CISCO DiffSvr QoS

AmericasHeadquarters:CiscoSystems,Inc.,170WestTasmanDrive,SanJose,CA95134-1706USA©2007CiscoSystems,Inc.Allrightsreserved.OverviewofDiffServforQualityofServiceFirstPublished:April30,2007LastUpdated:April30,2007ThismodulecontainsanoverviewofimplementingDifferentiatedServices(DiffServ)onyournetwork.DiffServisasetofend-to-endqualityofservice(QoS)capabilities.End-to-endQoSistheabilityofthenetworktodeliverservicerequiredbyspecificnetworktrafficfromoneendofthenetworktoanother.CiscoIOSQoSsoftwaresupportsthreetypesofservicemodels:best-effortservices,IntegratedServices(IntServ),andDifferentiatedServices.FindingSupportInformationforPlatformsandCiscoIOSandCatalystOSSoftwareImagesUseCiscoFeatureNavigatortofindinformationaboutplatformsupportandCiscoIOSandCatalystOSsoftwareimagesupport.ToaccessCiscoFeatureNavigator,goto•InformationAboutDifferentiatedServices,page379�WheretoGoNext,page399�AdditionalReferences,page399InformationAboutDifferentiatedServicesThissectioncontainsthefollowingconceptsaboutDifferentiatedServices:�DifferentiatedServicesDefinition�DSFieldDefinition�Per-HopBehaviors�BenefitsofImplementingDifferentiatedServices�DifferentiatedServicesComponents�DifferentiatedServicesFeatureSetsOverviewofDiffServforQualityofServiceInformationAboutDifferentiatedServices380CiscoIOSQualityofServiceSolutionsConfigurationGuide�SampleDiffServImplementation�AccountingFunctionalityandDiffServDifferentiatedServicesDefinitionDifferentiatedServicesisamultipleservicemodelthatcansatisfydifferingQoSrequirements.WithDifferentiatedServices,thenetworktriestodeliveraparticularkindofservicebasedontheQoSspecifiedbyeachpacket.Thisspecificationcanoccurindifferentways,forexample,usingthe6-bitdifferentiatedservicescodepoint(DSCP)settinginIPpacketsorsourceanddestinationaddresses.ThenetworkusestheQoSspecificationtoclassify,mark,shape,andpolicetrafficandtoperformintelligentqueueing.DifferentiatedServicesisusedforseveralmission-criticalapplicationsandforprovidingend-to-endQoS.Typically,DifferentiatedServicesisappropriateforaggregateflowsbecauseitperformsarelativelycoarseleveloftrafficclassification.DSFieldDefinitionAreplacementheaderfield,calledtheDSfield,isdefinedbyDifferentiatedServices.TheDSfieldsupersedestheexistingdefinitionsoftheIPversion4(IPv4)typeofservice(ToS)octet(RFC791)andtheIPv6trafficclassoctet.SixbitsoftheDSfieldareusedastheDSCPtoselectthePer-HopBehavior(PHB)ateachinterface.Acurrentlyunused2-bit(CU)fieldisreservedforexplicitcongestionnotification(ECN).ThevalueoftheCUbitsisignoredbyDS-compliantinterfaceswhendeterminingthePHBtoapplytoareceivedpacket.Per-HopBehaviorsRFC2475definesPHBastheexternallyobservableforwardingbehaviorappliedataDiffServ-compliantnodetoaDiffServBehaviorAggregate(BA).WiththeabilityofthesystemtomarkpacketsaccordingtoDSCPsetting,collectionsofpacketswiththesameDSCPsettingthataresentinaparticulardirectioncanbegroupedintoaBA.PacketsfrommultiplesourcesorapplicationscanbelongtothesameBA.Inotherwords,aPHBreferstothepacketscheduling,queueing,policing,orshapingbehaviorofanodeonanygivenpacketbelongingtoaBA,asconfiguredbyaservicelevelagreement(SLA)orapolicymap.ThefollowingsectionsdescribethefouravailablestandardPHBs:�DefaultPHB�Class-SelectorPHB(asdefinedinRFC2474)�AssuredForwardingPHB(asdefinedinRFC2597)�ExpeditedForwardingPHB(asdefinedinRFC2598)FormoreinformationaboutdefaultPHB,seeRFC2474,DefinitionoftheDifferentiatedServicesField(DSField)intheIPv4andIPv6Headers.OverviewofDiffServforQualityofServiceInformationAboutDifferentiatedServices381CiscoIOSQualityofServiceSolutionsConfigurationGuideDefaultPHBThedefaultPHBessentiallyspecifiesthatapacketmarkedwithaDSCPvalueof000000(recommended)receivesthetraditionalbest-effortservicefromaDS-compliantnode(thatis,anetworknodethatcomplieswithallofthecoreDiffServrequirements).Also,ifapacketarrivesataDS-compliantnode,andtheDSCPvalueisnotmappedtoanyotherPHB,thepacketwillgetmappedtothedefaultPHB.FormoreinformationaboutdefaultPHB,seeRFC2474,DefinitionoftheDifferentiatedServicesField(DSField)intheIPv4andIPv6Headers.Class-SelectorPHBTopreservebackward-compatibilitywithanyIPprecedenceschemecurrentlyinuseonthenetwork,DiffServhasdefinedaDSCPvalueintheformxxx000,wherexiseither0or1.TheseDSCPvaluesarecalledClass-SelectorCodePoints.(TheDSCPvalueforapacketwithdefaultPHB000000isalsocalledtheClass-SelectorCodePoint.)ThePHBassociatedwithaClass-SelectorCodePointisaClass-SelectorPHB.TheseClass-SelectorPHBsretainmostoftheforwardingbehaviorasnodesthatimplementIPPrecedence-basedclassificationandforwarding.Forexample,packetswithaDSCPvalueof11000(theequivalentoftheIPPrecedence-basedvalueof110)havepreferentialforwardingtreatment(forscheduling,queueing,andsoon),ascomparedtopacketswithaDSCPvalueof100000(theequivalentoftheIPPrecedence-basedvalueof100).TheseClass-SelectorPHBsensurethatDS-compliantnodescancoexistwithIPPrecedence-basednodes.FormoreinformationaboutClass-SelectorPHB,seeRFC2474,DefinitionoftheDifferentiatedServicesField(DSField)intheIPv4an