计算机类数据通信实验中的套接字编程中英文翻译外文翻译外文文献翻译.docx

计算机类数据通信实验中的套接字编程中英文翻译外文翻译外文文献翻译.docx

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计算机类数据通信实验中的套接字编程中英文翻译外文翻译外文文献翻译

附原文：

SOCKETPROGRAMMINGINTHEDATACOMMUNICATIONSLABORATORY

WilliamE.Toll

ComputingandSystemSciences

TaylorUniversity

Upland,IN46989

btoll@css.tayloru.edu

ABSTRACT

Althoughmanydatacommunicationscoursesaretaughtwithnoprogrammingcontentcoursesdesignedforcomputersciencemajorsshouldincludeprogramming.ManydatacommunicationscourseswithaprogrammingcomponentmakeuseofserialportsonPCswhilesomedealwithdetailednetworklayerprojects.UNIXsocketprogrammingallowsthestudentstodealwiththesameissuesandproblems,butinacontextthatismorelikelybeusefulandthatismoreinteresting.Inaddition,ifsocketclassesareusedwithC++,onlyasmuchdetailofsocketoperationasdesiredneedbepresented.

lNTRODUCTION

DatacommunicationsisastandardpartofmostMISandCSprograms.Theactualimplementationofthecoursevarieswidelyasevidencedbythevarietyoftextbooksavailable.Manytexts,whetherorientedtowardMISorCS,providelittleornolaboratoryactivity.MISprogramstendtoemphasizemanagementofdatacommunicationsandnetworks.Recentnewslistspostingsindicateanemphasisonusingdatacommunicationsandinvestigationsofthetypesandstylesofcommunicationavailable.National,orinternational,cooperativeprojectsarepopular.CSprogramsmayuseverytechnicaltextsorabroadtextsuchasusedbytheauthor[1]whereprinciples,designapproaches,andstandardsareemphasized.Obviously,anengineeringprogramwouldhaveamuchmoreextensiveanddetailedcourse（s）toinvestigatethephysicalandstructuralaspectsofdatacommunication.ThecoursetaughtbytheauthorisrequiredofallCSmajors.Studentsinthecoursemaybeinanyofthespecializedtracks（artificialintelligence,businessinformationsystems,graphicsorscientificprogramming）aswellasamoregenericCSmajor.

Thepossibletypesoflaboratoryexperiencesarealsobroad.The“globalcooperationmodel”teacheshowdatacommunicationsworksbyforcingstudentstousesophisticatedcommunicationsmechanismsandprovidesabasisforexplaininghowthesesystemsfunction.Itispossible,basedontheavailableresources,toconsiderdesignalternativesbyallowingstudentstoexploredifferentphysicalorlogicaltypesofcommunication.

Attheotherextremeareexercisesthatemphasizelow-levelphysicalunderstandingofdatacommunications-almostanengineeringapproach.AtypicalexamplewouldbetheuseofserialportsonPCs.Inadditiontowritingcodetomanipulatethephysicalhardware,manymorecomplicatedconceptscanbestudied.Inmaterialtheauthorhasusedinthepast,filetransferassignmentsusingamodifiedBiSynchprotocolandtokenringsareimplemented[2].Analternativelow-levelapproachismodeledbytheNetCpsoftware[3].Thislaboratoryapproachinvolvesalargescaleprojectbasedondevelopingthe0S11S0datalinklayer.

Noneoftheseapproachesprovidepracticalhands-onhardwareexperience.Inadditiontotheexercisesdescribedinthispaper,theauthorassignsaprojectinvolvinginstallationofhardwareandsoftwaretoaddaPCtoanetwork.Aservercanbeinstalledandconfiguredforextracredit.Suchaprojectwascontinuedwhenthesocketmodelwasadopted.

Studentsplacedinpracticalorintheirfirstjobhaveconsideredthedatacommunicationscourseimportant.TheyhavetypicallynotbelievedthePCserialportprogrammingtobeimportant,however.Theapproachpresentedhereisdesignedtoprovideabroadoverviewofdatacommunicationandnetworkissuestoourstudents.Thegoalsforthelaboratorypartofthedatacommunicationscoursearepresentedlaterinthepaper.

UNIXSOCKETS

Simplystatedsocketsareamechanismbywhichmessagesmaybesentbetweenprocessesonthesameordifferentmachines.Iftheprocessesareonthesamemachine,thesocketsmaybeusedaspipes.Internetsocketsallowcommunicationbetweenprocessesrunningondifferentmachines.Thesystemcallsarethesameasfilei/o.AtypicalapproachtosocketprogrammingistocreateaprocessthatopensaSewersocketportandlistensforanotherprocesstoattemptconnection.Aclientcanopenasocket,withthesameportnumberastheserversocket,requestingconnectiontotheservice.Whentheserverhearstherequestaconnectionisestablished.Communicationcannowproceedwithread（）andwrite（）

.

Therearemanytypesofstandardprotocols.TwoofthemostcommonareUDP（userdatagramprotocol）andTCP（transmissioncontrolprotocol）.Bothprotocolstransmitpacketsofinformationbetweenprocessesviaasocket.UDPdoesnotprovideaguaranteethatdatawillbereceivedorthatamultiplepackettransmissionwillbereceivedinorder.TCPisastreamprotocolthatisreliableandsequenced.TotheprogrammerinputandoutputonaTCPsocketappearsasabytestreamfromaterminalorafile.IfTCPdatacannotbetransmittedsuccessfullywithinareasonableamountoftime,anerrorisindicated.ThereislessoverheadinvolvedinUDP,butprogrammingmustbemuchmoresophisticatediforderlymessagereceiptisimportant.

Thesocketconnectionbetweentwoprocessesisaconnectionbetweenhost.portpairswheretheportnumberindicatesaparticularservicethatismadeavailable.ManyoftheservicescommonlyavailableviaTCPsocketsarerecognizableacronyms:

SMTP（SimpleMailTransportProtocolusedfore-mail）,NNTP（NetworkNewsTransportProtocol,usedforUsenetnews）andFIT（FileTransportProtocol）.Telnetandrshareadditionalsocketservices.UNIXprovidesamechanismwherebythenameofanavailableservicesistranslatedtoaportnumber.

Socketsarealsousedfortheinterprocesscommunicationnecessaryinconcurrentorparallelprocessing.Therefore,parallelprocessingassignmentsaswellasdatacommunicationsprojectscanbebuiltonthesameframework.

ADVANTAGESOFSOCKETS

OneobviousdisadvantageofusingsocketprogrammingforthedatacommunicationslabisthatthereislessdirecthardwareinteractionthanwithPCserialports.However,mostgraduateswillnotbeinsituationswheresuchdetailedknowledgewillbeimportant.EvenwiththeserialportapproachtheconceptshaveremainedsomewhatabstracttomanystudentsThesocketbasedapproachhastheadvantagethattheabstractconceptsofsockets（andpracticalusessuchasmail,telnet,etc.）becomemuchmoreconcrete.

OneadvantagethatPCbasedlabshavehadinthepastisthattheywereinexpensive.However,thereareatleasttwofactorsthatbalancethisadvantage.OneisthatUNIXworkstationsarenowcommonlyavailable.PClabscanbeconvertedtoworkstationsbyinstallingfreeversionsofUNIX.Sincemost,ifnotall,socketassignmentsgiveninadatacommunicationscoursearenotcomputeintensiveanddonotrequireagraphicalinterface,workstationsneednotbededicatedtothecourseaswouldbetrueofPCs.AnotherfactoristhateventhoughPCsarerelativelyinexpensive,whathappenspracticallyisthatolder,lessreliable,machinesareassignedtoadedicatedprojectsuchasadatacommunicationslab.Ourexperiencewasthatthemachineswecould“afford”tousewerequiteunreliable.

Althoughthe“highlevel”natureofsocketprogramminghasbeenstressedasanadvantage,itispossibletomaketheassignmentsasdetailedasdesired.Socketprogrammingwithoutanysupportsoftwarecanrequireagreatdealof“lowlevel”understandingandmanipulation.OnesimplemodificationwouldbetobaseassignmentsonUDPpacketsratherthanTCPpackets.Muchadditionalprogramming（errorcheckingviaCRCS,sequencenumbers,acknowledgmentofreceiptnegativeacknowledgmentforreceiptofabadpacket）wouldbenecessary.WitheitherUDPorTCPpackets,properlydesignedhandshakingmechanismsmaybenecessaryforsuchapplicationsasfiletransfer.

Withserialportassignments,lecturetimewasdevotedtosuchlowlevelconceptsascontrol,statusanddataregisters,andparalleltoserialconversion.Withasocketbasedapproachanalogousconceptssuchaspacketheadersandnetworkandmachinebyteordercanbediscussed.Ifdesiredmanyofthetopicsappropriateforserialportcommunicationcanberequiredforsocketprogramsandmanyofthesameassignmentscanbegiven.Evenifhighlevelapplicationsareassigned,thestudentsmuststillunderstandthedifferencesbetweenstreamsandbuffers.

ADVANTAGESOFC++SOCKETCLASSES

Manyreferencesprovidedetailsofsocketcommunication[4,5,6].Thesereferencesprovideexamplesandideasforassignments.AUofthedetailofestablishingcommunications,convertingthecommunicationintoabufferedstreamanderrorcheckingcanbedonewithUNIXsystemcalls.Muchlowlevelunderstandingmayberequiredtowriteapplicationsthatarestable.

AwelldesignedsetofC++classescanbeconstructedwhichwillprovidethefullpowerofsocketswhilerequiringsimplesemantics.Itispossibletowriteclientstoestablishedservers,eventdrivenservers,pollingservers,etc.

TheauthorprovidedthestudentswithasetofC++socketclasseswrittenandcopyrightbyGnanasekaranSwaminathanoftheElectricalEngineeringDepartmentoftheUniversityofVirginia（gs4t@virginia.edu）[7].TheseroutineshavebeenwrittentoworkwithGNUlibg++andappearthesameastheiostreamlibrary.Theyareavailablefromftp.virginia.edu.Theseclasseshavefunctionedverywellfortheassignmentsgiven.Theinterfaceisthesameastheiostreamlibraryandprovidestype-safeinputandoutput,TherearesockstreamclassesintheUDPandTCPdomainsaswellasapipestreamclass.Thesockbufclassesarederivedfromthestreambufclassofthelibg++iostreamlibrary.Thus,studentsmustlearnaboutstreamsandbuffersfornon-socketinputandoutput.