The PCF based P-CSCF restoration is an optional mechanism.
This mechanism is executed when a terminating request does not proceed due to a P-CSCF failure, as long as there are no other registration flows for this terminating UE using an available P-CSCF.
When P-CSCF failure is detected, the S-CSCF sends a P-CSCF restoration indication through an alternative P-CSCF and the PCF thus initiates IMS PDU session update procedure, so as to trigger the UE re-selects P-CSCF and registers again to IMS.
As the failure of the previous P-CSCF is detected, the S-CSCF forwards the Terminating INVITE message to an alternative P-CSCF, as specified in clause 5.5. The S-CSCF may include the address of the failed P-CSCF, if available. The alternative P-CSCF is chosen by local configuration.
The alternative P-CSCF shall send an Rx AAR message or shall invoke the Npcf_PolicyAuthorization_Create service operation with the P-CSCF restoration indication to the associated PCF. The associated PCF is chosen as specified in TS 23.503. The alternative P-CSCF includes, if received, the address of the failed P-CSCF in the request. The PCF shall send an Rx AAA or shall respond the Npcf_PolicyAuthorization_Create request to the P-CSCF.
If the requirements listed at clause 22.214.171.124 are fulfilled, the SMF initiates the P-CSCF address list update procedure to trigger the P-CSCF reselection by the UE as specified in clause 126.96.36.199; if the SMF received the address of the failed P-CSCF in the notification from PCF, it may take it into account to build the list of available P-CSCFs that should be sent to the UE.
If the requirements listed at clause 188.8.131.52 are fulfilled, the SMF initiates the DHCP based P-CSCF selection triggering procedure to trigger the P-CSCF reselection by the UE as specified in clause 184.108.40.206.
If IMS service is required in EPC with 5GC interworking as specified in TS 23.501, HSS+UDM and PGW-C+SMF are involved in the P-CSCF procedures.
After mobility from 5GC to EPC happens for a UE as specified in TS 23.502, the MME via S6a (or AAA for WLAN via SWx) registers itself to HSS+UDM and PGW-C+SMF registered itself via N10 in the HSS+UDM when the UE was in 5GC. In this case, when P-CSCF restoration indication is received from S-CSCF, the HSS+UDM shall, either:
send a P-CSCF restoration indication to the MME serving the UE as specified in clause 5.4"HSS-based P-CSCF restoration for 3GPP access", or to the AAA for WLAN as specified in clause 5.6.2"Basic mechanism for the HSS-based solution", or
send a P-CSCF restoration indication via N10 interface to the PGW-C+SMF serving IMS PDN connection as described in clause 220.127.116.11 step 6, and the PGW-C+SMF thus initiates IMS PDN connection update procedure in EPC or the IMS PDN connection deactivation with reactivation required, to trigger the UE re-selects P-CSCF and registers again to IMS as described in clause 18.104.22.168 from step 8 to 10, in clause 22.214.171.124 from step 8 to 10, or in clause 126.96.36.199 step 8 to 11.
If the IMS service is required to be served by untrusted WLAN where N10 is used instead of S6b interface as specified in TS 23.502, the AAA registers itself in HSS+UDM via SWx interface, and PGW-C+SMF registers itself in HSS+UDM via N10.
When P-CSCF restoration indication is received, the HSS+UDM shall send a P-CSCF restoration indication via N10 interface to the PGW-C+SMF serving the IMS PDN connection as described in clause 5.9 to continue with the P-CSCF restoration.