2006-SigMetrics-libra --- TCP Libra --- balancing flows over heterogeneous propagation scenarios

TCPLibra:balancing owsoverheterogeneous

propagationscenarios

–SubmissiontoSigMetrics2006–

AnonymousduetoDoubleBlindSubmission

ABSTRACT

ThemajorityofInternettraf creliesontheTransmissionControlProtocol(TCP1)devisedintheearly1970stoprovideareliableandfairdatatransferacrosstheARPANET.Usersthatdownloadlargemultimedia lesfromremoteservers,say,areexpectedtoreceivethesameshareof”bandwidth”(i.e.,sametransferrate)iftheysharethesamebottleneck.Unfortunately,withcurrentTCP,fairnessdoesnotapplywhentheroundtripdelayRTTisverydif-ferentamongsessions.Forinstance,supposeinapopularinternetcaf`einNewYorkCityseveralusersaresimultaneouslydownload-ingmultimedia lesfromvariousservers.Mostoftheserversarelocal,however,onecustomerisdownloadingalarge lefromare-moteserverinAustralia.Allcustomerssharethesame11MBPSWiFibottleneck.ThecustomerconnectedtoAustraliawillmakenovisibleprogressuntilallthecustomersdownloadingfromlocalserversaredone!OnecaneasilyimagineseveralotherapplicationswhereRTT-fairnessisamust.HereweproposeanewversionofTCP,namelyTCPLibra,whichguaranteesfairsharingregardlessofRTT.”Libra”inLatinmeans”scale”,thusindicatinggoodbal-anceamongcompetingsessions.

InthispaperwedescribethedesignofTCPLibraandprovethatitisindeedRTT-fair.ThekeyelementofTCPLibraisthewindowadjustmentalgorithmthatcompensatesforRTTdifference.ThealgorithmisderivedbymodelingTCPperformanceasa”utilityfunction”andbyoptimizingthisfunctionsuchthattheresultisin-dependentofRTT.ThisleadstoasolutionthatprovidesfairnessamongTCP owsthatsharethesamebottlenecklinkregardlessofRTT.Remarkably,TCPLibraachievesfairnesswhilestillmain-tainingthroughputef ciencyandfriendlinesswithrespecttoTCPNewReno.Moreover,TCPLibraisa”sender-side-only”modi -cationofTCP,thusgreatlysimplifyingitsdeployment.Inthepa-per,fairnessandstabilitypropertiesareprovedanalyticallyandareextensivelytestedviasimulationusingpopularbenchmarks.Thesensitivitytovariousdesignparametersiscarefullyanalyzed.Acomparisonisalsocarriedoutwithother”RTT-fair”TCPversionsreportedintheliterature.

1

WithTCP,unlessspeci ed,werefertoTCPNewReno.

CategoriesandSubjectDescriptors

C.2[ComputerSystemsOrganisation]:Computer-CommunicationNetworks;C.2.2[ComputerSystemsOrganisation]:Computer-CommunicationNetworks—NetworkProtocols;C.2.6[ComputerSystemsOrganisation]:Computer-CommunicationNetworks—Internetworking(C.2.2.)

GeneralTerms

TCP,CongestionControl,Optimization,PerformanceEvaluation

1.INTRODUCTION

TheInternetsuccessisbasedontheabilitytoprovideareliablemediumforinformationexchange.InthecurrentInternetthetraf- ccontrolfunctionalitiesareprovidedbytheTransmissionCon-trolProtocol(TCP)inanend-to-endfashion.TCPhasbeende-signedtoprovideaconnectionoriented,reliable,andfairserviceintheARPANET[48],initially,andintheInternetlater.Reliabil-ityandcongestioncontrolembodythetwomajorissuesaddressedbyTCP[20].Toachievethesecondgoal,TCPadaptsthesendingratetoavoidnetworkover oworfallintoservicestarvation.TCPCongestionControlhasbeenstudiedbytheresearchcommunityforthelast25yearsleadingtoseveralTCPvariantsforcongestioncontrolwithandwithouttheexplicitinterventionofthenetworklayer(asurveycanbefoundin[11]).

ThecongestioncontrolschemeimplementedbyTCPfallsintheAIMD(AdditiveIncrease,MultiplicativeDecrease)classofalgo-rithms.Thecurrentlydeployedschemeconsiderspacketlossasanindicatorofnetworkcongestionandreactswithadrasticreductionofthesendingrate[2].ThelossrateexperiencedovertheInter-netcanbewellbeyondtheoptimaloperatingconditionsforTCPthusleadingtopoornetworkutilization[39].TCPNewRenoandTCPSackpartiallycorrectthisproblemandimproveef ciencyinthelossrecoveryphase.However,theoverallperformanceisstillfarfromoptimalonlargepipesizes,pathswherethebandwidth-delayproductislarge.Practicalapproachesto”bigpipe”scala-bilityarethe ne-tuningofNewRenoparameters(i.e.receiveradvertisedwindowsandnetworkbuffers),theuseofjumbopack-ets(i.e.8Kbytepackets)andtheopeningofmultipleTCPsessionsinparallel.

Alloftheseapproachesweredirectedtoenhanceutilization.Noneofthemdirectlyaddressestheissuesoffairnessandfriendliness-thelatterreferringtothebehaviortowardslegacycompeting ows.OneparticulartypeofunfairnesswhichisintrinsicinlegacyTCPisRTTunfairness.Werecallthatincongestionavoidancethecon-gestionwindowincreaseslinearlywithRTT.Asaconsequence,

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