G.2 Reference models

23.2283GPPIP Multimedia Subsystem (IMS)Release 18Stage 2TS

This clause describes various reference models which can be used for NAT traversal.

G.2.1 IMS-ALG and IMS Access Gateway model

Figure G.1 presents the general reference model for IMS access when both the signalling and media traverses NAT devices. Figure G.2 presents the general reference model when IP address translation is needed between the IP‑CAN and the IMS domain. The IMS network architecture is the same for both cases. The NAT integrated with the IMS Access Gateway is under operator control in this reference model.

Figure G.1: Reference model for IMS access when both the signalling and media traverses NAT

Figure G.2: Reference model for IMS access when NAT is needed between the IP‑CAN and the IMS domain

G.2.2 ICE and Outbound reference model

Figure G.2a presents the general reference model for IMS access when both the signalling and media traverses NAT devices. Functional elements with dashed lines represent optional functionality. The transport of the Gm signalling is also subject to the policy enforcement.

Figure G.2a: Reference model for ICE and Outbound Methodology

The STUN Function shown within the P‑CSCF is a limited STUN Server for supporting STUN keep-alive messages as described in clause G.5.3.2.

For deployments where the IMS Access gateway (or other media manipulating functional entities, such as a MRFP, are used (see clause G.2.1), such functional entities shall be placed on the network side of the STUN server and STUN relay server (i.e. not between the UE and the STUN server or STUN relay server) as shown in figure G.2a. Otherwise they will prevent STUN messages from reaching the STUN Relay/Server outside of a session.