互联网基本工作(1)

CHAPTERInternetworkingBasics1-11InternetworkingBasicsThischapterworkswiththenextsixchapterstoactasafoundationforthetechnologydiscussionsthatfollow.Inthischapter,somefundamentalconceptsandtermsusedintheevolvinglanguageofinternetworkingareaddressed.Inthesamewaythatthisbookprovidesafoundationforunderstandingmodernnetworking,thischaptersummarizessomecommonthemespresentedthroughouttheremainderofthisbook.Topicsincludeflowcontrol,errorchecking,andmultiplexing,butthischapterfocusesmainlyonmappingtheOpenSystemsInterconnect(OSI)modeltonetworking/internetworkingfunctionsandsummarizingthegeneralnatureofaddressingschemeswithinthecontextoftheOSImodel.WhatisanInternetwork?Aninternetworkisacollectionofindividualnetworks,connectedbyintermediatenetworkingdevices,thatfunctionsasasinglelargenetwork.Internetworkingreferstotheindustry,products,andproceduresthatmeetthechallengeofcreatingandadministeringinternetworks.Figure1-1illustratessomedifferentkindsofnetworktechnologiesthatcanbeinterconnectedbyroutersandothernetworkingdevicestocreateaninternetwork:Figure1-1Differentnetworktechnologiescanbeconnectedtocreateaninternetwork.FDDITokenRingWANEthernet12308OpenSystemsInterconnection(OSI)ReferenceModelInternetworkingTechnologyOverview,June19991-2HistoryofInternetworkingThefirstnetworksweretime-sharingnetworksthatusedmainframesandattachedterminals.SuchenvironmentswereimplementedbybothIBM’sSystemNetworkArchitecture(SNA)andDigital’snetworkarchitecture.Localareanetworks(LANs)evolvedaroundthePCrevolution.LANsenabledmultipleusersinarelativelysmallgeographicalareatoexchangefilesandmessages,aswellasaccesssharedresourcessuchasfileservers.Wide-areanetworks(WANs)interconnectLANsacrossnormaltelephonelines(andothermedia),therebyinterconnectinggeographicallydispersedusers.Today,high-speedLANsandswitchedinternetworksarebecomingwidelyused,largelybecausetheyoperateatveryhighspeedsandsupportsuchhigh-bandwidthapplicationsasvoiceandvideoconferencing.Internetworkingevolvedasasolutiontothreekeyproblems:isolatedLANs,duplicationofresources,andalackofnetworkmanagement.IsolatedLANSmadeelectroniccommunicationbetweendifferentofficesordepartmentsimpossible.Duplicationofresourcesmeantthatthesamehardwareandsoftwarehadtobesuppliedtoeachofficeordepartment,asdidaseparatesupportstaff.Thislackofnetworkmanagementmeantthatnocentralizedmethodofmanagingandtroubleshootingnetworksexisted.InternetworkingChallengesImplementingafunctionalinternetworkisnosimpletask.Manychallengesmustbefaced,especiallyintheareasofconnectivity,reliability,networkmanagement,andflexibility.Eachareaiskeyinestablishinganefficientandeffectiveinternetwork.Thechallengewhenconnectingvarioussystemsistosupportcommunicationbetweendisparatetechnologies.Differentsites,forexample,mayusedifferenttypesofmedia,ortheymightoperateatvaryingspeeds.Anotheressentialconsideration,reliableservice,mustbemaintainedinanyinternetwork.Individualusersandentireorganizationsdependonconsistent,reliableaccesstonetworkresources.Furthermore,networkmanagementmustprovidecentralizedsupportandtroubleshootingcapabilitiesinaninternetwork.Configuration,security,performance,andotherissuesmustbeadequatelyaddressedfortheinternetworktofunctionsmoothly.Flexibility,thefinalconcern,isnecessaryfornetworkexpansionandnewapplicationsandservices,amongotherfactors.OpenSystemsInterconnection(OSI)ReferenceModelTheOpenSystemsInterconnection(OSI)referencemodeldescribeshowinformationfromasoftwareapplicationinonecomputermovesthroughanetworkmediumtoasoftwareapplicationinanothercomputer.TheOSIreferencemodelisaconceptualmodelcomposedofsevenlayers,eachspecifyingparticularnetworkfunctions.ThemodelwasdevelopedbytheInternationalOrganizationforStandardization(ISO)in1984,anditisnowconsideredtheprimaryarchitecturalmodelforintercomputercommunications.TheOSImodeldividesthetasksinvolvedwithmovinginformationbetweennetworkedcomputersintosevensmaller,moremanageabletaskgroups.AtaskorgroupoftasksisthenassignedtoeachofthesevenOSIlayers.Eachlayerisreasonablyself-contained,sothatthetasksassignedtoeachlayercanbeimplementedindependently.Thisenablesthesolutionsofferedbyonelayertobeupdatedwithoutadverselyaffectingtheotherlayers.InternetworkingBasics1-3CharacteristicsoftheOSILayersThefollowinglistdetailsthesevenlayersoftheOpenSystemInterconnection(OSI)referencemodel:•Layer7—Applicationlayer•Layer6—Presentationlayer•Layer5—Sessionlayer•Layer4—Transportlayer•Layer3—Networklayer•Layer2—DataLinklayer•Layer1—PhysicallayerFigure1-2illustratestheseven-layerOSIreferencemodel.Figure1-2TheOSIreferencemodelcontainssevenindependentlayers.CharacteristicsoftheOSILayersThesevenlayersoftheOSIreferencemodelcanbedividedintotwocategories:upperlayersandlowerlayers.TheupperlayersoftheOSImodeldealwithapplicationissuesandgenerallyareimplementedonlyinsoftware.Thehighestlayer,application,isclosesttotheenduser.Bothusersandapplication-layerprocessesinteractwithsoftwareapplicationsthatcontainacommunicationscomponent.ThetermupperlayerissometimesusedtorefertoanylayeraboveanotherlayerintheOSImodel.ThelowerlayersoftheOSImodel