A Feedback Based Scheme for Improving TCP Performance in Ad-Hoc Wireless Networks

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A Feedback Based Scheme for Improving TCP Performance in Ad-Hoc Wireless Networks
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  AFeedbackBasedSchemeForImprovingTCP PerformanceInAd-HocWirelessNetworks   KartikChandran,SudarshanRaghunathan,S.Venkatesan,RaviPrakash  ComputerScienceProgram UniversityofTexasatDallasRichardson,TX75083-0688  f  chandran,rsud,venky,ravip  g  @utdallas.edu  Abstract Ad-hocnetworksconsistofasetofmobilehoststhatcommunicateusingwirelesslinks,withouttheuseofothercommunicationsupportfacilities(suchasbasestations).Thetopologyofanad-hocnetworkchangesduetothemovementofmobilehosts,which mayleadtosuddenpacketlossesanddelays.TransportprotocolslikeTCP,whichhavebeenbuiltmainlyforxednetworks,misinterpretthislossascongestionandinvokecongestioncontrol.Thisleadstounnecessaryretransmissionsandlossofthroughput.Toovercomethisproblem,weproposeafeedbackscheme,wherebythesourcecan distinguishbetweenroutefailureandnetworkcongestion.Themainideaistoinform thesourcebyaRouteFailureNotication(RFN)whentherouteisdisruptedallowing thesourcetofreezeitstimersandstopsendingpackets(neworretransmitted)aslongasthesourcecannotreachthedestination.Whentherouteisre-established,thesource,onbeinginformedthroughaRouteRe-establishmentNotication(RRN),resumesbyun-freezingtimersandcontinuingpackettransmissions.Thesimulated performanceofTCPonad-hocnetworkswithandwithoutfeedbackiscomparedand reported.Weobservedthatintheeventofroutefailures,astheroutere-establishmenttimeincreases,theuseoffeedbackprovidessignicantgainsinthroughputaswellassavingsinunnecessarypackettransmissions.Severalfurtherenhancementsand directionsforfutureworkarealsosketched.  ThisworkwassupportedinpartbytheTexasAdvancedTechnologyProgramunderGrantNo.9741-052andbyNSFunderGrantNo.CCR-9796331. 1   1Introduction  Ad-hocnetworksconsistofasetofmobilehostscommunicatingamongstthemselvesusing wirelesslinks,withouttheuseofanyothercommunicationsupportfacilities(suchasbase stations).Theyarealsocalledmobileradionetworksormulti-hopwirelessnetworks.Two mobilehosts(MHs)aresaidtobewithinrangeandsaidtobeneighborsofeachotherifeachcanreceivetheother'stransmission.EveryMHbehavesinaco-operativefashionby actingasarouterallowingpacketsdestinedtootherMHstopassthroughit.Thus,ifMH  h  1 needstosendapacketto  h  2 and  h  2 isnotwithintherangeof h  1 ,thenthepacketmustbesenttooneof h  1 'sneighbors,say  h  i ,and  h  i willforwardthatpackettoitsneighbor,and soon,untilitreachesthedestination, h  2 .Inthisexample, h  i actsasarouter.Thetopologyofanad-hocnetworkchangeseverytimeamobilehost'smovementresultsinestablishmentofnewwirelesslinks(mobilehostmoveswithintherangeofanother)orlinkdisconnections(mobilehostmovesoutoftherangeofanotherwhichwaswithinitsrange).Therateoftopologychangeisdependentontheextentofmobilityofthehostsand thetransmissionrangeofthehosts.RoutesareheavilydependentontherelativelocationofMHs.Hence,routesmayberepeatedlyinvalidatedinasporadicandarbitraryfashiondue tothemobilityofhosts.Themobilityofasinglenodemayaectseveralroutesthatpassthroughit.ForexampleinFigure1,thetworoutes,onefroms1tod1andtheotherfrom s2tod2,arechangedwhenMH  i movesclosetoMH  h  .Ad-hocnetworkshavebeenstudiedextensivelyinthecontextofroutingandanum-berofroutingprotocolshavebeenproposedforad-hocnetworksincludingdistancevectorschemes16],linkreversal6],TORA15],dynamicsourcerouting11],routingusinga virtualbackbone7],zonerouting9]andclusterbasedrouting13].Inthispaper,weconsidertheproblemofmaintaining  reliablestream-orientedend-to-endcommunication  inad-hocnetworks,similartothatprovidedbyTCPoverthe Internet.Theend-to-endcommunicationproblemhasbeenstudiedinthecontextofcellularwirelessnetworksandanumberofmodicationsandextensionstoregularTCPforsuch networkshavebeenproposed1,2,3,4,18,19].ItisdesirabletouseTCPdirectlyeven inad-hocnetworksinordertoprovideseamlessportabilitytoapplicationslikeletransfer,emailandWWWbrowserswrittenusingstandardTCPlibraries.Hence,itisofinterestto 2   cef h jai k  S1S2D2D1S1S2D2D2D1 c gi ied f db bagh Original Routes between source-destn pairsNew Routes betweenSource-destn pairs Figure1:EectofMobilityonRoutesinAd-HocNetworksstudythebehaviorofTCPinthecontextofad-hocnetworksandevaluatetheeectofthe dynamictopologyonTCP'sperformance.Thisisdonetodeterminewhetheritisfeasibleto useTCPwithoutsubstantialchangesinthead-hocnetworkdomain.Ourpreliminarystudiesandresultsindicatethatasaresultoffrequentandunpredictableroutedisruptions,TCP'sperformanceisindeedsubstantiallydegradedbothintermsofthroughputandgoodput(theratioofusefuldatareceivedatthedestinationandthetotaldatatransmittedbythe source)3].Wethereforeproposea   feedbackbased  schemeforovercomingthisproblem.OursimulationexperimentsshowthattheuseoffeedbackmechanismsalongwithTCPcanresultinsubstantialperformanceimprovements.Section2ofthepaperdescribesthead-hocnetworkmodelandassumptions.Section 3discussesthebehaviorofTCPinad-hocnetworksandSection4explainsourproposed approachforimprovingTCPperformance.InSection5,oursimulationmodel,experimentsandobservationsarepresentedanddiscussed,followedbyconclusionsandproposedfuture extensionsinSection6. 2ModelandAssumptions  Fortheforthcomingdiscussions,wemakethefollowingassumptions:3   1.Anad-hocnetworkconsistsof n  MHs,eachofwhichisequippedwithwirelesscom-municationcapability.EachMHbroadcastsitspacketsonthewirelesschannels.No assumptionismadeaboutthemediumaccessprotocol.2.Thewirelesslinksarebidirectional.Thisimpliesthatallthemobilehostshavethe sametransmissionrange.Otherwise,the  weaker  hostscanreceivethetransmissionsof stronger  hostsbutnotviceversa,iftheyaresucientlyfaraway.3.Areliabledatalinklayerprotocolisimplementedovertheunreliablewirelesslinksby using,say,linklevelacknowledgementandretransmission.Therefore,weassumethatifapacketcannotbedeliveredatthelinklayer,thehigherlayerswillbedulyinformed.4.AsuitableroutingprotocolfromamongthosementionedinSection1isimplementedto establishandmaintainroutesbetweenasourceandadestination.Wewillrequirethe routingprotocoltoperformcertainactionsinadditiontoforwardingpackets,which mayincludesendingfeedbackmessagestotransportentities.Theroutingprotocolmaymaintainredundantroutesbetweendierentsourcesanddestinations.5.Whenapacketcannotbesentonalinkdespitethepresenceofareliablelinklayerprotocol,wetreatthesituationasthefailureofthelinkduetomobility.Therouting protocolisthenresponsibleforadaptingandmaintainingroutesbetweenallsourcesanddestinations.Weassumethatwhenaroutefailureoccurs,a   nitetimeelapses untiltherouteisrestored  andcommunicationcanberesumed.6.Allpacketscarrythesourceanddestinationid'ssothatthenetworklayercanidentify thesourceanddestinationaddressofeachpacket.Atransportconnectionismadebetweenasourceandadestination.Thesourcehasase-riesofpacketsthataretobetransmittedtothedestination.Notethatthistransportconnec-tionisunidirectional-packetsgofromsourcetodestination(andacknowledgementsgofrom thedestinationtothesource).Ingeneral,thetransportconnectionmaybeunidirectionalorbidirectional.Foreaseofexposition,wewilltreattheconnectiontobeunidirectional.4   3BehaviorofTCPinAd-hocNetworks  TCPisareliable,stream-orientedtransportlayerprotocolwhichhasbeendesignedforuse overxednetworksliketheInternet5].Ithasbeenestablishedthatpacketerror/lossratesovertheInternetduetotransmissionerrorsareoftheorderof1%10].Inotherwords,packetsarerarelylost.Routefailuresanddisruptionsareveryinfrequentasthenetwork isxed.Therefore,packetloss,whichisdetectedbyTCPasatimeout,isinterpretedto beasymptomofcongestioninthenetwork.Inresponse,TCPinvokescongestioncontrolmechanisms10,12,17].Inotherwords, TCPcannotdistinguishbetweencongestiononthe onehandandpacketlossduetotransmissionerrorsorroutefailuresontheother  .ThisinabilityofTCPtodistinguishbetweentwodistinctproblemsexhibitingthesamesymptom willresultinperformancedegradationinad-hocnetworks,asdescribedlaterinthesection.Inanad-hocnetwork,packetlossesarefrequentintheerror-pronewirelessmedium.However,theeectoftheselossescanbereducedusingreliablelinklayerprotocols.Route failures,whichcanoccurfrequentlyandunpredictablyduringthelifetimeofatransportsession,dependingontherelativemotionofMHsinthenetwork,aremorediculttohandle.Ingeneral,wheneverthemobilityofanMHinvalidatesaroute,there-establishmentofthe routebytheunderlyingroutingprotocoltakesaniteamountoftime.Duringthisperiod oftime,nopacketcanreachthedestinationthroughtheexistingroute.Thiswillresultin thequeuingandpossiblelossofpackets/acknowledgements,whichwillbeinterpretedbythe transportprotocolatthesourceascongestion.Consequentlythesourcewill:1. Retransmit unacknowledgedpacketsupontimingout.2.Invoke  congestioncontrol mechanismsthatincludeexponentialbackooftheretrans-missiontimersandimmediateshrinkingofthewindowsize,thusresultinginreduction ofthetransmissionrate.3.Enteraslowstartrecoveryphasetoensurethatthecongestionhasreducedbefore resumingpackettransmissionatthenormalrate.Thisisundesirableforthefollowingreasons:5 
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