计算机网络英文版课件：3-3 可靠数据传输

可靠数据传输PrinciplesofReliabledatatransferimportantinapp.,transport,linklayerstop-10listofimportantnetworkingtopics!characteristicsofunreliablechannelwilldeterminecomplexityofreliabledatatransferprotocol(rdt)PrinciplesofReliabledatatransferimportantinapp.,transport,linklayerstop-10listofimportantnetworkingtopics!characteristicsofunreliablechannelwilldeterminecomplexityofreliabledatatransferprotocol(rdt)PrinciplesofReliabledatatransferimportantinapp.,transport,linklayerstop-10listofimportantnetworkingtopics!characteristicsofunreliablechannelwilldeterminecomplexityofreliabledatatransferprotocol(rdt)Reliabledatatransfer:gettingstartedsendsidereceivesiderdt_send():

calledfromabove,(e.g.,byapp.).Passeddatatodelivertoreceiverupperlayerudt_send():

calledbyrdt,totransferpacketoverunreliablechanneltoreceiverrdt_rcv():

calledwhenpacketarrivesonrcv-sideofchanneldeliver_data():

calledbyrdttodeliverdatatoupperReliabledatatransfer:gettingstartedWe’ll:incrementallydevelopsender,receiversidesofreliabledatatransferprotocol(rdt)consideronlyunidirectionaldatatransferbutcontrolinfowillflowonbothdirections!usefinitestatemachines(FSM)tospecifysender,receiverstate1state2eventcausingstatetransitionactionstakenonstatetransitionstate:wheninthis“state”nextstateuniquelydeterminedbynexteventeventactionsRdt1.0:reliabletransferoverareliablechannelunderlyingchannelperfectlyreliablenobiterrorsnolossofpacketsseparateFSMsforsender,receiver:sendersendsdataintounderlyingchannelreceiverreaddatafromunderlyingchannelRdt2.0:channelwithbiterrorsunderlyingchannelmayflipbitsinpacketchecksumtodetectbiterrorsthequestion:howtorecoverfromerrors:acknowledgements(ACKs):receiverexplicitlytellssenderthatpktreceivedOKnegativeacknowledgements(NAKs):receiverexplicitlytellssenderthatpkthaderrorssenderretransmitspktonreceiptofNAKnewmechanismsinrdt2.0(beyondrdt1.0):errordetectionreceiverfeedback:controlmsgs(ACK,NAK)rcvr->senderrdt2.0:FSMspecificationrdt2.0:operationwithnoerrorsWaitforcallfromabovesnkpkt=make_pkt(data,checksum)udt_send(sndpkt)extract(rcvpkt,data)deliver_data(data)udt_send(ACK)rdt_rcv(rcvpkt)&¬corrupt(rcvpkt)rdt_rcv(rcvpkt)&&isACK(rcvpkt)udt_send(sndpkt)rdt_rcv(rcvpkt)&&isNAK(rcvpkt)udt_send(NAK)rdt_rcv(rcvpkt)&&corrupt(rcvpkt)WaitforACKorNAKWaitforcallfrombelowrdt_send(data)Lrdt2.0:errorscenarioWaitforcallfromabovesnkpkt=make_pkt(data,checksum)udt_send(sndpkt)extract(rcvpkt,data)deliver_data(data)udt_send(ACK)rdt_rcv(rcvpkt)&¬corrupt(rcvpkt)rdt_rcv(rcvpkt)&&isACK(rcvpkt)udt_send(sndpkt)rdt_rcv(rcvpkt)&&isNAK(rcvpkt)udt_send(NAK)rdt_rcv(rcvpkt)&&corrupt(rcvpkt)WaitforACKorNAKWaitforcallfrombelowrdt_send(data)LWhathappensifACK/NAKcorrupted?senderdoesn’tknowwhathappenedatreceiver!can’tjustretransmit:possibleduplicateHandlingduplicates:senderretransmitscurrentpktifACK/NAKgarbledsenderaddssequencenumbertoeachpktreceiverdiscards(doesn’tdeliverup)duplicatepktrdt2.0hasafatalflaw!Sendersendsonepacket,thenwaitsforreceiverresponsestopandwaitrdt2.1:sender,handlesgarbledACK/NAKsrdt2.1:receiver,handlesgarbledACK/NAKsrdt2.2:aNAK-freeprotocolsamefunctionalityasrdt2.1,usingACKsonlyinsteadofNAK,receiversendsACKforlastpktreceivedOKreceivermustexplicitlyincludeseq#ofpktbeingACKedduplicateACKatsenderresultsinsameactionasNAK:retransmitcurrentpktrdt2.2:sender,receiverfragmentsrdt3.0:channelswitherrorsandlossNewassumption:underlyingchannelcanalsolosepackets(dataorACKs)checksum,seq.#,ACKs,retransmissionswillbeofhelp,butnotenoughApproach:senderwaits“reasonable”amountoftimeforACKretransmitsifnoACKreceivedinthistimeifpkt(orACK)justdelayed(notlost):retransmissionwillbeduplicate,b