The present document investigates the SIP signalling interworking between entities of IM CN subsystems behaving as specified in the 3GPP profile of SIP and SDP in TS 24.229
, with related call flow examples in TS 24.228
, and SIP network entities external to the IM CN subsystems, which may not adhere to the 3GPP profile of SIP and SDP.
The present document assumes that GPRS access and service based local policy using the Go interface is applied.
Non-GPRS access to IMS may have implications on the TR, which are not yet discussed.
The considered SIP network entities external to the IM CN subsystems may feature different SIP capabilities, such as the support of arbitrary SIP options.
The document focuses on scenarios where the non-3GPP UA does not support one or more of the following SIP extensions:
"Integration of Resource Management and SIP" RFC 3312
"The Session Initiation Protocol UPDATE Method" RFC 3311
"Reliability of Provisional Responses in SIP" RFC 3262
The present document focuses on the preconditions, the update and 100rel extensions because only these extensions imply interworking issues since they require the end-to-end cooperation of both UAs.
Security interworking may also have implications on the TR, which are not yet discussed.
The present document does not make any a priori assumptions where a possible interworking is performed within the IM CN subsystem. Any SIP network entity within the IM CN subsystem may take part in the interworking. The network entities that may become involved in a certain interworking topic are identified for each of these topics separately.
The present document features a discussion of topics, where an interworking is possibly required. Aspects of the 3GPP profile of SIP and SDP, which obviously do not require any interworking, are not discussed. An assessment of the impact and probability of occurrence of the discussed scenarios is also provided.
Problems due to network elements within the IM CN subsystem, which do not or only partly satisfy the 3GPP profile of SIP and SDP, in particular non 3GPP compliant SIP UAs, are out of scope of the present document.
The present document is dedicated exclusively to issues inherent in the SIP and SDP signalling. Related topics in a wider sense, such as IPv6 to IPv4 address translation or user plane transcoding are out of scope.
For brevity, in what follows the above SIP extensions are only mentioned if a SIP UA does not make use of them. Otherwise, it is understood that the UA makes use of them.