SIP.docx
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SIP.docx
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SIP
1、Introduction
TherearemanyapplicationsoftheInternetthatrequirethecreation
andmanagementofasession,whereasessionisconsideredan
exchangeofdatabetweenanassociationofparticipants.The
implementationoftheseapplicationsiscomplicatedbythepractices
ofparticipants:
usersmaymovebetweenendpoints,theymaybe
addressablebymultiplenames,andtheymaycommunicateinseveral
differentmedia-sometimessimultaneously.Numerousprotocolshave
beenauthoredthatcarryvariousformsofreal-timemultimedia
sessiondatasuchasvoice,video,ortextmessages.TheSession
InitiationProtocol(SIP)worksinconcertwiththeseprotocolsby
enablingInternetendpoints(calleduseragents)todiscoverone
anotherandtoagreeonacharacterizationofasessiontheywould
liketoshare.Forlocatingprospectivesessionparticipants,and
forotherfunctions,SIPenablesthecreationofaninfrastructureof
networkhosts(calledproxyservers)towhichuseragentscansend
registrations,invitationstosessions,andotherrequests.SIPis
anagile,general-purposetoolforcreating,modifying,and
terminatingsessionsthatworksindependentlyofunderlyingtransport
protocolsandwithoutdependencyonthetypeofsessionthatisbeing
established.
2OverviewofSIPFunctionality
SIPisanapplication-layercontrolprotocolthatcanestablish,
modify,andterminatemultimediasessions(conferences)suchas
Internettelephonycalls.SIPcanalsoinviteparticipantsto
alreadyexistingsessions,suchasmulticastconferences.Mediacan
beaddedto(andremovedfrom)anexistingsession.SIP
transparentlysupportsnamemappingandredirectionservices,which
supportspersonalmobility[27]-userscanmaintainasingle
externallyvisibleidentifierregardlessoftheirnetworklocation.
SIPsupportsfivefacetsofestablishingandterminatingmultimedia
communications:
Userlocation:
determinationoftheendsystemtobeusedfor
communication;
Useravailability:
determinationofthewillingnessofthecalled
partytoengageincommunications;
Usercapabilities:
determinationofthemediaandmediaparameters
tobeused;
Sessionsetup:
"ringing",establishmentofsessionparametersat
bothcalledandcallingparty;
Sessionmanagement:
includingtransferandterminationof
sessions,modifyingsessionparameters,andinvoking
services.
SIPisnotaverticallyintegratedcommunicationssystem.SIPis
ratheracomponentthatcanbeusedwithotherIETFprotocolsto
buildacompletemultimediaarchitecture.Typically,these
architectureswillincludeprotocolssuchastheReal-timeTransport
Protocol(RTP)(RFC1889[28])fortransportingreal-timedataand
providingQoSfeedback,theReal-Timestreamingprotocol(RTSP)(RFC
2326[29])forcontrollingdeliveryofstreamingmedia,theMedia
GatewayControlProtocol(MEGACO)(RFC3015[30])forcontrolling
gatewaystothePublicSwitchedTelephoneNetwork(PSTN),andthe
SessionDescriptionProtocol(SDP)(RFC2327[1])fordescribing
multimediasessions.Therefore,SIPshouldbeusedinconjunction
withotherprotocolsinordertoprovidecompleteservicestothe
users.However,thebasicfunctionalityandoperationofSIPdoes
notdependonanyoftheseprotocols.
SIPdoesnotprovideservices.Rather,SIPprovidesprimitivesthat
canbeusedtoimplementdifferentservices.Forexample,SIPcan
locateauseranddeliveranopaqueobjecttohiscurrentlocation.
Ifthisprimitiveisusedtodeliverasessiondescriptionwrittenin
SDP,forinstance,theendpointscanagreeontheparametersofa
session.Ifthesameprimitiveisusedtodeliveraphotoofthe
calleraswellasthesessiondescription,a"callerID"servicecan
beeasilyimplemented.Asthisexampleshows,asingleprimitiveis
typicallyusedtoprovideseveraldifferentservices.
SIPdoesnotofferconferencecontrolservicessuchasfloorcontrol
orvotinganddoesnotprescribehowaconferenceistobemanaged.
SIPcanbeusedtoinitiateasessionthatusessomeotherconference
controlprotocol.SinceSIPmessagesandthesessionstheyestablish
canpassthroughentirelydifferentnetworks,SIPcannot,anddoes
not,provideanykindofnetworkresourcereservationcapabilities.
Thenatureoftheservicesprovidedmakesecurityparticularly
important.Tothatend,SIPprovidesasuiteofsecurityservices,
whichincludedenial-of-serviceprevention,authentication(bothuser
touserandproxytouser),integrityprotection,andencryptionand
privacyservices.
SIPworkswithbothIPv4andIPv6.
3Terminology
Inthisdocument,thekeywords"MUST","MUSTNOT","REQUIRED",
"SHALL","SHALLNOT","SHOULD","SHOULDNOT","RECOMMENDED","NOT
RECOMMENDED","MAY",and"OPTIONAL"aretobeinterpretedas
describedinBCP14,RFC2119[2]andindicaterequirementlevelsfor
compliantSIPimplementations.
4OverviewofOperation
ThissectionintroducesthebasicoperationsofSIPusingsimple
examples.Thissectionistutorialinnatureanddoesnotcontain
anynormativestatements.
ThefirstexampleshowsthebasicfunctionsofSIP:
locationofan
endpoint,signalofadesiretocommunicate,negotiationofsession
parameterstoestablishthesession,andteardownofthesessiononce
established.
Figure1showsatypicalexampleofaSIPmessageexchangebetween
twousers,AliceandBob.(Eachmessageislabeledwiththeletter
"F"andanumberforreferencebythetext.)Inthisexample,Alice
usesaSIPapplicationonherPC(referredtoasasoftphone)tocall
BobonhisSIPphoneovertheInternet.AlsoshownaretwoSIPproxy
serversthatactonbehalfofAliceandBobtofacilitatethesession
establishment.Thistypicalarrangementisoftenreferredtoasthe
"SIPtrapezoid"asshownbythegeometricshapeofthedottedlines
inFigure1.
Alice"calls"BobusinghisSIPidentity,atypeofUniformResource
Identifier(URI)calledaSIPURI.SIPURIsaredefinedinSection
19.1.Ithasasimilarformtoanemailaddress,typically
containingausernameandahostname.Inthiscase,itis
sip:
bob@,whereisthedomainofBob'sSIP
serviceprovider.AlicehasaSIPURIofsip:
alice@.
AlicemighthavetypedinBob'sURIorperhapsclickedonahyperlink
oranentryinanaddressbook.SIPalsoprovidesasecureURI,
calledaSIPSURI.Anexamplewouldbesips:
bob@.Acall
madetoaSIPSURIguaranteesthatsecure,encryptedtransport
(namelyTLS)isusedtocarryallSIPmessagesfromthecallertothe
domainofthecallee.Fromthere,therequestissentsecurelyto
thecallee,butwithsecuritymechanismsthatdependonthepolicyof
thedomainofthecallee.
SIPisbasedonanHTTP-likerequest/responsetransactionmodel.
Eachtransactionconsistsofarequestthatinvokesaparticular
method,orfunction,ontheserverandatleastoneresponse.In
thisexample,thetransactionbeginswithAlice'ssoftphonesending
anINVITErequestaddressedtoBob'sSIPURI.INVITEisanexample
ofaSIPmethodthatspecifiestheactionthattherequestor(Alice)
wantstheserver(Bob)totake.TheINVITErequestcontainsanumber
ofheaderfields.Headerfieldsarenamedattributesthatprovide
additionalinformationaboutamessage.Theonespresentinan
INVITEincludeauniqueidentifierforthecall,thedestination
address,Alice'saddress,andinformationaboutthetypeofsession
thatAlicewishestoestablishwithBob.TheINVITE(messageF1in
Figure1)mightlooklikethis:
...
.proxyproxy.
..
Alice's....................Bob's
softphoneSIPPhone
||||
|INVITEF1|||
|--------------->|INVITEF2||
|100TryingF3|--------------->|INVITEF4|
|<---------------|100TryingF5|--------------->|
||<--------------|180RingingF6|
||180RingingF7|<---------------|
|180RingingF8|<---------------|200OKF9|
|<---------------|200OKF10|<---------------|
|200OKF11|<---------------||
|<---------------|||
|ACKF12|
|------------------------------------------------->|
|MediaSession|
|<================================================>|
|BYEF13|
|<-------------------------------------------------|
|200OKF14|
|------------------------------------------------->|
||
Figure1:
SIPsessionsetupexamplewithSIPtrapezoid
INVITEsip:
bob@SIP/2.0
Via:
SIP/2.0/UDP;branch=z9hG4bK776asdhds
Max-Forwards:
70
To:
Bob bob@> From: Alice alice@>;tag=1928301774 Call-ID: a84b4c76e66710@ CSeq: 314159INVITE Contact: alice@> Content-Type: application/sdp Content-Length: 142 (Alice'sSDPnotshown) Thefirstlineofthetext-encodedmessagecontainsthemethodname (INVITE).Thelinesthatfollowarealistofheaderfields.This examplecontainsaminimumrequiredset.Theheaderfieldsare brieflydescribedbelow: Viacontainstheaddress()atwhichAliceis expectingtoreceiveresponsestothisrequest.Italsocontainsa branchparameterthatidentifiesthistransaction. Tocontainsadisplayname(Bob)andaSIPorSIPSURI (sip: bob@)towardswhichtherequestwasoriginally directed.DisplaynamesaredescribedinRFC2822[3]. Fromalsocontainsadisplayname(Alice)andaSIPorSIPSURI (sip: alice@)thatindicatetheoriginatoroftherequest. Thisheaderfieldalsohasatagparametercontainingarandomstring (1928301774)thatwasaddedtotheURIbythesoftphone.Itisused foridentific
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