J.3 Dynamic assignment of AS by S‑CSCF caching

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

J.3.1 Concept of Dynamic assignment of AS by S‑CSCF caching

The proposed solution "Dynamic assignment of AS by S‑CSCF caching" is based on standard SIP session control combined with a new S‑CSCF caching functionality. This solution is re-using the DNS (IETF RFC 1035) mechanism, and supports only the ISC interface.

J.3.2 Procedures related to Dynamic assignment of AS by S‑CSCF caching

Figure J.3.2.1 shows the procedure for allocating an AS by the first request of a service to an IMS registered user:

Figure J.3.2.1: Assignment of AS via DNS query during first service request

1. After IMS registration a user sends an initial request to the S‑CSCF for requesting a service (served by an AS).

2. The S‑CSCF performs the DNS query on the server name and resolves one (or a prioritised list) of the IP address(es), which represents a physical or logical AS.

3. The S‑CSCF caches the IP address of the assigned AS and stores it during the IMS registration period of the user.

4. The S‑CSCF routes the request to the assigned AS. (Depending on the service the AS could read/write/store user data, e.g., using Sh interface).

Figure J.3.2.2 shows how subsequent service requests are routed directly to the assigned AS during the registration period of the IMS user:

Figure J.3.2.2: S‑CSCF has stored assigned AS for following service requests

5. The IMS user requests the service again and sends an initial request to the S‑CSCF.

6. The S‑CSCF has stored the IP Address (or a prioritised list) of the assigned AS. There is no longer need to perform a DNS query.

7. The S‑CSCF routes the request to the assigned AS. (Depending on the service the AS can reuse prior stored user data).

The AS pre-assignment and storage could be also done after downloading the service profile during the user registration procedure.

Annex K (normative):
Inter-IMS Network to Network Interface between two IM CN subsystem networks