Report of sa wg2 meeting #114

5.4 IMS and IMS-Related Maintenance

This agenda item was convened by Krisztian Kiss (Apple).

TD S2 160024 LS from GSMA RiLTE: LS on SIP deregistration in GERAN and UTRAN (no VOPSSI). (GSMA RiLTE) (Revision of TD S2 154274).
Abstract: GSMA NG RILTE has been analysing preferred methods of saving on IMS resources used for IMS deployments providing only voice over LTE. In particular, it was the view of some operators that when the UE leaves E-UTRAN coverage and enters either a GERAN or a UTRAN that does not indicate support for voice over PS sessions (i.e. UE does not receive a Voice Over PS Session Indicator (VOPSSI) during the RAU) then there is unnecessary use of IMS resources i.e. while the UE remains registered in IMS even though there is no longer a need to be registered in IMS.
A number of existing solutions (including releasing the PDN connection to the IMS well-known APN and waiting for an IMS reregistration time-out to occur that would implicitly deregister the UE) and new solutions were considered within RILTE. However not all solutions were acceptable to all operators for various reasons and some operators expressed an interest for a more immediate release of IMS resources by UEs than that of the implicit deregistration functionality.
RILTE discussed the behaviour whereby, based on HPLMN policy, the UE specifically performs a SIP deregistration when moving out of E UTRAN coverage and into either GERAN coverage or UTRAN coverage not indicating VOPSSI.
Note that the use of HPLMN policy to control this feature was considered critical in order to accommodate those subscribers using IMS-based services other than, or in addition to, Voice over LTE (i.e. those that can work over GERAN/UTRAN) as well as those PLMN operators who prefer the UE to remain registered in IMS even though there is no longer a need (whereby the SIP registration timeout period eventually takes care of deregistration and/or the network releases the IMS PDN connection). It was also mentioned that HPLMN policy enables the HPLMN to accommodate for use cases other than the voice over LTE use case. GSMA NG RILTE could not identify any specification of the above behaviour within 3GPP technical specifications.
Since the VoLTE profile (GSMA PRD IR.92) is only a profile of 3GPP specifications, such behaviour currently cannot be accommodated for by GSMA, despite the operator interest in this behaviour of UEs.
Action: GSMA NG RILTE request SA WG2 and CT WG1 to confirm if based on HPLMN policy, the UE specifically performs a SIP deregistration when moving out of E UTRAN coverage and into either GERAN coverage or UTRAN coverage not indicating VOPSSI is already specified, and if so, inform GSMA NG RILTE where. If it is not specified, GSMA NG RILTE request SA WG2 and CT WG1 to consider specifying the required functionality for the above defined behaviour.