计算机网络复习-计算机网课件ch2p2p

Peer-to-Peer(P2P)networksandapplicationsClient/ServerWellpowerful,serverisadataClientsrequestfromVerysuccessfulWWW(HTTP),Webservices,

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Client/ServerScalabilityishardtoPresentsasinglepointofRequiresUnusedresourcesatthenetworkP2PsystemstrytoaddresstheseWhatis“P2Pisaclassofapplicationsthattakeadvantageofresources–storage,cycles,content,humanpresence–availableattheedgesoftheInternet.Itis“AdistributednetworkVariousdefinitionsseemtoagreesharingofdirectcommunicationbetweenequalsnocentralizedP2PAllnodesarebothclientsandserversProvideandAnynodecaninitiateNodesareNocentralizeddata

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P2P&BecauseaccessingthesedecentralizedresourcesmeansoperatinginanenvironmentofunstableandunpredictableIPaddresses,P2PnodesmustoperateoutsidetheDNSsystemandhavesignificant,ortotalautonomyfromcentralservers”（此时不需要域名，直接通过**ComparedwithClient-serverFile学生

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学生ComparedwithClient-serverFile与传统的客户端-服务器方式相学生学生 ？ 学生学生P2PFile学生学生学生学生学生学生学生学学生学生学生 学5 P2P 学5 率作理统率作理统P2PFileDistributionP2PScalabilityofP2PArchitectureP2PBitTorrentSearchingforInformationinaP2PCommunityCentralizedIndex集中式管理QueryFloodingHierarchicalOverlayCaseStudy:P2PInternetTelephonywithPeer-to-PeerContentPopularfilesharingP2PNapster,Gnutella,Kazaa,LargescalesharingofUserAmakesfiles(music,video,etc.)ontheircomputeravailabletoothersUserBconnectstothenetwork,searchesforfilesanddownloadsfilesdirectlyfromuserAIssuesofcopyrightFileDistribution:Server-ClientvsQuestion:HowmuchtimetodistributefilefromoneservertoNpeers?

us:serveruploadsizesizeuNNetworkabundantdi:peeriF F Network(withabundantbandwidth)serversequentiallysendsNcopies:NF/usclientitakesF/ditimetodownloadTimetodistributeFtoNclientsusingclient/server

=dcs=max{NF/us,F/min(di)iincreaseslinearlyini(forlargeF F Network(withabundantbandwidth)servermustsendonecopy:F/ustimetoclientitakesF/ditoNFbitsmustbedownloaded(aggregate)fastestpossibleuploadrate:us+ddP2P=max{F/us,F/min(di)i,NF/(us+ui)Server-clientvs.P2P:3Client-21005NMinimumDistributionClientuploadrate=3Client-21005NMinimumDistributionFiledistribution:Efficientcontentdistributionsystemusingfileswarming.Usuallydoesnotperformallthefunctionsofatypicalp2psystem,likesearching.CacheLogicestimated(around2003orso)thatBitTorrentTrafficaccountsforroughly35%ofalltrafficontheInternet.BitTorrent比特型例子BT原理普通的HTTP/FTP下载使用TCP/IP协议，BitTorrent协议是构于TCP/IP协议之上的一个P2P文件传输协议BitTorrent协议本身也包含了很多具体的内容协议和扩展协议，并不断扩充中BitTorrent比特根据BitTorrent协议，文件发布者会根据要发布的文件生成提一个.torrent文件，即种子文件，也简称为“种子”Tracker：收集下载者信息的服务器，并将此信息提供给其载者，使下载者们相互连接起来，传输数做种：发布者提供下载任务的全部内容的行为；下载者下载完后继续提供给他人下载的行为BitTorrent比特.torrent文件就是被下载文件的“索引Filedistribution:P2Pfiletracker:tracksparticipatingin

torrent:groupofpeersexchangingchunksofafileobtainof

filedividedinto256KBpeerjoininghasnochunks,butwillaccumulatethemoverregisterswithtrackertogetlistofconnectstosubsetofpeerswhiledownloading,peeruploadschunkstopeersmaycomeandoncepeerhasentirefile,itmay(selfishly)leave(altruistically)BT:FileToshareafileorgroupoffiles,apeerfirsta.torrentfile,asmallfilethatmetadataaboutthefilestobeshared,Informationaboutthetracker,thecomputercoordinatesthefilePeersfirstobtaina.torrentfile,andthenconnecttothespecifiedtracker,whichtellsthemfromwhichotherpeerstodownloadthepiecesofthefile.OverallAWebA

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BT:The.torrentTheURLofthePieces<hash1,hash2,…,hashPieceNameoftheLengthoftheBT:TheIPaddress,port,peerStateinformation(CompletedorReturnsarandomlistofBT:Pullingatanygiventime,differentpeershavesubsetsoffileperiodically,apeer(Alice)askseachneighborlistofchunksthattheyAlicesendsrequestsforhermissingBT:PiecesandSub-Apieceisbrokenintosub-pieces...typically16KBinsizeUntilapieceisassembled,onlydownloadthesub-piecesofthatpieceonlyThispolicyletspiecesassembleBT:WhentransferringdataoverTCP,alwayshaveseveralrequestspendingatonce,toavoidadelaybetweenpiecesbeingsent.Atanypointintime,somenumber,typically5,arerequestedsimultaneously.Everytimeapieceorasub-piecearrives,arequestissentBT:PieceTheorderinwhichpiecesareselectedbydifferentpeersiscriticalforgoodperformanceIfaninefficientpolicyisused,thenpeersmayendupinasituationwhereeachhasallidenticalsetofeasilyavailablepieces,andnoneofthemissingones.Iftheoriginalseedisprematurelytakendown,thenthefilecannotbecompletelydownloaded!Whatare“goodpolicies?”BT:ChunkStrictFirstRarestGeneralRandomFirstSpecialcase,attheEndgameSpecialRarestPieceDeterminethepiecesthataremostrareamongyourpeers,anddownloadthosefirst.ThisensuresthatthemostcommonlyavailablepiecesarelefttilltheendtoBT:SendingAlicesendschunkstofourneighborssendingherchunksatthehighestre-evaluatetop4every10every30secs:randomlyselectanotherpeer,startssendingchunksnewlychosenpeermayjointop“optimisticallyunchoke…障碍Chokingisatemporaryrefusaltoupload.ItisoneofBitTorrent’smostpowerfulideatodealwithfreeriders(thosewhoonlydownloadbutneverTodiscovercurrentlyunusedconnectionsarethantheonesbeingToprovideminimalservicetonewWithhigheruploadrate,canfindbettertradingpartners&getfileWithhigheruploadrate,canfindbettertradingpartners&getfilefaster!Alice“optimisticallyunchokes” esoneofBob’stop-fourproviders;Bob esoneofAlice’stop-fourUpload-OnlyOncedownloadiscomplete,apeerhasnodownloadratestouseforcomparisonnorhasanyneedtousethem.Thequestionis,whichnodestouploadto?Policy:Uploadtothosewiththebestuploadrate.Thisensuresthatpiecesgetreplicatedfaster,andnewseedersarecreatedfast下面 软件界搜索BT资以狗狗影视为搜索BT资以狗狗影视为P2PFileDistributionP2PScalabilityofP2PArchitectureP2PBitTorrentSearchingforInformationinaP2PCommunityCentralizedIndex集中式管理QueryFloodingHierarchicalOverlayCaseStudy:P2PInternetTelephonywithP2P:searchingforIndexinP2Psystem:mapsinformationtopeerlocation(location=IPaddress&portnumber).Filesharing(ege-Indexdynamicallytracksthelocationsoffilesthatpeersshare.PeersneedtoindexwhattheyPeerssearchindextodeterminewherefilescanbefound

InstantIndexmapsusernamestolocations.WhenuserstartsIMapplication,itneedstoinformindexofitsPeerssearchindextodetermineIPaddressofuser.Unstructured(queryStructured(DistributedHashCentralizedoriginal“Napster”whenpeerconnects,informscentralIP Alicequeriesfor“Hey Alicerequestsfilefrom

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3Centralized filetransferisdecentralized,filetransferisdecentralized,butlocatingcontentishighly Lowper-nodeLimitedbandwidthShortlocationHighsuccessFaultSinglepointof

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Napster:howdoesitApplication-level,client-serverprotocoloverpoint-pointFourConnecttoNapsterUploadyourlistoffiles(push)toGive tosearchthefulllistSelect“best”ofcorrectanswers.Filelistis

searchatUserpingshoststhathavedata.Looksforbesttransferrate.

UserretrievesQueryfullynocentralusedbyEachpeerindexesthefilesitmakesavailableforsharing(andnootherfiles)

overlaynetwork:edgebetweenpeerXandYifthere’saTCPallactivepeersedgesformoverlayedge:virtualphysical)givenpeertypicallyconnectedwith<10overlayneighborsGnutella:QueryFileQuerymessagesentoverexistingTCPpeersQuerysentover

Gnutella:PeerjoiningpeerAlicemustfindanotherpeerinGnutellanetwork:uselistofcandidatepeersAlicesequentiallyattemptsTCPconnectionswithcandidatepeersuntilconnectionsetupwithBobFlooding:AlicesendsPingmessagetoBob;BobforwardsPingmessagetohisoverlayneighbors(whothenforwardtotheirneighbors….)peersreceivingPingmessagerespondtoAlicewithPongmessageAlicereceivesmanyPongmessages,andcanthensetupadditionalTCPconnectionsSearchingbyIfyoudon’thavethefileyouwant,query7ofyourIftheydon’thaveit,theycontact7oftheirneighbors,foramaximumhopcountof10.Requestsareflooded,thereisnotreeNoloopingbutpacketsmaybereceivedtwice.Reversepathfool.*TTL=

TTL=TTL=TTL=YGnutellaLimitedper-nodeFaultHighbandwidthLongtimetolocateNoguaranteeonsuccessPossiblyunbalanced

Hierarchicalbetweencentralizedindex,queryfloodingeachpeeriseitherasupernodeorassignedtoasupernodeTCPconnectionbetweenpeeranditssupernode.TCPconnectionssomepairsofsuperSupernodetracksinits

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