4 General

29.1623GPPInterworking between the IM CN subsystem and IP networksRelease 17TS

4.1 General interworking overview

The IM CN Subsystem interworks with SIP (IETF RFC 3261 [2]) based IP Multimedia networks. These IP Multimedia networks include:

– SIP User Agents (SIP UAs); and

– SIP Servers.

As such, the IM CN Subsystem has to be able to interwork to all of these above functional entities in the IP multimedia network, as there is a possibility that they all may be involved in an IM session. The general interworking model is shown in figure 1. The SIP based Multimedia networks may use IP version 4 (IETF RFC 791 [9]) or IP version 6 (IETF RFC 2460 [10]).

Figure 1: Interworking Model for IM CN Subsystem to IP Multimedia Network

The PS UE uses the CSCF in order to communicate with the external IP multimedia network entities.

The CS UE uses a MSC Server to communicate with external IP multimedia network entities.

If no IP version interworking or no NAT/NAPT between different realms is required, the CSCF and the MSC Server can communicate with SIP UAs in an external IP multimedia network directly.

If no IP version interworking or no NAT/NAPT between different realms is required, the CSCF and the MSC Server can also communicate with SIP proxies in an external IP multimedia network directly, which in turn can then communicate with SIP UAs.

To provide the IP version interworking or NAT/NAPT between different realms the functions of an IMS-ALG and a TrGW may be inserted between the CSCF or the MSC Server and the external IP Multimedia Network by configuration. The IMS-ALG and the TrGW may be implemented as a part of other physical entities in the IMS.

NOTE: Other methods to provide IP version interworking are outside the scope of this release.

4.2 Interworking scenarios

3GPP specifications design the IM CN subsystem elements and interfaces to exclusively support IPv6. 3GPP TS 23.221 [3] details the interoperability scenarios that an UE may experience when interworking with an external PDN. All of these IP transport layer interworking scenarios can apply to the application layer interworking scenarios detailed in subclause 4.2.1.

4.2.1 UE with 3GPP SIP profile capability connecting to an external SIP device

The procedures used by an UE with 3GPP SIP profile to connect to an external SIP device, which may lack 3GPP SIP profile capabilities, have been analysed in Release 6 within 3GPP TR 29.962 [13] and are specified in 3GPP TS 24.229 [1].