Point-to-point transmission

Introduction

Apurepeer-to-peernetworkhasnoconceptofclientorserver,onlyequalnodesatthesamelevel,andatthesametimeactsasaclientandserverforothernodesonthenetwork.Thisnetworkdesignmodelisdifferentfromtheclient-servermodel,inwhichcommunicationusuallygoestoandfromacentralserver.Somefunctions(suchassearch)ofsomenetworks(suchasNapster,OpenNAP,orIRC@find)useaclient-serverstructure,anduseaP2Pstructureforotherfunctions.NetworkslikeGnutellaorFreenetuseapureP2Pstructuretoaccomplishalltasks.

Point-to-pointtransmissionstructure

Theso-calledpoint-to-pointtransmissionisalsostore-and-forwardtransmission.Itisapoint-to-pointconnectionmethodthatconnectsvariouscomputers.Thenetworkstructureofthispoint-to-pointtransmissionisusuallyUsedinlong-distancenetworksandmetropolitannetworks,thetopologicalstructureofthenetworkincludesstar,ring,tree,andmesh.

1.Star

Inthecaseofastarstructure,eachnodedeviceisconnectedtothecentralnode(startop)throughaconnectingline(suchasacable),andthedistancebetweenadjacentnodesisCommunicationmustpassthroughthecentralnode.Thiskindofstarstructureismainlyusedforhierarchicalmaster-slavenetwork,adoptingcentralizedcontrol,andthecentralnodeisthecontrolcenter.Theadvantageofthisstructureisthatthecostislowwhenaddingnodes,butthedisadvantageisthatwhenthecentralnodeequipmentisoutoforder,theentiresystemisparalyzed,sothereliabilityispoor.

2.Tree-shaped

Thiskindoftree-shapednetworkisalsocalledacentralizednetworkofmulti-processingcenters.Itscharacteristicisthatalthoughtherearemultiplecomputingcentersinthenetwork,thereislittleinformationflowbetweeneachcomputingcenter.Themaininformationflowisbetweentheterminalandtheconnectedcomputer,andbetweenthecomputingcentersupanddownaccordingtothetree-likeappearancestructure.,Eachmainloanmachinecanhandlebusinessindependently,butthetopmaincomputerhastheabilitytomanagetheentirenetwork.Theso-calledunifiedmanagementishierarchicalmanagementthroughthemaincomputersatalllevels.Inthissense,itisacentralizednetworkbasedonhierarchicalmanagement,suitableforvariousstatisticalmanagementsystems.Theadvantageofthisnetworkconfigurationisthatthecommunicationlineconnectionisrelativelysimple,thenetworkmanagementsoftwareisnotcomplicated,andthemaintenanceisconvenient.Thedisadvantageisthattheresourcesharingabilityispoorandthereliabilityispoor.Ifthehostfails,theterminalsconnectedtothehostcannotwork..

3.Ring

Intheringstructure,thepositionofeachmaincomputertobuythemachine,etc.,thecommunicationequipmentandlinesinthenetworkarerelativelyeconomical.Theinformationflowinthenetworkisdirectional.Sincethereisnochannelselectionproblem,thenetworkmanagementsoftwareisrelativelysimple.Thedisadvantageofthisstructureisthatthenetworkthroughputispoor,anditisnotsuitableforlargeinformationtraffic.Itissuitableforasmall-scalenetwork,forexample,alocalareanetworkinaschool.

4.Meshshape

Thiskindofnetworkstructuredoesnothavestrictrulesandconfigurationsforthedistributionofpoints,andtherearemultiplelinksbetweennodestochoosefrom.Therefore,whenacertainlineornodefails,itwillnotaffectthenormaloperationoftheentirenetwork,andhashighreliability.Itshowsthecharacteristicsofthenetworkintermsofcost,throughput,responsetimeandreliability.Eachnodeisusuallyconnectedtomultipleothernodes,soeachnodehasthefunctionsofchannelselectionandinformationflowcontrol,andthenetworkmanagementsoftwareismorecomplicated.Whensettingupthemaincomputernode,thelinklocation,linkcapacity,networkresponsetime,networkthroughputandnetworkreliabilityshouldbeconsideredfromthetopologicalconfiguration.Inshort,thisconfigurationhashighreliabilityandconvenientresourcesharing.Thedisadvantageisthatthenetworksoftwareismorecomplicatedandthecostishigher.Generally,thisunconstrainedmeshstructureisrarelyusedinlocalareanetworks,becausethisunconstrainedmeshstructurecannotachievelocalnetworkhigh-frequencyaccess,low-delaytransmission,andtechnicallyeasytomasterandusecharacteristics.Withthiskindofmeshstructure,thescaleandfunctionsofthenetworkinterfacemachineofthelocalareanetworkhavetobeincreased,whichisnoteconomicallycost-effective.

Application

Accordingtothenetworktopology:

StructuralP2P:

Point-to-pointhaslinkinformationtoeachother,formingspecificrulesforeachotherTopology.

Whenaresourceneedstoberequested,itissearchedaccordingtothetopologyrules,andifitexists,itmustbefound.

SuchasChord,CAN.

UnstructuredP2P:

Point-to-pointhaslinkinformationtoeachother,forminganirregularnetworktopologywitheachother.

Whenaresourcepointneedstoberequested,itissearchedbybroadcast,usuallyTTLisset,evenifitexists,itmaynotbefound.

SuchasGnutella.

LooselystructuredP2P:

Point-to-pointhaslinkinformationtoeachother,forminganirregularnetworktopologywitheachother.

Whenaresourceneedstoberequested,itisestimatedandsearchedbasedonexistinginformation,whichisbetweenstructuredP2PandunstructuredP2P.

SuchasFreenet.

AdvantagesofP2Pnetwork

AnimportantgoalofP2Pnetworkistoenableallclientstoprovideresources,includingbandwidth,storagespaceandcomputingpower.Therefore,whenanodejoinsandrequestsforthesystemincrease,thecapacityoftheentiresystemalsoincreases.ThisisnotpossiblewiththeClient-Serverstructurewithasetoffixedservers,becauseintheabove-mentionedstructure,theincreaseofclientsmeansslowerdatatransmissionforallusers.ThedistributednatureoftheP2Pnetworkalsoincreasestherobustnessoffailurepreventionbyreplicatingdataonmultiplenodes,andinapureP2Pnetwork,nodesdonotneedtorelyonacentralindexservertodiscoverdata.Inthelattercase,thesystemwillnotcrashatasinglepoint.

WhenP2PisusedtodescribetheNapsternetwork,thepeer-to-peerprotocolisconsideredimportant.However,inpractice,theachievementoftheNapsternetworkisthatthepeernodes(justliketheendofthenetwork)uniteacentralindextofulfill.Thisallowsittolocateavailablecontentquicklyandefficiently.Thepeer-to-peerprotocolisjustageneralmethodtoachievethis.

Relatedapplications

eMulepoint-to-pointtechnologyhasmanyapplications.Itisverycommontosharefilescontainingvariousformatsofaudio,video,data,etc.,andinstantdata(suchasIPphonecommunication)canalsobetransmittedusingP2Ptechnology.

Somenetworksandcommunicationchannels,suchasNapster,OpenNAP,andIRC@find,useamaster-slavearchitecturetohandlesometasks(suchassearchfunctions)ontheonehand,andP2Patthesametime.Structuretohandleothertasks.Somenetworks,suchasGnutellaandFreenet,usetheP2Pstructuretohandlealltasks,andaresometimesconsideredtobetrueP2Pnetworks.AlthoughGnutellaalsousesadirectoryservertofacilitatenodestoobtainthenetworkaddressesofothernodes.

AcademicP2Pnetwork

Recently,thedevelopersofPennsylvaniaStateUniversityhaveteamedupwiththeMITOpenKnowledgeInitiative,researchersfromSimonFraserUniversity,andThereisasecond-generationInternetP2Pworkinggroup,whichisdevelopinganacademicapplicationofP2Pnetwork.ThisprojectiscalledLionShare,basedonthesecondgenerationofnetworktechnology,morespecificallytheGnutellamodel.Themainpurposeofthisnetworkistoallowusersfrommanydifferentacademicinstitutionstoshareacademicmaterials.TheLionSharenetworkusesahybridP2Pnetworktype,whichisamixtureofGnutella'sdecentralizedP2PnetworkandthetraditionalC/Snetwork.Usersofthisprogramcanuploadfilestoaserver,regardlessofwhethertheuserisonline,theycancontinuetoshare.Thisnetworkalsoallowsuseinsharedcommunitiesthataremuchsmallerthannormal.

ThemaindifferencebetweenthisnetworkandotherP2PnetworkscurrentlyinuseisthattheLionSharenetworkdoesnotallowanonymoususers.Thepurposeofthisistopreventcopyrightedmaterialsfrombeingsharedonthenetwork,whichalsoavoidslegaldisputes.Anotherdifferenceistheselectivesharingofindividualfilesfordifferentgroups.Theusercanindividuallyselectwhichuserscanreceivethisfileorgroupoffiles.

Theacademiccommunityneedsthistechnologybecausemoreandmoremultimediafilesareusedintheclassroom.Moreandmoreprofessorsusemultimediafiles,likeaudiofiles,videofilesandslideshows.Passingthesefilestostudentsisadifficulttask,anditismucheasierifyouuseanetworklikeLionShare.

Disputes

Legalaspects

InAmericanlaw,the"Betamaxjudgment"precedentinsiststhatcopying"technology"isnotinherentlyillegal,iftheyaresubstantiveNon-infringinguse.ThisdecisionbeforethewidespreaduseoftheInternetisappliedtomostdatanetworks,includingP2Pnetworks,becausethedisseminationofapproveddocumentsisalsopossible.Thesenon-infringingusesincludethedistributionofopensourcesoftware,publicdomaindocumentsandworksnotcoveredbycopyright.Otherjudicialdepartmentscanviewthissituationinasimilarway.

Infact,mostofthefilessharedonP2Pnetworksarecopyrightedpopularmusicandmovies,includingvariousformats(MP3,MPEG,RM,etc.).Inmostjurisdictions,sharingthesecopiesisillegal.Thishascausedmanyobservers,includingmostmediacompaniesandsomeP2Padvocates,tocriticizethisnetworkasahugethreattotheexistingdistributionmodel.Studiesthatattempttomeasureactualmonetarylossesaresomewhatunclear.Althoughtheexistenceofthesenetworksonpapercausedalotoflosses,infact,sincetheestablishmentofthesenetworks,theactualincomehasnotchangedmuch.Regardlessofwhetherthisthreatexists,theRecordingAssociationofAmericaandtheMotionPictureAssociationofAmericaarespendingalotofmoneytryingtolobbylawmakerstoestablishnewlaws.Somecopyrightownersalsopaycompaniestohelplegallychallengeuserswhoengageinillegalsharingoftheirmaterials.

DespitetheBetamaxverdict,P2Pnetworkshavebecometargetsofattacksbyartistsandrepresentativesofcopyrightlicensingorganizations.ThisincludesindustryorganizationssuchastheRecordingAssociationofAmericaandtheMotionPictureAssociationofAmerica.TheNapsterservicewasforcedtoshutdownduetoacomplaintfromtheRecordingAssociationofAmerica.Inthiscase,Napsterdeliberatelyboughtandsoldtheseaudiovisualfilesthatwerenotlicensedfromthecopyrightowner.

Withtheexpansionofmediacompanies’effortstocombatcopyrightinfringement,thesenetworkshaverapidlyandcontinuouslyadjusted,makingitdifficulttoremovethembothtechnicallyandlegally.Thisleadstouserswhoreallyviolatethelawbeingtargeted,becausealthoughtheunderlyingtechnologyislegal,itsabusebyindividualswhospreaditinawayofinfringingcopyrightisobviouslyillegal.

AnonymousP2Pnetworkallowsthepublicationofmaterials,nomatterlegalorlegal,thereislittleornolegalresponsibilityinvariousjurisdictions.Manypeoplesaythatthiswillleadtomoreillegalmaterialsmoreeasilyspread,andeven(somepeoplepointedout)promoteterrorism,requiringregulationintheseareas.Othersobjectedthatthepotentialforillegalusecannotpreventtheuseofthistechnologyforlegitimatepurposes,andthepresumptionofinnocencemustbeapplied,asothernon-P2Panonymousservices,suchase-mail,havesimilarcapabilities.

Importantcase:

U.S.law

SonyCorp.vsUniversalCityStudios(Betamaxverdict)

MGMvsGrokster

Securityaspects

ManyP2Pnetworkshavebeenconstantlyattackedbypeoplewithvariouspurposes.Examplesinclude:

Poisoningattacks(providingfileswithdifferentcontentsanddescriptions)

Denialofserviceattacks(makingthenetworkrunveryslowlyorevencompletelycrashing)

Betrayalattacks(Usersorsoftwareusethenetworkbutdidnotcontributetheirownresources)

Insertvirusesinthedata(forexample,thedownloadedortransmittedfilesmaybeinfectedwithvirusesorTrojanhorses)

P2PTrojanhorseofthesoftwareitself(forexample,thesoftwaremaycontainspyware)

Filtering(networkoperatorsmaytrytoprohibitthetransmissionofdatafromP2Pnetworks)

Identityattacks(forexample,trackingUsersonthenetworkareconstantlyharassingorattackingthemlegally)

Spam(suchassendingunsolicitedinformationonthenetwork--notnecessarilyadenialofserviceattack)

IftheP2Pnetworkiscarefullydesignedandencryptiontechnologyisused,mostoftheattackscanbeavoidedorcontrolled.Infact,P2PnetworksecurityiscloselyrelatedtotheByzantineGeneralsissue.However,whenmanynodestrytodestroyit,almostanynetworkwillfail,andmanyprotocolswillfailbecauseoffewerusers.

OnApril23,2007,CAissuedaninformationsecurityalert,pointingoutthatFoxy,BitComet,eDonkey,µTorrent,Ares,Azureus,BearShare,Lphant,Shareaza,Hamachi,exeemlite,Fpsetup,Morpheus,Therearesecuritythreatsto14P2PsoftwaresuchasiMesh.ThepotentialthreatsourcesoftheseP2Psoftwareincludethepossibilityofoverwritingfiles,renamingfiles,deletingfiles,andimplantingmaliciousprogramsbythirdparties.

FutureofComputingTechnology

Technically,apureP2Papplicationmustimplementonlyapeer-to-peerprotocol,withouttheconceptofaserverandaclient.ButsuchpureP2Papplicationsandnetworksarerare.MostofthenetworksandapplicationscalledP2Pactuallycontainorrelyonsomenon-peerunits,suchasDNS.Atthesametime,realapplicationsalsousemultipleprotocols,sothatnodescanactasclients,servers,andpeernodesatthesametimeortime-sharing.Completelydecentralizedpeer-to-peernetworkshavebeeninuseformanyyears,suchasUsenet(1979)andFidoNet(1984).

ManyP2Psystemsusestrongerpeers(calledSuperNodes)asservers,andthoseclientnodesareconnectedtoasuperpeerinastar-likemanner.

Inthelate1990s,inordertopromotethedevelopmentofpeer-to-peernetworkapplications,SunSun(SUN)addedsomeclassestoJavatechnology,allowingdeveloperstodevelopdecentralizedreal-timechatappletsandapplications.Thiswasbeforeinstantmessagingbecamepopular.ThisworknowhastheJXTAprojecttocontinuetoachieve.

P2Psystemsandapplicationshaveattractedalotofattentionfromcomputerscienceresearch.SomeoutstandingresearchprojectsincludeChordProject,ARPANET,thePASTstorageutility,P-Grid(aspontaneouslyorganizedemergingcoveragenetwork),AndCoopNetcontentdistributionsystem.

Restrictions

ThePeople’sRepublicofChina

P2PtechnologyisinablankstateintermsofChineselaw,andtheprincipleisnotsubjecttoofficialrestrictionsbytheChinesegovernment.However,becauseP2Ptechnologywilloccupyalargeamountofnetworkbandwidth,andbecauseofthecurrentstatusofChina'snetworkfacilitiesandChina'sattitudetonetworkmanagement,therearevaryingdegreesofrestrictionsonP2Pcommunicationmethods.

Currently,allmajorISPsinChinalimitthespeedofnetworkaccess,usuallybelow512K/s.Measureswillbetakenforbandwidth-consumingapplicationssuchasP2P.CommonmethodsincludelimitingthenumberofTCPconnections.BlockP2Pprotocol,limitdownload/uploadspeed,etc.However,sincemostoftheseactionsarecarriedoutinsecret,manyP2PusersinChinaclaimthatthishasseriouslyviolatedtheirrighttoknowanduse.InChina,themostrestrictedP2PsoftwareisBitTorrentseriessoftwareandeMule.

Inaddition,outofconsiderationofdomesticindustrysupportandinformationcontrol,ChinawillalsorestrictsomeP2Pinstantmessagingsoftware.TheMinistryofInformationIndustryofChinahasissuedadocumentrequestingthatVOIPcanonlybetestedindesignatednetworkoperators,andskypeisblockedinsomeareas,butsomeofficialshavedeniedthisandstatedthatitisonlyforPC-Phone[2].

Japan

InJapan,accordingtoJapan'scurrentcopyrightlaw,allwebsitesinJapanthatprovidefreecommercialsoftware/orotherdatadownloadsareillegal.Seriousviolationscanbesentencedtofixed-termimprisonmentandafine.BothBitTorrentseriessoftwareandeMulearebanned.However,P2PtechnologyisstillpopularinJapan.ThankstotheinventionofShareandWinnysoftware.BothsoftwareusesIPencryptionanddataencryptiontechnology.Atpresent,millionsofpeopleinJapanusethesetwosoftwaretoexchangeanimation,games,music,softwareandotherdata,butitisclearthatthepolicehavecontrolledthemethodoftracingusers..OnMay9,2008,threeworkerswhopostedalargenumberofcopyrightedworksonsharewerearrested.Thepopularityofthesetwosoftwarehasalsobroughtsomenegativeproblems,suchastheleakageofconfidentialinformationandthespreadofcomputerviruses.