11 Roles for session release

24.2923GPPIP Multimedia (IM) Core Network (CN) subsystem Centralized Services (ICS)Release 17Stage 3TS

11.1 Introduction

This clause specifies session release procedures for when an ICS UE releases a session using a service control signalling path over Gm and when a non-ICS UE has IMS service control via an MSC server enhanced for ICS. The associated procedures for the SCC AS are also specified in this clause; specifically, the CS bearer release procedures when using Gm are described. The clause also specifies the SCC AS procedures when it detects the loss of service control signalling path over Gm. The session release procedures specific to an MSC server enhanced for ICS are also described.

11.2 ICS UE

11.2.1 General

This clause specifies the procedures for session release by an ICS UE.

11.2.2 ICS UE using Gm

The ICS UE shall support session release suitable for ICS via Gm reference point as specified in 3GPP TS 24.229 [11].

If the ICS UE uses a CS bearer, the UE shall release the resources in accordance with 3GPP TS 24.008 [7] for 3GPP systems, to release the CS bearer.

NOTE: The order of releasing the CS or PS resources is an implementation issue.

11.2.3 ICS UE using CS

The ICS UE shall implement the bearer release towards SCC AS suitable for ICS via CS domain in accordance with 3GPP TS 24.008 [7] for 3GPP systems.

11.2.4 ICS UE using I1

3GPP TS 24.294 [11B] specifies the session release procedures for an ICS UE using I1.

11.3 MSC Server enhanced for ICS

The MSC Server enhanced for ICS shall implement session release as specified in 3GPP TS 29.292 [24].

11.4 SCC AS

11.4.1 General

The following subclauses describe the procedures at the SCC AS for session release. In such scenarios, the SCC AS serves the terminating ICS UE. The SCC AS shall follow procedures specified in 3GPP TS 24.229 [11] with the additional procedures described in this specification in subclauses 11.4.2 and 11.4.3. These subclauses describe the procedures for the SCC AS when using service control over Gm and CS, respectively.

11.4.2 SCC AS for service control over Gm

When the SCC AS receives a SIP BYE request the SCC AS shall:

1) determine if the SIP BYE request was originated due to release of a service control session or if the SIP BYE requested was originated in the CS domain as a result of ICS UE bearer release procedures; and

2) if the SIP BYE request was received from an endpoint involved in the session to be released, this indicates a user’s desire to release the service control session forward the SIP BYE request towards the other endpoint involved in the session.

The SCC AS distinguishes the SIP BYE requests generated by the MGCF and the SIP BYE requests from the ICS UE by the associated dialog IDs.

When the SCC AS receives a SIP BYE request from the ICS UE, the SCC AS shall:

– forward the SIP BYE request towards the remote leg in accordance with the procedures defined in 3GPP TS 24.229 [11]; and

– if the CS bearer is not used by any other session, send a SIP BYE request towards the MGCF, in accordance with the procedures defined in 3GPP TS 24.229 [11], to release the CS bearer.

When the SCC AS receives a SIP BYE request from the MGCF, the SCC AS shall do the following for every session associated with the ICS UE which has a CS bearer:

– if the session includes other PS media in addition to the CS media, the SCC AS shall send a SIP re-INVITE or a SIP UPDATE request towards

– the access leg, removing the CS media by setting the port number to zero for the CS media "m=" line set to "PSTN" as described in IETF RFC 7195 [36]; and

– the remote leg, removing the corresponding audio media by setting the port number to zero; and

– if the session only contains CS media, the SCC AS shall send a SIP BYE request towards both the access and remote legs in accordance to the procedures defined in 3GPP TS 24.229 [11].

11.4.3 SCC AS procedure upon loss of Gm service control

If the SCC AS detects the ICS UE is not reachable over Gm service control, the SCC AS shall for every held session associated with the ICS UE send a SIP BYE request towards the other UE involved in the session in accordance with the procedures defined in 3GPP TS 24.229 [11].

NOTE: The exact mechanism for detecting loss of Gm by SCC AS is implementation dependent.

11.4.4 SCC AS for service control over I1

3GPP TS 24.294 [11B] specifies the session release procedures for service control using I1.