张鹏毕设英文翻译.docx

张鹏毕设英文翻译.docx

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张鹏毕设英文翻译

西安邮电学院

毕业设计（译文）

论文题目：

P2P技术在多媒体服务中的应用研究

院（系）：

通信与信息工程学院

专业：

通信工程

班级：

通工0614班

学生姓名：

张鹏

导师姓名：

杨小宝职称：

工程师

Directory

Peer-to-Peer（P2P）communicationacrossmiddle-boxes1

3.1　Relaying1

3.2　Connectionreversal1

3.3　UDPholepunching3

3.3.1　PeersbehinddifferentNATs3

3.3.2　PeersbehindthesameNAT5

3.3.3　PeersseparatedbymultipleNATs6

3.3.4　Consistentportbindings7

3.4　UDPportnumberprediction7

3.5　SimultaneousTCPopen9

目录

利用代理服务器的P2P通信1

3.1　转发1

3.2　反向连接1

3.3　UDP打洞技术2

3.3.1　处于不同NAT之后的客户端通信2

3.3.2　客户端都处于相同的NAT之后3

3.3.3　客户端分别处于多层NAT之后4

3.3.4　保持端口绑定5

3.4　UPD端口号预言5

3.5　同时开放TCP连接6

Peer-to-Peer（P2P）communicationacrossmiddle-boxes

——TechniquesforP2PCommunicationovermiddle-boxes

Thissectionreviewsindetailthecurrentlyknowntechniquesforimplementingpeer-to-peercommunicationoverexistingmiddleboxes,fromtheperspectiveoftheapplicationorprotocoldesigner.

3.1　Relaying

Themostreliable,butleastefficient,methodofimplementingpeer-to-peercommunicationinthepresenceofamiddle-boxistomakethepeer-to-peercommunicationlooktothenetworklikeclient/servercommunicationthroughrelaying.Forexample,supposetwoclienthosts,AandB,haveeachinitiatedTCPorUDPconnectionswithawell-knownserverShavingapermanentIPaddress.　Theclientsresideonseparateprivatenetworks,however,andtheirrespectivemiddle　boxespreventeitherclientfromdirectlyinitiatingaconnectiontotheother（asinFIG1-1）.

FIG1-1ServerRelaying

Insteadofattemptingadirectconnection,thetwoclientscansimplyusetheserverStorelaymessagesbetweenthem.Forexample,tosendamessagetoclientB,clientAsimplysendsthemessagetoserverSalongitsalready-establishedclient/serverconnection,andserverSthensendsthemessageontoclientBusingitsexistingclient/serverconnectionwithB.

Thismethodhastheadvantagethatitwillalwaysworkaslongasbothclientshaveconnectivitytotheserver.Itsobviousdisadvantagesarethatitconsumestheserver'sprocessingpowerandnetworkbandwidthunnecessarily,andcommunicationlatencybetweenthetwoclientsislikelytobeincreasedeveniftheserveriswell-connected.TheTURNprotocol[TURN]definesamethodofimplementingrelayinginarelativelysecurefashion.

3.2　Connectionreversal

Thesecondtechniqueworksifonlyoneoftheclientsisbehindamiddlebox.Forexample,supposeclientAisbehindaNATbutclientBhasagloballyroutableIPaddress,asintheFIG1-2:

FIG1-2Connectionreversal

ClientAhasprivateIPaddress10.0.0.1,andtheapplicationisusingTCPport1234.ThisclienthasestablishedaconnectionwithserverSatpublicIPaddress18.181.0.31andport1235.NATAhasassignedTCPport62000,atitsownpublicIPaddress155.99.25.11,toserveasthetemporarypublicendpointaddressforA'ssessionwithS:

therefore,serverSbelievesthatclientAisatIPaddress155.99.25.11usingport62000.ClientB,however,hasitsownpermanentIPaddress,138.76.29.7,andthepeer-to-peerapplicationonBisacceptingTCPconnectionsatport1234.

NowsupposeclientBwouldliketoinitiateapeer-to-peercommunicationsessionwithclientA.BmightfirstattempttocontactclientAeitherattheaddressclientAbelievesitselftohave,namely10.0.0.1:

1234,orattheaddressofAasobservedbyserverS,namely155.99.25.11:

62000.Ineithercase,however,theconnectionwillfail.Inthefirstcase,trafficdirectedtoIPaddress10.0.0.1willsimplybedroppedbythenetworkbecause10.0.0.1isnotapubliclyroutableIPaddress.Inthesecondcase,theTCPSYNrequestfromBwillarriveatNAT-Adirectedtoport62000,butNATAwillrejecttheconnectionrequestbecauseonlyoutgoingconnectionsareallowed.

AfterattemptingandfailingtoestablishadirectconnectiontoA,clientBcanuseserverStorelayarequesttoclientAtoinitiatea"reversed"connectiontoclientB.ClientA,uponreceivingthisrelayedrequestthroughS,opensaTCPconnectiontoclientBatB'spublicIPaddressandportnumber.NATAallowstheconnectiontoproceedbecauseitisoriginatinginsidethefirewall,andclientBcanreceivetheconnectionbecauseitisnotbehinda　middle-box.

Avarietyofcurrentpeer-to-peersystemsimplementthistechnique.　Itsmainlimitation,ofcourse,isthatitonlyworksaslongasonlyoneofthecommunicatingpeersisbehindaNAT:

intheincreasinglycommoncasewherebothpeersarebehind　NAT-s,　themethodfails.　Becauseconnectionreversalisnotageneralsolutiontotheproblem,　itisNOTrecommendedasaprimarystrategy.Applicationsmaychoosetoattemptconnectionreversal,butshouldbeabletofallbackautomaticallyonanothermechanismsuchasrelayingifneithera"forward"nora"reverse"connectioncanbeestablished.

3.3　UDPholepunching

Thethirdtechnique,andtheoneofprimaryinterestinthisdocument,iswidelyknownas"UDPHolePunching."UDPholepunchingreliesonthepropertiesofcommonfirewallsandconeNATstoallowappropriatelydesignedpeer-to-peerapplicationsto"punchholes"throughthemiddle-boxandestablishdirectconnectivitywitheachother,evenwhenbothcommunicatinghostsmayliebehindmiddle　boxes.　Thistechniquewasmentionedbrieflyinsection5.1ofRFC3027[NAT-PROT],andhasbeeninformallydescribedelsewhereontheInternet[KEGEL]andusedinsomerecentprotocols[TEREDO,ICE].Asthenameimplies,unfortunately,thistechniqueworksreliablyonlywithUDP.

Wewillconsidertwospecificscenarios,andhowapplicationscanbedesignedtohandlebothofthemgracefully.　Inthefirstsituation,representingthecommoncase,twoclientsdesiringdirectpeer-to-peercommunicationresidebehindtwodifferentNAT-s.　Inthesecond,　thetwoclientsactuallyresidebehindthesameNAT,butdonotnecessarilyknowthattheydo.

3.3.1　PeersbehinddifferentNATs

SupposeclientsAandBbothhaveprivateIPaddressesandliebehinddifferentnetworkaddresstranslators.　Thepeer-to-peerapplicationrunningonclientsAandBandonserverSeachuseUDPport1234.AandBhaveeachinitiatedUDPcommunicationsessionswithserverS,　causingNATAtoassignitsownpublicUDPport62000forA'ssessionwithS,andcausingNATBtoassignitsport31000toB'ssessionwithS,respectively.

FIG1-3AandBcommunicationsessionswithserverS

NowsupposethatclientAwantstoestablishaUDPcommunicationsessiondirectlywithclientB.　IfAsimplystartssendingUDPmessagestoB'spublicaddress,138.76.29.7:

31000,thenNATBwilltypicallydiscardtheseincomingmessages（unlessitisafullconeNAT）,becausethesourceaddressandportnumberdoesnotmatchthoseofS,withwhichtheoriginaloutgoingsessionwasestablished.　Similarly,ifBsimplystartssendingUDPmessagestoA'spublicaddress,thenNATAwilltypicallydiscardthesemessages.

SupposeAstartssendingUDPmessagestoB'spublicaddress,however,andsimultaneouslyrelaysarequestthroughserverStoB,askingBtostartsendingUDPmessagestoA'spublicaddress.A'soutgoingmessagesdirectedtoB'spublicaddress（138.76.29.7:

31000）causeNATAtoopenupanewcommunicationsessionbetweenA'sprivateaddressandB'spublicaddress.Atthesametime,B'smessagestoA'spublicaddress（155.99.25.11:

62000）causeNATBtoopenupanewcommunicationsessionbetweenB'sprivateaddressandA'spublicaddress.OncethenewUDPsessionshavebeenopenedupineachdirection,clientAandBcancommunicatewitheachotherdirectlywithoutfurtherburdenonthe"introduction"serverS.

TheUDPholepunchingtechniquehasseveralusefulproperties.Onceadirectpeer-to-peerUDPconnectionhasbeenestablishedbetweentwoclientsbehindmiddle　boxes,eitherpartyonthatconnectioncaninturntakeovertheroleof"introducer"andhelptheotherpartyestablishpeer-to-peerconnectionswithadditionalpeers,minimizingtheloadontheinitialintroductionserverS.　Theapplicationdoesnotneedtoattempttodetectexplicitlywhatkindofmiddle　boxitisbehind,ifany[STUN],sincetheprocedureabovewillestablishpeer-to-peercommunicationchannelsequallywellifeitherorbothclientsdonothappentobebehindamiddle　box.TheholepunchingtechniqueevenworksautomaticallywithmultipleNATs,whereoneorbothclientsareremovedfromthepublicInternetviatwoormorelevels

3.3.2　PeersbehindthesameNAT

Nowconsiderthescenarioinwhichthetwoclients（probablyunknowingly）happentoresidebehindthesameNAT,andarethereforelocatedinthesameprivateIPaddressspace.ClientAhasestablishedaUDPsessionwithserverS,towhichthecommonNAThasassignedpublicportnumber62000.ClientBhassimilarlyestablishedasessionwithS,towhichtheNAThasassignedpublicportnumber62001.

FIG1-4A,BestablishedaUDPsessionwithserverS

SupposethatAandBusetheUDPholepunchingtechniqueasoutlinedabovetoestablishacommunicationchannelusingserverSasanintroducer.　ThenAandBwilllearneachother'spublicIPaddressesandportnumbersasobservedbyserverS,andstartsendingeachothermessagesatthosepublicaddresses.　ThetwoclientswillbeabletocommunicatewitheachotherthiswayaslongastheNATallowshostsontheinternalnetworktoopentranslatedUDPsessionswithotherinternalhostsandnotjustwithexternalhosts.　Werefertothissituationas"loopbacktranslation,"becausepacketsarrivingattheNATfromtheprivatenetworkaretranslatedandthen"loopedback"totheprivatenetworkratherthanbeingpassedthroughtothepublicnetwork.　Forexample,whenAsendsaUDPpackettoB'spublicaddress,thepacketinitiallyhasasourceIPaddress