[26]presentsaframeworkforadaptingmultimediawebdocumentstooptimallymatchtheneedsoftherequestingclientdevicesusingamultimodal,multiresolutionrepre-sentationhierarchyforthecontentandaCustomizerthatpicksthebestcontentrepresentationthatprovidesmaxi-mumvaluetotheclient.Inthismodel,thecontentauthorprovidesthetranscodingpoliciesandcontrolstheadapta-tionandthisisdoneinanof inemanner(asopposedtoonrequestinmostproxybasedtranscodingsystems).[31]presentsaContent-awareActiveGateway(CAG)architec-tureallowingforonthe ycontentadaptation.Theyrelyontheabilityto lterspeci ctypesoftraf cbyidentifyingpat-3
ternsintheheaderorpayloadofpackets.Thegeneralideaistorunspecializedprograms(staticallyinstalledordown-loaded)onenterprise/residentialgatewaysthatcanregister lters(portnumber,IPaddressetc)onthepacketroutingfabric.These ltersareactivatedwhenpacketsmatchingthosecriteriaareencounteredandpassedtotheCAGcom-putationlayerforprocessingatclosetolinespeed.[9]isaproxybasedwebcontentadaptationforsupportingbrowsingbymobiledevicesoverwirelesslinksusinguserspeci edpreferences.[32]usesalinklevelredirectionin-frastructurecalledSelNetthattagspacketsatlinklevelwithfunctionidenti erstoenableaproxybasedcontentadapta-tion.[6]proposeaservercentriccontentadaptationframe-workcreatingservicespeci coverlaynetworksthroughtheuseofdynamicproxiesalongthedatapath.[30]pro-poseaclientcentriccontentadaptationusingWebStreamCustomizers(WSC)allowingforsystembasedandcontentbasedcustomization.Inadditiontoresearchprojects,sev-eralcommercialestablishmentshavebeenlaunchedwithcontentadaptationastheirprimarybusinessfocussuchasVoiceAgeNetworks,Volantis,Adamind,LightSurf,Sens-eStream,Mobixelletc.
Oneofthegoalsthatwehaveidenti edforourframeworkistosupportcontentadaptationfordatastreams.Ourap-proachofusingsemantictagsembeddedinthestreamwillallowcontentprovidersto“signal”theiradaptationpoliciesandrequirements.AsdiscussedinSection3,thiscanbebothin-bandorout-of-band.Anintelligentstreamproces-sorcanreasonoverthisinformationandusingitsexistingknowledgebase,theappropriatecontentadaptationservicescanbeapplied.
2.3CrossLayerTechniques
StrictLayeringhasbeentheprimarydesigntechniquefornetworkingstackswhichhasresultedinsimple,scal-ableandinteroperablesolutions.Proponentsofthelay-eredmodelsuchas[21]arguethatcrosslayeroptimiza-tionbreaksthearchitecturalsimplicityoflayeringandistooshortterminvisionandhighlyspeci c(totopology,tech-nology,applicationetc.)leadingtospaghettiimplementa-tionsandwillnotseewidespreaddeployment.Thecounterargumenthasbeenthatrestrictingtheinteractionsbetweenlayerstodataandwellde nedprimitivesimposesaratheruniformhandlingsemanticfordatastreamswhichisnotal-waysthemostef cient.Muchoftheworkoncrosslayeroptimizationshasfocusedonoptimizationsatthenetworkedgesprimarilyforwirelesstechnologies.In[17],inganEligibleRateEstimate,thequal-ityofthevideoisdynamicallyadjustedtooffertheuser,uninterruptedvideoatthebestqualitythatcanbesustained.MobiWeb[25]isaframeworkforsupportingadaptiveap-
Today’s model of networking primarily concentrates intelligence at the end hosts with the network itself offering a simple“best-effort”,“data agnostic ” communication medium. However, this paradigm has proven to be insufficient to meet todays needs co
plicationsthathavereal-timecharacteristicsoverwirelesslinks.Aninter-streampriorityschemeisusedtoaddresstheshortterm uctuationsofwirelessstreamsleavinglong-termadaptationtotheapplication.[33]presentsacrosslayeralgorithmthatadaptsthelinklevelARQtotheend-to-endpacketlossobservedbyTCP.TheideahereistoadapttheMAClayerretransmissionaccordingtoatargetlossrateusedasparametertodescribethedesiredQoSforaTCPconnection.[28]presentanapplicationlevelARQmecha-nismfor802.11MAC.Heretheapplicationdecideswhichpacketsrequireretransmissionandjustthoseareretrans-mitted.[24]presentsatransportlayersolutiontoenhancingnetworkQoSusingapplicationlevelinformation.Hereap-plicationscansplitdataintodifferentstreamswhichcanbedeliveredacrossdiverseroutessothatmorecriticaldataistransmittedoveramorereliableconnectionwhilethelesscriticaldatacanfollowalessreliableroute.[23]presentsanapproachtosupportcongestioncontrolledmulticastreal-timecommunicationusingself-organizedtranscodingtohandlelocalrepairinwirednetworks.Thetranscodingparametersareconstantlyupdatedtore ecttherealtimestateofthelinkbeingusedtoservethegrouptoprovideanetwork-friendlycongestioncontrol.
Ourproposedframeworkprovidesthenecessarysupportforimplementingcrosslayeroptimizationsatthelocalnodelevel.Oneoftheprimaryconcernsagainstcrosslayertech-niquesisthatexposingtoomuchinformationbetweenlay-ersmakesthesystemoverlycomplicated.Tothisend,ourframeworktakestheapproachofparameterizingtheinter-actionssimilarto[15].Thegoalistoexposemeaningfulparametersandtheirassociatedcostsateachlayerwhilekeepingtheactualmechanismofachievingthemhiddenwithinthatlayer.Inourframework,thelocalpolicyen-forcercanspecifyparameterstobeusedacrossthedifferentlayersofthestacktomeetapplicationneeds.Theobjec-tiveistokeeptheinteractionstractablewhilestillachievingtheef cienciesofcrosslayertechniques.Anadditionalandnecessarygoalistocoordinateinteractionsacrossanetworksincesometechniquessuchaslinklevelreliabilityneedtobeappliedatasystemleveltoreallyseethebene ts.Ourapproachtocrosslayeroptimizationsgoesonestepfurtherbyallowingforthespeci cationofsystemleveloptimiza-tionsthatcanspanmultiplenodes.Systemlevelinforma-tioncanbeexchangedbetweennodestoinformeachnodeofthedynamicsystemvariationsthatcanthendrivethelo-caloptimizationsthatareapplied.