Definition
Networkcongestion(congestion)referstothesituationwherethenetworktransmissionperformanceisreducedduetothelimitedresourcesofthestoreandforwardnodewhenthenumberofpacketstransmittedinthepacketswitchingnetworkistoolarge.Whenthenetworkiscongested,datalossgenerallyoccurs,thedelayincreases,andthethroughputdecreases.Inseverecases,itmayevenleadto"congestioncollapse"(congestioncollapse).Undernormalcircumstances,networkcongestionoccurswhenthenetworkperformanceisdegradedduetoexcessiveincreaseintheloadonthenetwork.Figure2.1canvisuallydescribetheprocessofnetworkcongestion.
Reasonsfornetworkcongestion
Becauseinaresourcesharingnetworkwithoutanypriornegotiationandpermissionrequestmechanism,severalIPpacketsarriveattherouteratthesametimeandexpecttopassthroughthesameoutputportThepossibilityofforwardingexists.Obviously,notallpacketscanbeprocessedatthesametime.Theremustbeaservicesequence.Thebufferontheintermediatenodeprovidescertainprotectionforthepacketswaitingtobeserved.However,ifthissituationhasacertaindegreeofpersistence,whenthebufferspaceisexhausted,therouterhastodiscardthepacket.Inthiscontinuousoverloadstate,networkperformancewilldropsharply.
Storagespacelimitation
Eachoutputporthasacertainstoragespace.Ifanoutputportisusedbyseveralinputdatastreams,theinputstreamThedatapacketswillbequeuedinthestoragespaceforoutput.Whentheportforwardingdatarateislowerthanthearrivalrateofthedatapacket,itwillcausethestoragespacetobefull,andthedatapacketsarrivinglaterwillbediscarded.Thisphenomenonofburstdataflowismore.Althoughtosomeextent,theincreaseinstoragespacecanrelievethepressureontheoutputport.However,theunlimitedincreaseinstoragespacewillcausethedatapacketstocompletetheforwarding,theyhavealreadytimedout.Thesourcebelievesthatthesedatapacketsarediscardedduringtransmissionandrequireretransmission,whichnotonlyreducesnetworkefficiency,butalsomakesnetworkcongestionmoreserious.Inaddition,inpracticalapplications,thestoragecapacitycannotbeincreasedindefinitely,whichdoesnotconformtotheactualmeaning.
Limitsonbandwidthcapacity
Ithasbeenprovedthroughpracticethatitisdifficultforlow-speedlinkstocopewiththeinputofhigh-speeddatastreams,resultinginnetworkcongestion.AccordingtoShannon'stheory,thetransmissionrateofthesourcemustbelessthanorequaltothechannelcapacity.Therefore,whenthesourcebandwidthismuchlargerthanthelinkbandwidth,thebandwidthbottleneckisformed,causingdatapacketstoqueueupatthenetworknode,causingnetworkcongestion.
Processorperformancelimitation
TheCPUintheroutermainlyperformsfunctionssuchasqueuinginthebufferarea,updatingtheroutingtable,andperformingroutingselection.Meetingtheneedsofhigh-speedlinkswillcausenetworkcongestion.
Inpracticalapplications,complexnetworkstructuresandunreasonableroutingprinciplescanalsocausenetworkcongestion.
Congestioncontrol
Thewaytosolvenetworkcongestioniscongestioncontrol.Congestioncontrolneedstoensurethatthenetworkcancarrythetrafficsubmittedbyusers.Thisisaglobalissuethatinvolvesmanyfactorssuchashostsandrouters.Theparameterstomeasurewhetherthenetworkiscongestedmainlyinclude:packetlossratecausedbylackofbuffers,averagequeuelength,numberofpacketsretransmittedovertime,averagepacketdelay,andpacketdelaychanges.Accordingtocybernetics,congestioncontrolmethodsaredividedintotwocategories:Thefirstcategoryisopen-loopcontrol,thatis,toavoidcongestionbydesigningagoodalgorithm.Whenperformingcongestioncontrol,thecurrentstateofthenetworkisnotconsidered.Commonlyusedalgorithmsare:
1.Source-endcongestioncontrolalgorithm
2.Linkcongestioncontrolalgorithm
Thesecondtypeisclosed-loopcontrol,thatis,givingfeedbackmechanism,Controlcongestionaccordingtothecurrentstateofthenetwork.Thereareseveralfeedbackmethods:
1.Sendanalarmdatagramtotheinformationsource.
2.Reserveabitorafieldinthestructureofthedatapackettoindicatethatcongestionoccurs.Oncecongestionoccurs,theroutersetsthecorrespondingbitsinalloutputdatagramstoalertneighbors.
3.Thehostoroilfilteractivelyandperiodicallysendsprobestocheckwhethercongestionoccurs.
Theworkingprocessofclosed-loopcongestioncontrolisasfollows:
1.Themonitoringsystemfindswhenandwherecongestionoccurs;
2.Whencongestionoccurs,Sendthecongestedmessagetothesitethatcantakeaction;
3.Adjustsystemoperationtosolvethecongestionproblem.