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word761Introduction
Quality-of-Service(QoS)guaranteesinpacketnetworksareoftenclassifiedaccording
totwocriteria.Thefirstcriterioniswhetherguaranteesareexpressedforindividual
end-to-endtrafficflows(per-flowQoS)orforgroupsofflowswiththesameQoSrequirements
(per-classQoS).Thesecondcriterioniswhetherguaranteesareexpressed
withreferencetoguaranteesgiventootherflows/flowclasses(relativeQoS),orwhether
guaranteesareexpressedasabsolutebounds(absoluteQoS).
EffortstoprovisionforQoSintheInternetintheearlyandmid-1990s,whichresulted
intheIntegratedServices(IntServ)servicemodel[3],focusedonper-flowabsolute
QoSguarantees.However,duetoscalabilityissuesandalaggingdemandfor
per-flowabsoluteQoS,theinterestinInternetQoSeventuallyshiftedtorelativeperclass
guarantees.Sincelate1997,theDifferentiatedServices(DiffServ)[2]working
grouphasdiscussedseveralproposalsforper-classrelativeQoSguarantees,e.g.,[4,
17].
WiththeexceptionoftheExpeditedForwardingservice[11],proposalsforrelative
per-classQoSdiscussedwithintheDiffServcontextdefinetheservicedifferentiation
qualitatively,inthesensethatsomeclassesreceivelowerdelaysandalowerlossrate
2J¨orgLiebeherrandNicolasChristin
thanothers,butwithoutquantifyingthedifferentiation.Recently,researchstudieshave
triedtostrengthentheguaranteesofrelativeper-classQoS,andhaveproposednew
buffermanagementandschedulingalgorithmswhichcansupportstrongernotionsof
relativeQoS[6,7,15,16].Probablythebestknownsucheffortistheproportionalservice
differentiationmodel[6,7],whichattemptstoenforcethattheratiosofdelaysor
lossratesofsuccessivepriorityclassesberoughlyconstant.Fortwopriorityclasses
suchaservicecouldspecifythatthedelaysofpacketsfromthehigher-priorityclassbe
halfofthedelaysfromthelower-priorityclass,butwithoutspecifyinganupperbound
onthedelays.
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