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3 Early Academic and Scientific Networks — The Roots and Architecture of the Internet

3 Early Academic and Scientific Networks — The Roots and Architecture of the Internet

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11.3EarlyAcademicandScientificNetworks—

TheRootsandArchitectureoftheInternet

AmidtheangstoftheColdWar,Americanscientistsatfar-flungresearch

institutionstoiledundergovernmentcontracts,seekingtopreservethe

militaryascendancyoftheUnitedStates.Atatimewhenwehadfallen

behindinthetechnologyrace,theUnitedStatesgovernmentcreatedan

organizationcalledtheAdvancedResearchProjectsAgency(ARPA).

Thesophisticatedcomputersthisorganizationneededtocarryoutits

work,however,werescarceandextremelycostly—evenbyPentagon

standards.Beforelong,itoccurredtosomeonethatbyestablishing

communicationlinksintothefewsupercomputersthatwerescatteredall

overtheUnitedStates,computationalresourcescouldbesharedby

innumerablelike-mindedresearchers.Moreover,thisnetworkwouldbe

designedwithsufficientredundancytoprovideforcontinuous

communication,evenifthermonuclearwarknockedoutalargenumber

ofnodesorcommunicationlines.TothisendinDecember1968,a

Cambridge,Massachusetts,consultingfirmcalledBBN(Bolt,Beranek

andNewman,nowGenuityCorporation)wasawardedthecontractto

constructsuchanetwork.InDecember1969,fournodes,theUniversity

ofUtah,theUniversityofCaliforniaatLosAngeles,theUniversityof

CaliforniaatSantaBarbara,andtheStanfordResearchInstitute,went

online.ARPAnetgraduallyexpandedtoincludemoregovernmentand

researchinstitutions.WhenPresidentReaganchangedthenameof

ARPAtotheDefenseAdvancedResearchProjectsNetwork(DARPA),

ARPAnetbecameDARPAnet.Throughtheearly1980s,nodeswere

addedatarateofalittlemorethanonepermonth.However,military

researcherseventuallyabandonedDARPAnetinfavorofmoresecure

channels.

In1985,theNationalScienceFoundationestablisheditsownnetwork,

NSFnet,tosupportitsscientificandacademicresearch.NSFnetand

DARPAnetservedasimilarpurposeandasimilarusercommunity,but

thecapabilitiesofNSFnetoutstrippedthoseofDARPAnet.Consequently,

whenthemilitaryabandonedDARPAnet,NSFnetabsorbedit,and

becamewhatwenowknowastheInternet.Bytheearly1990s,theNSF

hadoutgrownNSFnet,soitbeganbuildingafaster,morereliable



NSFnet.AdministrationofthepublicInternetthenfelltoprivatenational

andregionalcorporations,suchasSprint,MCI,andPacBell,tonamea

few.ThesecompaniesboughttheNSFnettrunklines,calledbackbones,

andmademoneybysellingbackbonecapacitytovariousInternetservice

providers(ISPs).

TheoriginalDARPAnet(andnowtheInternet)wouldhavesurvived

thermonuclearwarbecause,unlikeallothernetworksinexistenceinthe

1970s,ithadnodedicatedconnectionsbetweensystems.Information

wasinsteadroutedalongwhateverpathwayswereavailable.Partsofthe

datastreambelongingtoasingledialoguecouldtakedifferentroutesto

theirdestinations.Thekeytothisrobustnessistheideaofdatagram

messagepackets,whichcarrydatainchunksinsteadofthestreams

usedbytheSNAmodel.Eachdatagramcontainsaddressinginformation

sothateverydatagramcanberoutedasasingle,discreteunit.

AsecondrevolutionaryaspectofDARPAnetwasthatitcreatedauniform

protocolforcommunicationsbetweendissimilarhostsalongnetworksof

differingspeeds.Becauseitconnectedmanydifferentkindsofnetworks,

DARPAnetwassaidtobeaninternetwork.Asoriginallyspecified,each

hostcomputerconnectedtoDARPAnetbywayofanInterfaceMessage

Processor(IMP).IMPstookcareofprotocoltranslationfromthe

languageofDARPAnettothecommunicationslanguagenativetothe

hostsystem,soanycommunicationsprotocolcouldbeusedbetweenthe

IMPandthehost.Today,routers(discussedinSection11.6.7)have

replacedIMPs,andthecommunicationsprotocolsareless

heterogeneousthantheywereinthe1970s.However,theunderlying

principleshaveremainedthesame,andthegenericconceptof

internetworkinghasbecomepracticallysynonymouswiththeInternet.A

moderninternetworkconfigurationisshowninFigure11.2.Thediagram

showshowfourroutersformtheheartofthenetwork.Theyconnect

manydifferenttypesofequipment,makingdecisionsontheirownasto

howdatagramsshouldgettotheirdestinationsinthemostefficientway

possible.



Figure11.2:AnExample

Internetwork

TheInternetismuchmorethanasetofgooddatacommunication

specifications.Itis,perhaps,aphilosophy.Theforemostprincipleofthis

philosophyistheideaofafreeandopenworldofinformationsharing,

withthedestinyofthisworldbeingshapedcollaborativelybythepeople

andideasinit.Theepitomeofthisopennessisthemannerinwhich

Internetstandardsarecreated.Internetstandardsareformulatedthrough

ademocraticprocessthattakesplaceundertheauspicesoftheInternet

ArchitectureBoard(IAB),whichitselfoperatesundertheoversightofthe

not-for-profitInternetSociety(ISOC).TheInternetEngineeringTask

Force(IETF),operatingwithintheIAB,isalooseallianceofindustry

expertsthatdevelopsdetailedspecificationsforInternetprotocols.The

IETFpublishesallproposedstandardsintheformofRequestsfor

Comment(RFCs),whichareopentoanyone'sscrutinyandcomment.

ThetwomostimportantRFCs—RFC791(InternetProtocolVersion4)

andRFC793(TransmissionControlProtocol)—formthefoundationof

today'sglobalInternet.

TheorganizationofalltheISOC'scommitteesundermorecommittees

couldhaveresultedinatangleofbureaucracyproducinginscrutableand

convolutedspecifications.Butowingtotheopennessoftheentire

process,aswellasthetalentsofthereviewers,RFCsareamongthe

clearestandmostreadabledocumentsintheentirebodyofnetworking

literature.Itislittlewonderthatmanufacturersweresoquicktoadopt

Internetprotocols.Internetprotocolsarenowrunningonallsizesof



networks,bothpubliclyandprivatelyowned.Formerly,networking

standardswerehandeddownbyacentralizedcommitteeorthroughan

equipmentvendor.OnesuchapproachresultedintheISO/OSIprotocol

model,whichwediscussnext.











11.5NetworkProtocolsII—TCP/IPNetwork

Architecture

WhiletheISOandtheCCITTwerehagglingoverthefinerpointsofthe

perfectprotocolstack,TCP/IPwasrapidlyspreadingacrosstheglobe.

Bythesheerweightofitspopularitywithintheacademicandscientific

communicationscommunities,TCP/IPquietlybecamethedefactoglobal

datacommunicationstandard.

Althoughitdidn'tstartoutthatway,TCP/IPisnowaleanandeffective

protocolstack.Ithasthreelayersthatcanbemappedtofiveoftheseven

layersintheOSImodel.TheselayersareshowninFigure11.4.Because

theIPlayerislooselycoupledwithOSI'sDataLinkandPhysicallayers,

TCP/IPcanbeusedwithanytypeofnetwork,evendifferenttypesof

networkswithinasinglesession.Thesingularrequirementisthatallof

theparticipatingnetworksmustberunning—atminimum—Version4of

theInternetProtocol(IPv4).



Figure11.4:TheTCP/IPProtocolStack

VersustheOSIProtocolStack

TherearetwoversionsoftheInternetProtocolinusetoday,Version4

andVersion6.IPv6addressesmanyofthelimitationsofIPv4.Despite

themanyadvantagesofIPv6,thehugeinstalledbaseofIPv4ensures

thatitwillbesupportedformanyyearstocome.Someofthemajor

differencesbetweenIPv4andIPv6areoutlinedinSection11.5.5.But

first,wetakeadetailedlookatIPv4.



11.5.1TheIPLayerforVersion4



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