计算机网络英文版课件：2-7 BitTorrent

BitTorrentFiledistribution:BitTorrentEfficientcontentdistributionsystemusingfileswarming.Usuallydoesnotperformallthefunctionsofatypicalp2psystem,likesearching.CacheLogicestimated(around2003orso)thatBitTorrentTrafficaccountsforroughly35%ofalltrafficontheInternet.Author:BramCohenFiledistribution:BitTorrenttracker:trackspeersparticipatingintorrenttorrent:groupofpeersexchangingchunksofafileobtainlistofpeers

tradingchunkspeerBitTorrentfiledividedinto256KBchunks.peerjoiningtorrent:hasnochunks,butwillaccumulatethemovertimeregisterswithtrackertogetlistofpeers,connectstosubsetofpeers(“neighbors”)whiledownloading,peeruploadschunkstootherpeers.peersmaycomeandgooncepeerhasentirefile,itmay(selfishly)leaveor(altruistically)remainBT:FilesharingToshareafileorgroupoffiles,apeerfirstcreatesa.torrentfile,asmallfilethatcontains:metadataaboutthefilestobeshared,andInformationaboutthetracker,thecomputerthatcoordinatesthefiledistribution.Peersfirstobtaina.torrentfile,andthenconnecttothespecifiedtracker,whichtellsthemfromwhichotherpeerstodownloadthepiecesofthefile.BT:The.torrentfileTheURLofthetrackerPieces<hash1,hash2,…,hashn>PiecelengthNameofthefileLengthofthefileBT:TheTrackerIPaddress,port,peeridStateinformation(CompletedorDownloading)ReturnsarandomlistofpeersBT:PullingChunksatanygiventime,differentpeershavedifferentsubsetsoffilechunksperiodically,apeer(Alice)askseachneighborforlistofchunksthattheyhave.AlicesendsrequestsforhermissingchunksBT:PipeliningWhentransferringdataoverTCP,alwayshaveseveralrequestspendingatonce,toavoidadelaybetweenpiecesbeingsent.Atanypointintime,somenumber,typically5,arerequestedsimultaneously.Everytimeapieceorasub-piecearrives,anewrequestissentout.BT:PieceSelectionTheorderinwhichpiecesareselectedbydifferentpeersiscriticalforgoodperformanceIfaninefficientpolicyisused,thenpeersmayendupinasituationwhereeachhasallidenticalsetofeasilyavailablepieces,andnoneofthemissingones.Iftheoriginalseedisprematurelytakendown,thenthefilecannotbecompletelydownloaded!Whatare“goodpolicies?”BT:ChunkSelectionStrictPriorityFirstPriorityRarestFirstGeneralruleRandomFirstPieceSpecialcase,atthebeginningEndgameModeSpecialcaseBT:SendingChunkstit-for-tatAlicesendschunkstofourneighborscurrentlysendingherchunksatthehighestrate

re-evaluatetop4every10secsevery30secs:randomlyselectanotherpeer,startssendingchunksnewlychosenpeermayjointop4“optimisticallyunchoke”BT:ChokingChokingisatemporaryrefusaltoupload.ItisoneofBitTorrent’smostpowerfulideatodealwithfreeriders(thosewhoonlydownloadbutneverupload).Tit-for-tatstrategyisbasedongame-theoreticconcepts.BitTorrent:Tit-for-tat(1)Alice“optimisticallyunchokes”Bob(2)AlicebecomesoneofBob’stop-fourproviders;Bobreciprocates(3)BobbecomesoneofAlice’stop-fourprovidersWithhigheruploadrate,

canfindbettertradingpartners&getfilefaster!Upload-OnlymodeOncedownloadiscomplete,apeerhasnodownloadratestouseforcomparisonnorhasanyneedtousethem.Thequestionis,whichnodestouploadto?Policy:Uploadtothosewiththebestuploadrate.Thisensuresthatpiecesgetreplicatedfaster,andnewseedersarecreatedfastQuestionsaboutBTWhichfeaturescontributetotheefficiencyofBitTorrent?Whatistheeffectofbandwidthconstraints?IstheRa