREJ),SS=11NegativeACKforN(R)requestingthatN(R)beselectivelyretransmitted,ModifiedbyLangFei2010,UnnumberedFrames,SettingofModes:SABM:SetAsynchronousBalancedModeUA:acknowledgesacceptanceofmodesettingcommandsDISC:terminateslogicallinkconnectioInformationTransferbetweenstationsUI:UnnumberedinformationRecoveryusedwhennormalerror/flowcontrolfailsFRMR:framewithcorrectFCSbutimpossiblesemanticsRSET:indicatessendingstationisresettingsequencenumbersXID:exchangestationidandcharacteristics,ModifiedbyLangFei2010,ConnectionEstablishment&Release,SupervisoryframesusedtoestablishandreleasedatalinkconnectionInHDLCSetAsynchronousBalance