G.1 General

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

This clause specifies concepts of IMS service provisioning for the following scenarios:

1. When a device or devices that perform address and/or port translation are located between the UE and the P‑CSCF performing translation both of signalling and media packets.

2. When IP address and/or port translation is needed between the IP‑CAN and the IMS domain (e.g. different IP versions) on the media path only. This scenario covers the case when a device or devices that perform address and/or port translation are located on the media path only.

The IP address and/or port translation device can be a NAT or a NAPT as defined in IETF RFC 2663 [34]. Another type of translation is NA(P)T‑PT as specified in IETF RFC 2766 [33]. In the rest of this clause NAT will be used for all of the devices that perform one or more of NA(P)T and NA(P)T‑PT functions.

Note that the procedures of this Annex shall only be applied when they are necessary. If the terminal and/or the access network provide a transparent way of NAT traversal or no IP address translation is needed between the IP‑CAN and the IMS domain on the media path then the function as defined in this Annex shall not be invoked.

It is expected the NAT traversal methods of this Annex will co-exist. UE may support one or more of these methods. It shall be possible for an operator to use one or more of NAT traversal methods in its IMS domain. The selection of the method for a particular case shall depend on the UE’s capabilities, the capabilities of the network and policies of the operator.

Where possible, usage of these procedures shall not adversely impact usage of power saving modes in the UEs, i.e. when the NAT is integrated with the IMS Access Gate way which is under operator control, the reserved temporary addresses and port (binding) should be retained without requiring keep-alive messages from the UE. If the access type to IMS is GPRS, then the UE is not required to initiate any keep-alive messages, see clause E.6 for more information.

NOTE: A solution to allow power saving modes when non-operator controlled NATs are used is not defined in this version of the specification.

G.1.1 General requirements

The following list contains requirements that a NAT Traversal solution should satisfy:

– Support multiple UEs (on one or more devices) behind a single NAT;

– Support both inbound and outbound requests to and from UEs through one or more NAT device(s);

– Support the traversal of NATs between the UE and the IMS CN;

– Support uni-directional and bi-directional media flows;

– Minimize additional session setup delay.