计算机网络英文版课件：2-6 P2P应用

P2P应用WhatisP2P?“thesharingofcomputerresourcesandservicesbydirectexchangeofinformation”“P2Pisaclassofapplicationsthattakeadvantageofresources–storage,cycles,content,humanpresence–availableattheedgesoftheInternet.BecauseaccessingthesedecentralizedresourcesmeansoperatinginanenvironmentofunstableandunpredictableIPaddressesP2PnodesmustoperateoutsidetheDNSsystemandhavesignificant,ortotalautonomyfromcentralservers”“AdistributednetworkarchitecturemaybecalledaP2Pnetworkiftheparticipantsshareapartoftheirownresources.Thesesharedresourcesarenecessarytoprovidetheserviceofferedbythenetwork.Theparticipantsofsuchanetworkarebothresourceprovidersandresourceconsumers”WhatisP2P?Variousdefinitionsseemtoagreeon:sharingofresourcesdirectcommunicationbetweenequals(peers)nocentralizedcontrolClient/ServerArchitectureWellknown,powerful,reliableserverisadatasourceClientsrequestdatafromserverVerysuccessfulmodelWWW(HTTP),FTP,Webservices,etc.ServerClientClientClientClientInternetClient/ServerLimitationsScalabilityishardtoachievePresentsasinglepointoffailureRequiresadministrationUnusedresourcesatthenetworkedgeP2PsystemstrytoaddresstheselimitationsP2PArchitectureAllnodesarebothclientsandserversProvideandconsumedataAnynodecaninitiateaconnectionNocentralizeddatasource“TheultimateformofdemocracyontheInternet”“Theultimatethreattocopy-rightprotectionontheInternet”NodeNodeNodeNodeNodeInternetP2PNetworkCharacteristicsClientsarealsoservers

and

routersNodescontributecontent,storage,memory,CPUNodesareautonomous(noadministrativeauthority)Networkisdynamic:nodesenterandleavethenetwork“frequently”Nodescollaboratedirectlywitheachother(notthroughwell-knownservers)NodeshavewidelyvaryingcapabilitiesP2PGoalsandBenefitsEfficientuseofresourcesUnusedbandwidth,storage,processingpoweratthe“edgeofthenetwork”ScalabilityNocentralinformation,communicationandcomputationbottleneckAggregateresourcesgrownaturallywithutilizationReliabilityReplicasGeographicdistributionNosinglepointoffailureP2PGoalsandBenefits(cont.)EaseofadministrationNodesself-organizeBuilt-infaulttolerance,replication,andloadbalancingIncreasedautonomyAnonymity–PrivacynoteasyinacentralizedsystemDynamismhighlydynamicenvironmentad-hoccommunicationandcollaborationP2PApplicationsFilesharing(Napster,Gnutella,Kazaa,…)Multiplayergames(UnrealTournament,DOOM,…)Collaborativeapplications(ICQ,sharedwhiteboard,…)Distributedcomputation(Seti@home,…)Ad-hocnetworks(…)…P2PChallengesDecentralizationScalabilityandPerformanceAnonymityFairnessDynamismSecurityTransparencyFaultResilienceandRobustnessPopularfilesharingP2PSystemsNapster,Gnutella,Kazaa,Freenet,BitTorrent,…Largescalesharingoffiles.UserAmakesfiles(music,video,etc.)ontheircomputeravailabletoothersUserBconnectstothenetwork,searchesforfilesanddownloadsfilesdirectlyfromuserAIssuesofcopyrightinfringementResearchAreasPeerdiscoveryandgroupmanagementDataplacementandsearchingReliableandefficientfileexchangeSecurity/privacy/anonymity/trustFileDistribution:Server-ClientvsP2PQuestion:HowmuchtimetodistributefilefromoneservertoNpeers?usu2d1d2u1uNdNServerNetwork(withabundantbandwidth)File,sizeFus:serveruploadbandwidthui:peeriuploadbandwidthdi:peeridownloadbandwidthFiledistributiontime:server-clientusu2d1d2u1uNdNServerNetwork(withabundantbandwidth)FserversequentiallysendsNcopies:NF/us

timeclientitakesF/ditimetodownloadincreaseslinearlyinN(forlargeN)=dcs=max{

NF/us,F/min(di)

}iTimetodistributeF

toNclientsusingclient/serverapproachFiledistributiontime:P2Pusu2d1d2u1uNdNServerNetwork(withabundantbandwidth)Fservermustsendonecopy:F/us

timeclientitakesF/ditimetodownloadNFbitsmustbedownloaded(aggregate)fastestpossibleuploadrate:us+SuidP2P=max{

F/us,F/min(di),NF/(us+Sui)

}iServer-clientvs.P2P:exampleClientuploadrate=u,F/u=1hour,us=10u,dmin≥usP2PReviewTwokeyfunctionsofP2PsystemsSharingcontentFindingcontentSharingcontentDirecttransferbetweenpeersAllsystemsdothisStructuredvs.unstructuredplacementofdataAutomaticreplicationofdataFindingcontentCentralized(Napster)Decentralized(Gnutella)Probabilisticguarantees(DHTs)IssueswithP2PFreeRiding(FreeLoading)