ASAP-TR-06-6

ASAP:anAS-AwarePeer-RelayProtocolforHighQualityVoIPShansiRen,LeiGuo,andXiaodongZhangDepartmentofComputerScienceandEngineeringTheOhioStateUniversity{sren,lguo,zhang}@cse.ohio-state.eduAbstractPeer-to-peer(P2P)technologyhasbeensuccessfullyap-pliedinInternettelephonyorVoiceoverInternetProtocol(VoIP),suchastheSkypesystem,whereP2Pisusedforbothsearchingclientsandrelayingvoicepackets.Select-ingoneormultiplesuitablepeerstorelayvoicepacketsisacriticalfactorforthequality,scalabilityandcostofaVoIPsystem.Inthispaper,wefirstpresenttwosetsofintensiveInternetmeasurementresultstoconfirmthebenefitsgainedbypeerrelaysinVoIP,andtoinvestigatetheperformanceoftheSkypesystem.Weobtainthefollowingresults:(1)manyrelaypeerselectionsaresuboptimal;(2)thewaitingtimetoselectarelaynodecanbequitelong;and(3)therearealargenumberofunnecessaryprobes,resultinginheavynetworktraffictolimitscalabilityoftheVoIPsystem.Ourfurtheranalysisshowsthattwomainreasonscausetheseproblems.First,thepeerselectionsdonottakeAutonomousSystem(AS)topologyintoconsideration,andsecond,thecomplexcommunicationrelationshipsamongpeersarenotwellutilized.Motivatedbyourmeasurementsandanalysis,weproposeanAS-awarepeer-relayprotocolcalledASAP.OurobjectiveistosignificantlyimproveVoIPqualityandsystemscalabilitywithlowoverhead.Ourintensiveevalu-ationbytrace-drivensimulationshowsASAPishighlyef-fectiveandeasytoimplementontheInternetforbuildinglargeandscalableVoIPsystems.1IntroductionWiththecontinuousincreaseofInternetbandwidthandrapidadvancementofP2Papplications,VoIPtechnologyhasbecomeacommunicationalternativeformanyInternetusers.WeenvisionthatInternettelephonywillsoonbe-comeapopularvoicecommunicationvehicleduetoitslowcostandconveniencetomanyInternetendusers.VoIPtech-nologyhasbeeninvestigatedfromdifferentperspectivesre-cently([17,19,20]).P2PtechnologyhasquicklyemergedinInternettele-phony,whereP2Pisusedforbothsearchingclientsandrelayingvoicepackets.Littleworkhasbeendoneonpeernodeselectionstorelayvoicepackets,whichisacriticalfactorforthequality,scalabilityandcostofaVoIPsys-tem.Therearesomeoverlayroutingprojects,representedbyMIT’sResilientOverlayNetwork(RON)[4],andUni-versityofWashington’sScalableOne-hopSourceRout-ing(SOSR)[11],whichareproposedtomitigateInternetpathfailurethroughoneintermediarynodeoverlayrouting.However,theyarenotspecificallydesignedforVoIPsys-tems,thus,arenotsuitabletoVoIPpeerrelayselections.Tothebestofourknowledge,SkypeisthefirstandonlycommercialsoftwarethathasusedP2Ptechnologyinbothusersearchandvoicepacketrelay.MillionsofpeopleareenthusiasticallyusingtheSkypeVoIPsysteminmanyap-plications.WewilladdressseveralimportanttechnicalissuesofP2PsupportedVoIPsystemsinthispaper.HavingconductedintensiveInternetmeasurementsbasedonBorderGatewayProtocol(BGP)routingtablesandotherdynamicinformation,weshowthebenefitsgainedandim-portantrolesplayedbytherelayingpeersinaVoIPsystem.Thismeasurementstudyestablishesatechnicalfoundationforbuildingalargeandscalablepeer-relayedVoIPsystem.WehavelookedintotheSkyperoutingpathselectionsbyrunningextensiveexperimentsviaitsconversationsessions.WehaveidentifiedseveralperformancelimitsoftheSkypesystem:(1)manyrelaypeerselectionsaresuboptimal;(2)thewaitingtimetoselectarelaynodecanbequitelong;and(3)alargenumberofunnecessaryprobesaregenerated,resultinginheavynetworktraffictolimitscalabilityoftheVoIPsystem.Withtheselimits,SkypemaynotguaranteeastableVoIPqualityandahighscalabilitytoalargenumberofclients.Infact,Skypeusers,includingourselves,haveexperiencedsomequalityandscalabilitydifficultiesfromtimetotimeinpractice.Toourknowledge,thesekindsofmeasurementshavenotbeensystematicallyreportedintheresearchliterature.OurfurtheranalysisshowstwomainreasonsbehindourfindingsinSkype.First,thepeerselectionsdonottaketheAutonomousSystem(AS)intoconsideration,andsecond,complexcommunicationrelationshipsamongpeersarenotwellutilizedinpeerselections.Motivatedbyourmeasurementsandanalysis,weproposeanAS-awarepeer-relayprotocolcalledASAP.Ourtrace-drivensimulationshowsthatASAPsignificantlyimprovesVoIPqualityandsystemscalabilitywithlowoverhead.ItisalsoeasytoimplementASAPontheInternetforbuildinglargeandscalableVoIPsystems.2VoIPApplicationanditsQualityRequire-mentBasedontheunderlyingtransmissionmedia,VoIPap-plicationsaregenerallyoperatedinapureorhybridenvi-ronment.HybridVoIPapplicationspartlyusethepublictelephoneswitchnetworkforlocalaccesses,andpartlyuseIPnetworksforwideareadatatransmission.PureVoIPap-plications,suchasSkype,solelyrelyonIPnetworks.Ontheotherhand,theVoIPcommunicationprocessconsistsofsignalingandvoicepackettransmission.Inthispaper,wefocusonthevoicepackettransmissionofpureVoIPappli-cations.VoIPisareal-timeandinteractiveInternetapplica-tion,andgenerallyhasthefollowingqualityrequirements.VoIPSpeechQualityRequirement.MeanOpinionScore(MOS)isasubjectivemetricwidelyusedtoevaluatehu-manfeelingspeechqualityandisgivenonascaleof1-5,asdefinedin[1].HighMOSmeanshighspeechquality,andaMOSbelow3.6likelycauseslisteners’dissatisfac-tion.ForaVoIPapplication,itsspeechqualityisaffectedbymanyfactors,suchascodec