A failure in P-CSCF is detected via Gi/sGi by the GGSN/PDN-GW. The GGSN/PDN-GW informs to all UEs associated to the failed P-CSCF address that the P-CSCF is not available.
The PDN-GW shall send a PMIP UPN message (MN ID, APN, PDN connection ID, PCO, and Additional parameters) as specified in TS 29.275 and RFC 7077. The PCO contains a P-CSCF failure Indicator. The Notification reason shall indicate that there is a P-CSCF failure.
If the SGW supports the PMIP Update Notification message, it shall send Update Bearer Request message with the P-CSCF failure Indicator in the PCO to the MME as part of the PGW initiated bearer modification without QoS update procedure as specified in TS 23.401. Once the Update Bearer Response message is received, the SGW shall response with a PMIP UPA message (MN ID, APN, PDN connection ID, PCO, and Additional parameters) as specified in TS 29.275 and RFC 7077.
If the PGW knows the SGW does not support the PMIP Update Notification procedure, the PGW may release the PMIP binding with cause code "Reactivation Requested".