Application. Generic Conference Control (T.124) Multipoint Communications Service (T.122/T.125) Network Specific Transport Protocols (T.

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1 ATRANSPORT-INDEPENDENTCOMPONENTFORA GROUPANDSESSIONMANAGEMENTSERVICEIN GROUPCOMMUNICATIONSPLATFORMS ComputerEngineeringandNetworksLaboratory(TIK) ErikWilde,MuraliNanduri,BernhardPlattner SwissFederalInstituteofTechnology(ETHZurich) CH{8092Zurich tionrecently.thispaperfocusesonamodelandthearchitectureofasystemwhich Groupcommunicationsisanareaofresearchwhichhasreceivedalotofatten- ABSTRACT supportsgroupcommunicationsbyprovidinggroupandsessionmanagementfunctionality.thissystemisanextensionofdirectoryserviceswhichareusedwithunicast communications.newfunctionalityisneededforthedynamicsofgroupcommunications(membersofaconnectionmaychangeoverthelifetimeoftheconnection)and whichrepresenttherelevantobjects.usersandgroupsrepresentrealworldusersand increasedcomplexityofrelations.amodelisdescribedwhichdenessixobjecttypes theirrelations.sessionsandowsdescribeongoinggroupcommunications.flowtemplatesandcerticatesprovidemechanismsformanagementandsecurityissues.the dierentgroupcommunicationplatforms.ashortsketchoftheimplementationis architecturepresentedinthispaperistransport-independent,ieitcanbeusedwithin giveninthelastsection. Overthepastyears,surveysbyvariousauthors[1,2,3]haveshowntheincreasingneed formultipointcommunicationsandtherequirementsforplatformssupportingthistype 1.INTRODUCTION ofcommunications.additionally,therequirementformultimediadatatransportisalso formanagementissues.thispaperpresentsanapproachtothemanagementplane dierentplanes,onedealingwiththeactualdatatransferandanotherbeingresponsible whichsupportsmultipointmultimediadatatransportservicesistheseparationinto appliedtomultipointcommunications.onecommonapproachfordesigningaplatform acomponentwhichisreusablefordierenttransportplatforms.thisapproachhas fromthetransportinfrastructurebeingusedandshouldthereforebeaddressedinside thefollowingadvantages. whichisbasedontheassumptionthatmanyproblemstobesolvedareindependent

2 Reducedimplementationcosts.Becauseofthetransport-independencyofsuch acomponent,itcouldbeusedindierenttransportplatformswithouthaving Transport-independentnaming.Thenametoaddressmappingisoneofthetasks toimplementthefunctionalityforeachplatform.thisleadstoareductionof implementationcostsfornewplatformsusingthiscomponent. collaborativeapplicationhasitsownnamespacesothattheuseofmorethan component,thenitispossibletousethesamenamesforaddressingfordierent ofthemanagementplane.ifnamingisimplementedbyatransport-independent transportplatforms.thiswouldeliminatethesituationoftoday,whereeach Sessiondirectoryfunctionality.Asessiondirectorysimilartothewell-known onecollaborativeapplicationcanbecomeaverycomplicatedprocessintermsof mbonesessiondirectorywouldbepossible,anditwouldbeamoregeneraldirectory.itwouldnotonlylistthesessionsandtherespectiveapplications,butalso thetransportinfrastructurebeingused.thiswayitwouldbeeasytoimplement collaborativeapplicationswhichareabletousedierenttransportinfrastruc- userandgroupmanagement. separatedfromtransport-dependentissuesandimplementedinaseparateandreusable Theapproachtakeninthispaperillustratesthatthereisfunctionalitywhichcanbe tures. component.inordertoelaboratethishypothesis,therestofthepaperisstructured asfollows.section2givesashortoverviewoverotherworkinthisareaandrelated activities.section3thendescribesourmodelofhowsuchacomponentcouldbeused insideatransportinfrastructure.movingontoamodelofhowsuchacomponent couldbedesigned,section4describesourarchitectureofsuchamodel.section5gives ashortoverviewoverimplementationissues,andsection6thenconcludesthepaper. mostoftheworkconcentratesonmulticastprotocols(iethetaskoftransmittingdata Groupcommunicationshasbecomeaveryactiveeldinthelastyears.However, 2.RELATEDWORK distributedmultimediaapplications. multimediamulticastprotocols,andhigh-levelissues,suchastoolkitsforprogramming overanetworktomorethanonereceiverwithoutwastingnetworkresources),especially standardizationeorts.therestofthissectionwilldescribesomeoftheseprojects.a tionframeworksarenotsowide-spread,althoughthereareresearchactivitiesandalso moredetaileddescriptionhasrecentlybeenpublishedbymautheetal.[4]. Developmentsintheareaofgroupandsessionmanagementforgroupcommunica- 2.1.Researchactivities amultimediamailservice(mmm)andamultimediatransportservice(mmt).mmt al.[5,6]isanexampleforanapplicationorienteddevelopmentofacommunications platform.mmcisapartoftheberkommultimediateleservices,whichalsoinclude TheBERKOMMultimediaCollaborationService(MMC)describedbyAltenhofenet

3 isapuretransportserviceanddoesnotoeranygroupmanagementservices.itis basedonavariantofxtpwhichiscalledxtp-lite.mmccontainsvariouscomponents,ofwhichconferenceinterfaceagents(cias),conferencemanagers(cms),antionbetweenthesecomponentsisrealizedbytwoprotocols,thecdaccessprotocol (CDAP)andtheCMaccessprotocol(CMAP).TheseprotocolsareRPC-basedand aconferencedirectory(cd)arerelevantforthescopeofthispaper.communica- thearchitecturedescribedinthispaper.however,theserviceprovidedisapplication specicandthereforecanberegardedasonepossibleapplicationofthegeneralconcept ofagroupandsessionmanagementservicedescribedinthispaper. builtontopoftheisorosemechanisms.thearchitectureofmmcissimilarto concernedwiththesupportofgroupcommunication,especiallyformultimediadata. tectureqos-adescribedbycampbelletal.[9].thetransportaspectsofgcomms OneofthecomponentsofthearchitectureproposedforGCommSistheQoSarchi- GCommSbyMautheetal.[7,8]isaprojectatLancasterUniversitywhichis abstractlevel.currently,nospecicationisavailable,althoughatransportservice(the aredenedindetail,butthegroupmanagementservicesareonlydescribedonavery so-calledm-connectionservice)hasbeenimplemented.gcommsthereforecanberegardedasoneofthemanyprojectswheregroupmanagementisnecessarytoprovidea well-functioningplatform,butisnotspeciedindetail.otherexamplesforplatforms indetailarethexamparchitecturedescribedbyrodriguesandverssimo[10],prism bythetoutains[11],ortenetdescribedbyferrarietal.[12,13]. conceptintermsoffunctionality.thetransportgroupmanagementdenedforthe CIOtransportservicehasmanysimilaritiestothemodelwedescribeinthispaper.A useroftheciomulti-peertransportserviceusestwodierentcomponentsforaccessing CIOmulti-peercommunicationsasdescribedbyHenckel[14,15]isaveryinteresting wheregroupmanagementisidentiedasanimportantcomponentbutnotinvestigated arehandledwithtwocompletelyseparateprotocols.however,ciotransportgroup thetransportserviceandthetransportgroupmanagementservice.communications theciotransportservice),andithasnotbeenimplemented.furthermore,sincethe managementislimitedtoonecommunicationsplatform(iedependsontheusageof X.500directoryserviceisproposedasabasisforthetransportgroupmanagement 2.2.Standardizationbodies service,itmaybeimpossibletohavenoticationssenttousers. ITU'sT.120seriesofrecommendations[16]isanexampleforastandardizedapplication basisofthet.120infrastructurearethenetworkspecictransportprotocolsdened int.123[17],whichatthemomentsupportdatatransferusingintegratedservices digitalnetworks(isdn),circuitswitcheddigitalnetworks(csdn),publicswitched architecturewhichalsoincorporatesgroupandsessionmanagementfunctionality.the communicationsservicet.122/t.125[18,19],whichdenesanetworkindependent toincludefuturebroadbandnetworksareunderstudy.t.123isusedbythemultipoint digitalnetworks(psdn),andpublicswitchedtelephonenetworks(pstn).extensions addressing(onetoall,onetosub-group,andonetoone),andmultipointrouting servicewithexibledatatransfermodes(broadcastandrequest/response),multipoint (shortestpathstoeachreceiveranduniformsequencing).recommendationt.124[20]

4 Application Generic Conference Control (T.124) Multipoint Communications Service (T.122/T.125) Network Specific Transport Protocols (T.123) thendenesagenericconferencecontrolwhichusest.122'smultipointcommunicationsservice.theabstractservicesoftheconferencecontrolincludecreate,query, Figure1:ArchitectureoftheITUT.120infrastructurerecommendations T.120 Infrastructure Recommendations tions,whichthenuset.124andt.122services.however,theapplicabilityofthese recommendationsislimitedbecauseoftheconcentrationonconferencing.thet.120 Theseservicesprovideapowerfulenvironmentforimplementingconferencingapplica- join,invite,add,lock,unlock,disconnect,terminate,ejectuser,andtransferservices. seriesofrecommendationscanthereforeberegardedasaspecicexamplewhichshould bekeptinmindwhendesigningmoregeneralgroupandsessionmanagementservices. groups.wewillonlyconsidertheworkgoingoninjtc1/sc6,whichiscurrentlythe mostactivewithrespecttogroupcommunications.iso'smultipeertaxonomy[21] (whichisalsodescribedinapaperbymathyetal.[22])givesageneralarchitectural InISO,groupcommunicationisdealtwithindierentcommitteesandworking modelforgroupcommunications,whichmaybeusedwithindierentosilayers.conceptsdenedinthistaxonomydescribeagroup(asetofentities),groupmemberships (entitiesbelongingtoagroup),populationcharacteristics(asetofattributesagroup mayhave),groupassociations(datatransferrelationships),andconversations(thebasiccomponentsofgroupassociations).theseconceptscanbecombinedtogetamodel whichisveryexibleintermsofgroupcommunications.however,aswithmanyother sibletoexperimentallyevaluatetheusabilityofthismodel.theisodraftdocument onanenhancedcommunicationstransportservice[23]incorporatesmostconceptsdescribedinthemultipeertaxonomy.thenamingandaddressingmodeldenedinthis andgroupassociationshavetobetakenintoaccount. standardizationactivities,noimplementationsoftheseconceptsexist,soitisnotpos- draftismorecomplexthaninpoint-to-pointtransportservices,becauseconversations saryineverysystemsupportinggroupcommunications.figure2showshowsucha Groupandsessionmanagementcanbeidentiedasonecomponentwhichisneces- 3.AMODELFORGROUPANDSESSIONMANAGEMENT componentmaybeintegratedintothecommunicationsplatform(thecomponentislabeledgua,anamewhichwillbeexplainedlater).themodelisindependentfromthe

5 Application API Security Management Data GUA Resource Management communicationsplatformbeingused,althoughitismainlyinuencedbydiscussions Figure2:Groupandsessionmanagementinsideacommunicationsplatform withinthedacapoproject.thisproject,wherethemaingoalsareecientcommunicationprotocolsachievedbydynamicconguration,developedtheneedforgroupand Transport Infrastructure sessionmanagementafterbroadeningthemodelfrompoint-to-pointcommunications groupcommunicationsplatform.thesetasksmaybedividedintosecurityissues, topoint-to-multipointconnections. management.thefollowingparagraphswilldiscusstheseissuesingreaterdetail. nametoaddressmapping,qosissues,andthecorefunctionality,groupandsession Agroupandsessionmanagementcomponentisusedfordierenttaskswithina Securityissuesareaddressedintheformofidentication,authentication,authorization,andcertication.Thesearethesecurityissueswhichareimportant itselfviaqosparameters,whereauthenticand/orprivatecommunicationsmay inthecontextofgroupandsessionmanagementandtheprovisionofcertied softwarecomponents.securityissuesmayalsobeaddressedforcommunications Nametoaddressmappingisthefunctionalitywhichenablesusersofanycommunicationssystemtousenamesinsteadofaddresses,ietouseanabstractformof namingentities(communicationendpoints).thismakesitpossibletouselogicalnamesindependentfromlocationsandconnectionswhicharemappedonto Saltzer[24]describestheconceptofnaming,addressing,andbindingforthe addresseswhichcanbeusedforaddressinginagiventransportinfrastructure. berequired.however,thistypeofsecurityfallsintothetopicofqosissues. QoSissuesareimportantwhenspeakingofactualconnections.Thewell-known Internet. seenasonespecialcaseofageneralcharacterizationwhichmaybedenedfor toallowtheutilizationofyettobedenedtypes.szyperskiandventre[25] eachconnection.theparametersbeingusedshouldbeasversatileaspossible multimediaqosparameterslikethroughput,errorrate,delay,orjitter,canbe

6 Groupandsessionmanagementisnecessaryformakingthenamingspace(ie andcarleetal.[26]describethisconceptofqosparametersformultipoint thenameswhichmaybeused)exibleandcongurable.typically,groupsof communicationsingreaterdetail. usersarechangingovertime,soitmustbepossibletocreatenewgroups,to changegroupmembers,andtodeletegroups.furthermore,thecommunications insideagroup(whichwecallasession)shouldalsobemanageablebyusersof thecommunicationplatform.itshouldalsobepossibletocreatespecialsessions, sessionswhichareopentoanyparticipant(opensessions).anotherimportant eganonymoussessions(wherethesendersand/orreceiversarenotknown)or Thearchitectureweproposeforthisfunctionalityisthatofadirectoryservice, modications. functionalityinthisareaistheprovisionofauthorizationsforgroupandsession accessprotocoltothedirectoryservice.withrespecttothisdesign,ourproposed whereeachuserofthedirectoryserviceusesacomponentwhichisimplementingthe groupandsessionmanagementserviceissimilartotheinternetdomainnamesystem (DNS)denedbyMockapetris[27,28],orthedirectoryservicestandardizedbyISO anditu,alsoknownasthex.500service[29,30].however,becauseinourcase sessionmanagementsystem(gms)andisavailablethroughagmsuseragent(gua). neveracts),andspecialfunctionality(suchasoperationsforjoiningandleavinggroups weneedmorethanapurelookupservice(wherethedirectorysystemonlyreactsand andsessions),anewserviceisrequired.thisnewserviceisprovidedbythegroupand deneanarchitecturewhichmaythenbeimplementedandtested.thefollowing ThegeneralGMSmodelinventedinthepreviouschaptermaybefurtherrenedto 4.GMS{GROUPANDSESSIONMANAGEMENTSYSTEM section,wherethedierentstepsanddenitionsofqosusageareexplained. architecture,thegmsobjecttypesarespecied.qosissuesarethetopicofthelast sectionsdescribedierenttopicsofthearchitecture.afterageneraldescriptionofthe 4.1.Architecture whichisusedbythegmssystemagents(gsa)tocommunicateandthusimplement bygmsuseragents(gua),andisimplementedbythegmssystemprotocol(gsp), thatthegmsserviceisavailablethroughagmsaccessprotocol(gap)whichisused ThearchitectureoftheGMSissimilartothatofotherdirectoryservices,inthesense thegms.theoverallviewofthisarchitectureisshowningure3. GMSuseragent(GUA).GUAsarethecomponentsoutsidetheGMSwhichare machinewhichimplementsthegmsaccessprotocol.assuch,theguaprovidesaninterfacewhichmaybeusedbyothercomponentsofthecommunication usedtoaccessthegms.typically,aguaisnotmuchmorethanaprotocol platform.itperformsencodinganddecodingofprotocolmessagesandalsoimplementssomelocalparameterchecking.theguaisnotdirectlyaccessibleto

7 GUA Application Data GSA GSA GSA GSA GMS GSP GSP GSP GUA Application Data GUA Application Data GAP GAP GAP Figure3:GMSarchitecture applicationprogrammers,becausethecommunicationplatformdesignersshould befreetodenetheplatform'sapiinacoherentwaywithouthavingtoaccept apredenedguaapiforapplications. GMSsystemagent(GSA).GSAsarethecomponentswhich,intheirentirety, makeupthegms.eachgsaisacomponentofthisdistributedsystemandcommunicateswitheitherothergsas(usingthegmssystemprotocol),orguas (usingthegmsaccessprotocol).inthegmssystem,datastorageisdistributed, sothateachgsamaystoresomelocaldatawhichmaybequeriedbyanother GSA.GSAsaregroupedintodomains,whereGSAscommunicatinginsideonedomainusetheintra-domainGMSsystemprotocol,whilecommunicationscrossing domainboundariesusetheinter-domaingmssystemprotocol. ThetwomaincomponentsoftheGMSarchitecture,GMSuserandsystemagents, arecommunicatingusingspecialprotocols.theseprotocolsareusedforaccessing thegms(viaguas)respectivelyforcommunicationsinsidethegms.thefollowing paragraphswillgiveamoredetaileddescriptionofthesetwoprotocols. GMSaccessprotocol(GAP).GUAsareusedforaccessingtheGMSusingentry pointsprovidedbygsas.theseentrypointsmustbeaccessedusinggap,which isanasymmetricalprotocol.thecompletespecication[31]containsdenitions ofusedobjecttypes(whicharedescribedinsection4.2ofthispaper),relations betweenobjects,pdudenitions,andstatetransitiondiagramsdescribingthe behaviorofthetwocommunicatingagents.figures4and5showthestatetransitiondiagramsoftheguaandgsasideoftheprotocol.thenotationofevents andactionsusedforthetransitionsistakenfromitu'sacsespecication[32]. Becausethenumberofstatesismuchbiggerthantheonesshowninthesegures,onlythetopmostlevelofthediagramsisshown,whichillustratesconnection setupandtear-down.doubleboundedboxesrepresentstateswhichcontaincompletestatediagramsthemselves.dashedlinesseparatestatediagramswhichare executedinparallel.furthermore,itispossiblethattheremaybeseveralparallel entitiesoftheguaboundstate,eachrepresentingoneuser'sconnectiontothe

8 BindGuaReq/ BindGuaRQ idle GUA binding UnbindGuaRE+/ UnbindGuaCnf+ GUA unbinding UnbindGuaReq/ UnbindGuaRQ GUA control idle BindGuaRQ/ BindGuaRE+ UnbindGuaRQ/ UnbindGuaRE+ Figure5:GAP/GSAstatediagram GUA control UnbindGuaRQ/ UnbindGuaRE- BindUserRQ/ BindUserRE- UnbindGuaRE-/ UnbindGuaCnf- BindGuaRE-/ BindGuaCnf- Figure4:GAP/GUAstatediagram BindGuaRE+/ BindGuaCnf+ GUA bound BindGuaRQ/ BindGuaRE- GMS.ThiswayitispossiblethatseveraluserscanbeconnectedtotheGMS GUA bound usingonlyonegua,whichisnecessaryforcommunicationframeworkswhere thetwogures,representingconnectionsetupandtear-downbetweenguaand GAPitselfcanbeseparatedintothreephases.Therstphaseisillustratedin onlyoneguaexistsforeachmachine. challengeswithmultipleiterations)canbeused.aftersuccessfulauthentication, weakandstrongmechanisms(fromnoneorstandardunixpasswordsuptorsa andauthenticated.authenticationishandledinaverygenericway,sothat GSA.Thesecondphaseistheauthenticationphase,whereauserisidentied thethirdphase,theuserboundstate(notshowninthegures),isenteredand servicesforaccessingthegms'sdatacanbeused.theseservicesincludecreate, GSAbecomesactive(egwhensendingnotications,invitations,orrenegotiation requests). initiatedbythegua(ietheuser),buttherearealsosomesituationswherethe modify,delete,join,andleavefortheappropriateobjects.mostgaprequestsare GMSsystemprotocol(GSP).GSAsarecomponentsoftheGMSwhichstoredata oftwodierentparts,anintra-domainprotocolandaninter-domainprotocol. andprovideaccesstothegms.gsascommunicateusinggsp.gspismadeup DependingontherelationoftwoGSAs,thecommunicationbetweenthemis

9 ThesedescriptionsofGMScomponents(GUAandGSA)andtheprotocolsused service,itisnotdescribedingreaterdetail. usingoneofthetwovariants.becausegspisnotvisibleforusersofthegms forcommunicationsbetweenthesecomponentsgiveanoverviewofthegmssystem. Thefollowingtwosectionswilldealwithtwoaspectsofthedatabeingstoredand exchanged,whicharetheobjecttypesavailableandthewayqosissuesarehandled. 4.2.GMSobjecttypes overviewoftheobjecttypes,theirattributes,possiblerelationsbetweenobjects,and thegeneralconceptbehindeachtype. DetaileddescriptionsaregivenintheGAPspecication[31],herewewillonlygivean AlldenitionsoftheobjecttypesavailableinsidetheGMSarespeciedinASN.1. User.AuserisapersonorentityusingGMS.Eachuserhasanidentity(a name)andoneormorewaysofauthenticatinghimself.thisauthenticationmay varyfromnoauthenticationatall(itissucienttousetherightname)tosophisticated,hardware-orientedauthenticationschemeswithmultiplechallenge iterations.theauthenticationmethodbeingusedisimportantforsomeoperations,whichmayonlybeperformedifacertainlevelofauthenticationhasbeen usedwhenbindingtothegms. Auserobjectcontainsinformationaboutauser,suchashisrealworldname,a description,his address,andalistofthebindingsofauser,iethelistof activegmsconnectionsauserhas.therelationsofauserobjectdescribewhich objectstheuserowns,ofwhichgroupsand/orsessionstheuserisamanagerof, Group.Groupcommunicationsneedaexiblewayofhandlinggroupsofusers. forwhichowsheisasenderand/orreceiver. ofwhichgroupsheisamemberof,ofwhichsessionsheisaparticipantof,and GMSgroupsmayconsistofusersand/orgroups,dependingonthedenition ofthegroup.joiningandleavingagroupdependsonthegroup'sjoinpolicy managersand/ormembers.groupsmayalsobestatic(opposedtodynamic), andauthenticationrequirements.joinsandleavesmaybenotiedtoagroup's denedattimeofthegroup'screation). whichmeansthatnojoinsorleavesarepossible(groupmembersthenmustbe Eachgroupobjectmaycontainagroup'srealworldname(egthenameofa address,andtheaccessrights,whichdeterminewhoisauthorizedtomodifythe group,managersofthegroup,membersofthegroup,andsessionsassociated attributesofthegroup.possiblerelationsforagroupspecifyanownerofthe companyoracompany'sdepartment),adescriptionofthegroup,agroup'smail FlowTemplate.Forseveralapplicationsandcommunicationplatformsitisuseful withthatgroup. tohaveanumberofpredenedpossibilitiestosetupconnections.flowtemplates containinformationaboutdatatypeswhichmaybecarriedbyaowofthattype, thenecessarytransportservice,datawhichisneededtosetupaowofthattype,

10 alsobecreatedwithoutusingaowtemplate. whichcanbeusedtogiveadescriptionoftheowtemplate.however,owsmay informationaboutuni-orbidirectionalservices,andasetofqosparameters, Flow.Aowisoneconnectionfordatatransport.Dependingontheow'sdefinition,itiseitheruni-orbidirectional,hasalimitednumberofsendersand/or receivers,andarenegotiationpolicy,whichdetermineswhoisauthorizedtoinitiateqosrenegotiationsforthatow.flowsarecreatedwhenasessioniscreated andaredeletedwhenasessionisdeleted.joiningaowtakesplacewhena sessionisjoined,andaowisleftwhenthesessionofaowisleft. (theaddressinginformationisalsopartoftheowanddiersdependingon Eachowhasrelationswhichdescribetheuserssendingandreceivingthatow tocreatearelationshipbetweenowswhichdenotesasynchronizationbetween thetypeofaddressingbeingused),dependencieswithotherows(incasedata ows(apopularexampleforthisareindividuallytransmittedaudioandvideo isbeingsentwhichneedsdatabeingsentonotherowsforbeinginterpreted correctly,suchashierarchicallycodeddatalikempeg-2[33]).itisalsopossible Session.Themainmetaphorfordatatransferisasession.Eachsessionisusedto thesynchronizationmustbedoneinsidethecommunicationsplatform. datawhichmustbesynchronizedforplayback),however,theimplementationof logicallygroupanumberofowsandtocreateanabstractionformanagement, authorization,andadmissioncontrolforows.theowsofasessionarecreated whenthesessioniscreatedanddeletedwhenthesessionisdeleted,iethereisno possibilitytodynamicallyaddorremoveowsfromanexistingsession.however, Sessionsmayhaveapplicationspecicinformation,whichconsistsofanapplicationidenticationandapplicationspecicdata,whichmaybeinterpretedbythe levelausermusthavetosuccessfullyjoinasession(providedheisauthorized suciently).authorizationisbasedonthesession'sjoinpolicywhichmaybe open(everyonemayjoin),group(onlymembersofthegroupassociatedwiththe conrm). sessionmayjoin),ormanaged,whichmaybeeitherrelative(agivenpercent- whenjoiningasession,notallowsofthesessionsmustbejoined,souserscan choosewhichowstouse(provideddependenciesbetweenowsarerespected). application.furthermore,thedurationofasessionmaybegivenwitheitherstart orendtimesorboth.inaddition,itispossibletospecifywhichauthentication Asession'srelationsdescribetheownerandmanagersofasession,whichare ageofmanagersmustconrm)orabsolute(agivennumberofmanagersmust importantformodifyingthesessionobjectanddeterminingwhomayjointhe session(ifthejoinpolicyissettomanaged).theparticipantsofasessionare alluserswhohavejoinedthesession(excepttheuserswhohaveleftalready). Certicate.Applicationswithspecialsecurityrequirementsmayhavetheneed Asession'sowsareallowswhicharepartofthissession.Alastrelation determines,whichgroup(ifany)thesessionisassociatedwith. tostorecerticatesinsidethegms,whichareusedforcheckingdataidentity

11 andintegrity.certicatesincludethetype(whichmaybeapredenedtypeor checkingthedata.theonlyrelationacerticatehasistheonewithitsowner. anddataandsignatures,whichcontaintheinformationswhichisnecessaryfor andthepossibilitytodeneowntypes),thecerticate'svalidity,asimplename, anyothertype),thenametype(whichalsohasanumberofpredenedvalues modied,anddeletedwithgapservices.however,becauseanumberofattributes andrelations(suchastheaddressinginformationattributeandthesendersrelationof ows)needstobemodiedaccordingtotheirsemantics,theyareimplicitlymodied TheseobjecttypesareavailableforusageinsidetheGMSandmaybecreated, toaowhejoinedandmodifythesendersrelationaccordingly). byusinggapservices(suchasthejoinsessionservice,whichaddsasender'saddress 4.3.QoSissues issuescanbeidentiedhere,theqosparametertypes(deninghowqoscanbe makeitsuitableforthesupportofmultimediamultipointcommunications.twomain isalsotakenintoaccount.gmsmustbeabletosupportqosaspectsinawaywhich QoSisaveryimportantaspectofmultipointcommunications,especiallyifmultimedia howqoscanbeapplied). dened)andtheproceduresavailabletomanipulateparametersofthesetypes(dening values,andrealvalues.unsortedvaluesareasetofpredenedvalues,whicharenot inanyparticularorder(anexampleforthisisaqosparameterwhichdenesacoding algorithm,whereitisnotpossibletoarrangethedierentalgorithmsinanyorder). GMShasfourQoSparametertypes,whichareunsortedvalues,sortedvalues,integer becauseitispossibletoarrangetheminanorder(anexampleforthistypeofqos SortedValuesarealsopredenedvalues,butthesevaluesaregivenasasequence, parameteristheselectionofaerrordetectionalgorithm,whichmaybegivenasa sequenceofnone,crc8,crc16,andsomemoresophisticatedalgorithms).integer andrealvaluesrepresentthetwobasictypesofnumberswhichmaybeused(which maybeusedforthroughputorerrorprobabilitynumbers). Theinterpretationoftheparameter'svaluesdependsentirelyontheparameter'sname. avoidmisinterpretations.forthisreason,anumberofpredenedqosparametersis ItisthereforeimportantforGMSuserstoagreeonnamesforQoSparametersto Anyparameterdenedforaowtemplateorowmaybeofoneofthesetypes. available,wherethesemanticsofaparameterareclearlydened(takenfromtheiso draftdocumentforaqosframework[34]).additionalparametersmaybeusedat anytime,althoughitisstronglyrecommendedtousethepredenedtypeswhenever possible. nipulatingqosparameters.giventhegmsobjecttypes(asdescribedinsection4.2), fourdierenteventswhereqoscomeintoplaycanbedescribed. ThesecondaspectofQoS,asmentionedabove,aretheproceduresavailableforma- Flowtemplatecreation.Whenaowtemplateiscreated,itmayalsobegiven anumberofqosparameters.dependingonthetransportinfrastructure,the numberofparametersalreadynecessaryorevenknownatthispointintimemay

12 Sessioncreation(owcreation).Everydatatransferisrepresentedbyaow, alreadybedened,ifpossible. dieralot.forthedierenttypesofqosparameters,values(resp.limits)may whichiscreatedwhenthesessionitisapartofiscreated.eachow'sqos forrenegotiationsmaybedened.forunorderedvaluesqosparameters,thereis Optionally,aweakestlimitforjoiningtheowandstrongestandweakestlimits isthevalueusedforjoiningparticipantsifnolocalmodicationsarerequested). parametersaredenedbytheirnameandtypeandatleastadefaultvalue(which Sessionjoining.Whenjoiningasession,theQoSparametersbeingusedare assetsofvalues. noorderofthevalues,thereforethejoinandrenegotiationvaluesmustbegiven takenfromtheow'sqosdenitions.accordingtotheuser'srequirements,a oftheweakestlimitforjoiningtheow.becausetheactualqosestablishment weakervaluethanthedefaultmaybeselected,aslongasitdoesnotfallshort liesoutsidethescopeofgms(whichisonlyresponsibleforstoringthevalues), QoSrenegotiation.QoSrenegotiationistheprocessofdeningnewdefaultvalues rametersaccordingtothevaluesprovidedbythegms. itisrequiredthattransportinfrastructuresusingthegmscontroltheqospa- participants(sendersandreceivers)ofaowareinformedofqosrenegotiations andweakestlimitsforqosparametersofanyrenegotiableow.thisrenegotiationmaybelimitedbytherenegotiationlimitsofaqosparameter,ifpresent.all alongwiththeasn.1denitionscanbefoundinthegapspecication[31].however,itshouldalwaysbekeptinmindthatqosestablishmentandrenegotiationare distributionofqosvaluesandrenegotiationnotications. taskstobeperformedbythetransportinfrastructure,whilethegmsisonlyusedfor AdetailedandcompletedescriptionoftheQoSparametersandtheirmanipulation ofthatow. Theimplementationofthearchitecturedescribedinsection4isoneofthekeyaspects ofthegmswork.sofar,researchhaseitherconcentratedonimplementingtransport 5.IMPLEMENTATION components,oronmerelyspecifyingmanagementservices.gmsisbeingimplemented protocols,onimplementingapplicationstogetherwithapplication-specicmanagement atthetimeofwritingandwillbeintegratedinatleasttwocommunicationplatforms. ThesewillbetheextendedDaCaPoplatformdevelopedatourlab,whichhasbeen protocolswithqosguarantees.furthermore,verysmallgroupcommunicationplatformsbasedonipmulticastandatm/unimulticastfacilitiesshouldbeimplemented modiedtoincludegroupcommunications,andamultipointcommunicationsframework(mcf,describedbybaueretal.[35]),whichaimsatimplementingmulticast thattheapplicationofthegmsconceptwithindierentcommunicationplatforms toprovetheapplicabilityofthegmsconcepttodierenttransportservices.wehope aswellasthedevelopmentofapplicationsontopofthesecommunicationplatforms

13 Written Code Parameter + State Checking Programming Interface Finite State Machine State Machine Management Figure6:GMSuseragent(GUA)design Packet Coding ASN.1 Encoding and Decoding and Decoding Network Adaptation managementserviceintermsofprotocols,objecttypes,andperformanceissues. willleadustoabetterunderstandingoftherequirementsforagroupandsession laris2)usingc.twotoolsarebeingused,oneistheasn.1compilersnaccdescribed bysampleandneufeld[36,37],whichisperformingallthecodinganddecodingof incomingpdus,theothertoolisthecommercialsoftwarestatemate,whichisbeing TheimplementationofaGUAiscurrentlybeingdoneinaUnixenvironment(So- accordingtogure2)andtheinterfacetothetransportsystemaresubjecttochange usedforimplementingthestatemachinewhichhandlestheprotocol.theprogramminginterface(whichisusedbyothercomponentsofthecommunicationplatform componentsareisolatedfromtherestofthecodeandeasilyreplaceable. forintegrationoftheguawithindierentcommunicationsplatforms,thereforethese groupandsessionmanagementsystem(gms).gmsconsistsoftwocomponents,gms Inthispaperwehavepresentedthemodelandarchitectureforatransport-independent 6.CONCLUSIONS useragents(guas),andgmssystemagents(gsas).thesecomponentscommunicate usingthegmsaccessprotocol(gap,forgua-gsacommunications)respectivelythe GMSsystemprotocol(GSP,forGSA-GSAcommunications).TheGMSserviceisdesignedasadistributednameservicewithadditionalfunctionality,suchasnotications. Thereisasetofdenedobjecttypeswhichmaybeusedtomodelinformationabout municationplatforms.becausethedesignofobjecttypesandoperationsisindependent areavailableandmakethedesignofsecurecommunicationplatformspossible. users,groups,sessions,andows.mechanismsforauthenticationandauthorization fromaspecictransportservice,thegmsservicemaybeusedbyvariouscommunicationplatforms.thecurrentimplementationwillbeintegratedintotwoplatform, TheimplementationoftheGUAcomponentpermitstheeasyintegrationintocom- extendtheguawithregardtothetransportinfrastructurebeingused. incorporationoftheguacomponentintodierentplatforms.itisalsoplannedto thedacaposystemdevelopedatourinstitute,andamultipointcommunications framework,whichwillalsobedevelopedatourinstitute.futureplansincludethe Generated Code

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