Internetworking Basics.docx

Internetworking Basics.docx

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InternetworkingBasics

InternetworkingBasics

ChapterGoals

InternetworkingBasics

WhatIsanInternetwork?

HistoryofInternetworking

InternetworkingChallenges

OpenSystemInterconnectionReferenceModel

CharacteristicsoftheOSILayers

Protocols

OSIModelandCommunicationBetweenSystems

InteractionBetweenOSIModelLayers

OSILayerServices

OSIModelLayersandInformationExchange

OSIModelPhysicalLayer

OSIModelDataLinkLayer

OSIModelNetworkLayer

OSIModelTransportLayer

OSIModelSessionLayer

OSIModelPresentationLayer

OSIModelApplicationLayer

InformationFormats

ISOHierarchyofNetworks

Connection-OrientedandConnectionlessNetworkServices

InternetworkAddressing

DataLinkLayerAddresses

MACAddresses

MappingAddresses

NetworkLayerAddresses

HierarchicalVersusFlatAddressSpace

AddressAssignments

AddressesVersusNames

FlowControlBasics

Error-CheckingBasics

MultiplexingBasics

StandardsOrganizations

Summary

ReviewQuestions

ForMoreInformation

ChapterGoals

∙Learnwhatmakesupaninternetwork.

∙LearnthebasicsoftheOSImodel.

∙Learnthedifferencesbetweenconnection-orientedandconnectionlessservices.

∙Learnaboutthedifferenttypesofaddressesusedinaninternetwork.

∙Learnaboutflowcontrolanderror-checkingbasics.

InternetworkingBasics

Thischapterworkswiththenextsixchapterstoactasafoundationforthetechnologydiscussionsthatfollow.Inthischapter,somefundamentalconceptsandtermsusedintheevolvinglanguageofinternetworkingareaddressed.Inthesamewaythatthisbookprovidesafoundationforunderstandingmodernnetworking,thischaptersummarizessomecommonthemespresentedthroughouttheremainderofthisbook.Topicsincludeflowcontrol,errorchecking,andmultiplexing,butthischapterfocusesmainlyonmappingtheOpenSystemInterconnection（OSI）modeltonetworking/internetworkingfunctions,andalsosummarizingthegeneralnatureofaddressingschemeswithinthecontext

oftheOSImodel.TheOSImodelrepresentsthebuildingblocksforinternetworks.Understandingtheconceptualmodelhelpsyouunderstandthecomplexpiecesthatmakeupaninternetwork.

WhatIsanInternetwork?

Aninternetworkisacollectionofindividualnetworks,connectedbyintermediatenetworkingdevices,thatfunctionsasasinglelargenetwork.Internetworkingreferstotheindustry,products,andproceduresthatmeetthechallengeofcreatingandadministeringinternetworks.Figure1-1illustratessomedifferentkindsofnetworktechnologiesthatcanbeinterconnectedbyroutersandothernetworkingdevicestocreateaninternetwork.

Figure 1-1:

DifferentNetworkTechnologiesCanBeConnectedtoCreateanInternetwork

HistoryofInternetworking

Thefirstnetworksweretime-sharingnetworksthatusedmainframesandattachedterminals.SuchenvironmentswereimplementedbybothIBM'sSystemsNetworkArchitecture（SNA）andDigital'snetworkarchitecture.

Local-areanetworks（LANs）evolvedaroundthePCrevolution.LANsenabledmultipleusersinarelativelysmallgeographicalareatoexchangefilesandmessages,aswellasaccesssharedresourcessuchasfileserversandprinters.

Wide-areanetworks（WANs）interconnectLANswithgeographicallydisperseduserstocreateconnectivity.SomeofthetechnologiesusedforconnectingLANsincludeT1,T3,ATM,ISDN,ADSL,FrameRelay,radiolinks,andothers.NewmethodsofconnectingdispersedLANsareappearingeveryday.

Today,high-speedLANsandswitchedinternetworksarebecomingwidelyused,largelybecausetheyoperateatveryhighspeedsandsupportsuchhigh-bandwidthapplicationsasmultimediaandvideoconferencing.

Internetworkingevolvedasasolutiontothreekeyproblems:

isolatedLANs,duplication

ofresources,andalackofnetworkmanagement.IsolatedLANsmadeelectroniccommunicationbetweendifferentofficesordepartmentsimpossible.Duplicationofresourcesmeantthatthesamehardwareandsoftwarehadtobesuppliedtoeachofficeordepartment,asdidseparatesupportstaff.Thislackofnetworkmanagementmeantthatnocentralizedmethodofmanagingandtroubleshootingnetworksexisted.

InternetworkingChallenges

Implementingafunctionalinternetworkisnosimpletask.Manychallengesmustbefaced,especiallyintheareasofconnectivity,reliability,networkmanagement,andflexibility.Eachareaiskeyinestablishinganefficientandeffectiveinternetwork.

Thechallengewhenconnectingvarioussystemsistosupportcommunicationamongdisparatetechnologies.Differentsites,forexample,mayusedifferenttypesofmediaoperatingatvaryingspeeds,ormayevenincludedifferenttypesofsystemsthatneedtocommunicate.

Becausecompaniesrelyheavilyondatacommunication,internetworksmustprovideacertainlevelofreliability.Thisisanunpredictableworld,somanylargeinternetworksincluderedundancytoallowforcommunicationevenwhenproblemsoccur.

Furthermore,networkmanagementmustprovidecentralizedsupportandtroubleshootingcapabilitiesinaninternetwork.Configuration,security,performance,andotherissuesmustbeadequatelyaddressedfortheinternetworktofunctionsmoothly.Securitywithinaninternetworkisessential.Manypeoplethinkofnetworksecurityfromtheperspectiveofprotectingtheprivatenetworkfromoutsideattacks.However,itisjustasimportanttoprotectthenetworkfrominternalattacks,especiallybecausemostsecuritybreachescomefrominside.Networksmustalsobesecuredsothattheinternalnetworkcannotbeusedasatooltoattackotherexternalsites.

Earlyintheyear2000,manymajorwebsiteswerethevictimsofdistributeddenialofservice（DDOS）attacks.TheseattackswerepossiblebecauseagreatnumberofprivatenetworkscurrentlyconnectedwiththeInternetwerenotproperlysecured.Theseprivatenetworkswereusedastoolsfortheattackers.

Becausenothinginthisworldisstagnant,internetworksmustbeflexibleenoughtochangewithnewdemands.

OpenSystemInterconnectionReferenceModel

TheOpenSystemInterconnection（OSI）referencemodeldescribeshowinformationfromasoftwareapplicationinonecomputermovesthroughanetworkmediumtoasoftwareapplicationinanothercomputer.TheOSIreferencemodelisaconceptualmodelcomposedofsevenlayers,eachspecifyingparticularnetworkfunctions.ThemodelwasdevelopedbytheInternationalOrganizationforStandardization（ISO）in1984,anditisnowconsideredtheprimaryarchitecturalmodelforintercomputercommunications.TheOSImodeldividesthetasksinvolvedwithmovinginformationbetweennetworkedcomputersintosevensmaller,moremanageabletaskgroups.AtaskorgroupoftasksisthenassignedtoeachofthesevenOSIlayers.Eachlayerisreasonablyself-containedsothatthetasksassignedtoeachlayercanbeimplementedindependently.Thisenablesthesolutionsofferedbyonelayertobeupdatedwithoutadverselyaffectingtheotherlayers.ThefollowinglistdetailsthesevenlayersoftheOpenSystemInterconnection（OSI）referencemodel:

∙Layer7—Application

∙Layer6—Presentation

∙Layer5—Session

∙Layer4—Transport

∙Layer3—Network

∙Layer2—Datalink

∙Layer1—Physical

Note   Ahandywaytorememberthesevenlayersisthesentence"Allpeopleseemtoneeddataprocessing."Thebeginningletterofeachwordcorrespondstoalayer.

∙All—Applicationlayer

∙People—Presentationlayer

∙Seem—Sessionlayer

∙To—Transportlayer

∙Need—Networklayer

∙Data—Datalinklayer

∙Processing—Physicallayer

Figure1-2illustratestheseven-layerOSIreferencemodel.

Figure 1-2:

TheOSIReferenceModelContainsSevenIndependentLayers

CharacteristicsoftheOSILayers

ThesevenlayersoftheOSIreferencemodelcanbedividedintotwocategories:

upperlayersandlowerlayers.

TheupperlayersoftheOSImodeldealwithapplicationissuesandgenerallyareimplementedonlyinsoftware.Thehighestlayer,theapplicationlayer,isclosesttotheenduser.Bothusersandapplicationlayerprocessesinteractwithsoftwareapplicationsthatcontainacommunicationscomponent.ThetermupperlayerissometimesusedtorefertoanylayeraboveanotherlayerintheOSImodel.

ThelowerlayersoftheOSImodelhandledatatransportissues.Thephysicallayerandthedatalinklayerareimplementedinhardwareandsoftware.Thelowestlayer,thephysicallayer,isclosesttothephysicalnetworkmedium（thenetworkcabling,forexample）andisresponsibleforactuallyplacinginformationonthemedium.

Figure1-3illustratesthedivisionbetweentheupperandlowerOSIlayers.

Figure 1-3:

TwoSetsofLayersMakeUptheOSILayers

[page]

Protocols

TheOSImodelprovidesaconceptualframeworkforcommunicationbetweencomputers,butthemodelitselfisnotamethodofcommunication.Actualcommunicationismadepossiblebyusingcommunicationprotocols.Inthecontextofdatanetworking,aprotocolisaformalsetofrulesandconventionsthatgovernshowcomputersexchangeinformationoveranetworkmedium.AprotocolimplementsthefunctionsofoneormoreoftheOSIlayers.

Awidevarietyofcommunicationprotocolsexist.SomeoftheseprotocolsincludeLANprotocols,WANprotocols,networkprotocols,androutingprotocols.LANprotocolsoperateatthephysicalanddatalinklayersoftheOSImodelanddefinecommunicationoverthevariousLANmedia.WANprotocolsoperateatthelowestthreelayersoftheOSImodelanddefinecommunicationoverthevariouswide-areamedia.Routingprotocolsarenetworklayerprotocolsthatareresponsibleforexchanginginformationbetweenrouterssothattherouterscanselecttheproperpathfornetworktraffic.Finally,networkprotocolsarethevariousupper-layerpr