D.3 LI for capability discovery

33.1273GPPLawful Interception (LI) architecture and functionsRelease 18TS

D.3.1 Background

The capability or service discovery mechanism in RCS is a process which enhances service usability by allowing a RCS user to exchange its own RCS service capabilities and to understand the RCS service capabilities of another RCS user, at certain points in time.

When available, the RCS specification provides two alternative mechanisms to perform the capability discovery:

– SIP OPTIONS exchange: The SIP OPTIONS end-to-end message is used by one RCS user (e.g., User A) to query the capabilities (services which the other user has available) of the other RCS user (e.g., User B). The SIP OPTIONS message passes the information about which capabilities are supported by User A and the response contains information about which capabilities are supported by User B. Using this method, both users get updated information in a single transaction.

– Presence: In this case, instead of performing an end-to-end transaction, the capabilities are queried against a presence server which is part of the RCS Server as defined in GSMA RCC.07 [35] clause 2.6.1.2.

When the SIP OPTIONS request is used, user A includes user A’s RCS capabilities and the IMPU of user B. The response is any of the following:

– SIP 200 OK including at least, one of the tags assigned to the RCS Services. User B is an RCS user. The capabilities returned in the SIP 200 OK response are considered as the current communication options with user B.

– SIP 200 OK not including any of the tags used by RCS services. User B is registered to IMS, but not with an RCS client. User B is not an RCS user. Only the non-RCS communication services (e.g. voice calls, SMS, MMS, etc.) are indicated as available.

– SIP 480 TEMPORARY UNAVAILABLE or 408 REQUEST TIMEOUT returned by the network if user B is an IMS (and potentially thus an RCS) user, but is currently not registered.

– 404 Not Found or 604 Does Not Exist Anywhere. User B is not considered as an IMS user nor an RCS user. Only the non-RCS communication services (e.g., voice calls, SMS, MMS, etc.) are indicated as available.

When presence is used:

– After user A has registered to IMS, User A publishes their RCS capabilities in a Presence document that is published by using the SIP PUBLISH request. If changes are required in the published capabilities (e.g., due to RAT change), a new PUBLISH request is sent. When the client/device is switched off, the published capabilities are removed from the presence server before deregistering from IMS by sending another PUBLISH request.

– When User A wants to use RCS, User A requests the RCS capabilities of user B by sending SIP SUBSCRIBE requests. The Presence server of User B sends a SIP NOTIFY request to User A containing the RCS capabilities of User B.

D.3.2 Architecture

The figure 7.13.2-1 without the CC-POI in the RCS Server provides the architecture for LI for capability discovery.

In a normal deployment, if SIP OPTIONS are used for capability discovery, the Messaging Server performs the function of the RCS Server in figure 7.13.2-1.

In a normal deployment, if presence is used for capability discovery, the presence server and S-CSCF performs the function of the RCS Server in figure 7.13.2-1.