P2P流媒體傳輸外文翻譯文獻P2P流媒體傳輸外文翻譯文獻(文檔含中英文對照即英文原文和中文翻譯)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.譯文：基于P2P的流媒體傳輸P2P的構(gòu)建無論我們對P2P如何定義，P2P都會作為一個新穎的且行之有效的處理方式出現(xiàn)在我們面前。許多人并不認同“新穎”這一觀點，人們對于P2P的興趣僅僅是在集中的和分散的系統(tǒng)之間搖擺。這個周期可以描述為：分別處理，消除瓶頸集中管理，提高效率盡管如此，因特網(wǎng)上可用資源的特別拐點，在此時已經(jīng)使P2P系統(tǒng)顯示出了明顯的延展性和資源互換能力。本文簡要介紹了一種不同以往的看待系統(tǒng)中可用資源的方式，文中還指出P2P系統(tǒng)在內(nèi)容傳送方面的適用性。P2P技術(shù)廣泛用于描述各種各樣的網(wǎng)絡(luò)系統(tǒng)，這些網(wǎng)絡(luò)系統(tǒng)一般運行于顯示、會議和應用層面上，盡管自組網(wǎng)以及其他系統(tǒng)在物理層面上應用了相同的概念。特別地，P2P分為內(nèi)容傳送、協(xié)作、緩沖、業(yè)務(wù)流程自動化、供應鏈管理、網(wǎng)格計算、分布式計算、用戶間交流、數(shù)據(jù)部署、用戶通信、用戶社區(qū)、自組網(wǎng)和因特網(wǎng)本身。或許P2P在構(gòu)建方面應該考慮更多一些，而不是一個特定的技術(shù)或業(yè)務(wù)。一方面來看，如果一個問題要用P2P來解決，是要問，“如果每個客戶在這一系統(tǒng)中還可以提供他們所需的服務(wù)，是否能從中獲利？”但是并非總是要求有利可圖。例如，許多數(shù)據(jù)庫應用需要集中注意力在管理簡易和安全方面。特殊的P2P范例人們常說的ICQ與Napster就是開創(chuàng)P2P先河的兩個例子。二者為DNS提供了替換系統(tǒng)并為之命名，（部分P2P定義的一個屬性）同時二者還為用戶提供直接交流的服務(wù)，提供與P2P相聯(lián)系的“人對人”的交流方式。目前，有各種其他的系統(tǒng)提供較早的關(guān)于轉(zhuǎn)向P2P系統(tǒng)的例子。如果P2P被看作從客戶服務(wù)端到等功率的計算機系統(tǒng)，那么任何提供更高比率的服務(wù)器系統(tǒng)均可視為P2P。Quake曾經(jīng)是（現(xiàn)在仍然是）一種網(wǎng)上流行的三維游戲。Quake（以及后來的Quakeworld）提供的客戶端—服務(wù)器系統(tǒng)用于同步視頻游戲。這些服務(wù)器通常是高帶寬，高功率系統(tǒng)，但由于網(wǎng)上視頻游戲的需求，客戶端也是高帶寬，高功率的系統(tǒng)。在Quake中，服務(wù)器被嵌入在客戶應用中，用以模糊區(qū)分客戶和服務(wù)器，并允許任何節(jié)點（由用戶選擇）被作為客戶或服務(wù)器。上千萬在線的Quake球員中，存在著數(shù)以萬計的服務(wù)器，并開始以相同比例的客戶伺服器在互聯(lián)網(wǎng)中拉平。Shoutcast是作為一個插件安裝在一款受歡迎的MP3播放機上，使得流媒體MP3音頻實現(xiàn)多HTTP連接。用戶可以用Shoutcast創(chuàng)造一個電臺（在互聯(lián)網(wǎng)上），廣播自己收藏的MP3。這項服務(wù)允許從客戶的應用，以及增加方便的角度，為最終用戶配置這個服務(wù)器。P2P資源存儲CPU帶寬存儲和CPU循環(huán)往往是P2P系統(tǒng)中最常被引用的兩種資源。不過，有一些孤立資源，可通過P2P方式更好地描述一系列的優(yōu)化。帶寬是一種瞬時資源，即不可再造的。同理，飛機上的空座位是不能被取回再作資源的。當帶寬可以得到且未被使用時，意味著它正在失去時效。消費者網(wǎng)路擴展的第一階段涉及客戶的帶寬和服務(wù)器帶寬，它們之間存在巨大差距，但由于寬帶平均分配了這一資源，使之成為P2P系統(tǒng)。撥號潛伏期/近似性撥號可以看成是一種資源。當P2P中的‘p’做“人”來講（如在即時通信的情景