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A Model for Web Services Discovery With QoS

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AModelforWebServicesDiscoveryWithQoS
SHUPINGRAN
CSIROMathematicalandInformationSciencesGPOBox664,Canberra,ACT2601,AustraliaShuping.Ran@csiro.au
________________________________________________________________________
Webservicestechnologyhasgeneratedalotinterest,butitsadoptionratehasbeenslow.Thispaperdiscussesissuesrelatedtothisslowtakeupandarguesthatqualityofservicesisoneofthecontributingfactors.ThepaperproposesanewWebservicesdiscoverymodelinwhichthefunctionalandnon-functionalrequirements(i.e.qualityofservicesaretakenintoaccountfortheservicediscovery.TheproposedmodelshouldgiveWebservicesconsumerssomeconfidenceaboutthequalityofserviceofthediscoveredWebservices.

AdditionalKeyWordsandPhrases:Webservicesdiscovery,qualityofservices,UDDI,UDDIextension,tModel,model.
________________________________________________________________________

1.INTRODUCTION
Webservicestechnologyisbecomingincreasinglypopularbecauseofitspotentialinmanyareas.ItisanewtypeofcomponentsthatcanbeinvokedovertheInternet.Thispresentsapromisingsolutionforaddressingplatforminteroperabilityproblemsfacedbysystemintegrators.TheflexibilityofthisnewcomponenttypealsofacilitatesservicecompositionusingexistingWebservices,promotingcomponentre-usewhichhasbeenadreamforthesoftwareengineeringindustry.Becauseofitspotentialforservicecomposition,agentresearchcommunityhasalsoexploreditforcomposingagent’sbehaviors[BuhlerandVidal2003,Mcilraithet.al.2001].
Webservicestechnologyisnowovertwoyearsold.Althoughithasalotofpotential,buttheadoptionratehasbeenveryslow.AccordingtoGartnerresearchpresentedatGartnerSymposiumITxpo2001[PlammerandAndrews2001],Webservicestechnology’srealtakeupisby2005.Therearemanyfactorsthatmaycontributetothisslowtakeup,suchasperceivedlackofsecurityandtransactionsupport[DuWaldtandTrees2002].AlthoughthereareemergingstandardsintheseareassuchasWS-Coordination[BEA,IBMandMicrosoft2002a],WS-Transaction[BEA,IBMandMicrosoft2002b],WS-Security[IBM,MicrosoftandVerisign2002]etc.,acoherentpicturewithfullsupportinalltheseareasisyettobeseen.AnotherveryimportantissueisthequalityoftheWebservices[DuWaldtandTrees2002,Rao2002,Borck2001].Atthepresenttime,UniversalDescription,DiscoveryandIntegrationofWebservices(UDDI[OASIS2002]basedlookupsforWebservicesarebasedonthefunctionalaspectsofthedesiredWebservices.Figure1presentsthispublish-find-bindmodel.WebservicestechnologyhasyettoaddressquestionssuchashowwillIknowtheWebservicewillmeetmyperformancerequirementssuchas2msresponsetime?WilltheWebservicebereliableformymission-criticalsystem’simplementation?Untilthesequestionshavebeenaddressed,itisunrealistictoexpectthatabusinesswouldwanttosearchforaWebservicebasedontheexpectedfunctionalrequirementsinanUDDIregistryand
Permissiontomakedigital/hardcopyofpartofthisworkforpersonalorclassroomuseisgrantedwithoutfeeprovidedthatthecopiesarenotmadeordistributedforprofitorcommercialadvantage,thecopyrightnotice,thetitleofthepublication,anditsdateofappear,andnoticeisgiventhatcopyingisbypermissionoftheACM,Inc.Tocopyotherwise,torepublish,topostonservers,ortoredistributetolists,requirespriorspecificpermissionand/orafee.

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invokethatservicewithouttheassuranceofknowingtheexpectedqualityofservicewouldbemetbeforehand.
Toaddresstheseproblems,thispaperproposesanewservicediscoverymodelwherequalityofserviceistakenasconstraintswhensearchingforWebservices.ThiswouldgivesomeconfidencetotheWebserviceconsumersaboutthequalityoftheservicetheyareabouttoinvoke.
UDDIRegistry
Find(RetrieveWSDLWebServiceConsumer
Request/Response
Register(PublishWSDLWebServiceProvider
Fig.1.CurrentWebservicespublish-find-bindmodel.
2.EXTENDINGUDDIMODEL2.1ANewModel
ThecurrentproposedWebservicespublishanddiscoverymodel(Figure1islargelyunregulatedbasedonUDDIregistries.48%oftheproductionUDDIregistry(tModelstestedonlyhaslinksthatareunusable.Thesepointerscontainmissing,brokenorinaccurateinformation[Clarke2001].Thisisoneexampleillustratingtheimportanceofaddressingqualityofservice(QoSissues.TheothershortcomingofthecurrentUDDImodelisthatitlimitstheservicediscoverytofunctionalrequirementsonly.ItisforeseeablethattheremaybemorethanoneWebservicesavailablethatcanmeetthefunctionalrequirementswithdifferentqualityofserviceattributes.Thereforetheabilityofincorporatingqualityofserviceintoservicediscoveryprocessbecomesveryimportant.ToovercometheseshortcomingsweproposeanewmodelshowninFigure2.Theproposedframeworkisaregulatedmodelthatcanco-existwiththecurrentde-regulatedUDDIregistries.Thecurrentde-regulatedregistriescanofferservicestopeopletowhomthequalityofserviceisnotimportant.Theregulatedregistriesbasedonthemodelpresentedherecanservetotheapplicationsneedingqualityofserviceassurance.Therearefourrolesinthisproposedmodel:Webservicesupplier,Webserviceconsumer,WebserviceQoScertifier,andthenewUDDIregistry.Asbefore,theWebserviceprovideroffersWebservicebypublishingtheserviceintotheregistry;theWebserviceconsumerneedstheWebserviceofferedbytheprovider;thenewUDDIregistryisarepositoryofregisteredWebserviceswithlookupfacilities;thenewcertifier’sroleistoverifyserviceprovider’sQoSclaimsdescribedbelow.TheproposednewregistrydiffersfromthecurrentUDDImodelbyhavinginformationaboutthefunctionaldescriptionoftheWebserviceaswellasitsassociatedqualityofserviceregisteredintherepository.LookupcouldbemadebyfunctionaldescriptionofthedesiredWebservice,withtherequiredqualityofserviceattributesaslookupconstraints.ThenewroleinthismodelistheWebserviceQoScertifierthatdoesnotexistintheoriginalUDDImodel.ThecertifierverifiestheclaimsofqualityofserviceforaWebservicebeforeitsregistration.ThedetailsofWebserviceregistration,discoveryandinvocationarediscussedbelow.
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NewUDDIRegistry
Find(RetrieveWSDLandCertifiedQoS
WebServiceConsumer
Request/Response
Register(PublishWSDLandCertifiedQoSWebServiceSupplier
VerifyQoS
WebServiceQoSCertifier
CertifyQoS
Fig.2.AnewWebservicesregistrationanddiscoverymodel.
2.2WebServicesRegistrationUnderNewModel
Intheproposedmodel,aWebserviceproviderneedstosupplyinformationaboutthecompany,thefunctionalaspectsoftheprovidedserviceasrequestedbythecurrentUDDIregistry,aswellastosupplyqualityofserviceinformationrelatedtotheproposedWebservice.Theclaimedqualityofserviceneedstobecertifiedandregisteredintherepository.
TheWebserviceproviderfirstneedstocommunicateitsQoSclaimtotheWebserviceQoScertifier.Thecertifiercheckstheclaimsandeithercertifiesordowngradetheclaim.Theoutcomeissentbacktotheproviderwithcertificationidentificationinformation.Thisinformationisalsoregisteredinthecertifier’srepositoryidentifiedbyacertificationId.ThecertifierprovidesasetofWebservicesforanyinterestedpartiestoaccessitsrepositoryaboutQoSclaimsforverificationpurposes.AftertheQoScertificationbeenissuedbythecertifier,thesupplierthenregisterswiththeUDDIregistrywithbothfunctionaldescriptionoftheserviceanditsassociatedcertifiedqualityofserviceinformation.TheUDDIregistrycommunicateswiththecertifiertochecktheexistenceofthecertification.Aftersuccessfulchecking,theregistrythenregisterstheserviceinitsrepository.
2.3WebServiceDiscoveryandInvocationUnderNewModel
AconsumerofaWebservicehascertainfunctionalandqualityofservicerequirements,suchas“responsetimenotgreaterthan2mswithcostlessthan$100perinvocation”.TheconsumersearchestheUDDIregistryforaWebservicewiththerequiredfunctionalityasusual;theycanalsoaddconstraintstothesearchoperation.Onetypeofconstraintistherequiredqualityofservice.IfthereweremultipleWebservicesintheUDDIregistrywithsimilarfunctionalities,thenthequalityofservicerequirementwouldenforceafinersearch.ThesearchwouldreturnaWebservicethatofferstherequiredfunctionalitywiththedesiredsetofqualityofservice.IfthereisnoWebservicewiththesequalities,feedbackisgiventotheconsumer.Theconsumercanthenreducetheirqualityofserviceconstraintsorconsideringtrade-offsbetweenthedesiredqualitiesofservice.OnceaWebserviceisfound,theWSDLandthecertifiedQoSinformationisretrievedbytheconsumer.TheconsumercanverifytheQoSclaimswiththecertifier©ACM2003
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usingthecertificationId.Oncetheconsumerishappywiththeirfindings,theycaninvoketheWebserviceaspercurrentmodel.
3.EXTENDINGUDDIDATASTRUCTURE
businessEntity:Informationaboutthepartywhopublishinformationaboutafamilyofservices

publisherAssertion:Informationabouta
relationshipbetweentwoparties,assertedbyoneorbothfingerprints
businessService:Descriptiveinformationaboutaparticularservice

qualityInformation:Descriptionofqualityofserviceinformationaboutaparticularservice
bindingTemplate:Technicalinformationaboutaserviceentrypointandconstructionspecsfingerprints

tModel:Descriptionofspecificationforservicesortaxonomies.Basisfortechnicalfingerprints

Fig.3.UDDIdatastructuretypesincludingthenewproposedqualityinformationdatatype.
TheinformationthatmakesupanexistingUDDIregistrationconsistsoffivedatastructuretypes[UDDICommittee2002]:businessEntity,businessService,bindingTemplate,publisherAssertionandtModel.Thesearepresentedasun-shadedboxesinFigure3.ThesefivetypesmakeupthecompleteamountofinformationprovidedwithinthecurrentUDDIservicedescriptionframework.EachoftheseXMLstructurescontainsanumberofdatafieldsthatserveeitherabusinessortechnicaldescriptivepurpose.[UDDICommittee2002]explainseachofthesestructuresandthemeaningandplacementofeachfield.ThesestructuresaredefinedintheUDDIVersion2.0APIschema.Theschemadefinesapproximately25requestsand15responses,eachofwhichcontainthesestructures,referencestothesestructures,orsummaryversionsofthesestructures.
InordertorealizetheproposedextensiontotheUDDImodeldiscussedinSection2,weproposetoaddanewdatastructuretype.ItispresentedasashadedboxinFigure3.Thisdatastructuretyperepresentsdescriptionofqualityofserviceinformationaboutaparticularservice.DifferentcategoriesofqualityofserviceinformationcanbeprovidedunderthisqualityInformationdatastructure,suchasavailability,reliability[Gunther1998]etc.Section5discussesthepotentialcategoriesofQoSinmoredetail.ThisproposeddatastructureisunderthebusinessServicedatastructuretype,inadditiontobindingTemplatedatastructuretype,whichprovidesbindinginformationforaparticularservice.
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LikebindingTemplate,thisnewdatastructurealsoreferstotModelsdefinedintheUDDIregistry.UnlikebindingTemplatereferstotModelsasreferencetointerfacespecificationsofservices,qualityInformationreferstotModelsasreferencestoqualityofservicetaxonomieswhichalsoneedtobedefinedintheextendedUDDIregistry.ThesetaxonomiesdefinethenewterminologiesorconceptsabouttheproposedQoSinformation,whichdonotexistintheexistingUDDIregistries.Figure4showsanexampleofoneoftheseproposedtModels.ThetModeldefinesthetermqualityInformationinthiscase.
uudi-org:qualityInformation
QualityofServiceInformation

http://www.uddi.org/specification.html

keyName="uddi-org:types"keyValue="categorization"
tModelKey="uuid:c1acf26d-9672-4404-9d70-39b756e62ab4"/>
keyName="uddi-org:types"keyValue="checked"
tModelKey="uuid:c1acf26d-9672-4404-9d70-39b756e62ab4"/>
keyName="uddi-org:types"keyValue="specification"
tModelKey="uuid:c1acf26d-9672-4404-9d7039b756e62ab4"/>
Fig.4.tModelforqualityofserviceinformation(qualityInformation.



xmlns="urn:uddi-org:api"maxRows="100">Stockquote
0.9
Fig.5.SOAPrequestforservicediscovery
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4.PUTTINGITTOGETHER–ASERVICEDISCOVERYEXAMPLE
Figure5showsaservicediscoveryrequestexampleusingSOAP,wheretherequiredserviceisrelatedtoStockQuotewithadesiredQoSattributeavailabilityatleastof0.9(probabilityoftheserviceavailable90%,seenextsection.Figure6showsthecorrespondingSOAPresponse.Inthiscase,therearetwoservicessatisfyingtherequiredqualityofservice:StockQuoteandStockQuotesfromtwodifferentservicesuppliersidentifiedbybusinessKeyfieldsb42b5fef-85df-4fbf-b468-62a356089ea8andb6cb1cf0-3aaf-11d5-80dc-002035229c64respectively.TheWebservicerequestercanthendecidetochooseonefromthislistofserviceprovidersafterreceivingtheresponse.



operator="www.ibm.com/services/uddi"truncated="false">
serviceKey="9021cb6e-e8c9-4fe3-9ea8-3c99b1fa8bf3"businessKey="b42b5fef-85df-4fbf-b468-62a356089ea8">StockQuote
0.99
serviceKey="74154900-f0b0-11d5-bca4-002035229c64"businessKey="b6cb1cf0-3aaf-11d5-80dc-002035229c64">StockQuotes0.91

Fig.6.SOAPresponsetotheSOAPrequestforservicediscovery.

5.QUALITYOFSERVICEDESCRIPTION
QualityofServiceresearchhasbeenanactiveresearchareaforseveraldomains.Theterm“qualityofservice”hasbeenusedforexpressingnon-functionalrequirementsfordifferentareassuchasnetworkresearchcommunity[Cruz1995,SalamatianandFdida2001]andinrealtimeissues[Clark,ShenkerandZhang1992].ThereissomeresearcheffortindefiningQoSindistributedsystems.TheirinterestsareprimarilyonhowtoexpresstheQoSforasystem,andhowtheserequirementsarepropagatedtotheresourcemanagertofulfilltheQoSrequirements[Tien,VillinandBac2000].[Stephanieet.al.1997]presentsalayeredmodelforrepresentingQoSfortelecommunicationapplications.Itpresentsservicequalityfunction,QoSschemamappingandprice-QoStrade-off.©ACM20036

[FrølundandKoistinen1998]presentsaQoSspecificationlanguage.Theyadvocateitsusefordesigningdistributedobjectsystem,inconjunctionwiththefunctionaldesign.[Sabataet.al.1997]categorizestheQoSfromdifferentviewpoints:application,systemandresource.ItspecifiesQoSintermsofmetricsandpolicy.AlltheseresearchcategorizesanddefineQoSfromtheirperspectivewithsomeoverlapbetweenthem.ThereisnotgreatconsensusaboutasetofQoSimportanttodistributedsystems.ThereisevenlessresearchdoneontheQoSforservice-orientedarchitecture,althoughQoSisanimportantaspectasdiscussedintheintroductionSection.
BecauseWebservicescanbeprovidedbythirdpartiesandinvokeddynamicallyovertheInternet,theirQoScanvarygreatly.ThereforeitisimportanttohaveaframeworkcapturingtheQoSprovidedbythesupplierandfortheQoSrequiredbythecustomer,andultimatelythematchbetweenthetwowhendiscoveringtheWebservicebestmatchtherequiredQoS.
TheinternationalqualitystandardISO8402(partoftheISO9000(ISO90002002describesqualityas”thetotalityoffeaturesandcharacteristicsofaproductorservicethatbearonitsabilitytosatisfystatedorimpliedneeds.”Wedefinequalityofserviceasasetofnon-functionalattributesthatmayimpactthequalityoftheserviceofferedbyaWebserviceinthecontextofthispaper.
TherearemanyaspectsofQoSimportanttoWebservices.WearestartingtoorganizethemintoQoScategories.Eachcategoryneedstohaveasetofquantifiableparametersormeasurements.Furtherresearchisneededinthisarea.Forillustrationpurposes,thecategoriesaredefinedhere.Theydescribedbriefly,followedbyaquestionasanexampletoshowwhattypeofquestionstheparticularQoScanaddress.Tofacilitatethedescription,thecategoriesaregroupedintodifferenttypes,i.e.QoSrelatedtoruntime,transactionsupport,configurationmanagementandcostandsecurity.
5.1RuntimeRelatedQoS
Scalability–Thecapacityofincreasingthecomputingcapacityofserviceprovider’scomputersystemandsystem’sabilitytoprocessmoreoperationsortransactionsinagivenperiod.Itisrelatedtoperformance.
Q:WillthesystemscaleuptohandleXtransactionspersecond?Thisiscloselyrelatedtothroughputandperformance.
Capacity–Limitofconcurrentrequestsforguaranteedperformance.Q:Howmanyconcurrentconnectionsdoestheservicesupport?
Performance–ameasureofthespeedincompletingaservicerequest.Itismeasuredby:
Responsetime–theguaranteedmax(oraverageormintimerequiredtocompleteaservicerequest(relatedtocapacity[Gunther1998].
Latency–Timetakenbetweentheservicerequestarrivesandtherequestisbeingserviced.
Q:Whatistheaveragedelayonservicingarequest?
Throughput–Thenumberofcompletedservicerequestsoveratimeperiod.Throughputisrelatedtolatency/capacity.
Reliability–Theabilityofaservicetoperformitsrequiredfunctionsunderstatedconditionsforaspecifiedperiodoftime[InstituteOfElectricalAndElectronicsEngineers1990].Itcanbemeasuredby:Meantimebetweenfailure(MTBF,MeanTimetoFailure(MTF,andMeanTimeToTransition(MTTT.Itiscloselyrelatedtoavailability[Gunther1998].
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Availability–Itistheprobabilitysystemisupandrelatedtoreliability.Itcanbemeasuredas[Gunther1998]:
〈upTime〉〈upTime〉=A=
(〈upTime〉+〈downTime〉〈totalTime〉Where:
isthetotaltimethesystemhasbeenupduringthemeasurementperiod.
isthetotaltimethesystemhasbeendownduringthemeasurementperiod.
isthetotalmeasurementtime,isthesumofand.Q:WhatisthechancefortheserviceisavailablewhenIinvokeit?
Robustness/Flexibility–Itisthedegreetowhichaservicecanfunctioncorrectlyinthepresenceofinvalid,incompleteorconflictinginputs.
Q:Willtheservicestillworkifincompleteparametersareprovidedtotheservicerequestinvocation?
Exceptionhandling–Sinceitisnotpossiblefortheservicedesignertospecifyallthepossibleoutcomesandalternatives(especiallywithvariousspecialcasesandunanticipatedpossibilities,exceptionscanbeexpected.Exceptionhandlingishowtheservicehandlestheseexceptions.Itcanbeinabrutaloragracefulway.
Q:HowwilltheservicestillworkcorrectlyifIgivelessnumberofparametersthanitrequires?
Accuracy–Definestheerrorrateproducedbytheservice.
Q:Howmanyerrorsdoestheserviceproduceoveraperiodoftime?
5.2TransactionSupportRelatedQoS
Integrity–Transactionscanbegroupedintoaunitinordertoguaranteetheintegrityofthedataoperatedonbythesetransactions.Theunitcaneitherbesuccessfulwherealltransactionsintheunit“commit”orall“rollback”totheiroriginalstateincaseofatransactionfailure.ThisisdescribedbytheACIDproperties:Atomicity(executesentirelyornotatall,consistency(maintainstheintegrityofthedata,isolation(individualtransactionsrunasifnoothertransactionsarepresentanddurability(theresultsarepersistent.
Atwo-phasecommitcapabilityisthemechanismtoguaranteetheACIDpropertiesfordistributedtransactionsrunningovertightlycoupledsystemsasiftheywereasingletransaction.ItismoredifficultintheWebservicesenvironment,asthetransactionsmayinvolvemorethanonebusinesspartnerwiththepossibilityoftransactionsspanningoverlongtime(hoursordays–LongRunningTransactions(LRT.ThetransactionintegrityisstilldescribedbyACIDproperties,althoughitisamuchhardertoachieveinthiscase.Itmayrequiredifferentmechanisms[Peryret2002].5.3ConfigurationManagementandCostRelatedQoS
Regulatory–Itisameasureofhowwelltheserviceisalignedwithregulations.Q:Howalignedistheservicewithappropriateregulations?
SupportedStandard–Ameasureofwhethertheservicecomplieswithstandards(e.g.industryspecificstandards.Thiscanaffecttheportabilityoftheserviceand©ACM2003
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interoperabilityoftheservicewithothers.OneexampleisISO8583,whichisastandardforcreatingandreadingfinancialtransactionmessagesincludingPointofSale(POStransactions[ISO].
Q:Howmuchdoestheserviceadheretoapplicablestandards?Orwhatstandardsdoestheservicecomply?
Stability/changecycle–Ameasureofthefrequencyofchangerelatedtotheserviceintermsofitsinterfaceand/orimplementation.
Q:Howstableistheservice,howoftenitchanges(interfaceandimplementation?
Guaranteedmessagingrequirements–doesitensuretheorderandpersistenceofthemessages?
Cost–Itisameasureofthecostinvolvedinrequestingtheservice.Q:Whatisthecostbasedon(perrequestorpervolumeofdata?
Completeness–Ameasureofthedifferencebetweenthespecifiedsetoffeaturesandtheimplementedsetoffeatures.
Q:Howmanyofthespecifiedfeaturesarecurrentlyavailable?
5.4SecurityRelatedQoS
Itmeasuresofthetrustworthinessandmechanismssecurityimplemented.
Authentication–Howdoestheserviceauthenticateprincipals(usersorotherserviceswhocanaccessserviceanddata?
Authorization–Howdoestheserviceauthorizeprincipalssothatonlythemcanaccesstheprotectedservices?
Confidentiality–Howdoestheservicetreatthedata,sothatonlyauthorizedprincipalscanaccessormodifythedata?
Accountability–Canthesupplierbeholdaccountablefortheirservices?
TraceabilityandAuditability–Isitpossibletotracethehistoryofaservicewhenarequestwasserviced.
Dataencryption–Howdoestheserviceencryptdata?
Non-Repudiation–Aprincipalcannotdenyrequestingaserviceordataafterthefact.Q:Howdoestheserviceproviderensurethesesecurityrequirements?
6CONCLUSIONSANDDISCUSSION
ThispaperdiscussedissuesrelatedtoWebservicestechnology’sslowtakeupandproposedthatqualityofservicesisoneoftheissuescontributingfactors.ThepaperproposedanewWebservicesdiscoverymodelinwhichthefunctionalandnon-functionalrequirements(i.e.qualityofservicearetobetakenintoaccountfortheservicediscovery.Anewroleisintroducedintothisframework–theCertifier(s.TheyverifytheQoSclaimsfromtheWebservicesuppliers.Theirroleisverysimilartoratingagenciesinotherdomainssuchasthefinancialsector,serviceindustryetc.ThepaperalsoproposedanextensiontoUDDI’sdatastructuretypesthatcouldbeusedforimplementingtheproposedextendedUDDImodel.
Inorderfortheproposedframeworktoberealized,weneedtoestablishasetofmetricstoquantifyeachQoScategoryproposedhereandtheirassociatedmodelsfortheirrepresentation.FurtherresearchisneededinestablishingthematchingalgorithmsbetweenthedesiredandsuppliedQoS.ItisanticipatedthataconsumerofaservicemaynotneedalltheQoScategories.Thematchingalgorithmsneedtotakethisintoaccount.Tofullyexploitthepotentialoftheproposedframework,incorporatingthesemantic©ACM2003
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modelingoftheQoScategoriesisnecessary.ThedetailsfortheCertifiersalsoneedtobeexploredfurther.
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