TCPIP tutorial RFC 1180.docx
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TCPIP tutorial RFC 1180.docx
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TCPIPtutorialRFC1180
TCP/IPtutorial
RFC1180
∙Document
∙IESGEvaluationRecord
∙IESGWriteups
∙History
Documenttype:
RFC-Informational(LegacyStream)
Errata
Published:
1991-01
Otherversions:
plaintext,pdf,html
IPRDisclosures|DependenciestothisRFC
NetworkWorkingGroupT.Socolofsky
RequestforComments:
1180C.Kale
SpiderSystemsLimited
January1991
ATCP/IPTutorial
StatusofthisMemo
ThisRFCisatutorialontheTCP/IPprotocolsuite,focusing
particularlyonthestepsinforwardinganIPdatagramfromsource
hosttodestinationhostthrougharouter.Itdoesnotspecifyan
Internetstandard.Distributionofthismemoisunlimited.
TableofContents
1.Introduction................................................1
2.TCP/IPOverview.............................................2
3.Ethernet....................................................8
4.ARP.........................................................9
5.InternetProtocol...........................................12
6.UserDatagramProtocol......................................22
7.TransmissionControlProtocol...............................24
8.NetworkApplications........................................25
9.OtherInformation...........................................27
10.References..................................................27
11.RelationtootherRFCs......................................27
12.SecurityConsiderations.....................................27
13.Authors'Addresses..........................................28
1.Introduction
ThistutorialcontainsonlyoneviewofthesalientpointsofTCP/IP,
andthereforeitisthe"barebones"ofTCP/IPtechnology.Itomits
thehistoryofdevelopmentandfunding,thebusinesscaseforits
use,anditsfutureascomparedtoISOOSI.Indeed,agreatdealof
technicalinformationisalsoomitted.Whatremainsisaminimumof
informationthatmustbeunderstoodbytheprofessionalworkingina
TCP/IPenvironment.Theseprofessionalsincludethesystems
administrator,thesystemsprogrammer,andthenetworkmanager.
ThistutorialusesexamplesfromtheUNIXTCP/IPenvironment,however
themainpointsapplyacrossallimplementationsofTCP/IP.
Notethatthepurposeofthismemoisexplanation,notdefinition.
Ifanyquestionarisesaboutthecorrectspecificationofaprotocol,
pleaserefertotheactualstandardsdefiningRFC.
Socolofsky&Kale[Page1]
RFC1180ATCP/IPTutorialJanuary1991
ThenextsectionisanoverviewofTCP/IP,followedbydetailed
descriptionsofindividualcomponents.
2.TCP/IPOverview
Thegenericterm"TCP/IP"usuallymeansanythingandeverything
relatedtothespecificprotocolsofTCPandIP.Itcaninclude
otherprotocols,applications,andeventhenetworkmedium.Asample
oftheseprotocolsare:
UDP,ARP,andICMP.Asampleofthese
applicationsare:
TELNET,FTP,andrcp.Amoreaccuratetermis
"internettechnology".Anetworkthatusesinternettechnologyis
calledan"internet".
2.1BasicStructure
Tounderstandthistechnologyyoumustfirstunderstandthefollowing
logicalstructure:
----------------------------
|networkapplications|
||
|...\|/..\|/...|
|----------|
||TCP||UDP||
|----------|
|\/|
|--------|
||IP||
|------*------|
||ARP|||
|-----||
|\||
|------|
||ENET||
|---@--|
----------|-----------------
|
----------------------o---------
EthernetCable
Figure1.BasicTCP/IPNetworkNode
Thisisthelogicalstructureofthelayeredprotocolsinsidea
computeronaninternet.Eachcomputerthatcancommunicateusing
internettechnologyhassuchalogicalstructure.Itisthislogical
structurethatdeterminesthebehaviorofthecomputeronthe
internet.Theboxesrepresentprocessingofthedataasitpasses
throughthecomputer,andthelinesconnectingboxesshowthepathof
Socolofsky&Kale[Page2]
RFC1180ATCP/IPTutorialJanuary1991
data.ThehorizontallineatthebottomrepresentstheEthernet
cable;the"o"isthetransceiver.The"*"istheIPaddressandthe
"@"istheEthernetaddress.Understandingthislogicalstructureis
essentialtounderstandinginternettechnology;itisreferredto
throughoutthistutorial.
2.2Terminology
Thenameofaunitofdatathatflowsthroughaninternetis
dependentuponwhereitexistsintheprotocolstack.Insummary:
if
itisonanEthernetitiscalledanEthernetframe;ifitisbetween
theEthernetdriverandtheIPmoduleitiscalledaIPpacket;ifit
isbetweentheIPmoduleandtheUDPmoduleitiscalledaUDP
datagram;ifitisbetweentheIPmoduleandtheTCPmoduleitis
calledaTCPsegment(moregenerally,atransportmessage);andifit
isinanetworkapplicationitiscalledaapplicationmessage.
Thesedefinitionsareimperfect.Actualdefinitionsvaryfromone
publicationtothenext.Morespecificdefinitionscanbefoundin
RFC1122,section1.3.3.
Adriverissoftwarethatcommunicatesdirectlywiththenetwork
interfacehardware.Amoduleissoftwarethatcommunicateswitha
driver,withnetworkapplications,orwithanothermodule.
Thetermsdriver,module,Ethernetframe,IPpacket,UDPdatagram,
TCPmessage,andapplicationmessageareusedwhereappropriate
throughoutthistutorial.
2.3FlowofData
Let'sfollowthedataasitflowsdownthroughtheprotocolstack
showninFigure1.ForanapplicationthatusesTCP(Transmission
ControlProtocol),datapassesbetweentheapplicationandtheTCP
module.ForapplicationsthatuseUDP(UserDatagramProtocol),data
passesbetweentheapplicationandtheUDPmodule.FTP(File
TransferProtocol)isatypicalapplicationthatusesTCP.Its
protocolstackinthisexampleisFTP/TCP/IP/ENET.SNMP(Simple
NetworkManagementProtocol)isanapplicationthatusesUDP.Its
protocolstackinthisexampleisSNMP/UDP/IP/ENET.
TheTCPmodule,UDPmodule,andtheEthernetdriveraren-to-1
multiplexers.Asmultiplexerstheyswitchmanyinputstooneoutput.
Theyarealso1-to-nde-multiplexers.Asde-multiplexerstheyswitch
oneinputtomanyoutputsaccordingtothetypefieldintheprotocol
header.
Socolofsky&Kale[Page3]
RFC1180ATCP/IPTutorialJanuary1991
123...n123...n
\|/|\||/^
\||/|\||/|
-------------flow----------------flow
|multiplexer|of|de-multiplexer|of
-------------data----------------data
||||
|v||
11
Figure2.n-to-1multiplexerand1-to-nde-multiplexer
IfanEthernetframecomesupintotheEthernetdriveroffthe
network,thepacketcanbepassedupwardstoeithertheARP(Address
ResolutionProtocol)moduleortotheIP(InternetProtocol)module.
ThevalueofthetypefieldintheEthernetframedetermineswhether
theEthernetframeispassedtotheARPortheIPmodule.
IfanIPpacketcomesupintoIP,theunitofdataispassedupwards
toeitherTCPorUDP,asdeterminedbythevalueoftheprotocol
fieldintheIPheader.
IftheUDPdatagramcomesupintoUDP,theapplicationmessageis
passedupwardstothenetworkapplicationbasedonthevalueofthe
portfieldintheUDPheader.IftheTCPmessagecomesupintoTCP,
theapplicationmessageispassedupwardstothenetworkapplication
basedonthevalueoftheportfieldintheTCPheader.
Thedownwardsmultiplexingissimpletoperformbecausefromeach
startingpointthereisonlytheonedownwardpath;eachprotocol
moduleaddsitsheaderinformationsothepacketcanbede-
multiplexedatthedestinationcomputer.
DatapassingoutfromtheapplicationsthrougheitherTCPorUDP
convergesontheIPmoduleandissentdownwardsthroughthelower
networkinterfacedriver.
Althoughinternettechnologysupportsmanydifferentnetworkmedia,
Ethernetisusedforallexamplesinthistutorialbecauseitisthe
mostcommonphysicalnetworkusedunderIP.ThecomputerinFigure1
hasasingleEthernetconnection.The6-byteEthernetaddressis
uniqueforeachinterfaceonanEthernetandislocatedatthelower
interfaceoftheEthernetdriver.
Thecomputeralsohasa4-byteIPaddress.Thisaddressislocated
atthelowerinterfacetotheIPmodule.TheIPaddressmustbe
uniqueforaninternet.
Socolofsky&Kale[Page4]
RFC1180ATCP/IPTutorialJanuary1991
ArunningcomputeralwaysknowsitsownIPaddressandEthernet
address.
2.4TwoNetworkInterfaces
Ifacomputerisconnectedto2separateEthernetsitisasinFigure
3.
----------------------------
|networkapplications|
||
|...\|/..\|/...|
|----------|
||TCP||UDP||
|----------|
|\/|
|--------|
||IP||
|------*----*------|
||ARP||||ARP||
|-----||-----|
|\||/|
|------------|
||ENET||ENET||
|---@-----@--|
----------|-------|---------
||
|---o---------------------------
|EthernetCable2
---------------o----------
EthernetCable1
Figure3.TCP/IPNetworkNodeon2Ethernets
Pleasenotethatthiscomputerhas2Ethernetaddressesand2IP
addresses.
Itisseenfromthisstructurethatforcomputerswithmorethanone
physicalnetworkinterface,theIPmoduleisbothan-to-m
multiplexerandanm-to-nde-multiplexer.
Socolofsky&Kale[Page5]
RFC1180ATCP/IPTutorialJanuary1991
123...n123...n
\||/|\||/^
\||/|\||/|
-------------flow----------------flow
|multiplexer|of|de-multiplexer|of
-------------data----------------data
/||\|/||\|
/||\v/||\|
123...m123...m
Figure4.n-to-mmultiplexerandm-to-nde-multiplexer
Itperformsth
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