For that we use TLS as a transport layer protocol. The format for a SIPS URI is the same, except that the scheme is “sips” instead of sip. Note, ...
EveryoneknowsthatSIPheaderslikeR-URI,To,From,P-Asserted-Identity,Path,Routeandotherscontain UniformResourceIdentifiers(URI)– sipuriorteluri.Butdoyouknowwhatformatswecanuse,withwhatparameters?AsURIisoneoftheIPcommunication’scornerstones,itworthtohavesomebetterunderstanding.
SIPURIinVoLTE
Let’sstartfromthebeginning. SIPURIisdefinedinRFC3261,TELURIinRFC3966 (thisRFCdefinesalsomodemandfaxURLschemes).
SIPURIhasasimilarformtoanemailaddress.Itcontainstypicallyausernameandahostname,[email protected],whererealtimecommunication.infoisthedomainofaSIPserviceprovider.TELURIissimplyatelephone(landlineormobile) numberas tel:+611234567890.ItisneededmainlytosupportCSrelatedscenariosandMobileNumberPortability(MNP).Inpracticewecanseevariousformsofsip-uris:
sip:[email protected]
sip:[email protected]
sip:ims.mnc000.mcc000.3gppnetwork.org
tel:+611234567890
sip:[email protected];user=phone;npdi
tel:4567890;phone-context=+61123
tel:7890;phone-context=operator.com
sip:[email protected]:5060;transport=udp
Lastbutnotleastwehavealsoa SIPSURI,whichspecifiesthattheresourceistobecontactedsecurely.ForthatweuseTLSasatransportlayerprotocol.TheformatforaSIPSURIisthesame,exceptthattheschemeis“sips”insteadofsip. Note,thatanyresourcedescribedbyaSIPURIcanbe“upgraded”toaSIPSURIbyjustchangingthescheme,ifitisdesiredtocommunicatewiththatresourcesecurely.
SIPURISyntax
ASIPorSIPSURIidentifiesacommunicationsresource. Itsgeneralform,inthecaseofaSIPURI,is:
sip:user:password@host:port;uri-parameters?headers
user:Theidentifierofaparticularresourceatthehostbeingaddressed.
E.g.toaddressacontextwehavecreatedforaparticularsessiononS-CSCFwe’dusesipuri
sip:[email protected]
The“userinfo”ofaURIconsistsofthisuserfield,thepasswordfield,andthe@signfollowingthem.TheuserinfopartofaURIisoptionalandmaybeabsent,forinstanceSIPREGISTERinR-URIdoesn’thavetheuserpart.
Request-Line:REGISTERsip:ims.mnc000.mcc000.3gppnetwork.orgSIP/2.0
Method:REGISTER
Request-URI:sip:ims.mnc000.mcc000.3gppnetwork.org
Request-URIHostPart:ims.mnc000.mcc000.3gppnetwork.org
password:wedon’tusepasswordsinVoLTEnetworksinSIPProtocol
host:ThehostisprovidingtheSIPresource.Theterm“host”inthiscontextreferseithertoadomainoraparticularSIPserver.Thehostpartcontainseitherafully-qualifieddomainname(FQDN)ornumericIPv4orIPv6address.TheusageofFQDNisrecommendedwheneverpossible.(HoweverinenterpriseenvironmentwecanseemostlyaddressesbasedonnumericIPs.)
port:Theportnumberwheretherequestistobesent.URIparameters:ParametersaffectingarequestconstructedfromtheURI.
ThesignificanceofnumericIPandportinformationinsipuriforroutingisexplainedin SIPIllustrated5:SessionRoutingorDNSarticles.
URIparameterstaketheform:
parameter-name"="parameter-value
parameters
comp:compression(sigcomp)
user:allowstodistinguishtelephonenumbersfromusername,seebelow
maddr:indicatestheserveraddresstobecontactedforthisuser
method:methodoftheSIPrequestconstructedfromtheURIcanbespecifiedwiththemethodparameter
lr:Indicateslooserouting.
ttl:TimeToLive
other(user-defined)parametersareallowed–SIPelementshavetosilentlyignoreanyuri-parametersthattheydonotunderstand!
TELURISyntax
“tel”URIhasthefollowingsyntax:
tel:telephone-subscriber
The‘telephone-subscriber’partoftheURIindicatesthenumber.ThephonenumberisrepresentedineitherGlobal –E.164orLocalnotation.Allphonenumbershavetousetheglobalformunlesstheycannotberepresentedassuch.Numbersfromprivatenumberingplans,emergency(“911”,“112”),andsomedirectory-assistancenumbers(e.g.,“411”)andother“servicecodes”(numbersoftheformN11intheUnitedStates)cannotberepresentedinglobal(E.164)formandneedtoberepresentedasalocalnumberwithacontext.Localnumbersaretaggedwitha‘phone-context‘.
GlobalTELURI:
tel:+1-234-555-0123
Alocalphonenumbervalidwithin “operator.com”:
tel:0123;phone-context=operator.com
Alocalphonenumberthatisvalidwithinaparticularphoneprefix:
tel:555-1234;phone-context=+1-123-456
URIComparasion
Thereismorethanonewayhowtowriteanaddress. AndweneedtoknowwhethertwoSIPorSIPSURIsareequivalent.E.g.S-CSCFneedstocomparebindingsinContactURIsinREGISTERrequests.Themostimportantrulesare:
ASIPandSIPSURIareneverequivalent.
ComparisonoftheuserinfoofSIPandSIPSURIsiscase-sensitive.
ComparisonofallothercomponentsoftheURIiscase-insensitiveunlessexplicitlydefinedotherwise.
TheorderingofparametersandheaderfieldsisnotsignificantincomparingSIPandSIPSURIs.
AnIPaddressthatistheresultofaDNSlookupofahostnamedoesnotmatchthathostname.
FortwoURIstobeequal,theuser,password,host,andportcomponentsmustmatch.
SIPURIsandtelURLs,MobileNumberPortability
ATELURLcanbeconvertedtoaSIPorSIPSURI.Theentiretelephone-subscriberportionofthetelURL,includinganyparameters,isplacedintotheuserinfopartoftheSIPorSIPSURI.
SIPURIinitsuserpartcancontainalphanumericcharacters.E.g.example1@[email protected]’resupposedtotreattheusernameasatelephonenumberornot? AuserURIparameterexiststodistinguishtelephonenumbersfromusernamesthathappentolookliketelephonenumbers.Iftheuserstringcontainsatelephonenumberformattedasatelephone-subscriber,theuserparametervalue“phone”isresent.
(Butevenwithoutthisparameter,recipientsofSIPandSIPSURIscanstillinterprettheuserpartasatelephonenumberiflocalrestrictionsonthenamespaceforusernameallowit.)
tel:+123-555-1234567becomes
sip:[email protected];user=phone
or
sips:[email protected];user=phone
SoifTASrecievesarequest
INVITEsip:[email protected];user=phoneSIP/2.0
itmeans,treatthesipuriasateluri
tel:+19023412345
Thereasonisthatwhenwe’recallingsomeone,wehavenoideawhatnetworkheorshebelongsto.Weknowonlythenumber.AndthankstoMNPwecan’tevenderivethisinformationfromthenumber.ThatisataskforTASorS-CSCFtodoanENUMdip.
TherelatedspecificationisRFC4694,whichspecifiesadditionalteluriparameterssuchas “rn“,“npdi“,or“cic“.
IMSandIdentities
IMSServers
I-CSCFsusedinregistrationareallocatedSIPURIs,whicharealsoprovisionedinDNS.IngeneralI-CSCFisrepresentingthewholedomain.
Queries
operator.com:typeNAPTR,classIN
Name:operator.com
Type:NAPTR(Namingauthoritypointer)
Authoritativenameservers
operator.com:typeSOA,classIN,mnamedns_01.operator.com
...
;orderprefflagsserviceregexpreplacement
INNAPTR1050"s""SIPS+D2T"""_sips._tcp.icscf.ims.operator.com.
INNAPTR2050"s""SIP+D2T"""_sip._tcp.icscf.ims.operator.com.
INNAPTR10050"s""SIP+D2U"""_sip._udp.t.ims.operator.com.
OtherIMSelementshavetheirownspecificSIPURIs,whichcanalsocontainausername.
sip:pcscf.operator.com
sip:@pcscf-site_a.operator.com
Iftheuserpartexists,itisanessentialpartoftheaddressandcan’tbeomittedwhencopyingormovingtheaddress.Howtheseaddressesareassignedtothelogicalentitiesisuptothenetworkoperator.Forexample,asingleSIPURImaybeassignedtoallI-CSCFs,andtheloadsharedbetweenvariousphysicalboxesbyunderlyingIPcapabilities,orseparateSIPURIsmaybeassignedtoeachI-CSCF,andtheloadsharedbetweenvariousphysicalboxesusingDNSSRVcapabilities.
;;PriorityWeightPortTarget
INSRV015060icscf1.ims.operator.com
INSRV025060icscf2.ims.operator.com
IMSSubscribers
Thesubscriberisallocatedaprivateuseridentity(IMPI)bythehomenetworkoperator.ThisIMPIisavailabletotheSIPapplicationwithintheUE.Dependingona deviceandtechnology,variousarrangementsexistwithintheUEforretainingthisinformation:
whereanISIMispresent,IMPIisstoredwithintheISIM
wherenoISIMispresentbutUSIMispresent,theprivateuseridentityisderived
ifeitherISIMnorUSIMispresent,butIMSCredentials(IMC)ispresent,IMSIissetwithinIMC(e.gforWifi)
whenneitherISIMnorUSIMnorIMCispresent,theprivateuseridentityisavailabletotheUEviaothermeans(e.gforWebRTCusingWebRTCGWretrievesIMPIfromaconfigurationserver)
Inasimilarwaythesubscriberisallocatedoneormorepublicuseridentities (IMPUs)bythehomenetworkoperator.
ForeachTELURI,thereisatleastonealiasSIPURIinthesetofimplicitlyregisteredpublicuseridentitiesthatisusedtoimplicitlyregistertheassociatedtelURI.
P-Associated-URI:,
TheIMPUscanbesharedacrossmultipleUEs. IftheTELURIisasharedpublicuseridentity,thentheassociatedaliasSIPURIisasharedpublicuseridentitytoo.AndalsoifthealiasSIPURIisasharedpublicuseridentity,thentheassociatedTELURIisalsoasharedpublicuseridentity.
Registration
OnsendinganunprotectedREGISTERrequest,theUEcreatestheheaderfieldsasfollows:
aFromheaderfieldsettotheSIPURIthatcontains:
IMPUtoberegistered;
aToheaderfieldsettotheSIPURIthatcontains:
IMPUtoberegistered;
aContactheaderfieldsetincludesSIPURI(s)containingtheIPaddressorFQDNoftheUEinthehostpartparameter.
Emergency
IdentificationoftheUEtoaPSAPwithpointofpresenceintheCSdomainisnotpossibleifaTELURIisnotincludedinthesetofimplicitlyregisteredIMPUs.IftheincludedTELURIisassociatedeitherwiththefirstentryinthelistofIMPUsprovisionedintheUEorwiththetemporaryIMPU,thenaPSAPcanuniquelyidentifytheUEthattheemergencyregistrationisperformed.
MindthattheEmergencyregistrationisnotalwayssupported!
E-CSCFsthemselvesareallocatedmultipleSIPURIs.TheSIPURIconfiguredintheP-CSCF,ASorIBCFtoreachtheE-CSCFisdistinctfromtheonegivenbytheE-CSCFtotheEATFsuchthatEATFcanreachtheE-CSCF.
NNI
The“iotl”SIPURIparametercontainingstrafficleginformation.It canbeusedby IBCF andtransitnetworkstomakepolicydecisionsrelatedtoe.g.mediaanchoring,screeningofSIP
signalling,insertionofmediafunctions(e.g.transcoder)andcharging.
Route:,
VoLTEIdentifiers
Contactheader
UEincludesauserpartintheURIofthecontactaddresssuchthattheuserpartisgloballyuniqueanddoesnotrevealanyprivateinformation.Togeneratethisuserpart,theUEcanuseatime-basedUUID(UniversalUniqueIdentifier)generatedasperRFC4122.
VoLTEPublicUserIdentities
AllIMPUsprovidedintheimplicitregistrationsetusedforVoLTEbytheIMSarealiasuseridentitiesandincludeaTELURI.TheIMPUisassignedtotheimplicitregistrationsetusedforVoLTEanditisusedbytheUEwhenregisteringforVoLTE:
WhenISIMisused,theIMPUinthefirst(oronly)recordinthe ElementaryFileintheISIM(3GPPTS31.103);
orthetemporarypublicuseridentityderivedfromtheIMSI(3GPPTS24.229).
NootherimplicitregistrationsetcanbeusedforVoLTE.
ForVoLTEIMSthesupportforISIMbasedauthenticationintheUEismandatory. TheUEandIMScorenetworkalsosupporttheproceduresforUSIMbasedauthenticationifthereisnoISIMpresentontheUICC.
MoreoverthisincludessupportfortheP-Associated-URIheadertohandlebarredIMPUs.
PublicServiceIdentifiers(PSIs)
ThesupportofPSIsisdefinedin3GPPTS23.003,andincludesthefollowingtypesofaddresses:
AlphanumericSIP-URIs
sip:[email protected]
MSISDNrepresentedasaSIPURI
sip:[email protected];user=phone
MSISDNrepresentedasatelURI
tel:+447700900123
Note:SomemorerequirementsforsupportofPublicUserIdentitiesinthenetworkarespecifiedinIR.65.
Localnumbers
AsperIR.92theUEsetsthedialstringcontainingthelocalnumbertotheuserpartofSIPURIintheRequestURI,andsetthe“user=phone”parameter,withthe“phone-context” TELURIparametertotheuserpart.
forhomelocalnumberstheUEmustsetthe“phone-context”parametertothehomedomainname,asitisusedtoaddresstheSIPREGISTERrequest.
forgeo-localnumberstheUEmustsetthe“phone-context”parameterwithadditionalvisitednetworkinformationwhenavailable.
Relatedarticles:
SIPIllustrated:SIPby
sip
How
toreadtcpdump– Registration
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6thoughtson“SIPURIOverview”
oneuserhasa3devicewithdifferentIpaddress.inthatcaseifuserhasalreadyregisteredwithonedevice.myquestionisthatsupposesameuserisgoingtoregisterwithotherdeviceinIMSnetwork.
Doesfirstdevicegetderegisterautomatically.pleaseshareyourcommentsonthis.
appreciateyourcommentsinadvance.
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No,youcanhavemultipledevicesregisteredfor1user.AlltheregistrationswillbehandledbythesameS-CSCF.
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whatisthesignificanceofPSI(Publicserviceidentity)
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Apublicserviceidentityidentifiesaservice,oraspecificresourcecreatedforaserviceonanapplicationserver.E.g.thiswaywecantriggersomefunction,likeachatbotorconferenceservice:
sip:[email protected]
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Hi
CantheSCSCFmodifytheURItoincorporatespecialserviceprefixcodestoidentifyaservice(eg:translatingBpartynumberintoxxxB)andhowisthatthencorrelatedbacktotheoriginatingINVITEviathePCSCFforCDRbillingpurposes?
Thanks!
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AlthoughthisistypicallydonebyAS,ingeneralanynetworkelementcanchangeaURI(supposewe’retalkingaboutR-URI).Solet’ssaythatyouchangeR-URIonS-CSCFandthenTASwillapplyOnlineCharging.ButthatshouldbebasedonP-Served-Useridentity.OfflinechargingthenshoulduseP-Charging-Vector.StillweshouldbecarefulwithR-URImanipulationandratherusesometagsifpossible.
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