Concept
Thephysicalmemoryisdividedintofixed-sizeblockscalledframes.Incontrast,logicalmemoryisdividedintoblocksofthesamesize,calledpages.
Networktransmission
Framesareformedbyspecificsoftwarecallednetworkdrivers,andthensenttothenetworkcablethroughthenetworkcard,andreachtheirdestinationmachinethroughthenetworkcable.Theoppositeprocessisperformedononeend.TheEthernetcardofthereceivingmachinecapturestheseframes,tellstheoperatingsystemthattheframeshavearrived,andthenstoresthem.Itisinthistransmissionandreceptionprocessthatthesnifferwillbringsecurityproblems.
"Frame"dataconsistsoftwoparts:frameheaderandframedata.Theframeheaderincludesthelocationofthereceiver'shostphysicaladdressandothernetworkinformation.Theframedataareacontainsadatabody.Toensurethatthecomputercaninterpretthedatainthedataframe,thetwocomputersuseacommoncommunicationprotocol.ThecommunicationprotocolusedbytheInternetisreferredtoasIP,orInternetProtocol.TheIPdatabodyiscomposedoftwoparts:thedatabodyheaderandthedataareaofthedatabody.ThedatabodyheaderincludesIPsourceaddressandIPdestinationaddress,andotherinformation.ThedataareaofthedatabodyincludesUserDataProtocol(UDP),TransmissionControlProtocol(TCP),andotherinformationaboutthedatapacket.Thesedatapacketsallcontainadditionalprocessinformationandactualdata.
TheminimumframelengthofEthernet
OnceaCSMA/CDconflictoccurs,itmustbedetectedbyeverynode.Ethernethasspecifiedtheminimumtransmissiongapandminimumframelengthtoavoidconflictsasmuchaspossible.Ifthedistancebetweenthetwonodesisverylongandtheframeissmall,thereceiverstartstosendtheframebeforetheframereachesthedestination.Inthisway,thereceiver'sframehasbeensentandthedataisstillbeingtransmittedinthechannel,andithasnotyetreachedthedestinationnode.Sincethereceivingnodehasnotreceivedtheinformation,itmistakenlybelievesthatthechannelisidle,andmaytransmitdataoutwards,whichwillcausenewchannelconflicts.Inthesendingsection,itwasmistakenthatthesendingwassuccessful.Signaltransmissionistime-delayed,soittakesacertainamountoftimetodetectcollisions,whichdeterminesthattheEthernetmusthaveaminimumframelengthlimit.
ThemaximumframelengthofEthernet
MTUreferstothemaximumdatapacketsizethatcanbepassedonthecommunicationprotocol,usuallyrelatedtothecommunicationinterface.
Datacommunicationinthenetworkdividesthedataintomultiplesegmentssuitablefortransmissioninthenetwork.Thesedataaresmallenoughtoensurethatthelargestdatacanalsopassthroughthesmallestforwardinglinkinthenetwork.Thefragmentationprocessoccursinthe3rdand4thlayersoftheOSImodel.The4thlayeristhemostimportantlayerintheOSImodel.Itmainlycontrolsthetransmissionbasedonthewindow,nottheMTU.
Duringthedatatransmissionprocess,thesenderandreceiveraresentfastorslowthroughthetrialofQOS.Intheprocessofnetworktransmission,iftoolargedataisfound,thelargerdatapacketwillbeforcedtogroup.InEthernet,thelargestprotocoldataunitis1500bytes.Ofcourse,thisdatadoesnotincludetheframeheaderpartofthedata.Inthegroupingprocess,eachfragmenthasasequencenumber,thepurposeisthatafterthereceiverreceivesthedata,itcanbereorganizedcorrectly.Thelargesttransmissionunitinthenetworkistoensurethatalldatacanpassthroughalllinknodesduringnetworklinktransmission.
Becausethechannelissharedbyallnodes,thedataframeistoolong,eachframewilltakealongtimeduringthedatatransmissionprocess,andsomenodeswillwaittoolong;thedataarrivesatthereceiverAfterthepoint,thesizeofthereorganizationbufferofthetargetnodeislimited,andsomesentdatamayexceedthebuffersizeofthereceivingendbyalot,causingbufferoverflow.Inordertopreventnodesfromoccupyingtoolong,aruleof1,500byteshasbeenformulatedinthenetworktoensurethefairnessofchannelownership.