rfc791Internet Protocol.docx
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rfc791Internet Protocol.docx
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rfc791InternetProtocol
INTERNETPROTOCOL1
1.INTRODUCTION1
1.1.Motivation2
1.2.Scope2
1.3.Interfaces2
1.4.Operation2
2.OVERVIEW3
2.1.RelationtoOtherProtocols3
2.2.ModelofOperation4
2.3.FunctionDescription5
2.4.Gateways7
3.SPECIFICATION7
3.1.InternetHeaderFormat7
3.2.Discussion17
3.3.Interfaces23
APPENDIXA:
Examples&Scenarios24
APPENDIXB:
DataTransmissionOrder28
GLOSSARY29
REFERENCES31
PREFACE
ThisdocumentspecifiestheDoDStandardInternetProtocol.ThisdocumentisbasedonsixearliereditionsoftheARPAInternetProtocolSpecification,andthepresenttextdrawsheavilyfromthem.Therehavebeenmanycontributorstothisworkbothintermsofconceptsandintermsoftext.Thiseditionrevisesaspectsofaddressing,errorhandling,optioncodes,andthesecurity,precedence,compartments,andhandlingrestrictionfeaturesoftheinternetprotocol.
JonPostel
Editor
September1981
RFC:
791
Replaces:
RFC760
IENs128,123,111,
80,54,44,41,28,26
INTERNETPROTOCOL
DARPAINTERNETPROGRAM
PROTOCOLSPECIFICATION
1.INTRODUCTION
1.1.Motivation
TheInternetProtocolisdesignedforuseininterconnectedsystemsofpacket-switchedcomputercommunicationnetworks.Suchasystemhasbeencalleda"catenet"[1].Theinternetprotocolprovidesfortransmittingblocksofdatacalleddatagramsfromsourcestodestinations,wheresourcesanddestinationsarehostsidentifiedbyfixedlengthaddresses.Theinternetprotocolalsoprovidesforfragmentationandreassemblyoflongdatagrams,ifnecessary,fortransmissionthrough"smallpacket"networks.
1.2.Scope
Theinternetprotocolisspecificallylimitedinscopetoprovidethefunctionsnecessarytodeliverapackageofbits(aninternetdatagram)fromasourcetoadestinationoveraninterconnectedsystemofnetworks.Therearenomechanismstoaugmentend-to-enddatareliability,flowcontrol,sequencing,orotherservicescommonlyfoundinhost-to-hostprotocols.Theinternetprotocolcancapitalizeon(利用)theservicesofitssupportingnetworkstoprovidevarioustypesandqualitiesofservice.
1.3.Interfaces
Thisprotocoliscalledonbyhost-to-hostprotocolsinaninternetenvironment.Thisprotocolcallsonlocalnetworkprotocolstocarrytheinternetdatagramtothenextgatewayordestinationhost.
Forexample,aTCPmodulewouldcallontheinternetmoduletotakeaTCPsegment(includingtheTCPheaderanduserdata)asthedataportionofaninternetdatagram.TheTCPmodulewouldprovidetheaddressesandotherparametersintheinternetheadertotheinternetmoduleasargumentsofthecall.Theinternetmodulewouldthencreateaninternetdatagramandcallonthelocalnetworkinterfacetotransmittheinternetdatagram.
IntheARPANETcase,forexample,theinternetmodulewouldcallonalocalnetmodulewhichwouldaddthe1822leader[2]totheinternetdatagramcreatinganARPANETmessagetotransmittotheIMP.TheARPANETaddresswouldbederivedfromtheinternetaddressbythelocalnetworkinterfaceandwouldbetheaddressofsomehostintheARPANET,thathostmightbeagatewaytoothernetworks.
1.4.Operation
Theinternetprotocolimplementstwobasicfunctions:
addressingandfragmentation.
Theinternetmodulesusetheaddressescarriedintheinternetheadertotransmitinternetdatagramstowardtheirdestinations.Theselectionofapathfortransmissioniscalledrouting.
Theinternetmodulesusefieldsintheinternetheadertofragmentandreassembleinternetdatagramswhennecessaryfortransmissionthrough"smallpacket"networks.
Themodelofoperationisthataninternetmoduleresidesineachhostengagedininternetcommunicationandineachgatewaythatinterconnectsnetworks.Thesemodulessharecommonrulesforinterpretingaddressfieldsandforfragmentingandassemblinginternetdatagrams.Inaddition,thesemodules(especiallyingateways)haveproceduresformakingroutingdecisionsandotherfunctions.
Theinternetprotocoltreatseachinternetdatagramasanindependententityunrelatedtoanyotherinternetdatagram.Therearenoconnectionsorlogicalcircuits(virtualorotherwise).
Theinternetprotocolusesfourkeymechanismsinprovidingitsservice:
TypeofService,TimetoLive,Options,andHeaderChecksum.
TheTypeofServiceisusedtoindicatethequalityoftheservicedesired.Thetypeofserviceisanabstractorgeneralizedsetofparameterswhichcharacterizetheservicechoicesprovidedinthenetworksthatmakeuptheinternet.Thistypeofserviceindicationistobeusedbygatewaystoselecttheactualtransmissionparametersforaparticularnetwork,thenetworktobeusedforthenexthop,orthenextgatewaywhenroutinganinternetdatagram.
TheTimetoLiveisanindicationofanupperboundonthelifetimeofaninternetdatagram.Itissetbythesenderofthedatagramandreducedatthepointsalongtheroutewhereitisprocessed.Ifthetimetolivereacheszerobeforetheinternetdatagramreachesitsdestination,theinternetdatagramisdestroyed.Thetimetolivecanbethoughtofasaselfdestructtimelimit.
TheOptionsprovideforcontrolfunctionsneededorusefulinsomesituationsbutunnecessaryforthemostcommoncommunications.Theoptionsincludeprovisionsfortimestamps,security,andspecialrouting.
TheHeaderChecksumprovidesaverificationthattheinformationusedinprocessinginternetdatagramhasbeentransmittedcorrectly.Thedatamaycontainerrors.Iftheheaderchecksumfails,theinternetdatagramisdiscardedatoncebytheentitywhichdetectstheerror.
Theinternetprotocoldoesnotprovideareliablecommunicationfacility.Therearenoacknowledgmentseitherend-to-endorhop-by-hop.Thereisnoerrorcontrolfordata,onlyaheaderchecksum.Therearenoretransmissions.Thereisnoflowcontrol.
ErrorsdetectedmaybereportedviatheInternetControlMessageProtocol(ICMP)[3]whichisimplementedintheinternetprotocolmodule.
2.OVERVIEW
2.1.RelationtoOtherProtocols
Thefollowingdiagramillustratestheplaceoftheinternetprotocolintheprotocolhierarchy:
+------++-----++-----++-----+
|Telnet||FTP||TFTP|...|...|
+------++-----++-----++-----+
||||
+-----++-----++-----+
|TCP||UDP|...|...|
+-----++-----++-----+
|||
+--------------------------+----+
|InternetProtocol&ICMP|
+--------------------------+----+
|
+---------------------------+
|LocalNetworkProtocol|
+---------------------------+
ProtocolRelationships
Figure1.
Internetprotocolinterfacesononesidetothehigherlevelhost-to-hostprotocolsandontheothersidetothelocalnetworkprotocol.Inthiscontexta"localnetwork"maybeasmallnetworkinabuildingoralargenetworksuchastheARPANET.
2.2.ModelofOperation
Themodelofoperationfortransmittingadatagramfromoneapplicationprogramtoanotherisillustratedbythefollowingscenario:
Wesupposethatthistransmissionwillinvolveoneintermediategateway.
Thesendingapplicationprogrampreparesitsdataandcallsonitslocalinternetmoduletosendthatdataasadatagramandpassesthedestinationaddressandotherparametersasargumentsofthecall.
Theinternetmodulepreparesadatagramheaderandattachesthedatatoit.Theinternetmoduledeterminesalocalnetworkaddressforthisinternetaddress,inthiscaseitistheaddressofagateway.Itsendsthisdatagramandthelocalnetworkaddresstothelocalnetworkinterface.
Thelocalnetworkinterfacecreatesalocalnetworkheader,andattachesthedatagramtoit,thensendstheresultviathelocalnetwork.
Thedatagramarrivesatagatewayhostwrappedinthelocalnetworkheader,thelocalnetworkinterfacestripsoffthisheader,andturnsthedatagramovertotheinternetmodule.Theinternetmoduledeterminesfromtheinternetaddressthatthedatagramistobeforwardedtoanotherhostinasecondnetwork.Theinternetmoduledeterminesalocalnetaddressforthedestinationhost.Itcallsonthelocalnetworkinterfaceforthatnetworktosendthedatagram.
Thislocalnetworkinterfacecreatesalocalnetworkheaderandattachesthedatagramsendingtheresulttothedestinationhost.
Atthisdestinationhostthedatagramisstrippedofthelocalnetheaderbythelocalnetworkinterfaceandhandedtotheinternetmodule.
Theinternetmoduledeterminesthatthedatagramisforanapplicationprograminthishost.Itpassesthedatatotheapplicationprograminresponsetoasystemcall,passingthesourceaddressandotherparametersasresultsofthecall.
ApplicationApplication
ProgramProgram
\/
InternetModuleInternetModuleInternetModule
\/\/
LNI-1LNI-1LNI-2LNI-2
\/\/
LocalNetwork1LocalNetwork2
TransmissionPath
Figure2
2.3.FunctionDescription
ThefunctionorpurposeofInternetProtocolistomovedatagramsthroughaninterconnectedsetofnetworks.Thisisdonebypassingthedatagramsfromoneinternetmoduletoanotheruntilthedestinationisreached.Theinternetmodulesresideinhostsandgatewaysintheinternetsystem.Thedatagramsareroutedfromoneinternetmoduletoanotherthroughindividualnetworksbasedontheinterpretationofaninternetaddress.Thus,oneimportantmechanismoftheinternetprotocolistheinternetaddress.
Intheroutingofmessagesfromoneinternetmoduletoanother,datagramsmayneedtotraverseanetworkwhosemaximumpacketsizeissmallerthanthesizeofthedatagram.Toovercomethisdifficulty,afragmentationmechanismisprovidedintheinternetprotocol.
Addressing
Adistinctionismadebetweennames,addresses,androutes[4].Anameindicateswhatweseek.Anaddressindicateswher
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