P2P流媒体传输外文翻译文献

P2P流媒体传输外文翻译文献P2P流媒体传输外文翻译文献(文档含中英文对照即英文原文和中文翻译)原文：Peer-to-PeerStreamingMediaDeliveryPeer-to-PeerArchitectureWhateverdefinitionshavebeenputuponit,peer-to-peerisaneffectiverallyingcryforanewwayofdoingthings.Manydonotconsideritanewway;ithasbeenarguedthatthecurrentinterestinpeer-to-peerismerelythependulumswingbetweencentralizedanddecentralizedsystems.Thatcyclecanbedescribedas:1.Decentralizetoremovebottlenecks2.CentralizetoincreaseefficiencyNonetheless,theparticularinflectionpointofresourcesavailableontheInternetatthistimehasallowedpeer-to-peersystemstoexhibitremarkablescalabilityandresourceexchange.Thispaperbrieflydescribesadifferentwayoflookingattheresourcesavailableinthesesystems;itthenillustratestheapplicabilityofpeer-to-peersystemstocontentdelivery.Peer-to-peerisusedbroadlytodescribeavarietyofnetworksystemsthatgenerallyrunatthepresentation,session,andapplicationlayers,althoughad-hocnetworksandothersystemsusethesameconceptsfromthephysicallayerup.Specifically,peer-to-peerspanscontentdelivery,collaboration,caching,businessprocessautomation,supplychainmanagement,gridcomputing,distributedcomputation,business-to-businessexchanges,datadeployment,usertousercommunication,usercommunities,ad-hocnetworks,andtheInternetitself.Perhapspeer-to-peershouldbeconsideredmoreofanarchitecturalapproachthanaspecifictechnologyorbusinessapproach.Anwayofseeingifaproblemissusceptibletoapeer-to-peerapproachistoask,“Ifeveryclientinthissystemcouldalsoprovidetheservicetheyconsume,wouldtherebeabenefit?”Itisnotalwaysthecasethatthereisabenefit;manydatabaseapplicationsrequirecentralizationforsecurityandsimplicityofadministration,forexample.UnusualPeer-to-PeerExamplesTheoft-citedICQandNapsteraretwopioneeringpeer-to-peerexamples.BothprovideanalternativesystemtoDNSfornaming,(anattributeofsomepeer-to-peerdefinitions)andbothprovidetheabilityforuserstodirectlycommunicate,providingthe“person-to-person”aspectalsoassociatedwithpeer-to-peer.Thereare,however,avarietyofothersystemsprovidingearlierexamplesoftheshifttowardspeer-to-peersystems.Ifpeer-to-peerisconsideredasaqualitywithagradientscale,rangingfromclient-servertoamoreequilateralpowerofcomputingsystems,anysystemthatprovidesahigherratioofserverstoclientscouldberegardedaspeer-to-peer.Quakewas(andremains)a3donlinemultiplayervideogame.Quake(andlaterQuakeWorld)providedaclient-serversystemforsynchronizedvideogaming.Theserverswereusuallyhighbandwidth,highpoweredsystems,butduetothedemandsofonlinevideogaming,soweretheclients.AndinQuake,theserverwasactuallyembeddedintheclientapplication,blurringthedistinctionbetweenclientsandserversandallowinganynodetoact(byuserselection)asaclientoraserver.AmongmillionsofonlineQuakeplayers,thereeventuallyexistedtensofthousandsofserversontheInternet,andsotheratioofclientstoserversbegantoevenout.ShoutCastwasdesignedasaplug-intoapopularMP3playerthatenabledlivestreamingofMP3audiooveranHTTPconnection.UserscoulduseShoutCasttocreatearadiostation(ontheInternet)basedontheirMP3collection.Thisallowedservingfromaclientapplication,andincreasedtheeaseforanenduserinconfiguringthisserver.Peer-to-peerResourcesStorageCPUBandwidthStorageandCPUcyclestendtobethetworesourcesmostcommonlycitedinpeer-to-peersystems.Thereare,however,severalresourcesthat,isolated,canbeusedtobetterdescribetherangeofoptimizationsavailablethroughpeer-to-peerapproaches.Bandwidthisaresourcethatistransientandnon-recapturable.Inthesamewaythatunusedairlineseatscannotberecapturedasaresource,bandwidth,whenavailableandunused,islosttotime.ThefirststageofconsumerInternetexpansioninvolvedagreatdisparitybetweenclientbandwidthandserverbandwidth,butasbroadbandequalizesthisresource,itbecomesavailabletoapeer-to-peersystem.PresenceLatency/ProximityPresencecanbeviewedasaresource.Whenthe‘p’inp2pstandsfor“person”(asininstantmessagingscenarios),presenceistheresourceofthatpersonbeingonlineandavailableforcommunicationatthattime.Thisenablesonlinecollaborationbecauseitprovidesataglancenotificationofavailability.LatencyandproximityaretworelativelyunremarkedresourcesthatarekeytointeractivesimulationsontheInternet.Quakeprovidesanexampleofalargenumberofservers(Quakeservers)locatedwithanalternativenamespacesystem(GameSpy)andthensortedbylatency.ThetensofthousandsofQuakeserversprovidedapooloftheservicecalled“Quake”,butthatservicewasineffectiveifthenumberofhopsbetweentheserverandtheclients,orthelatency,orthepacketloss,wastoohigh.Itwasnotthestoragespaceofthemachinesthatwasbeingutilized(althoughitwassomeoftheCPUcycles),butitwastheproximityoftheservertotheclientthatwasthepreciousresource.Napsterusedonlymegabytesofclient-servertraffictomanage,direct,andcontrolterabytesofpeer-to-peertraffic.Thestorageofthesepeerswasnotable,buttherealfeatwasoffloadingthebandwidthrequirementstothepeersconsumingtheresourcesandcoordinatingtheirrelativelyseamlessinterchange.AcentralizedNapsterwouldhavebeentechnicallytrivialtoimplement,butprohibitivelycostlyandremarkablydifficulttoscale.Aconsistentthemeinpeer-to-peersystemstodateisthattheyputadditionalcodeattheclientlevelandthuswhereitcandodifferentthingsthanifitwascentralized.Astrengthofpeer-to-peersystemsisthattheydistributecodethatcanprovideservicesatamorestrategiclocation.Forinstance,somepeer-to-peersystemsroutetrafficbetweenpeers.TheseserversprovideCPUcycles(toperformtheservice),proximity(iftheroutingalgorithmisbasedonlowhopcountforinstance),andbandwidth(byprovidingtherouting).Thiscombinationofvariousresourcesshowswhywebservicesnowtendtobeincludedindiscussionsofpeer-to-peerarchitectures.ContentDeliveryCostsTheprimaryresourcecontentionontheInternetisoverbandwidth.Thecostsofbandwidth,especiallybandwidthwiththequalityofservicegoalsnecessarytosupportonlineaudioandvideo,doesnotdropasdramaticallyasthecostofcomputerhardware.Anotherinterestingaspectofbandwidthisthatconsumerstendtopayaflatfeeoralowfeeforamoderateamountofbroadbandbandwidth,whereasenterprisetendstopaylarger,variablecostsfortheirbandwidth.Thesetwofactorspresentanopportunityforsystemsthatcansubstitutelowcosthardwareintohighervaluebandwidth,orcansubstitutefixed-costconsumerbandwidthforvariablecostenterprisebandwidth.Enoughlarge,earlystreamingcompanieshavefailedbecauseoftheoverwhelmingcostofstreamingbandwidth.Would-beInternet“televisionstations”weretechnicallyfeasiblebutcompletelyimpracticalfromacoststandpoint.ThesupplychainofInternetvideoinparticularisquitebroken:Contentprovidersareslowtoadvertisetheirservicesbecausetheycannotaffordthebandwidthcostsofanincreasedaudience.Similarly,bandwidthproviderscatertocustomerswhotendnottousetheirbandwidth.Thissituationtendstocreateunprofitable,shrinkingcontentproviderswhopaytoomuchforbandwidththeydonotuse.Thisbandwidthneedappliestobothstatic(webpage)anddynamic(streamingmedia)content;shoppingforbandwidthandconstraininghecostscanbedifficultandcanresultinhighlyvariablequalityofserviceonthepartofcontentproviders.Thisisanopportunityforpeer-to-peertechnology.译文：基于P2P的流媒体传输P2P的构建无论我们对P2P如何定义，P2P都会作为一个新颖的且行之有效的处理方式出现在我们面前。许多人并不认同“新颖”这一观点，人们对于P2P的兴趣仅仅是在集中的和分散的系统之间摇摆。这个周期可以描述为：分别处理，消除瓶颈集中管理，提高效率尽管如此，因特网上可用资源的特别拐点，在此时已经使P2P系统显示出了明显的延展性和资源互换能力。本文简要介绍了一种不同以往的看待系统中可用资源的方式，文中还指出P2P系统在内容传送方面的适用性。P2P技术广泛用于描述各种各样的网络系统，这些网络系统一般运行于显示、会议和应用层面上，尽管自组网以及其他系统在物理层面上应用了相同的概念。特别地，P2P分为内容传送、协作、缓冲、业务流程自动化、供应链管理、网格计算、分布式计算、用户间交流、数据部署、用户通信、用户社区、自组网和因特网本身。或许P2P在构建方面应该考虑更多一些，而不是一个特定的技术或业务。一方面来看，如果一个问题要用P2P来解决，是要问，“如果每个客户在这一系统中还可以提供他们所需的服务，是否能从中获利？”但是并非总是要求有利可图。例如，许多数据库应用需要集中注意力在管理简易和安全方面。特殊的P2P范例人们常说的ICQ与Napster就是开创P2P先河的两个例子。二者为DNS提供了替换系统并为之命名，（部分P2P定义的一个属性）同时二者还为用户提供直接交流的服务，提供与P2P相联系的“人对人”的交流方式。目前，有各种其他的系统提供较早的关于转向P2P系统的例子。如果P2P被看作从客户服务端到等功率的计算机系统，那么任何提供更高比率的服务器系统均可视为P2P。Quake曾经是（现在仍然是）一种网上流行的三维游戏。Quake（以及后来的Quakeworld）提供的客户端—服务器系统用于同步视频游戏。这些服务器通常是高带宽，高功率系统，但由于网上视频游戏的需求，客户端也是高带宽，高功率的系统。在Quake中，服务器被嵌入在客户应用中，用以模糊区分客户和服务器，并允许任何节点（由用户选择）被作为客户或服务器。上千万在线的Quake球员中，存在着数以万计的服务器，并开始以相同比例的客户伺服器在互联网中拉平。Shoutcast是作为一个插件安装在一款受欢迎的MP3播放机上，使得流媒体MP3音频实现多HTTP连接。用户可以用Shoutcast创造一个电台（在互联网上），广播自己收藏的MP3。这项服务允许从客户的应用，以及增加方便的角度，为最终用户配置这个服务器。P2P资源存储CPU带宽存储和CPU循环往往是P2P系统中最常被引用的两种资源。不过，有一些孤立资源，可通过P2P方式更好地描述一系列的优化。带宽是一种瞬时资源，即不可再造的。同理，飞机上的空座位是不能被取回再作资源的。当带宽可以得到且未被使用时，意味着它正在失去时效。消费者网路扩展的第一阶段涉及客户的带宽和服务器带宽，它们之间存在巨大差距，但由于宽带平均分配了这一资源，使之成为P2P系统。拨号潜伏期/近似性拨号可以看成是一种资源。当P2P中的‘p’做“人”来讲（如在即时通信的情景