M-RPC:ARemoteProcedureCallServiceforMobileClients. DepartmentofComputerScience. Rutgers,TheStateUniversityofNewJersey. Piscataway,NJ08855
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1 M-RPC:ARemoteProcedureCallServiceforMobileClients AjayBakreandB.R.Badrinath DepartmentofComputerScience Rutgers,TheStateUniversityofNewJersey Piscataway,NJ08855 Abstract RemoteProcedureCall(RPC)isapopular paradigmfordesigningdistributedapplications.the existingrpcimplementationsdonotallowspecial treatmentofmobilehostsandwirelesslinkswhichcan beacauseofdegradedperformanceandinconvenience inthepresenceofdisconnections,movesandwirelesserrors.futureinformationorientedandlocation awaremobileapplicationswillalsoneedtheabilityto dynamicallybindmobileclientstolocalinformation servers.inthispaperwedescribem-rpcsystem whichprovidesacleanwayformobileclientstoaccessexistingrpcbasedservicesonthewirednetwork withtheadditionalfeaturesofdynamicbinding,disconnectedoperationandcallretriesfromthemobility supportrouter(msr). 1Introduction RemoteProcedureCall(RPC)isusedasabasisfor structuringmanyclientserverapplications.conventionalrpcimplementationshowever,assumethatall thehostsinanetworkarestationaryandarealways reachableexceptincaseoffailures.theapplications builtontopofconventionalrpcalsotendtoassume thatthenetworkismoreorlessafreelyavailableresourceandthatnetworkbandwidthisnotaconstraint. Suchapplicationsthusdonotattempttooptimizethe numberofmessagesexchangedbetweenaclientanda server.forexample,controlmessagesbetweenclients andaserverinnfs[32]torefreshlesystemhandleformasignicantportionoftheoveralllesystem trac.theclientsideofsuchanapplicationtypically getstheserveraddressusinganamelookupmechanismwhichisusedtoestablishaclienthandle,sothat allthesubsequentcallrequestsgotothesameserver. Sinceboththeclientandserverhostsarestationary, thereisrarelyanyneedtochangetheserverinsuch applications. Thepresenceofmobilehostswithinanetworkinvalidatesalltheaboveassumptions.Mobilehostscan movefromonecelltoanotherandcanremaindisconnectedfromthexednetworkforextendedperiods toconservescarcebatterypower.thewirelesslink connectingamobilehosttotherestofthenetwork usuallyhaslowerbandwidthandhighererrorrateas comparedtoawiredlink.availablebandwidthon wirelessisthusascarceresourceandinfuturemobile hostsareexpectedtoincurcostproportionaltothe connectiontimeonthewirelesslink.thismeansthat therpcbasedapplicationsformobileclientshaveto bestructuredsuchthattheamountofdatasenton thewirelessisminimized. Withanincreaseinthenumberofmobilehosts andthescaleofthedeployedsupportinfrastructure, broadcastbasedinformationservicesarelikelytobecomepopular[18].suchservicesareespeciallysuitableformobileclientssincewirelessisinherentlya broadcastmediumandamobilitysupportstationconsumesthesamepowerfortransmittingapacketonthe wirelesslinkregardlessofthenumberofreceiversin itscell.amobileclientapplicationshouldtherefore havethecapabilitytoswitchtoalocalinformation serverinitscurrentwirelesscelltosavethecostof accessinginformationservices.suchcapabilitieswill alsobeneededbyapplicationswhichaccesslocation dependentinformationsuchastrac,weather,availableservicesinageographicareaetc.[18]. InthispaperwedescribeM-RPC,whichisanRPC serviceformobileclients.anm-rpcclientcanaccess connectionlessrpcbasedservicesonthewirednetworkviaanagentlocatedatitscurrentmobilitysupportrouter(msr)whichprovidesthefollowingfunctionality: Supportfordynamicserverbinding Reliabletransportoverthewirelesslink CallretriesfromtheMSRand Partialsupportfordisconnectedoperation. 1
2 Inthenextsection,weexamineindetailtherequirementsofmobileclientsthatwebelieveshould besatisedbytherpclayer.section3describes them-rpcsystemthatwehavedevelopedformobileclient-serverapplications.section4outlinesthe implementationofm-rpccomponents.intheend, welistsomerelatedresearchworkandconcludingremarks. 2RPCandMobileClients RPC[9]isapopularparadigmfordesigningdistributedapplications.Clientapplicationscanaccess serversoveranetworkwiththesimpleandfamiliar interfaceofaprocedurecall.therpclayerhidesthe networkcommunicationfromtheclientandserverapplicationstherebymakingthedesignofdistributedapplicationseasier.sunrpc[31]isoneofthemostcommonlyusedrpcimplementationswhichalsoforms thebasisfornfs[32].inthefollowingdiscussion,we willuseaconventionalclient-serverapplicationbased onsunrpcforreference,eventhoughthediscussion isequallyapplicabletoothersimilarrpcimplementations. 2.1FlexibleTransportlayer SunRPCallowstheuseofbothconnectionless (UDP)[27]andconnectionoriented(TCP)[28]transportlayersfornetworkcommunication.Usingaconnectionlesstransportprotocolaordsdesignofsimple statelessserverswhichcanbereplicatedforhighavailability.inpractice,nfsimplementationsusingudp workquitewelloveralan,eventhoughudpdoes notprovidereliabledeliveryofdatagrams.thereliabilityneededfornfsisprovidedbyrpcandnfs levelretrymechanisms.usingtcpforrpcstyle request-responsekindofcommunicationcanbeawkwardbecause-i)itisastreamprotocolwhichdoes notpreserverecordboundariesandii)itbringsina lotofadditionalbaggageintermsofcongestionand owcontrolwhichisquiteunnecessary.connection orientedtransportalsorequirestheserverstomaintainstateforeveryopenconnectionwithaclient. Withmobileclientsthatwishtoaccessserviceson thewirednetworks,thechoiceofatransportprotocol forrpcbasedapplicationsisratherdicult.udpis clearlyinadequateforerror-pronewirelesslinks.tcp, althoughitprovidesreliabledelivery,suersfromthe problemsmentionedearlier.inaddition,inamobile environmenttcpperformancedegradesbecauseof wirelesserrorsandmoves[11,16,24].forrpcbased applicationsinmobileenvironment,wethereforeneed atransportprotocolthatprovidesreliabilityonthe wirelesslink,butatthesametimehasthesimplicity ofudpfortherelativelyerrorfreewiredlinks. 2.2Dynamicbindingandsubjectbased naming Dynamicserverbindingisusefulforclient-server applicationsinwirednetworkstohandleserverfailures[10].arelatedpropertyissubjectbasednaming[14]whereaclientisboundtoaserviceratherthan toaserversothatthesupportinginfrastructurecan switchserversatwillorevenemploymultipleservers toprovidethesameservicetransparentlyforthesake ofreliabilityand/oreciency.formobileapplicationsthatdependonlocationinformation,dynamic bindingtoalocalinformationserverbecomesessential.itmaybenecessarytochangetheclient-server bindinginsuchanapplicationoneverymovewhich makessubjectbasednamingespeciallyimportantfor mobilehosts.evenforcostreasonsitmaybeadvantageoustoswitchtoanewserverafterthemhmoves toanewcell.itisunrealistictoexpectmobileapplicationstochangeserverbindingsaftereverymove andanrpcsystemformobileclientsshouldperform thistaskautomatically. 2.3Disconnectedoperation CurrentRPCbasedapplicationsdonotexpect clientsorserverstostaydisconnectedinnormalcircumstancesandmostsuchapplicationswilltimeoutor exitwithahostunreachablemessageincaseeitherthe clientortheserverisdisconnected.withmobilehosts however,disconnectionandreconnectionisfairlyroutinewithcellcrossoversandalsotoconservethescarce batterypowerandshouldbehandledassuchbythe RPCsystem.Therearetwowaysinwhichamobile hostcangetdisconnectedfromthexednetwork: a)suddendisconnectionwithoutanywarningsuch aswhenthemhmovesoutofawirelesscellor simplygoesdown.intheformercase,themh maysoonemergeinanotheradjacentcelland reestablishcontactviaitsnewmsrwiththerest ofthewirednetwork. b)planneddisconnectioninwhichcasethemhcan actuallyinformitspeers(oratleastthecurrent MSR)beforedisconnectingsothatpreventiveactioncanbetakenbothattheMHandonthe wirednetworktolessentheeectofdisconnectiononapplications.suchpreventiveactionmay includecachinganylesonthemhwhichare mostlikelytobeusedduringthedisconnection period.planneddisconnectionmayoccurwhen anmhgoesintowhatisknownasthedozemode tosavebatterypower. 2
3 Oneobviouswaytohandledisconnectionistoallowtheindividualapplicationstotakecareofit.One exampleofsuchanapproachiscoda[21]whichattemptstohandledisconnectedoperationattheapplicationsupport(lesystem)layerandprovidesfor stand-aloneclientlesystemswhichcanbeintegrated withtheleserverafterreconnection.theaboveapproachhoweverisnotgeneralenoughtobesuitable forallkindsofmobileapplications.eachnewapplicationthereforeneedstoprovideitsownsupport fordetectingdisconnectionsorhastopaythepenalty ofdegradedperformanceinthepresenceoffrequent disconnections.abetterapproachistoprovidesupportfordisconnectedoperationatthetransportand RPClayers.SinceRPCisusedasthebasiccommunicationmechanismbymanyclient-serverapplications, thiswillallowabasicmechanismforhandlingdisconnectionsandmoveswhichcanbefurtherrenedby theindividualapplications,ifneeded. 2.4PresentationRequirements Sincewirelessbandwidthsareexpectedtoremain ascarceresourceforsometimetocome,itishighly desirablethattheamountofdatatransmittedonthe wirelesslinkbetweentheclientandserverbekepttoa minimum.also,itispossibleforawanderingmobile hosttondwirelesslinksofdierentspeeds,error rates,coststructuresandsecuritylevelsindierent cells.itisdesirablethereforethatthepresentation ofdatatransmittedoverwirelessbetunedtothelink characteristicsinthecurrentwirelesscell.asanexample,themobilehostmaywanttodeferreceptionof datafromtheserverincaseitndsitselftemporarily inacellwithpoorwirelessreceptionorhighsecurity risks.anrpcsystemformobileclientsshouldallowtheplacementofadatalterinthepathofthe wirelesstransmissiondynamicallytohandlesuchsituations.amoregeneralproblemformobileclients isdynamicadaptationtothenewenvironmentafter movingintoanewwirelesscell.thismayinvolve updatingthestateatthemobilehostsothatenvironmentvariablessuchasnearest-printeretc.contain correctinformation.anyinformationaboutthedata ltersinstalledonthewirelesslinkbetweenthemsr andthemhshouldalsobepropagatedtothenew MSRafteramovesothattheMHdoesnotneedto re-installsuchltersinthenewcell. 2.5CallRetries Inaconnectionlessclient-serverapplication,theretransmissionsoffailedRPCrequestsaretypicallyperformedbytheclientsidewhichusesaretrytimeout valueforthispurpose.withamobileclientusinglow bandwidthandcostlywirelesslink,callretryfromthe clientisclearlyundesirableespeciallyiftherequestis correctlydeliveredtothewirednetworkbutthereply waslostforsomereason. 2.6Targetapplications Itisimportanttonotethatthekindofmobileclient applicationsthatneedtheabovementionedcapabilitiesarethosewhichengageinarelativelylonglived interactionwiththeserversmakingaseriesofrpc requestsintheprocess.theaboverequirements,althoughuseful,donotaectinasignicantwaythose applicationswhichsimplymakeonerpcrequestto aserverandexit.someofthemobileapplications whichcanmakeuseofanrpcsystemsupportingthe abovementionedrequirementsarelistedbelow: 1.Queryingdatabasesonthexednetworke.g.regardingweather,trac,localnetworkandfacilitiesavailableinandaroundthecurrentwireless cell. 2.Client-client-servertypeapplicationse.g.reading fromaparticularxedhost(possiblywith someltersinplace),maintainingle-systemconsistencybetweenamobileunitandadesktop clientmachineoramongtheserver,mobileclient andadesktopmachine,runningremotesessions onadesktopclientfromthemobileetc. 3.Accessingbroadcastbasedcontinuousmonitoring kindofservicese.g.stockinformation,videoand audioviathecurrentmsr. 4.Groupcommunicationandmulticastbasedapplicationssuchasformingadhocnetworkswithmobilehostsinthesameand/oradjacentwireless cellsorparticipatinginagroupprotocolsuchas two-phasecommitprotocolfordistributedtransactions. 5.WidearealesystemsanalogoustoNFS. 6.Distributedremoteobjectinvocationfrommobilehosts.Thismayincludegettingareference toaremoteobjectandinvokingoperationson it.suchremoteobjectinvocationisthebasis forstructuringdistributedapplicationsinsystemslikeansa[2]andarjuna[29]whichusesan RPCsystemforcommunicationamongtheparticipatinghosts. 3OverviewofM-RPCSystem AnRPCsystemformobilehosts,whichwecall M-RPCandwhichsatisestherequirementsofthe mobileapplicationslistedaboveisdescribedhere.the 3
4 FH MH MSR-1 MSR-2 Server Client M-RPC Agent Connectionless Transport (UDP) Reliable Transport (RDP) RPC and Transport Layer Handoffs M-RPC Agent Figure1:OverviewofM-RPCSystem goalofm-rpcsystemistoextendrpclayerservices tomeetthenewrequirementsbroughtaboutbyhost mobilityandwirelesslinksinawaythatisbackward compatiblewiththeexistingrpcsystems.itisbased upontheindirectclient-servermodel[4]formobile hosts.therearetwomainreasonsforchoosingthe indirectmodelforstructuringthem-rpcsystem: 1.ThemobilehostanditsMSRareinthebestpositiontoknowaboutthepropertiesofthewireless linkandsincethemobilehasrelativelylessbatteryandcomputingpower,themsristhebest placetoimplementanydynamicschemeoflteringthedataonthewireless. 2.Sincemostoftheinternetworkisunawareofmobility,wecannotassumethattheserversonthe xednetworkwilladapttoanyspecializedstrategiesdevelopedformobileclients.thusthemsr canprovidecompatibilitywiththewirednetwork. Theindirectmodelplacesnorestrictionsonxed hostsexceptthemsrsthemselves,toadapttothe protocolsdevelopedformobilehostswhileatthesame timeallowingspecialzedprotocolstobeusedonthe wirelesslinkbetweenthemhanditsmsr.theindirectmodelalsoallowsanmhtostaydisconnected atwillwhilethemsrmaintainstherequiredprotocol stateforthemh.itisalsoeasierforthewirelesslink includingthemhandthemsrtoquicklyadaptto anychangesinthelinkcharacteristicsthanitisfor theotherxedhostscurrentlycommunicatingwith themh. Figure1showstheoverallstructureforM-RPC. TheM-RPCsystemhasbeendesignedformobile clientswhichaccessservicesfromtheserversonthe wirednetwork.wedonotyettakecareofmobile serversalthoughoursystemcanbeextendedtowork withmobileserversaswell.wealsoconcentrate mainlyonconnectionless(orrequest-response)servicesoeredbystationaryserverswhicharecommonly usedbyinformationorientedapplicationsandnfsalthoughtheideaspresentedhereextendequallywell toconnection-orientedservices.them-rpcsystem consistsofrpcagentsrunningonmobilitysupport routers(msrs)whichsubmitrpcrequeststothe xedhostsandcollectresultsfromthemonbehalfof locallyregisteredmhs.them-rpcagentsalsokeep trackofservicesoeredbyserversonthexednetwork sothattheycandynamicallybindclientsrequeststo oneormoreavailableservers.librarysupportisprovidedformobileclientapplicationswhichhidesthe communicationdetailswiththem-rpcagentsonthe MSRs. 3.1M-RPCtransportlayer WeuseanindirecttransportlayerfortheM-RPC system.suchanarrangementallowstheuseofreliabledataprotocol(rdp)withsequencedpacketdelivery(seqpacketmode)onthewirelesslinkwhile maintainingaconnectionless(udp)interfacewiththe stationary(andstateless)servers.usingrdponthe wirelesslinkprovidesprotectionagainstwirelesserrors andtransientdisruptionsinthelinklayerconnectivitywhichisnotrequiredonrelativelyrobustwired links.thusreliabilityisprovidedonlywhereneeded. UsingTCPfromtheMHallthewaytotheserverto providereliabilityontheotherhand,wouldaddextra overheadofmakingaconnectionwiththeserverand 4
5 wouldmakeitdiculttoswitchserversonamove. TransportlayerhandosoncellcrossoversbyanMH ensureseamlesstransferofrdpconnectionsbetween themhandthemsrrpcagenttothenewmsr's RPCagent.Ifconnection-oriented(TCP)interfaceis neededwiththeserveronthewirednetwork,i-tcp canbeusedinsteadofrdp/udpcombinationform- RPC. 3.2ClientInterface Theclientsideofadistributedapplicationrunning onamobilehostcanmakeuseofclientstubstocallremoteproceduresataserver.theseclientstubscanbe generatedbyastubgeneratorsuchasrpcgen.inthe M-RPCsystem,theclientstubsperformthetaskof marshalingandunmarshalingargumentsandresults respectively.figure2showsthem-rpcclientsideat themh.beforetheclientapplicationcanmakeany RPCrequests,ithastoobtainaclienthandlefromthe M-RPCsystem.Initsrequestforcreationofaclient handle,theclientprogramcaneitherspecifyaparticularserverwhichistobecontactedtoservicethe RPCrequestsoritcanspecifyaservicenameinstead. TheM-RPCsystemmapsaservicenametoaservice identierwhichisrecognizedbythem-rpcagentat themsr.arequestforcreationofclienthandleat themhresultsintheestablishmentofanrdpconnectionbetweenthem-rpcagentatthemsrand themh.them-rpcagentcreatesitsowncopyof theclienthandlewhichisusedtomakerpcrequests toaserveronthewirednetwork.ifaservicename wasspeciedinsteadofaserverhostaddress,them- RPCagentalsoperformsanynecessarybindingtoa serverthatisavailabletoprocessthespeciedservice request. 3.3M-RPCAgentattheMSR Figure3showsthestructureofM-RPCagentrunningateveryMSR.TheM-RPCagentisresponsible fortheexecutionofrpcrequestssubmittedbythe MHswithinitscells.Oncesucharequestisreliably deliveredtothem-rpcagentbythem-rpcsystem onthemh,theagenttakesovertheretransmissionof failedrequests.inamobilewirelessenvironment,a callrequestmadebyamobileclienttoaserveronthe wirednetworkusingunreliableconnectionlesstransportcanfailduetofollowingreasons: 1.Thecallrequestmessageislostonthewireless link. 2.Thecallrequestmessageislostonthewirednetwork. 3.Thecallrequestisdroppedordelayedatthe serverpossiblybecausetheserverisbusyservingotherclients. 4.Thereplymessageislostonthewirednetwork. 5.Thereplymessageislostonthewirelesslink. UsingconventionalSunRPC,thecallrequestwill beretransmittedbythemobileclientoverthewireless linkinalloftheabovecases.usingm-rpconthe otherhandrequiresaretransmissionoftherequest (bythereliabletransportlayer)fromthemhonlyin therstcase.inaddition,usingm-rpcsavesonthe wirednetworktracandserverloadincasethereply islostonthewirelesslink. TheUDPsocketusedbytheagenttocommunicatewiththeserverisboundtotheaddress(andport number)correspondingtothemhclientsothatthe serverseesthecallrequestscomingfromthemhaddress.them-rpcagentcanalsodynamicallybind callrequeststoasuitableserverunlessaspecicserver wasrequestedbythemh.theclient-serverbinding maybechangedbytheagentifneeded,inresponse toeventssuchasaserverfailureoracellcrossoverby themh.whenareplyisreceivedbytheagentforan RPCrequestsubmittedonbehalfofanMH,thereply isreliablydeliveredtotheclientprogramatthemh. 3.4BindingtoaService Bindingtoaservice(asopposedtoaserver)is transparentlyprovidedbym-rpcsystematthemh andtheagentatthemsr.whenbindingtoaservice isrequestedbyanmhapplication,aservice-mapper interfaceatthemhisconsultedtodetermineifthe serviceisavailableviathecurrentmsr.theservicemapperreturnsaserviceidentiertobeusedinthe bindingrequestwhichissenttothem-rpcagentat themsratthetimeofclienthandlecreation.the agentinturnconsultsitslocaldatabasetolocatean availableserverwhichprovidestherequestedservice andperformsaprovisionalbindingoftheclientrequestwiththatserver.suchbindingcanbechanged ifneededbythem-rpcagentforsubsequentrpc requestsfromthesameclientapplication.adynamic bindingschemeworkswellwithstateless(andpossiblyreplicated)serverswhichisexpectedtobethecase withfutureinformationbasedservices.withstateful andconnection-orientedservers,switchingtoadierentserverrequirestearingdowntheconnectionwith thepreviousserverandestablishmentofaconnection withthenewserver. 5
6 Client Application Create handle & Call requests Client handle, Call results & Error indication M-RPC Library Service name Service ID Service Mapper /etc/msrservices Connection requests Call results & to M-RPC agent & Error messages Call requests Indirect port 3.5ClientMobilityandHandos RDP/UDP registration Indirect Transport mhmicp Library Process viousmsraboutthemovement.them-rpcagent switchescells,themsrinitsnewcellinformsthepre- Reliable delivery of Call requests Beacons Registration atthepreviousmsrthentransfersthestateinforma- replies to & from agent tionrelatedtotheclienthandlesbelongingtothemh Figure2:Client(MH)SideofM-RPCSystem tothenewmsr.thehandoincludestransferofthe Disconnection & RDPconnections(onperclienthandle)betweenthe IfanM-RPCclientwithopenclienthandles Reconnection UX Server / Mach 3.0 Micro Kernel MHandtheM-RPCagenttotheagentatthenewcareofretransmittingthelostRPCrequestsorreplies. Events MSRandusessockethandomechanismsdescribedForlongerdisconnections,whichcanoccurwhenan in[6].asaresultofthehando,theagentatthe WaveLan Driver newmsrcreatesitsowncopiesofclienthandlesthatitswirelessinterfacetosaveonthebatterypower,the transportlayerbetweenthemhandthemsrtakes MHmovesoutofreceptionareaorwhenitturnso agentwithoutrequiringanyinterventionbytheclient. 3.6HandlingDisconnections andswitchableserverbindingisthusprovidedbythe belongtothemhandaudpsockettohandlecommu-m-rpcsystemprovidesthefollowingalternativesnicationwiththeserver.anysubjectbasedbindingmineifadierent(possiblynearer)servershouldberationcanbeprovidedatmultiplelevels.fortransientdisconnectionsthatoccurduetosuddenfading usedforsubsequentcallrequestsandretries.dynamic requesttothem-rpcagentatthemsr,the agentcancollectandstoretheresultuntilthe MHreconnectswhentheresultisreliablyhanded overtothemh. madebythepreviousmsrarere-evaluatedtodeter- 1.IftheMHdisconnectsaftersubmittinganRPC ofwirelesssignalorduetocellcrossovers,thereliable IntheM-RPCsystem,supportfordisconnectedop- 2.IftheM-RPCsystemontheclient(MH)sidereceivesanRPCrequestfromanapplicationprogramwhiletheMHisdisconnected,itcanimmediatelyreturntotheapplicationwithanerrormessageindicatingthedisconnection.Alternatively,theM-RPCsystemcantransfercontrol 6theclientsidealsoifthedisconnectionoccursaf- tertherpcrequestwassubmitted. toanapplicationspecichandlerwiththesame quest.thelattermechanismcanbeinvokedon argumentsthatweresuppliedwiththerpcre-
7 Server Lookup M-RPC Agent MHIn & MHOut notifications msrmicp Call requests & Call requests Process retries to Server from MH Call results & Results from error indication Server Figure3:M-RPCAgentattheMSR to MH Beaconing & Library MH registration Communication Communication with FH Server with MH Client UDP / IP over RDP / Mobile-IP UX Server / Mach 3.0 Micro Kernel cessible.currentlydisconnectionsupportinm-rpc errorintheconventionalrpcsystemsormaybea substitutefortheremoteprocedure.oneuseofsuch normallycauseatimeoutorserverhostunreachable fordisconnectedoperationwhichmayeitherprovide applicationspecichandlercanbetoretrievealocally cachedcopyofaleincasetheleserverisinac- gracefulrecoveryfromsuchaconditionwhichwould Applicationscanspecifyahandlertobeused besetupattheclientsideforeachremoteprocedure Ethernet Driver whichiscalledifacallrequesttimesout. islimitedtoallowingapplicationspecichandlersto WaveLan Driver typicalrpcserverinoursystem. 4Implementation tionalrpcsystems.thusm-rpcclientscanaccess nomodicationsform-rpcclients.figure4showsa networkbehavesexactlylikeaserverhostinconven- existingnetworkservicesfromsuchserverswhichneed 3.7ServerInterface WehaveimplementedM-RPCsystemonourmobilecomputingtestbedwhichconsistsofagroupof IntheM-RPCsystem,aserverhostonthewired Server Application Call results Call requests returned to from Clients Clients and Agents MSRssupportingmultiplecellswithinacampususingColumbiaMobileIPprotocol.TheMSRsandthe Library 7Figure4:Server(FH)SideofM-RPCSystem Connectionless Transport UDP / IP Unix Kernel Ethernet Driver
8 MHsinourtestbedare386and486PC-ATmachines equippedwith2mbpswavelanradiosforwireless communication.allthemachinesrunmach3.0microkernelanduxserverfromcmu(mk84/ux40). TheMSRsarealsoconnectedto10MbpsEthernet segmentsthatarelinkedtogetherbyarouterwhich isunawareofmobility.them-rpcsystemexports aninterfacethatissimilartosunrpcforclientand serverpartsofadistributedapplications.clientand serversidestubsgeneratedbytherpcgenprogramcan beusedwithoutmodicationsform-rpcclientsand servers. 4.1M-RPCClientLibrary TheclientsidelibraryforM-RPCconsistsofthe clientinterfaceinsunrpcaugmentedforindirect RDP/UDPtransportlayer.TheextrafunctionalityprovidedbyM-RPC(andtheindirectRDP/UDP layer)canbeinvokedbyrequestingaclienthandle creationusingtheclntcreatecallwithrdpasthe transportprotocol.thesubjectbasedbindingfacility isautomaticallyinvokediftherequestforthecreation ofaclienthandlesuppliesthenameofaserviceinsteadofatarget(server)host.oncetheclienthandle isobtainedbytheclientapplication,itremainsvalid untilitsdestructionbytheapplicationitself,tomake anynumberofremotecallsusingclntcalltotherequestedserver(orservice).theclntcreateinterface forrdp/udpcreatesanrdpconnectionwiththem- RPCagentatthecurrentMSRusingtheRDP/UDP indirecttransportlibrary.theconnectionrequestto themsragentcontainseitheraremoteservername ifspecied,orthemsraddressifbindingtoaservice isrequested.theportnumberattheremotehostcan besuppliedintherequestifknownoritcanbeleft uninitializedinwhichcasetheagentwillllitinafter contactingtheportmapperattheremotehost.the portnumbereldisalsousedtoholdtheserviceidentierincasebindingtoaserviceisrequested.buer spaceisallocatedforcallandreplymessagesandthe staticpartofcallheaderispre-serialized. 4.2RDP/UDPIndirectTransportLayer RDP/UDPindirecttransportlayerisimplemented inawaysimilartoi-tcp[6,7].theclientside ofm-rpclibraryusesaspecialrudpconnectcall fromtherdp/udpindirectlibraryinresponsetoa clntcreaterequestfromtheclientapplication.the rudpconnectcallrstmakesaconnectiontoawell knownportatthemsrtoobtainaportnumberon whichthem-rpcagentiswillingtoacceptanother connectionfromthemh.thenanotherrdpconnectionismadetothisnewlyobtainedportonwhich thedetailsabouttheremoteendpoint(serveraddress andportnumberorserviceidentierincasesubject basedbindingisdesired)aresentasadatagram.the rudpconnectcallreturnsafteraudpsockethas beenestablishedbythem-rpcagentonbehalfofthe MHclientapplicationtocommunicatewithaserver andtherdpconnectionwiththemsrisreadyat thispointfordatatransferinbothdirections. WhenanindirectRDP/UDPconnectionisestablished,themhmicpprocessontheMHisalsoinformedabouttheconnectionendpoints.ThisinformationissentbythemhmicpprocessintheinitialgreetingmessagetotheMSRimmediatelyafteramoveto anewcell.theinitialgreetingmessagereceivedby themsrmicpprocessatthemsrisforwardedtothe localm-rpcagentwhichpreparesrdp/udpskeletonsocketsinanticipationofahandorequestfrom thepreviousmsrwherethemhwasearlierlocated. Thedetailsofasockethandocanbefoundin[6]. 4.3M-RPCAgentatMSR TheM-RPCagentmanagestheindirectRDP/UDP transportconnectionsforeachclienthandlerequested byalocallyregisteredmh.foreverymhclienthandle,them-rpcagentalsomaintainsitsownhandle tomakerpccallrequeststoaserver.theudp socketusedbytheagentformakingsuchcallrequests isboundtotheipaddressofthemh,sothatthe aserver(onthewirednetwork)seesthecallrequest comingdirectlyfromthemh.them-rpcagentalso entertainsclienthandlecreationrequestsfromlocal MHsopeningaseparateRDPconnectionwiththeMH foreverysuchhandle.thusthem-rpcagentactslike aservertotheclientsideatthemhandasaclientto theserveronthewirednetworkinallrespectsexcept one:theagentdoesnotperformanyxdrconversion onthecallargumentsorresults.theagenthasno informationtodeterminethetypeofthecallargumentsandtheresultseither.thusonlythetwoend pointsi.e.theclientandtheserverperformmarshalingandunmarshalingofcallargumentsandresults. TheagentdoesperformsomelimitedXDRconversionsonthecallandreplyheadershowever,whichare necessarytomatchareplywiththecorrespondingcall request. Aftercreatingaclienthandleontherequestofand onbehalfofthemhclient,them-rpcagentstartsa workerthreadtoprocessanysubsequentcallrequests receivedfromthemhandthecorrespondingreplies receivedfromtheserver(fh).thisworkerthreadis alsoresponsibleforretransmittingfailedrpcrequests totheserver.aspeciallydesignatedthreadupdates aglobaltimerinrealseconds.thisglobaltimeris usedbyindividualworkerthreadstokeeptrackof 8
9 retrytimeouts.aworkerthreadremainsinoperation untiltheclienthandleattheagentisdestroyedbythe MHoruntilahandosignalisreceivedfortheMHin whichcasethethreadterminates. 4.4BindingServicestoServers Forsubjectbasedbinding,weprovidealibraryinterfacenamedgetmsrservbynameontheclientside whichmapsaservicenametoaserviceidentierto beusedintheclienthandlecreationrequestsentto themsragent.thislibraryfunctionisanalogousto thegetservbynameclibraryfunctionandconsults astandardlecalled/etc/msrservicesforthemapping.alternatively,anexternalservicemappercanbe usedwhichisresponsiblefordiscoveringservicesavailableinawirelesscellandmappingtheirnamestoserviceidentiers.onesuchschemecanbebasedupon periodicpublishing[17]ofavailableservicesbyeach MSRinitscellusingawellknownmulticastaddresstheservicemappercankeeptrackofavailableservices bylisteningtothismulticastaddress.inourcurrent scheme,theserviceidentierobtainedfromgetmsrservbynamecallisapseudoportnumberwhich isrecognizedbythem-rpcagent.choiceofsubject basedbindingisindicatedbytheuseofmsr'saddressinplaceofaserveraddresswhenthem-rpc librarysubmitstheclienthandlecreationrequestto them-rpcagent.theagentusesagetmsrservbyportfunctiontolocateaserverhostconsultingeither itslocalservicedatabaseoranamelookupservicein theprocess.theagentalsosetsaagintheclient handlecreatedonbehalfofthemhindicatingthat theserverbindingisswitchable. Figure5showsanexampleofswitchablebinding wherethemhasksforaprintservice.inthecell ofmsr-1,theprintrequestaresenttoprintserver- 1whichisthevalueoftheeld<remote-addr>in theagent'shandle.whenthemhmovestothecell ofmsr-2,thenewmsrnoticesthatbindingtothe PRINTserviceisswitchablebylookingattheRPC layerstatereceivedfrommsr-1.msr-2therefore bindstheagenthandletoprintserver-2,whichisthe localprintserverinitscell.thisnewserveraddressisputinthe<remote-addr>eldoftheagent handlesothatthefuturerpcrequestsaresentto PrintServer-2.NoticethattheMHdoesnotneedto takeanyactionforthechangeofservers. 4.5M-RPCHandos WhenanMHwithvalidclienthandlesswitches cells,them-rpcagentatthepreviousmsrtransfersthestateassociatedwiththemhclienthandles tothenewmsr.therpclayerhandoismadeafter networkandtransportlayerhandosarecompleted. Thehandosequenceisoutlinedbelow: 1.TheMHsendsinitialgreetingmessagetothe MSRinitsnewcell.Theinitialgreetingsent bythemhmicpprocessatthemhcontainsthe end-pointparametersofalltherdp/udpindirectconnectionsactivebetweenthemhandthe previousmsrforopenm-rpcclienthandles. ThegreetingalsoliststhepreviousMSRwhere themhwaslocatedbeforemovingtothenew cell. 2.ThemsrmicpprocessattheMSRupdatesitstablesmarkingthenewlyarrivedMHaslocaland sendsaforwardingpointertothepreviousmsr listedinthegreeting.acopyofthegreetingmessageisalsosenttothelocalm-rpcagentthus initiatinghigherlayerhando. 3.TheM-RPCagentestablishedskeletonsocketsfor RDPconnectionswiththeMHandUDPsockets boundtothemhaddressforcommunicationwith theserversonthewirednetworkfromtheendpointparameterslistedinthemhgreeting. 4.WhenthemsrmicpprocessatthepreviousMSR receivestheforwardingpointer,itupdatesitstablesmarkingthemhwhichmovedoutofitscell asremote.acopyoftheforwardingpointeris alsosenttothelocalm-rpcagent. 5.TheM-RPCagentsignalsalltheworkerthreads activeonbehalfofthe(nowremote)mhtoterminate.afterterminatingtheworkerthreads,the M-RPCagentmakesahandoconnectionwith thenewmsr'sm-rpcagentonawellknown TCPport.Thestateofeachclienthandleactive atthemsronbehalfofthemhisthentransferredtothenewmsr.foreveryclienthandle, theagentattheoldmsrsendsthestateofthe MHside(RDP)andFHside(UDP)socketsusing aspecialsiocgetstateioctl[6].aftersendingthestateofthetwosockets,rpclayerstate informationabouttheclienthandleissenttothe newmsr.thisinformationincludestheprogram andversionnumbersfortheremoteprogram(or service)andaagindicatingiftheexistingserver bindingisswitchableornot. 6.Ontheotherhand,theM-RPCagentatthe newmsrtakesanalogousstepstoreceivethe stateinformationfromtheoldmsr.thestate ofmhsideandfhsidesocketsisrstreceived andinstalledusingasiocsetstateioctl[6]. 9
10 Client Handle PRINT VERS0 RDP socket MSR addr SW_BINDING program & version socket pointer remote addr flags Agent Handle PRINT VERS0 UDP socket PrintServer-1 SW_BINDING MH Handoff Client Handle PRINT VERS0 program & version RDP socket socket pointer MSR addr remote addr SW_BINDING flags 7.TheagentatthenewMSRstartsaworkerthread dleisreadfromthehandoconnectionandalo- calcopyofthemhclienthandleiscreatedby theagent.additionalstepsrequiredtohandle switchableserverbindingsaredescribedlater. RDP connection questsfromthemhandrepliesfromtheserver perclienthandletoprocesssubsequentcallre- M-RPC (FH).Ifareplyforapendingcallarrivesfromthe o,suchareplyisdeliveredtotheskeletonudp server(addressedtothemh)duringthehand- socketatthenewmsrbythemobileiplayer whichcanbepickedupbythenewmsr'sm-rpc agentonceitobtainsthestateinformationfrom newmsrlocatesalltheclienthandlesforthemh thepreviousmsr.thushandosdonotcause thathaveswitchablebindings,andattemptstore- 4.6SwitchingServersonaMove switchescells,aspartofhandotheagentatthe WhenanMHwithopenM-RPCclienthandles anyextraneousretriesbythem-rpcagentdue bindthemto(possiblycloser)serversintheneigh- tolostreplies. 10 MSR-1 Agent Handle PRINT VERS0 UDP socket PrintServer-2 SW_BINDING To PrintServer-2 M-RPC catedschemesfordynamicbindingcanbedeveloped anysubsequentcallretrieswillbedirectedtothenew server.thisworksnewithourassumptionofstatelessserversandidempotentrequests.1moresophisti- arere-computedfromtheinformationavailableatthe newmsr,immediatelyaftereverymovewhichmeans calcopyoftheclienthandlewhichisusedinthe retriestotheserver.currently,theserverbindings sendtosystemcallsformakingrpcrequestsand Agent UDP byextendingourbasicmechanismhowever,forexampletoswitchserversonlywhenthenextcallrequest MSR-2 arrivesfromthemh. 4.7HandlingDisconnections curbecauseofcellcrossoversandtemporarylossof radiosignal.thereliablenatureoftransportprotocol onthewirelesslinkcoupledwithrpclevelbuertionofm-rpcsystemislimitedtohandlingtransient orshort-liveddisconnectionssuchasthosewhichocingofrepliesformsthebasisofhandlingdisconnectionsinoursystem.wealsoallowthemhapplica- Disconnectionsupportinthecurrentimplementa- semantics. 1SunRPCwithcallretriesdoesnotguaranteeexactly-once M-RPC Client Subsequently,informationabouttheclienthan- ClientFigure5:ExampleofSwitchableServerBinding RDP connection M-RPC Agent To PrintServer-1 UDP borhood.theserveraddressisrecordedinthelo-
11 tiontospecifyadisconnectionhandlerforeachprocedurethatispartoftheremoteprogramusingan extendedclntcontrollibraryfunction.thiscanbe doneonlyafteraclienthandlehasbeenestablished fortheremoteprogram.thedisconnectionhandler, ifestablished,iscalledwhenacallattempttimesout ontheclientside.thebasicmechanismsforhandling disconnectionsdescribedherewillbeextendedinthe nearfuturetocoverlongerdisconnectionsincluding theonesinitiatedbytheuser. 4.8M-RPCServerInterface AnyRPCserverexportingaconnectionless(UDP) serviceinterfaceundersunrpccanbeusedasa serverwithm-rpcclients.thusasingleservercan beusedtoserverequestsfromclientsusingregular SunRPCclientlibraryandfromM-RPCclients(via M-RPCagents).Thisparticularfeatureofbackward compatibilityisoneofthedesigncharacteristicsof protocollayersbasedontheindirectmodel.wedo notyethandleserversexportingaconnectionoriented interfacee.g.overtcp,butm-rpccanbeeasilyextendedtousei-tcpinsteadofrdp/udpwhichwill enablem-rpcclientstoaccesstcpbasedservicesas wellwiththeadditionalbenetsaordedbym-rpc. 5RelatedWork Useofagentsorintermediariesforprotocolsand applicationsbasedontheclient-servermodelisnota newphenomenon.thekkathetal.[33]recentlyadvocatedtheuseofaserverclerkontheclientmachineto separatecontrolanddatatracinanrpcbasedle system.intheirsystemtheclerkisatrustedrepresentativeoftheserverwhichcanaccessservermemory directlywithoutanycontroloverhead.bycontrast, them-rpcagentinoursystemisatrustedrepresentativeofthemobileclientandisauthorizedtomake RPCrequestsandcollectresultsonitsbehalf. Amethodfordynamicallyobtainingclient-service bindingswasproposedin[10].here,thebindingsare changedbyaclientagentwhenthecurrentserveris nolongeraccessible.bycontrast,inamobileenvironment,theserverbindingsmayhavetochangejustdue toamoveandnotnecessarilyduetofailures.hence, inourscheme,theagentatthemsrisresponsiblefor providingswitchablebindings.objectrequestbrokers(orbs)whicharepartoftheinfrastructurein CORBAspecications[26]alsobehaveasagentsmappingclientrequeststoservers.Agentbasedmanyto-manyRPCdescribedin[35]allowsintegrationof resultsfrommultipleserversandcallbacks.formobileenvironments,anagentbasedmodelformessage passingwassuggestedin[3].schemesforadaptive presentationofdatatomobileclientshavebeenpresentedrecentlyin[37]and[25].theneedforsubject basednamingandbindingindissemination-oriented protocolswasemphasisedin[14]. 6ConclusionandFutureWork WehavepresentedinthispaperaRemoteProcedureCallserviceformobileclientscalledM-RPC whichtakescareofsomeoftheuniquefeaturesof mobilewirelesscomputing.weoutlinedcertainrequirementsoffuturemobileclient-serverapplications whichwebelieveshouldbemetbytherpclayer.in particular,them-rpcsystemprovides: 1.Reliabletransportontheerrorpronewirelesslink evenwhenaconnectionlessinterfaceisdesired withaserveronthewirednetwork. 2.Subjectbasedbindingofclientrequeststoservers bythem-rpcagentatthemsrwhichisswitchableonamove. 3.RetransmissionsoffailedRPCrequestsfromthe MSRthussavingonwirelesstransmissionsfrom themh. 4.Partialsupportforlinklayerdisconnections. WealsodescribedourimplementationoftheM- RPCsystemwhichisbaseduponandcompatiblewith SunRPC.WealsoimplementedindirectRDP/UDP transportlayertocarrythem-rpcrequestsand replies.wetestedourimplementationforthesupportedfunctionalityusingclientandserverstubsgeneratedbyrpcgenstubgeneratoravailablepublicly withthesunrpcdistribution.ourimplementation ofm-rpchandosdemonstrateshowmultipleprotocollayerscanbehandedofromonemsrtoanother. Suchhandoscanbegeneralizedtomigrateapplicationlevelproxies. Inthenearfuture,weplantomeasureM-RPCperformancewithsimpleteststoquantifytheoverhead addedbytheinterposingagentatthemsr.other interestingmeasurementsincludecomparingm-rpc performancewiththatofsunrpcundersimulated lossconditionsonthewirelesslink.weareplanningto extendthem-rpcsystemtohaveextensivesupport fordisconnectedoperationwhichcanbenetunedby distributedapplicationsifneeded.wearealsoplanningtobuildsupportforthenoticationofdisconnectionandreconnectioneventstotheinterestedapplicationsandagentsatthemhandthemsrrespectively. SupportforadaptivepresentationofRPCresultsfrom 11
12 themsragenttothemhapplication,includingintegrationofresultsfrommultipleserversisneededas well. TheM-RPCsystemdescribedherewillbeusedas abasisforimplementingsupportforapplicationsusingmobileclient-agent-serverparadigm.suchsupport willincludeprovisionforhandingostateinformation relatedtoamobilehostfromoneagenttoanother whenthemhswitchescells.anexampleofsuchsupportisalesystemformobileclientswhichusesa cachingpolicybetweentheagentandthemhclient thatisdierentfromthecachingpolicysupportedby theleserver. Acknowledgements WewouldliketothankDanDuchampforpointing outsomeoftherelatedworkintheareaofdynamic bindingsinrpc. References [1]M.Accetta,R.Baron,W.Bolosky,D.Golub,R. Rashid,A.TevanianandM.Young,\Mach:a newkernelfoundationforunixdevelopment", Proc.oftheUSENIX1986SummerConference, July1986. [2]ArchitectureProjectsManagementLimited, \ANSA:Anengineer'sintroductiontothearchitecture",ANSAReleaseTR.03.02,November [3]A.AthanandD.Duchamp,\Agent-mediated messagepassingforconstrainedenvironments", Proc.oftheUSENIXSymposiumonMobileand Location-IndependentComputing,August1993. [4]B.R.Badrinath,A.Bakre,T.ImielinskiandR. Marantz,\HandlingMobileClients:ACasefor IndirectInteraction",Proc.ofthe4thWorkshop onworkstationoperatingsystems(wwos-iv), October1993. [5]B.R.Badrinath,A.AcharyaandT.Imielinski, \StructuringDistributedAlgorithmsforMobile hosts",proc.ofthe14thintl.conf.ondistributedcomputingsystems,pp.21{28,june [6]A.BakreandB.R.Badrinath,\Handoand systemsupportforindirecttcp/ip"inproc. ofthe2ndusenixsymposiumonmobile andlocation-independentcomputing,pp.11{24, April1995. [7]A.BakreandB.R.Badrinath,\I-TCP:Indirect TCPformobilehosts"ToappearinProc.ofthe 15thIntl.Conf.onDistributedComputingSystems,May1995. [8]A.Birell,\Securecommunicationusingremote procedurecalls",acmtrans.oncomputersystems,3(1),february1985. [9]A.BirellandB.Nelson,\Implementingremote procedurecalls",acmtrans.oncomputersystems,2(1),february1984. [10]R.ChangandC.V.Ravishankar,\Aserviceacquisitionmechanismfortheclient/servicemodel incygnus",proc.ofthe11thintl.conf.on DistributedComputingSystems,pp.90{97,May [11]R.CaceresandL.Iftode,\TheEectsofMobilityonReliableTransportProtocols",Proc.ofthe 14thIntl.Conf.onDistributedComputingSystems,pp.12{20,June1994. [12]D.Cheriton,\Exploitingrecursiontosimplify RPCcommunicationarchitectures",Proc.ofthe ACMSIGCOMM,pp.76{87,September1988. [13]D.Cheriton,\VersatileMessageTransactionProtocol",RequestforComments1045,February [14]D.Cheriton,\Dissemination-orientedcommunicationsystems",TechnicalReport,StanfordUniversity,PaloAlto,CA,1992. [15]E.C.CooperandR.P.Draves,\CThreads", TechnicalReportNo.CMU-CS-88{154,Schoolof ComputerScience,CarnegieMellonUniversity, Pittsburgh,PA,June1988. [16]A.DeSimone,M.C.ChuahandO.C.Yue, \Throughputperformanceoftransport-layerprotocolsoverwirelessLANs",Proc.ofGlobecom '93,December1993. [17]T.ImielinskiandS.ViswanathanandB.R. Badrinath,\EnergyEcientFilteringofDataon theair",inproc.oftheacmsigmodconference,may1994. [18]T.ImielinskiandB.R.Badrinath,\Mobilewirelesscomputing:Challengesindatamanagement",CommunicationsoftheACM,Vol.37,No. 10,October1994,pp.19{28. [19]J.Ioannidis,D.DuchampandG.Q.Maguire, \IP-basedprotocolsformobileinternetworking" Proc.ofACMSIGCOMM,pp.235{245,September
13 [20]J.IoannidisandG.Q.Maguire,\Thedesignand implementationofamobileinternetworkingarchitecture",proc.ofusenixwintertechnical Conference,January1993. [21]J.KistlerandM.Satyanarayanan,\Disconnected operationinthecodalesystem",acmtrans. oncomputersystems,10(1),february,1992. [22]M.Kojo,K.RaatikainenandT.Alanko,\ConnectingmobileworkstationstotheInternetover adigitalcellulartelephonenetwork",mobidata WorkshoponMobileandWirelessInformation Systems,November1994. [23]S.J.Leer,M.K.McKusick,M.J.Karelsand J.S.Quarterman,Thedesignandimplementation ofthe4.3bsdunixoperatingsystem,addison Wesley,1989. [24]P.Manzoni,D.GhosalandG.Serazzi,\Impactof mobilityontcp/ip:anintegratedperformance study",toappearintheieeejournalonselectedareasincommunications,1995. [25]B.Noble,M.PriceandM.Satyanarayanan, \Aprogramminginterfaceforapplication-aware adaptationinmobilecomputing",toappearin Proc.oftheFifthWorkshoponHotTopicsinOperatingSystems(HoTOS-V),May1995. [26]ObjectManagementGroup,\Commonobject requestbrokerarchitectureandspecication", OMGDocument ,December1991. [27]J.Postel,\Userdatagramprotocol",Requestfor Comments768,August1980. [28]J.Postel,\Transmissioncontrolprotocol",RequestforComments793,September1981. [29]K.Shrivastava,G.N.DixonandG.D.Parrington, \AnoverviewoftheArjunadistributedprogrammingsystem",IEEESoftware,pp.66{73,January1991. [30]SunMicrosystems,\XDR:ExternalDataRepresentationStandard",RequestforComments 1014,1987. [31]SunMicrosystems,\RPC:RemoteProcedure Call,ProtocolSpecication,Version2",Request forcomments1057,1988. [32]SunMicrosystems,\NFS:NetworkFileSystem ProtocolSpecication",RequestforComments 1094,1988. [33]C.A.Thekkath,H.M.LevyandE.D.Lazowska, \Separatingdataandcontroltransferindistributedoperatingsystems",Proc.ofthe6thIntl. Conf.onArchitecturalSupportforProgramming LanguagesandOperatingSystems,pp.2{11,October1994. [34]D.Velten,R.HindenandJ.Sax,\Reliabledata protocol",requestforcomments908,july1984. [35]G.WellingandB.R.Badrinath,\Anarchitecture ofathreadedmany-to-manyremoteprocedure call",proc.ofthe12thintl.conf.ondistributed ComputingSystems,pp.504{511,June1992. [36]R.YavatkarandN.Bhagwat,\Improvingendto-endperformanceofTCPovermobileinternetworks,"IEEEWorkshoponMobileComputing, December1994. [37]B.ZenelandD.Duchamp,\Intelligentcommunicationlteringforlimitedbandwidthenvironments",ToappearinProc.oftheFifthWorkshop onhottopicsinoperatingsystems(hotos-v), May
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