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1、UnderstandingthePerformanceofTCPPacingAmitAggarwalamit@cs.washington.eduDepartmentofComputerScienceandEngineeringUniversityofWashingtonSeattle,WA98195USAAbstractManyresearchershaveobservedthatTCP'scongestioncontrolmechanismscanleadtoburstytraf?c?owsonmodernhigh-speednetworks,withanegativeimpac

2、tonoverallnetworkef?ciency.Aproposedsolutiontothisproblemistoevenlyspace,or“pace”,datasentintothenetworkoveranentireround-triptime,sothatdataisnotsentinaburst.Inthispaper,wequantitativelyevaluatethisapproach.Weshowthatpacingoffersbetterfairness,throughput,andlowerdropratesinsomesituations.Howe

3、ver,contrarytoourinitialintuition,pacingoftenhassigni?cantlyworsethroughputthanregularTCPbecauseitissusceptibletosynchronizedlosses.Weproposeandevaluateapproachesforeliminatingthiseffect.1IntroductionTheTransmissionControlProtocol(TCP)isthede-factostandardforreliable,unicast,best-effortcommu-n

4、icationontheInternet.OnereasonforTCP'ssuccessisthatitsbehaviorhasproventobeextremelyrobustacrossawidevarietyofenvironments.AstheInternethasgrown,TCPhasevolvedtoadapttonewoper-atingconditionsandperformancedemands,addingcongestioncontrol[16],adaptivetimers[17],andsmallpacketavoidance[25].Morerec

5、ently,anumberofextensionssuchaslargerinitialwindows[1,2],selectiveacknowledgements[22],andothers[3,4,5,10,26,31]havebeenproposedtoimproveperformanceandcopewiththeproblemsposedbychangingInternetconditions.OnesuchproblemistheinteractionbetweenTCPandmodernhighbandwidthandhighlymultiplexednetworks

6、.BecauseTCPusesincomingacknowledgmentstoclockoutnewdata,called“ack-clocking”,TCPtendstoproduceveryburstypacket?ows.Intheory,whenaburstarrivesatabottleneckrouter,thebottleneckhastheeffectofspacingouttraf?conthedownstreamlink.Well-spacedacknowledgmentsthentriggerwell-spacednewdatatransmissions.I

7、npractice,whileack-clockingworkswellforlow-bandwidthlinksusedbyasmallnumberofTCP?ows,whenmany?owsaresharingahigh-bandwidthlink,each?ow'sdatapacketsprogresstogetherthroughthebottleneckcausingclumpedacknowledgments,inturncausingclumpednew