外文翻译-P2P视频点播系统的最新进展

0外文原文RecentAdvancesinPeer-to-PeerMediaStreamingSystemsABSTRACTRecently,thereisgreatinterestinusingthepeer-to-peer(P2P)networkinmediastreaming.AgreatnumberofP2Pmediastreamingsystemshavebeendeveloped.Inthispaper,wefirstgiveabriefsurveyonsomekeytechniquesandalgorithmsinthefieldofP2Pstreamingresearch.WealsoanalyzethemarketviewofP2Pstreamingmediaservice,andgiveabriefdescrip-tionaboutthecurrentmainstreamP2PstreamingsystemsdeployedinChina.I.INTRODUCTIONTherapiddevelopmentoftheInternethaschangedtheconven-tionalwaysthatpeopleaccessandconsumeinformation.Besidessendingandreceivinge-mails,browsingwebpages,anddownloadingdatafiles,peoplealsohopetocalltelephone,watchmovieandTV,andconductotherentertainmentsviathesameInternet.Theidealobjectiveisthatanyonecanaccessanything(contents)fromanywhereatanytime.ItiscommonlyconceivedthatthenextgenerationInternetshouldbeamulti-mediacommunicationnetworkbasedonthecoreofIPprotocol.Besidestraditionaldataservices,othermultimediacontentssuchasvoice,image,andvideo,wouldalsobedeliveredoverthesameIPnetwork,amongwhichthestreamingmediaservicewillplayaverymoreimportantrole.Streamingmediaenablesreal-timeandcontinuousdeliveryofvideoandaudiodatainafashionof“flow”,i.e.,oncethesenderbeginstotransmit,thereceivercanstartplaybackalmostatthesametimewhileitisreceivingmediadatafromthesender,insteadofwaitingfortheentiremediafiletobereadyinthelocalstorage.Unlikenormaldatafile,astreamingmediafileishuge,thusrequireshighchannelbandwidth.Moreover,streamingmediaalsocarriesstringentdemandinthetimingofpacketdelivery.Thelargesizeofthestreamingmediaaswellasitsdeliverytimingrequirementcausesastreamingmediaservertobeexpensivetosetupandrun.Intraditionalclient/server-basedmediastreamingsystems,all1clientsaccessthesameserverresource.Inthisscenario,ontheonehand,theprocessingpower,storagecapacity,andI/Othroughputoftheservermaybecomethebottleneck;ontheotherhand,largenumberoflong-distancenetworkconnectionsmayalsoleadtotrafficcongestion,thuscannotaffordbetterqualityofservice(QoS)comparablewiththatofothertradi-tionalInternetservices,suchasWWWandFTP,andcannotmeettheperformancerequirementsoflarge-scalereal-timemediastreamingapplications,especiallyintheaspectsofscalability,adaptability,fault-toleranceandrobustness.Toaddresstheseproblems,recentlyresearchershavepro-posedmanysolutions,suchasIPmulticastandCDN(contentdeliverynetwork).However,bothofthemneedsupportsfromspecialhardware.ForIPmulticastnetwork,large-scalemulticast-capableroutersmustberedeployedintheInternet.Forcontentdeliverynetwork,alargenumberofCDNserversshouldbeplacedatthenetworkedge,closetoanyreceiver,andcooperatewitheachothertodistributemultimediadata.Thecostsofinfrastructuresetupandadministrationareexpensive,andcannotresolvetheproblemsfundamentally.Inrecentyears,Peer-to-Peer(P2P)networkingtechnologyhasgainedtremendousattentionfrombothacademyandindustry.InaP2Psystem,peerscommunicatedirectlywitheachotherforthesharingandexchangeofdataaswellasotherresourcessuchasstorageandCPUcapacity,eachpeeractsbothasaclientwhoconsumesresourcesfromotherpeers,andalsoasaserverwhoprovidesserviceforothers.P2Psystemscanbenefitfromtheirfollowingcharacteristics:adaptation,self-organization,load-balancing,fault-tolerance,availabilitythroughmassivereplication,andtheabilitytopooltogetherandharnesslargeamountsofresources.Forexample,file-sharingP2Psystemsdistributethemaincostofsharingdata-bandwidthandstorage-acrossallthepeersinthenetwork,therebyallowingthemtoscalewithouttheneedforpowerfulandexpensiveservers.P2Psystemsareoriginallyappliedinnetworkfilesharing,andhaveachievedgreatsuccess,suchasNapster,Gnutella,Emule,andBitTorrent.However,differentfromgeneralP2Pfilesharing,P2Pmediastreamingposesmorestringenttimingandresourcerequirementsforreal-timemediadatatransmis-sionandrendering,thereforeitisneededtoprovidemorerestrictedfunctionsintherespectsofresourcemanagement,scheduling,andcontrol.2VariousP2Pmediastreamingsystemshavebeenproposedanddevelopedrecently.EveninChina,nowadaysthereareaboutmorethanadozenofP2Pstreamingapplicationsde-ployedintheInternet.Inthispaper,wefirstgiveabriefsurveyonsomekeyresearchissuesandalgorithmsofP2Pstreamingsystems,andthenanalyzeandsummarizethecurrentstatusanddevelopmenttrendofP2PstreamingmarketinChina.II.RESEARCHPROGRESSOFP2PMEDIASTREAMINGAsimpleandstraightforwardwayofP2Pstreamingimplemen-tationistousethetechniqueofapplication-layermulticast(ALM).WithALM,allpeernodesareself-organizedintoalogicaloverlaytreeovertheexistingIPnetworkandthestream-ingdataaredistributedalongtheoverlaytree.Thecostofprovidingbandwidthissharedamongthepeernodes,reducingtheburdenofthemediaserver.Inapplication-layermulticast,datapacketsarereplicatedandforwardedatendhosts,insteadofatroutersinsidethenetwork.ComparedwithIPmulticast,application-layermulticasthasseveraladvantages.Ontheonehand,sincethereisnoneedforsupportsfromrouters,itcanbedeployedgraduallybasedonthecurrentInternetinfrastructure;ontheotherhand,application-layermulticastismoreflexiblethanIPmulticast,andcanadaptdifferentdistributiondemandsofvariousupperlevelapplications.Thus,howtoconstructandmaintainanefficientALM-basedoverlaynetworkhasbecameoneofthekeyproblemsofP2Pstreamingresearch.Toaddressthisproblem,mainlythreequestionsshouldbeanswered.ThefirstrelatestotheP2Pnetworkarchitecture,i.e.,whattopologiesshouldtheoverlaynetworkbeconstructed?Thesecondconcernsroutingandschedulingofmediadata,i.e.,oncetheoverlaytopologyisdetermined,howtofindandselectappropriateupstreampeersfromwhichthecurrentpeerreceivestheneededmediadata?Thethirdismembershipmanagement,i.e.,howtomanageandadapttheunpredictablebehaviorsofpeerjoininganddeparture?Recently,severalP2Pstreamingsystemsandalgorithmshavebeenproposedtoaddresstheaboveissues.Fromtheviewofnetworktopology,currentsystemscanbeclassifiedintothreecategoriesapproximately:tree-basedtopology,forest-based(multi-tree)topology,and3meshtopology.InthefollowingwegiveabriefsummarizationofP2Pstreamingtechniquesaccord-ingtothisclassification.2.1Tree-basedtopologyThetypicalmodeloftree-basedP2PstreamingsystemisPeerCast.InPeerCast,nodesareorganizedasasinglemulticasttree,wheretheparentprovideserviceonlydirectlytoitssons.ThenodejoininganddeparturestrategiesusedinPeerCastaresimple.Fornodejoining,anewnodenfirstrequestservicesfromtherootnodeS.IftheShasenoughresources,itprovidesserviceforndirectly;otherwise,Sredirectstherequestofntooneofitssons.Thesonthenrepeatsthisprocess,untiltheparentofnisfound.Sinceeachnodeonlymaintainstheinformationofitsparentandsons,unbalancedtreemaybeconstructed.Generally,thereexistfourrouteselectionstrategiesinPeerCast:randomselection,round-robinselection,smartselectionaccord-ingtophysicalplacement,andsmartselectionaccordingtobandwidth.Toachieveabalancedmulticasttree,customroutingpolicyshouldbechosencarefullyforindividualpeernode.ZIGZAGisanothertree-basedP2Pstreamingsystemwhichcanconstructmorebalancedmulticasttree.ZIGZAGorganizesreceiversintoahierarchyofbounded-sizeclustersandbuildsthemulticasttreebasedonthat.Theconnectivityofthistreeisenforcedbyasetofrules,whichguaranteesthatthetreealwayshasaheighOandanodedegreeO(k),whereNisthenumberofreceiversandkisaconstant.Furthermore,theeffectsofnetworkdynamicssuchasunpredictablereceiverbehaviorsarehandledgracefullywithoutviolatingtherules.Thisisachievedrequiringaworst-casecontroloverheadofOreceiverandO(k)foranaveragereceiver.Othertree-basedP2PstreamingsystemsalsoincludeNICE,Overcast,andBayeux.2.2Forest-basedtopologyConventionaltree-basedmulticastisinherentlynotwellmatchedtoacooperativeenvironment.Thereasonisthatinanymulticasttree,theburdenofduplicatingandforwardingmulticasttrafficiscarriedbythesmallsubsetofthepeersthatareinteriornodes4inthetree.Mostofthepeersareleafnodesandcontributenoresources.Thisconflictswiththeexpectationthatallpeersshouldsharetheforwardingload.Toaddressthisproblem,forest-basedarchitectureisbeneficial,whichconstructsaforestofmulticasttreesthatdistributestheforwardingloadsubjecttothebandwidthcon-straintsoftheparticipatingnodesinadecentralized,scalable,efficientandself-organizingmanner.Atypicalmodelofforest-basedP2PstreamingsystemisSplitStream.ThekeyideaofSplitStreamistosplittheoriginalmediadataintoseveralstripes,andmulticasteachstripeusingaseparatetree.Peersjoinasmanytreesastherearestripestheywishtoreceiveandtheyspecifyanupperboundonthenumberofstripesthattheyarewillingtoforward.Thechallengeistoconstructthisforestofmulticasttreessuchthataninteriornodeinonetreeisaleafnodeinalltheremainingtreesandthebandwidthconstraintsspeci-fiedbythenodesaresatisfied.Thisensuresthattheforwardingloadcanbespreadacrossallparticipatingpeers.Forexample,ifallnodeswishtoreceivekstripesandtheyarewillingtoforwardkstripes,SplitStreamwillconstructaforestsuchthattheforwardingloadisevenlybalancedacrossallnodeswhileachievinglowdelayandlinkstressacrossthesystem.Stripingacrossmultipletreesalsoincreasestheresiliencetonodefailures.SplitStreamoffersimprovedrobustnesstonodefailureandsuddennodedepartureslikeothersystemsthatexploitpathdiversityinoverlays.SplitStreamensuresthatthevastmajorityofnodesareinteriornodesinonlyonetree.Therefore,thefailureofasinglenodecausesthetemporarylossofatmostoneofthestripes(onaverage).Withappropriatedataencodings,applicationscanmaskormitigatetheeffectsofnodefailuresevenwhiletheaffectedtreeisbeingrepaired.BesidesSplitStream,therearemanyotherforest-basedsystems.Examplesincludebuildingmesh-basedtree(Naradaanditsextensions,andBullet),leveraginglayeredcoding(PALS),andmultipledescriptioncoding(CoopNet).2.3MeshtopologyInconventionaltree-basedP2Pstreamingarchitectures,atthesametimeapeercanonlyreceivedatafromasingleupstreamsender.Duetothedynamicsandheterogeneityof5networkbandwidths,asinglepeersendermaynotbeabletocontributefullstreamingbandwidthtoapeerreceiver.Thismaycauseseriousperformanceproblemsformediadecodingandrendering,sincethereceivedmediaframesinsomeendusersmaybeincomplete.Inforest-basedsystems,eachpeercanjoinmanydifferentmulticasttrees,andreceivedatafromdifferentupstreamsenders.However,foragivenstripeofamediastream,apeercanonlyreceivethedataofthisstripefromasinglesender,thusresultsinthesameproblemlikethecaseofsingletree.Multi-senderschemeismoreefficienttoovercometheseproblems.Inthisscheme,atthesametimeapeercanselectandreceivedatafromadifferentsetofsenders,eachcontributingaportionofthestreamingbandwidth.Inaddition,differentfromthemulti-treesystems,thesendersetmembersmaychangedynamically,duetotheirunpredictableonline/offlinestatuschanges,andthetime-variablebandwidthandpacket-lossrateoftheInternet.Sincethedataflowhasnotafixedpattern,everypeercansendandalsoreceivedatafromeachother,thusthetopologyofdataplanelikesmesh.Themainchallengesofmeshtopologyarehowtoselectthepropersetofsendersandhowtocooperateandschedulethedatasendingofdifferentsenders.Examplesofmesh-basedmulti-senderP2PstreamingsystemincludeCollectCast,GnuStream,andDONet(CoolStreaming).CollectCastputsitsemphasismainlyonthejudiciousselec-tionofsenders,constantmonitoringofsender/networkstatus,andtimelyswitchingofsenderswhenthesenderornetworkfailsorseriouslydegrades.CollectCastoperatesentirelyattheappli-cationlevelbutinfersandexploitsproperties(topologyandperformance)oftheunderlyingnetwork.EachCollectCastsessioninvolvestwosetsofsenders:thestandbysendersandtheactivesenders.Membersofthetwosetsmaychangedynamicallyduringthesession.ThemajorpropertiesofCollectCastincludethefollowing:(1)itinfersandleveragestheunderlyingnetworktopologyandperformanceinformationfortheselectionofsenders.Thisisbasedonanovelapplicationofseveralnetworkperformanceinferencetechniques;(2)itmonitorsthestatusofpeersandconnectionsandreactstopeer/connectionfailureordegradationwithlowoverhead;6(3)itdynamicallyswitchesactivesendersandstandbysenders,sothatthecollectivenetworkperformanceoutoftheactivesendersremainssatisfactory.GnuStreamisareceiver-drivenP2PstreamingsystemwhichisbuiltontopofGnutella.Itfeaturesmulti-senderbandwidthaggregation,adaptivebuffercontrol,peerfailureordegradationdetectionandstreamingqualitymaintenance.GnuStreamisawareofthedynamicsandheterogeneityofP2Pnetworks,andleveragestheaggregatedstreamingcapacityofindividualpeersenderstoachievefullstreamingquality.GnuStreamalsoper-formsself-monitoringandadjustmentinthepresenceofpeerfailureandbandwidthdegradation.Recently,DONetimplementedamulti-sendermodelbyintroducingasimplerandstraightforwarddata-drivendesign,whichdoesnotmaintainanevenmorecomplexstructure.ThecoreofDONetisthedata-centricdesignofstreamingoverlay,andtheGossip-baseddatascheduleanddistributionalgorithm.Inthedata-centricdesignofDONet,anodealwaysforwardsdatatoothersthatareexpectingthedata,withnoprescribedroleslikefather/child,internal/external,andupstreaming/downstreaming,etc.Inotherwords,itistheavailabilityofdatathatguidestheflowdirections,whilenotaspecificoverlaystructurethatrestrictstheflowdirections.Thisdata-centricdesignissuitableforoverlaywithhighdynamicnodes.GossipalgorithmshaverecentlybecomepopularsolutionstomulticastmessagedisseminationinP2Psystems.Inatypicalgossipalgorithm,anodesendsanewlygeneratedmessagetoasetofrandomlyselectednodes;thesenodesdosimilarlyinthenextround,andsodoothernodesuntilthemessageisspreadtoall.Therandomchoiceofgossiptargetsachievesresiliencetorandomfailuresandenablesdecentral-izedoperations.Similartotherelatedwork,DONetemploysagossipingprotocolmembershipmanagement.Thedatasched-uleanddistributionmethodusedinDONetisalsopartiallymotivatedbythegossipconcept.Itusesasmartpartnerselectionalgorithmandalow-overheadschedulingalgorithmtointelligentlypulldatafrommultiplepartners,whichgreatlyreducesredundancy.Experimentsshowthat,comparedwithatree-basedoverlay,DONetcanachievemuchmorecontinuousstreamingwithcomparabledelay.7III.P2PSTREAMINGINCHINASincethefirstpracticalP2Pstreamingmediasystemwasborn,P2PstreamingservicehasexperiencedasignificantgrowthinChina,especiallyintheyear2005and2006.Accordingtoamarketreport,overmorethan12,000,000InternetusershaveaccessedP2PstreamingserviceordownloadedP2PstreamingsoftwareinChina.Itispredictedthatbytheendoftheyear2006,thisnumbercantakeagrowthtoabove25,000,000.Facingsuchalargepre-profitablemarket,tillnowthereareatleastover15organizationsthatareprovidingP2Porlikelystreamingservices.Withthemostrepresentative,PPlive,PPstream,Mysee,ROXandUUseehavetakenover80%ofthecurrentmarketshare.Intherestofthissection,wewillanalyzethemarketviewofP2Pstreamingmediaservice,andthengiveabriefintroductiontothecurrentmainstreamP2PmediastreamingsystemsdeployedinChina.TherearethreereasonswhichcauseP2PmediastreamingservicesopopularinChinainrecentyears.Firstly,thankstotherapidadvanceofaudioandvideocompressiontechnologies,userscaneasilyhaveaccesstostreamingmediainaverylowbitrate.Moreandmoremultimediaproductions,TVclips,andmoviesarefullofthewholeInternet.ThismakestheP2Pstreamingserviceproviderseasiertogetenoughmediasourcesforservicethanbefore.Withthevariousandabundantsupplyofmediacontents,serviceproviderscanattractmoreandmoreclients.Thelargertheclientnumber,theeasiertomaketestofsoftwareandservices.Secondly,comparedwiththetraditionalwayofwatchingvideofromtheInternet,suchasVOD,userscangetmoresatisfiedqualityofserviceincurrentbandwidth-limitednetworkenvironment.Finally,bythegrowthofuserssnetworkaccessbandwidth,theydemandonmoreluxuryexperience,notsimplyontextandpictures,butmoreonfluentandhigh-definitionvideos.UserstrendmakesalargeroomageforP2Pstreamingservicetogrow.AlthoughP2Pstreamingservicehasachievedaconsiderableuserexperienceanddefinitelyitwouldhaveabrightfuture,therearestillseveralissuesneedtopayattentionto.First,currentserviceprovidershavenotfoundanydistinctbusinessmodelsyet.Currently,almostallP2PsolutionvendorsareprovidingTVprogram/moviebroadcastingsfreeofcharge.Obviously,itisnotpracticalfortheserviceprovidertochargetheusersinthetimeof8promotingtheservice.Inthestartingperiod,developingusernumbersandgainingsubscribersarethekeypointsbutearningprofits.Second,P2Pstreamingserviceprovidersshouldfacethechallengeofcopyright.Aswevejustmentioned,someP2PvendorsprovideTV/moviebroadcastingusingthirdpartycontentswithoutcheckingtheirlegalstatus.Forlongtermdevelopment,serviceprovidersmustmakecooperationwithcontentproviderstomakeatwinwin.ThirdlyP2Pstreamingserviceprovidersmustfacethesur-veillancefromtheInternetserviceproviders(ISPs)andgovern-mentalauthorities.Ontheonehand,thepurposeofP2Pistomaximizetheusageofbandwidthresource,however,totheopposite,thebandwidthspewingcausedbysuchapplicationsoftenmakestheISPsfeelintolerable.ISPsusuallytakerejec-tiveactions,suchaslimitingtheapplicationbandwidthorevenblockingtheapplicationfromrunningontheInternet.However,limitingorblockingisnotthemostproperwaytosolvetheproblem,andtheconflictsbetweentheISPsandP2Pstreamingserviceproviderswillbeinexistenceforacertainwhile.Ontheotherhand,beingregardedasanewmediatrendontheweb,governmentalauthoritiesmusttakesurveillanceonP2Pstream-ingservicetoguaranteetheorderlinessoftheindustry.Bythetwosidesofsurveillance,P2Pstreamingserviceprovidersmustplaythegameprudentially.PP,investedbySoftBankHK,whichisacknowl-edgedasthenumberoneintermsofsubscribersinChina,wasfoundedintheearly2005.PPlivehasverystableplayingquality,anditseldomchangestheplayersstatetobufferingduringplaying.Whenwatchinganewchannel,theaveragewaitingtimefromsearchingtoplayingisabout35sto55s.PPliveprovidesover200channels,categorizedbyProvincialTVstations,Sports,Cartoon,Entertainment,HKfilms,Gaming,Moviesetc,butveryfewprogramsofoverseasTVstations.PPlivecurrentlyonlysupportsbroadcasting,andalmostalltheprogrambitrateisbetween300kbps400kbpswithmediacodeclikeWindowsMediaVideo(.wmv)orRealMedia(.rm).Itsprogramtimetableisbothshownonthewebsiteanddisplayedattheclientplayer.Advertisingcommercialsissupportedbytheclient.Worthtobementioned,PPlivebroadcastedSupergirlContestsin2005anditwasreportedthattheconcurrentonlineusershitarecordof500kforthefinalcontest.Thoughthepopularusersithas,somecontents9PPliveprovidesarelackofcopyright,whichmaybeahiddentroubleforitslongtermdevelopment.PPstream,whichisfoundedbytwoengineersinSichuanProvince,wasannouncedalsointheyear2005.ComparedwithPPlive,PPstreamhassimilarfunctionsbuthigher