毕业设计计算机专业翻译Word格式文档下载.docx
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毕业设计计算机专业翻译Word格式文档下载.docx
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AComparativeSurvey
文献、资料来源:
美国计算机汇刊网站
文献、资料发表(出版)日期:
1980.4.4
院(部):
计算机科学与技术学院
专业:
网络工程
班级:
网络102
姓名:
王旭
学号:
2010111215
指导教师:
秦松赵秀梅
翻译日期:
2014.3.1
外文文献:
AComparativeSurvey
Abstract:
Packetswitchingoffersattractiveadvantagesoverthemoreconventionalcircuit-switchedscheme,namely,flexibilityinsettingupuserconnectionsandmoreefficientuseofresourcesaftertheconnectionisestablished.However,ifuserdemandsareallowedtoexceedthesystemcapacity,unpleasantcongestioneffectsoccurwhichrapidlyneutralizethedelayandefficiencyadvantages.Congestioncanbeeliminatedbyusinganappropriatesetoftrafficmonitoringandcontrolprocedurescalledflowcontrolprocedures.Flowcontrolcanheexercisedatvariouslevelsinapacketnetwork.ThefollowinglevelsareidentifiedanddiscussedinthisPaper:
hoplevel,entry-to-exitlevel,networkaccesslevel,andtransportlevel.Foreachlevel,themostrepresentativetechniquesaresurveyedandcompared.Furthermore,theinteractionbetweenthedifferentlevelsisdiscussed.
I.INTRODUCTION
Apacket-switchednetworkmaybethoughtofasadistributedpoolofproductiveresourceswhosecapacitymustbeshareddynamicallybyacommunityofcompetingusers(or,moregenerally,processes)wishingtocommunicatewitheachother.Dynamicresourcesharingiswhatdistinguishespacketswitchingfromthemoretraditionalcircuitswitchingapproach,inwhichnetworkresourcesarededicatedtoeachuserforanentiresession.Thekeyadvantagesofdynamicsharingaregreaterspeedandflexibilityinsettingupuserconnectionsacrossthenetworkandmoreefficientuseofnetworkresourcesaftertheconnectionisestablished.
Theseadvantagesofdynamicsharingdonotcomewithoutacertaindanger,however.Indeed,unlesscarefulcontrolisexercisedontheuserdemands,theusersmayseriouslyabusethenetwork.Infact,ifthedemandsareallowedtoexceedthesystemcapacity,highlyunpleasantcongestioneffectsoccurwhichrapidlyneutralizethedelayandefficiencyadvantagesofapacketnetwork.Thetypeofcongestionthatoccursinanoverloadedpacketnetworkisnotunlikethatobservedinahighwaynetwork.Duringpeakhours,thedemandsoftenexceedthehighwaycapacity,creatinglargebacklogs.Further-more,theinterferencebetweentransittrafficonthehighwayandon-rampandoff-ramptrafficreducestheeffectivethroughputofthehighway,thuscausinganevenmorerapidincreaseinthebacklog.Ifthispositivefeedbacksituationpersists,trafficonthehighwaymaycometoastandstill.Thetypicalrelationshipbetweeneffectivethroughputandofferedloadinahighwaysystem(and,moregenerally,inmanyuncontrolled,distributeddynamicsharingsystems)isshowninFig.1.
Fig.1.Effectivethroughputversusofferedloadinanuncontrolled,distributeddynamicsharingsystem.
Byproperlymonitoringandcontrollingtheofferedloadmanyofthesecongestionproblemsmaybeeliminated.Inahighwaysystem,itiscommontocontroltheinputbyusingaccessramptrafficlights.Theobjectiveistokeeptheinterferencebetweentransittrafficandincomingtrafficwithinacceptablelimits,andtopreventtheincomingtrafficratefromexceedingthehighwaycapacity.
Similartypesofcontrolsareusedinpacketswitchednetworks,andarecalledflowcontrolprocedures.Asinthehighwaysystem,thebasicprincipleistokeeptheexcessloadoutofthenetwork.Thetechniques,however,aremuchmoresophisticatedsincetheelementsofthenetworkareintelligent,cancommunicatewitheachother,andthereforecancoordinatetheiractionsinadistributedcontrolstrategy.
Internalnetworkcongestionmayalsoberelievedbyreroutingsomeofthetrafficfromheavilyloadedpathstounderutilizedpaths.Itisimportanttounderstand,however,thatroutingcanreduceand,perhaps,delaynetworkcongestion;
itcannotpreventit.Wedonotdiscusstheinteractionsbetweenroutingandflowcontrolinthispaper.
Themainfunctionsofflowcontrolinapacketnetwork
1)preventionofthroughputdegradationandlossofefficiencyduetooverload,
2)deadlockavoidance,
3)fairallocationofresourcesamongcompetingusers,
4)speedmatchingbetweenthenetworkanditsattachedusers.
Throughputdegradationanddeadlocksoccurbecausethetrafficthathasalreadybeenacceptedintothenetworkexceedsthenominalcapacityofthenetwork.Topreventoverallocationofresources,theflowcontrolprocedureincludesasetofconstraintswhichcaneffectivelylimittheaccessoftrafficintothenetworkor,moreprecisely,toselectedsectionsofthenetwork.Theseconstraintsmaybefixed,ormaybedynamicallyadjustedbasedontrafficconditions.
Apartfromtherequirementofthroughputefficiency,networkresourcesmustbefairlydistributedamongusers.Unfortunately,efficiencyandfairnessobjectivesdonotal-wayscoincide.Forexample,referringbacktoourhighwaytrafficsituation,theeffectivethroughputoftheLongIslandExpresswaycouldbemaximizedbyopeningallthelanestotrafficfromtheIslandtoNewYorkCityduringthemorningrushhour,andintheoppositedirectionduringtheeveningrushhour.Thissolution,however,wouldalsomaximizethediscontentofthereversecommuters!
Inpacketnetworks,unfairnessconditionscanalsoarise;
buttheytendtobemoresubtleandlessobviousthaninhighwaynetworksbecauseofthecomplexityofthecommunicationsprotocols.Oneofthefunctionsofflowcontrol,therefore,istopreventunfairnessbyplacingselectiverestrictionsontheamountofresourcesthateachusermayacquire,inspiteofthenegativeeffectthattheserestrictionsmayhaveondynamicresourcesharingand,therefore,overallthroughputefficiency.
Flowcontrolcanbeexercisedatvariouslevelsinapacketnetwork.Thefollowinglevelsareidentifiedanddiscussedinthispaper.
1)HopLevel:
Thislevelofflowcontrolattemptstomaintainasmoothflowoftrafficbetweentwoneighboringnodesinacomputernetwork,avoidinglocalbuffercongestionanddeadlocks.
2)Entry-to-ExitLevel:
Thislevelofflowcontrolisgenerallyimplementedasaprotocolbetweenthesourceanddestinationswitch,andhasthepurposeofpreventingbuffercongestionattheexitswitch.
3)NetworkAccessLevel:
Theobjectiveofthislevelistothrottleexternalinputsbasedonmeasurementsofinternalnetworkcongestion.
4)TransportLevel;
Thisisthelevelofflowcontrolassociatedwiththetransportprotocol,i.e.,theprotocolwhichprovidesforthereliabledeliveryofpacketsonthe“virtual”connectionbetweentworemoteprocesses.Itsmainpurposeistopreventcongestionofuserbuffersattheprocesslevel.
Someauthorsreservetheterm“flowcontrol”forthetransportlevel,andrefertotheotherthreelevelsofcontrolascongestioncontrol.Thisterminologyisusedtoemphasizethephysicaldistinctionbetweenthefirstthreelevels,whicharerealizedinthecommunicationssubnetandthefourthlevel,whichisrealizedintheuserdevices.Inthispaper,wehavechosentousethetermflowcontrolforallfourlevels.
Thedesignofanefficientflowcontrolstrategyforapacketnetworkisacomplextaskinmanyways.Themostcriticalissueisthefactthatflowcontrolisamultilayerdistributedprotocolinvolvingseveraldifferentlevels.Ateachlevel,theflowcontrolimplementationmustbeconsistentandcompatiblewithotherprotocolfunctionsexistingatthesamelevel.Furthermore,theinteractionsbetweendifferentlevelsmustbecarefullystudiedinordertoavoidduplicationoffunctionsononehand,andlackofcoordinationontheother.
Thepurposeofthispaperistoprovideataxonomyofflowcontrolmechanismsbasedontheabovedefinedmultilevelstructure.First,wereviewproblems,functions,andperformancemeasuresofflowcontrol.Then,foreachlevelwesurveythemostrepresentativeflowcontrol,techniquesthathavebeenproposedand/orimplemented,providingaperformancecomparisonamongtechniquesatthesamelevel,anddiscussingtheinteractionbetweentechniquesatdifferentlevels.Finally,webrieflymentionsomenewflowcontrolissuesraisedbynovelcomputernetworkapplications.
II.FLOWCONTROL:
PROBLEMS,FUNCTIONS,ANDMEASURES
Ouroverallproblemistoidentifymechanismswhichpermitefficientdynamicsharingofthepoolofresources(channels,buffers,andswitchingprocessors)inapacketnetwork.Inthissection,wefirstdescribeandillustratethecongestionproblemscausedbylackofcontrol.Thenwedefinethefunctionsofflowcontrolandthedifferentlevelsatwhichthesefunctionsareimplemented.Finally,weintroduceperformancemeasuresfortheevaluationandcomparisonofdifferentflowcontrolschemes.
A.LossofEfficiency
Themaincauseofthroughputdegradationinapacketnetworkisthewastageofresources.Thismayhappeneitherbecauseconflictingdemandsbytwoormoreusersmaketheresourceunusable(e.g.,collisionsonarandomaccesschannel);
orbecauseauseracquiresmoreresourcesthanstrictlyneeded,thusstarvingotherusers(e.g.,aslowsinkfedbyafastsourcemaycreateabacklogofpacketswithinthenetworkwhichpreventsothertrafficfromgettingthrough).Thetworesourcesthataremostcommonly“wasted”inapacketnetworkarecommunicationscapacityandstoragecapacity.
Bufferwastageisanindirectconsequenceoflimitednodalstorage:
agivenend-to-endpacketstreammaybeblockedatanintermediatenodealongthepathbecauseallofthebuffershavebeen“hogged”by,otherstreams.Thismayhappenevenifchannelbandwidthisplentifulalong
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