qosDiffServcisco.docx
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qosDiffServcisco.docx
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qosDiffServcisco
DifferentiatedServices
DiffServ--TheScalableEnd-to-EndQoSModel
∙
WHITEPAPER
LastUpdated:
August2005
TheInternetischangingeveryaspectofourlives-business,entertainment,education,andmore.BusinessesusetheInternetandWeb-relatedtechnologiestoestablishIntranetsandExtranetsthathelpstreamlinebusinessprocessesanddevelopnewbusinessmodels.
BehindallthissuccessistheunderlyingfabricoftheInternet:
theInternetProtocol(IP).IPwasdesignedtoprovidebest-effortservicefordeliveryofdatapacketsandtorunacrossvirtuallyanynetworktransmissionmediaandsystemplatform.TheincreasingpopularityofIPhasshiftedtheparadigmfrom"IPovereverything,"to"everythingoverIP."Inordertomanagethemultitudeofapplicationssuchasstreamingvideo,VoiceoverIP(VoIP),e-commerce,EnterpriseResourcePlanning(ERP),andothers,anetworkrequiresQualityofService(QoS)inadditiontobest-effortservice.Differentapplicationshavevaryingneedsfordelay,delayvariation(jitter),bandwidth,packetloss,andavailability.Theseparametersform thebasisofQoS.TheIPnetworkshouldbedesignedtoprovidetherequisiteQoStoapplications.
Forexample,VoIPrequiresverylowjitter,aone-waydelayintheorderof150millisecondsandguaranteedbandwidthintherangeof8Kbps->64Kbps,dependentonthecodecused.Inanotherexample,afiletransferapplication,basedonftp,doesnotsufferfromjitter,whilepacketlosswill behighlydetrimentaltothethroughput.
Tofacilitatetrueend-to-endQoSonanIP-network,theInternetEngineeringTaskForce(IETF)hasdefinedtwomodels:
IntegratedServices(IntServ)andDifferentiatedServices(DiffServ).IntServfollowsthesignaled-QoSmodel,wheretheend-hostssignaltheirQoSneedstothenetwork, whileDiffServworksontheprovisioned-QoSmodel,wherenetworkelementsaresetuptoservicemultipleclassesoftrafficwithvarying QoSrequirements.Bothmodelscanbedrivenoffapolicybase,usingtheCoPS(CommonopenPolicyServer)protocol.CiscoIOS®SoftwaresupportsboththeIntServandDiffServmodelsofQoS,alongwithanoptionalCoPS-clientfunctionality.
IntServprovidesforarichend-to-endQoSsolution,usingend-to-endsignaling,state-maintenance(foreachRSVP-flowandreservation)andadmissioncontrolateachnetworkelement.DiffServ,ontheotherhand,addressestheclearneedforrelativelysimpleandcoarsemethodsofcategorizingtrafficintodifferentclasses,alsocalledClassofService(CoS),andappliesQoSparameterstothoseclasses.Toaccomplishthis,packetsarefirstdividedintoclassesbymarkingtheTypeofService(ToS)byteintheIPheader.A6-bitbit-pattern(calledtheDifferentiatedServicesCodePoint[DSCP])intheIPv4ToSOctetortheIPv6TrafficClassOctetisusedasshowninFigures1,2,and3.
Figure1.IPv4andIPv6Headers
Figure2.TheOriginalIPv4ToSByte
Figure3.DiffServCodepointField
Whenpacketsareclassifiedattheedgeofthenetwork,specificforwardingtreatments,formallycalledPer-HopBehavior(PHB),areappliedoneach networkelement,providingthepackettheappropriatedelay-bound,jitter-bound,bandwidth,etc.Thiscombinationofpacketmarkingandwell-definedPHBsresultsinascalableQoSsolutionforanygivenpacket,andanyapplication.InDiffServ,signalingforQoSiseliminated,andthenumberofstatesrequiredtobekeptateachnetworkelementisdrasticallyreduced,resultinginacoarse-grained,scalableend-to-endQoSsolution.
WHYDOWENEEDDIFFSERV?
IntServ,ItsStrengthsandShortcomings
TheIETFdefinedmodels,IntServandDiffServ,aretwowaysofconsideringthefundamentalproblemofprovidingQoSforagivenIPpacket.The IntServmodelreliesontheResourceReservationProtocol(RSVP)tosignalandreservethedesiredQoSforeachflowinthenetwork.Aflowis definedasanindividual,unidirectionaldatastreambetweentwoapplications,andisuniquelyidentifiedbythe5-tuple(sourceIPaddress,sourceportnumber,destinationIPaddress,destinationportnumber,andthetransportprotocol).TwotypesofservicecanberequestedviaRSVP(assumingallnetworkdevicessupportRSVPalongthepathfromthesourcetothedestination).Thefirsttypeisaverystrictguaranteedservicethatprovidesforfirmboundsonend-to-enddelayandassuredbandwidthfortrafficthatconformstothereservedspecifications.Thesecondtypeisacontrolledloadservicethatprovidesforabetterthanbesteffortandlowdelayserviceunderlighttomoderatenetworkloads.Itispossible(atleasttheoretically)toprovidetherequisiteQoSforeveryflowinthenetwork,provideditissignaledusingRSVPandtheresourcesareavailable.
However,thereareseveraldrawbackstothisapproach:
•Everydevicealongthepathofapacket,includingtheendsystemssuchasserversandPCs,needtobefullyawareofRSVPandcapableofsignalingtherequiredQoS.
•Reservationsineachdevicealongthepathare"soft,"whichmeanstheyneedtoberefreshedperiodically,therebyaddingtothetrafficonthe networkandincreasingthechancethatthereservationmaytimeoutifrefreshpacketsarelost.
•Maintainingsoft-statesineachrouter,combinedwithadmissioncontrolateachhopandincreasedmemoryrequirementstosupportalargenumber ofreservations,addstothecomplexityofeachnetworknodealongthepath.
•Sincestateinformationforeachreservationneedstobemaintainedateveryrouteralongthepath,scalabilitywithhundredsofthousandsof flows throughanetworkcorebecomesanissue.
Fortunately,manyoftheseshortcomingshavebeenremediedbytheintroductionof"RSVPRefreshReductionandReliableMessaging,""RSVP scalabilityenhancements,"ProxyRSVPandmanyotherfeatureenhancementsthatmakeRSVPmorescalableanddeployable.
OnLayer2QoSMechanisms
BeforetheIETFdefinedIP(Layer3)QoSmethods,theInternationalUnionforTelecommunications,Telecommunications(ITU-T),theAsynchronousTransferMode(ATM)Forum,andtheFrame-RelayForum(FRF)hadalreadyarrivedatstandardstoperformLayer2QoSinATM andFrame-Relaynetworks.TheATMstandardsdefineaveryrichQoSinfrastructurebysupportingtrafficcontracts,manyadjustableQoSknobs(suchasPeakCellRate[PCR],MinimumCellRate[MCR],etc.),signalingandConnectionAdmissionControl(CAC)[Ref-A].Alternatively,FrameRelayprovidesasimpleryetrichsetofmechanismstoprovideforaCommittedInformationRate(CIR),congestionnotificationandFrame-RelayFragmentation(FRF.12)[Ref-B].
ThoughtheserichQoSmechanismsexistinLayer2transporttechnologies,trueend-to-endQoSisnotachievableunlessaLayer3solutionisoverlaid.ServiceprovidersofferingbothATM/Frame-RelayandIPserviceswanttoproviderobustQoSsolutionstocustomers.MappingLayer3QoStoLayer2QoSisthefirststeptowardachievingacompletesolutionthatdoesnotdependonanyspecificLayer2technology.BothIntServand DiffServcanbeimplementedoverQoS-awaretransportssuchasATMandFrame-Relay.Forexample,theIntServcontrolled-loadservicecanimplementusingRSVP,overanATMVariableBitRate,Real-Time(VBR-rt)SwitchedVirtualCircuit(SVC).WithDiffServ,packetsmarkedwithdifferentToS-bytevaluescanbesentoverdifferentATMPVCsorSVCs.Forexample,highprioritytrafficmaygooveraVBR-nrtVC,andallother trafficmaygooveranAvailableBitRate(ABR)VC,withtheVBRVCcapableofpreemptingtheABRVCincaseofcongestionorfailure.Similarly,Frame-RelayTrafficShaping(FRTS)(slowingdowntherateoftransmissionbybuffering,inresponsetocongestionnotificationbytheFRswitches),FRF.12(packetfragmentationandinterleavingonlowspeedFRlinks),andothermechanismscanbeusedtocomplementIPQoS.
Atrueend-to-endQoSsolutioncomprisesbothLayer3andLayer2QoSandismediaindependent.IntroductionofaGigabitEthernetlinksomewherealongthepathofapacketposesnoproblemtodeliverQoS,astheLayer3QoSisstillpreservedandcanevenbeenhancedbymapping tothe802.1p(user-priority)QoSmechanismonEthernet(RFC-1349).CiscoIOSQoSfocusesondeliveringexactlythismodel-inter-operability/mappingsbetweenLayer2andLayer3QoSoverIP,ATM,Frame-Relay,PacketoverSONET(POS),Ethernet,etc.
ASimplerMiddleGround
Sinceper-flowQoSisdifficulttoachieveinanend-to-endnetworkwithoutaddingsignificantcomplexity,cost,andintroducingscalabilityissues,itnaturallyleadstothinkingaboutclassifyingflowsintoaggregates(classes),andprovidingappropriateQoSfortheaggregates.Forexample,allTCPflowscouldbegroupedasasingleclass,andbandwidthallocatedfortheclass,ratherthanfortheindividualflows.Inadditiontoclassifyingtraffic,signalingandstatemaintenancerequirementsoneachnetworknodeshouldbeminimized.TheIETFrealizedthis,anddefinedamechanismtousetheToSfieldintheIPv4headertoprioritizepacketsasshowninFigures1,2and3.Whenpacketsaremarkedwiththeappropriatepriority/IPprecedencebits,anynetworknodealongthepathofthepacketknowstherelativeimportance(prioritylevel)ofthepacket,andcanapplypreferentialforwardingtopacketsofhigherprioritylevels.
TheToS/IPPrecedenceSolution
TheIPv4ToSbyteintheIP-headerasshowninFigure1isdefinedinFigure2.ThethreeprecedencebitsaremainlyusedtoclassifypacketsattheedgeofthenetworkintooneoftheeightpossiblecategorieslistedinFigure2.Packetsoflowerprecedence(lowervalues)canbedroppedinfavorofhigherprecedencewhenthereiscongestiononanetwork.Eachpacketmaybemarkedtoreceiveoneoftwolevelsofdelay,throughput,andre
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