G.3 Network elements for employing the IMS-ALG and IMS Access Gateway

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

G.3.1 Required functions of the P‑CSCF

When supporting IMS communication for a UE residing behind a NAT or when IP address translation is needed between the IP‑CAN and the IMS domain on the media path only, the P‑CSCF may include the IMS-ALG function that is defined in Annex I of this specification. The following functions shall be performed in the P‑CSCF:

1) The P‑CSCF shall be able to recognize that the UE is behind a NAT device or IP address translation is needed between the IP‑CAN and the IMS domain on the media path only.

2) The IMS-ALG function in the P‑CSCF shall control the IMS Access Gateway, e.g. request transport addresses (IP addresses and port numbers) from the IMS Access Gateway, and shall perform the necessary changes of the SDP parameters.

3) The IMS-ALG function in the P‑CSCF shall perform the necessary changes of headers in SIP messages.

4) The IMS-ALG function in the P‑CSCF shall be able to support scenarios where IMS CN domain and IP‑CAN use the same IP version and where they use different IP versions.

5) The IMS-ALG function in the P-CSCF shall be able to request opening and closing of gates on the IMS Access Gateway.

6) The IMS-ALG function in the P-CSCF may configure the IMS Access Gateway to police the remote source address/port of the associated media flow(s).

7) The IMS-ALG function in the P-CSCF may configure the IMS Access Gateway to police the bandwidth/data rate of the associated media flow(s) (see TS 23.333 [73]).

8) The IMS-ALG may configure the IMS Access Gateway to set the differentiated service code point for egress packets to an explicit value or alternately to allow the differentiated service code point of the ingress packet to be copied into the corresponding egress packet. An IMS Access Gateway can also support differentiated service code point marking based on local configuration.

9) The IMS-ALG may request an IMS Access Gateway to detect and report inactive media flows.

G.3.2 Required functions of the IMS Access Gateway

The required functions of the IMS Access Gateway for NAT translation are the following:

1) It allocates and releases transport addresses according to the requests coming from the IMS-ALG function of the P‑CSCF.

2) It ensures proper forwarding of media packets coming from or going to the UE.

3) It shall support the scenarios where IMS CN domain and IP‑CAN use the same IP version and where they use different IP versions.

4) It shall support opening and closing of gates, under control of the IMS-ALG.

5) It shall support policing of the remote source address/port and bandwidth/data rate of media flows, as configured by the IMS-ALG.

6) It shall support the setting of the differentiated service code point for egress packets as configured by the IMS-ALG or else based on local configuration.

7) It may support detection and reporting of inactive media flows.

8) It shall support remote NAT traversal.

G.3.3 Iq reference point

The Iq reference point is between the P‑CSCF and the IMS Access Gateway. It conveys the information necessary for the IMS-ALG to activate the procedures defined in clause G.3.2. Those procedures are further detailed in TS 23.334 [74].