model OSI
Nejdůležitější věc při učení OSI modelu je to, co ve skutečnosti znamená.
Ifyouareanoperatingsystemonthenetwork.Thenetworkcardsworkingonthe1stand2ndlayerswillnotifyyouwhendataarrives.Thedriverhandlestheexportoflayer2frames,throughwhichyoucangetalitandflashinglayer3datapacket(hopefully).Asanoperatingsystem,youwillcallsomecommonlyusedapplicationstoprocesslayer3data.Ifthedataissentfrombelow,youknowthatitisadatapacketsenttoyou,oritisabroadcastdatapacket(unlessyouarealsoarouter,butdon’tworryaboutthisproblemforthetimebeing).Ifyoudecidetokeepthispacket,youwillopenitandtakeoutthelayer4packet.IfitisaTCPprotocol,theTCPsubsystemwillbecalledandopenthepacket,andthensendthelayer7datatotheapplicationwaitingonthetargetport.Thisprocessisover.
Whenitcomestorespondingtoothercomputersonthenetwork,everythinghappensinthereverseorder.The7-layerapplicationwillsendthedatatotheexecutoroftheTCPprotocol.Then,theTCPprotocoladdsadditionalfileheaderstothesedata.Inthisdirection,thevolumeofthedataislargereachstepforward.TheTCPprotocoladdsalegalTCPfieldtotheIPprotocol.Then,theIPprotocoldeliversthisdatapackettotheEthernet.TheEthernetthensendsthisdataasanEthernetframetothedriver.Then,thisdatapassesthroughthisnetwork.TherouteronthislinewillpartiallydecomposethepackettoobtaintheLayer3fileheaderinordertodeterminewherethepacketshouldbesent.IfthedestinationofthedatapacketisthelocalEthernetsubnet,theoperatingsystemwillreplacetheroutertoresolvetheaddressforthecomputerandsendthedatadirectlytothehost.
Specificintroduction
Itmeansopensysteminterconnection.TheInternationalOrganizationforStandardization(InternationalOrganizationforStandardization)developedtheOSI(OpenSystemInterconnection)model.Thismodeldividestheworkofnetworkcommunicationinto7layers,namelythephysicallayer,datalinklayer,networklayer,transportlayer,sessionlayer,presentationlayerandapplicationlayer.Layers1to4areconsideredlowerlayers,andtheselayersarecloselyrelatedtodatamovement.Layers5to7arehigh-levelsandcontainapplication-leveldata.Eachlayerisresponsibleforaspecificjob,andthentransferthedatatothenextlayer.
První vrstva je fyzická vrstva (tedy první vrstva v modelu OSI)
Itisoftenoverlookedintheclassroom.Itseemsverysimple.However,certainaspectsofthislayersometimesrequirespecialattention.Thephysicallayerisactuallywiring,opticalfibers,networkcardsandotherthingsusedtoconnecttwonetworkcommunicationdevicestogether.Evenacarrierpigeoncanbeconsideredalayer1device.Networktroubleshootingofteninvolveslayer1issues.Wecan'tforgetthelegendarystoryofconnectingtheentirefloorwithafive-categoryline.Becauseofficechairsareoftenpressedoverthecables,thenetworkconnectionappearsintermittent.Unfortunately,suchfailuresareverycommon,andittakesalongtimetotroubleshootsuchfailures.
Layer2istheDataLinkLayer(DataLinkLayer)
RunningprotocolssuchasEthernet.Remember,wewanttomakethisproblemsimpler.Themostimportantthinginlayer2isthatyoushouldunderstandwhatabridgeis.Theswitchcanberegardedasanetworkbridge,andpeoplecallitthatwaynow.Thebridgesallworkatlayer2,focusingonlyontheMACaddressontheEthernet.IfyouaretalkingaboutMACaddresses,switchesornetworkcardsanddrivers,youareinthelayer2category.Hubsbelongtothefirstlayerofthedomain,becausetheyarejustelectronicdevices,withouttheknowledgeofthesecondlayer.Therelatedissuesoflevel2havetheirownpartinthiswebinar,sowewillnotdiscussthedetailsofthisissueindetailfornow.Nowonlyneedtoknowthatthesecondlayerconvertsthedataframeintobinarybitsforthefirstlayertoprocess.
Třetí vrstva je Síťová vrstva
Theremaybemanydatalinksbetweentwocomputerscommunicatinginacomputernetwork,ortheymayalsopassManycommunicationsubnets.Thetaskofthenetworklayeristoselectappropriateroutingandswitchingnodesbetweennetworkstoensuretimelydatatransmission.Thenetworklayercomposestheframesprovidedbythedatalinklayerintodatapackets,andthepacketisencapsulatedwithanetworklayerheader,whichcontainslogicaladdressinformation-thenetworkaddressesofthesourcesiteandthedestinationsiteaddress.
IfyouaretalkingaboutanIPaddress,thenyouaredealingwithalayer3problem,whichisa"packet"problem,notalayer2"frame".IPispartofthelayer3problem,inadditiontosomeroutingprotocolsandaddressresolutionprotocol(ARP).Everythingaboutroutingishandledatlayer3.AddressresolutionandroutingareimportantpurposesofLayer3.
ThefourthlayeristheTransportLayerforprocessinginformation.Thedataunitsoflayer4arealsocalledpackets.However,whenyoutalkaboutspecificprotocolssuchasTCP,therearespecialnames.ThedataunitsofTCParecalledsegmentsandthedataunitsofUDPprotocolarecalled"datagrams."Thislayerisresponsibleforobtainingallinformation.Therefore,itmustkeeptrackofdataunitfragments,out-of-sequencedatapacketsandotherhazardsthatmayoccurduringtransmission.Anotherwaytounderstandlayer4isthatlayer4providesend-to-endcommunicationmanagement.SomeprotocolslikeTCPareverygoodatensuringthereliabilityofcommunication.Someprotocolsdonotcarewhethersomedatapacketsarelost.UDPprotocolisaprimeexample.
Pátý hráč je vrstva relace.
Thislayercanalsobecalledthemeetinglayerorthedialoguelayer.Inthehighlevelofthesessionlayerandabove,thedatatransmissionTheunitsarenolongernamedseparately,andarecollectivelyreferredtoasmessages.Thesessionlayerdoesnotparticipateinspecifictransmissions.Itprovidesmechanismsforestablishingandmaintainingcommunicationbetweenapplications,includingaccessverificationandsessionmanagement.Iftheserververifiestheuserloginiscompletedbythesessionlayer.
Šestá vrstva je prezentační vrstva.
Thislayermainlysolvestheproblemofgrammaticalrepresentationofuserinformation.ItconvertsthedatatobeexchangedfromanabstractsyntaxsuitableforacertainusertoatransfersyntaxsuitableforinternaluseintheOSIsystem.Thatistoprovideformattedrepresentationandconversiondataservices.Thepresentationlayerisresponsiblefordatacompressionanddecompression,encryptionanddecryption.
Theseventhlayeris"everything".Layer7isalsocalledthe"ApplicationLayer"(ApplicationLayer),whichisdedicatedtoapplications.Theapplicationlayerdeterminesthenatureofcommunicationbetweenprocessestomeetuserneedsandprovidesinterfaceservicesbetweenthenetworkanduserapplicationsoftware.Ifyourprogramrequiresdatainaspecificformat,youcaninventsomeformatyouwanttobeabletosendthedatatothedestination,andcreatealayer7protocol.SMTP,DNSandFTPareall7-layerprotocols.