reservationscheme.Thebasicmodelutilizesapacketclas-si er,scheduler,admissioncontrolandareservationsetupmechanism.ResourceReservationProtocol(RSVP)[12]isthesignalingprotocolusedtoconveya“ ow-spec”and“ lterspec”tointermediaryroutersdescribingthetraf ccharacteristicsandtheresourcereservationsthatneedtobemade.CloselycoupledtoIntServisthepolicyadmis-sioncontrolframework[36]allowinganetworkoperatortospecifypolicybasedadmissioncontrolrulesthatcanbelookedupandenforcedattheroutingelements.TwokeycomponentsarethePolicyDecisionPoint(PDP)andthePolicyEnforcementPoint(PEP).ThePEPreceivesrequestforresource(throughRSVP).ThePEPcontactsthePDPtodeterminewhatpolicytouse.ThePDPusesapolicydata-basetoconveyaparticularpolicydecisiontothePEP.EachPEPalsohasalocalPDPthatcanusedincasethePDPisnotreachable.Throughthismechanism,admissioncontrolpoliciessuchastimeofday,SLAs,prioritization,prepaidtransactions,senderspeci edrestrictionsetc.canbeen-forced.OneofthemaindrawbacksofIntServisthemain-tenanceofsoftstaterepresentingper owresourcereserva-tionsonrouterspreventingitfromscalingforlargenetworksizes.Also,the ltersarebasedonpacketheadersandhavelimitedexpressivity.
DifferentiatedServices(DiffServ)[27]takesadifferentap-proachtoenablingQoSguarantees.Theideahereistomoveawayfromaper- ow, negrainQoSmodeltoamorecoarsegrainedQoSmodeltoallowforscalingtolargenet-works.TheapproachfollowedistousetheIPv4TypeofService(TOS)ortheIPv6Traf cclassbytetoconveyag-gregateQoSrequirements.6bitsofthis eldareusedtospecify64DifferentiatedServicesCodePoints(DSCPs).EachDSCPmapstoaspeci cPerHopBehavior(PHB)ateveryintermediaryrouter.Packetsaremarkedtocorre-spondingDSCPsatthenetworkingressbasedonServiceLevelAgreements.OneofthemainproblemswithDiff-Servisthatsinceitiscoarsegrain,itisreallyusableonlybyISPsandnotbyendusers.Also,markingisstaticanddoesnotensurethatadequateresourcesareactuallyavail-abletohandletheQoSrequirements.ThereisalsolimitedexpressivityasthenumberofDSCPsislimited.
Forourframework,weuseacombinationofapproaches.WeuseaPEP/PDPlikeapproachtoallowforpoliciestobespeci edthatarelocallyenforcedandcanspeci edatasys-temlevelbyanetworkoperator.SimilartotheDiffServap-proach,werelyonpackettaggingtoofferdifferentlevelsofservice.However,unliketheDSCPtags,werelyoncustomtagsthatpointtosemanticmetadataaboutthecontent.Thismetadatacanberetrievedandreasonedovertodeterminewhatpoliciesneedtoenforcedforthatstream.Forexam-ple,apolicyspeci edbyanenterprisethatdropsallmusicMP3streams owingintotheirof cenetworkshouldstillallowanof cialpresentationcontainingMP3audiotoen-4
2.4ServiceDifferentiation
Goingbeyondthe“besteffort”paradigmofthecurrentInternethasbeenthegoalofseveralinitiatives.IntegratedServices(IntServ)[11]proposesanarchitecturefora negrainedQoSsystemthatisbasedonaper- owresource
Today’s model of networking primarily concentrates intelligence at the end hosts with the network itself offering a simple“best-effort”,“data agnostic ” communication medium. However, this paradigm has proven to be insufficient to meet todays needs co
terthenetwork.Approachesbasedonpacket lteringandheadercheckingcannotofferthislevelof exibilitysincetheylackinferencecapabilities.Likewise,applicationsrun-ningontopofourframeworkcanspecifytheirresourceneeds,withthisinputvaryingindetailfrombeinghighlyspeci ctojustcommunicatinghighlevelsemanticinfor-mationlikethe“ owoftypeJPEGwithframeratexandqualityy”.OurframeworkrunningontopofanIntservorDiffServnetworkwillbeabletoconverttheapplicationrequesttothecorrespondingrealizationmechanismsuchassignallinganRSVPreservationormappingthedatapacketstothecorrespondingDSCPasthecasemaybe.
3.1Node
Level
2.5SemanticMarkup
ResourceDescriptionFramework(RDF)[3]isaframe-workforrepresentingmetadataregardingwebcontentusingXMLastheencodingmechanism.RDFprovidesastan-dardframeworkforinterchanginginformationacrossappli-cations.RDFisbasedontheideaofidentifyingobjectsusingUniformResourceIdenti ers(URI)anddescribingresourcesintermsofsimplepropertiesandpropertyvalues.Astatementisatriplespecifyingasubject(usingURI),apredicate(representingaproperty)andanobject(repre-sentingthevalue).Thisresultsinstructuredgraphswithnodesforsubjectsandobjectswitharcsrepresentingthepredicates.RDFSchema[2]isthemechanismforspecify-ingvocabulariesthatcanbeusedtoformRDFstatements.Thevocabularygivestheactualmeaningtothestatement.Throughthis,objectsandtheirpropertiesandwhattheyre-fertocanbespeci ed.Morepowerfulschemade nitioncanbeprovidedusingOWL.
Providingmarkupforcontentisprevalentinmanyotherar-easalbeitnotalwaysusingastandardmachineprocessablemeanssuchasRDF.SessionDescriptionProtocol(SDP)-Anytimeisaspeci cationforaudio-videocontentmarkup.ID3tagsappliedtoMP3 lescanconveymetadatasuchastitlesandcomposers.ManyoftheseapproachesuseunstructuredXMLwhichisdif culttouseforconveyingknowledge.WehavechosentouseRDFforourframe-workmainlytoprovideastandardandsimplemechanismthroughwhichdatastreamscanbedescribed.Themainob-jectiveistokeepourframework exibletoallowfortheintroductionofnewconceptsasneededratherthanenforcearigidschematobefollowedbyallparticipants.