5.7 Interaction between P-CSCF restoration and NBIFOM

23.3803GPPIMS Restoration ProceduresRelease 17TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

5.7.1 Introduction

P-CSCF restoration and NBIFOM present interactions as NBIFOM can be applied to the IMS PDN connection which can be set on a 3GPP access or on a WLAN access or be simultaneously active over the two types of accesses. This impacts the P-CSCF restoration procedures for a network supporting both P-CSCF restoration and NBIFOM with IMS UEs supporting NBIFOM, as described in the hereafter clauses which cover:

– the HSS-based and the PCRF-based solutions;

– the cases where the IMS PDN connection is established over one access or over two accesses (3GPP access, WLAN access);

– the cases where the basic mechanism and/or the extended mechanism for P-CSCF restoration are supported by the network over only one access or on both accesses used by the IMS PDN connection;

– the capabilities of the IMS UEs with NBIFOM regarding the support or not of the extended mechanism for P-CSCF restoration over the 3GPP access, the trusted WLAN and/or the untrusted WLAN.

Regarding WLAN accesses, the statements in clause 5.7 are the same for trusted WLAN with multi-connection mode and untrusted WLAN. For a trusted WLAN with the single connection mode or the transparent single connection mode, only the basic P-CSCF restoration mechanism may apply.

The support of the basic mechanism or of the extended mechanism for P-CSCF restoration in clause 5.7 is meant as the support over the whole chain of involved functional elements over an access type at the creation of the IMS PDN connection over the concerned access, so, in particular, taking into account the UE capability for the extended mechanism over this access.

5.7.2 HSS-based solution

If the MME/SGSN is configured with the extended mechanism (i.e. the MME/SGSN forwards a received P-CSCF restoration indication to the PGW) and:

– If the IMS PDN connection is established over at least one access supporting the extended mechanism with the UE, the PGW, when receiving a P-CSCF restoration indication, shall select one access supporting the extended mechanism and send the list of available P-CSCF addresses over this access. The PGW shall ignore any second P-CSCF restoration indication which may be received shortly afterwards;

– If the IMS PDN connection is established over access(es) not supporting the extended mechanism with the UE, but over at least one supporting the basic mechanism, the PGW, when receiving a P-CSCF restoration indication (be it from the MME or the 3GPP AAA server), shall:

– select one access supporting the basic mechanism and release the IMS PDN connection over that access with the cause "reactivation requested". The PGW shall ignore any second P-CSCF restoration indication which may be received shortly afterwards. If the IMS PDN connection is established on both accesses with the single connection mode over the trusted WLAN, the PGW shall select the 3GPP access; and

– release the IMS PDN connection over the other access with a "local release" cause. The MME/SGSN or TWAN/ePDG in the other access shall release the corresponding bearer resources as specified in 3GPP TS 23.401 [17] or IETF RFC 5996 [26], but without signalling to the UE. Further, for the local release of the bearer resources by the MME/SGSN for a UE that is in connected mode, the MME/SGSN shall initiate the radio access bearer release procedure towards the serving eNB/NB.

If the MME/SGSN supports only the basic mechanism (i.e. the MME/SGSN does not forward the P-CSCF restoration indication to the PGW), the MME/SGSN, when receiving a P-CSCF restoration indication, shall release the IMS PDN connection with the cause "reactivation requested" sent to the UE or, if this is the last PDN connection of the UE, sends an explicit detach to the UE with the cause "reattach required" (as specified in clause 5.4.2.1). The MME/SGSN shall release the IMS PDN connection towards the PGW by sending a Delete Session Request to the PGW with the Release Over Any Access Indication. Upon receipt of such a message and indication, the PGW shall initiate a Delete Bearer Request procedure with a "local release" cause to tear down the PDN connection over the other access, if this is a NBIFOM PDN connection and resources still exist on that other access. The TWAN/ePDG shall then proceed with the local disconnection of the PDN connection, i.e. the TWAN/ePDG need not send signalling to the UE to release that access. The UE may, over the WLAN access, also receive a release of the IMS PDN connection or a list of available P-CSCFs.

Upon receipt of a request to release an IMS PDN connection with the cause "reactivation requested" over the 3GPP or the WLAN access, or a Detach request with the cause "reattach required" over the 3GPP access, the UE shall locally release the IMS PDN connection over the other access if the IMS PDN connection was established on both accesses, then re-establish the IMS PDN connection and do a new IMS registration. The UE should avoid doing two new IMS registrations in a row.

NOTE: When a Detach request with the cause "reattach required" over the 3GPP access occurs due to a P-CSCF restoration indication, there is no PDN connection other than the IMS PDN connection established over the 3GPP access, so any PDN connection established over the WLAN access other than the IMS PDN connection is maintained.

5.7.3 PCRF-based solution

If the IMS PDN connection is established over at least one access supporting the extended mechanism with the UE, the PGW, when receiving a P-CSCF restoration indication, shall select one access supporting the extended mechanism and send the list of available P-CSCF addresses over this access.

If the IMS PDN connection is established over access(es) not supporting the extended mechanism with the UE, but over at least one supporting the basic mechanism, the PGW, when receiving a P-CSCF restoration indication, shall:

– select one access supporting the basic mechanism and release the IMS PDN connection over that access with the cause "reactivation requested". If the IMS PDN connection is established on both accesses with the single connection mode over the trusted WLAN, the PGW shall select the 3GPP access; and