6.2.1 Gate Control & Local NA(P)T procedure

23.3343GPPIP Multimedia Subsystem (IMS) Application Level Gateway (IMS-ALG) - IMS Access Gateway (IMS-AGW) interface: Procedures descriptionsRelease 17TS

The session establishment and session release procedures are specified in 3GPP TS 23.228 [2] Annex G.4.3 and G.4.4.

Figure 6.2.1.2 depicts the signalling flow for a session setup from the IMS access network towards the IMS core network when the P‑CSCF invokes the IMS-ALG function for a session. The same signalling flow applies for a session setup from the IMS core network towards the IMS access network with the exception that terminations T1 and T2 are then exchanged.

Figure 6.2.1.1: H.248 Context Model

Figure 6.2.1.2: IMS-ALG and IMS-AGW interaction at session establishment

Upon receipt of a session initiation request, the IMS-ALG shall extract the offerer’s destination network address(es) and port number(s) from the signalling message body received from the calling party endpoint. It shall then request the IMS-AGW to allocate transport resources (T2) via the Reserve AGW Connection Point procedure. Upon receipt of the response from the IMS-AGW, the IMS-ALG shall modify the offerer’s destination address(es) and/or port(s) contained in the application signalling message body and propagate the session establishment toward the terminating party.

On receipt of the terminating end SDP in the session establishment response, the IMS-ALG shall pass the information to the IMS-AGW in the Configure AGW Connection Point procedure and shall request the IMS-AGW to allocate transport resources (T1) via the Reserve and Configure AGW Connection Point. Upon receiving the response from the IMS-AGW, the IMS-ALG shall modify the answerer’s destination address(es) and/or port(s) contained in the application signalling message body and pass the information to the originating party.

On session termination, the IMS-ALG shall request the IMS-AGW to release its transport resources via the Release AGW Termination procedure.