MME provides the following support for HSS-based P-CSCF restoration:

This section provides the MME's call flows for HSS-based P-CSCF Restoration.

Figure 1. Call flow for HSS-based P-CSCF Restoration

On receiving the P-CSCF Restoration indication from the HSS, the MME/SGSN from the received IMSI identifies the UE and finds the corresponding IMS APN. The support of this feature by the serving SGW/PGW is determined based on the local configuration at the MME. If the optional extension is not supported by the SGW/PGW, the MME releases the identified PDN connection towards the UE by executing PDN disconnection/detach procedure with NAS cause code "reactivation requested/ reattach required". Additionally, the MME/SGSN release the same PDN connection towards the SGW/PGW by sending Delete Session message. As a result of the release of the IMS PDN connection, the UE activates the IMS PDN connection to select an available P-CSCF and to perform a new initial IMS registration.

Figure 2. Call flow for HSS-based P-CSCF Restoration, continued...

The HSS-based P-CSCF basic mechanism is optionally extended by reusing part of the "Update PDP context/bearer at P-CSCF failure" mechanism. This in order to avoid the need to deactivate and reactivate the IMS PDN connection. PCO-based optional extension is based on the possibility for the P-GW/GGSN to know whether or not the UE supports the "Update PDP context/bearer at P-CSCF failure" mechanism.

The MME sends Modify Bearer to the P-GW for the associated PDN connection with a P-CSCF Restoration indication. The MME provides this indication to the P-GW through the S-GW. When Modify Bearer Request is received by the S-GW with the P-CSCF Restoration indication, this message is forwarded to PGW. PGW sends Update Bearer Request to the MME along with a list of available P-CSCF addresses within PCO IE to update the destination UE.

MME sends an Update EPS Bearer Context Request or Modify PDP Context Request to the UE, including the PCO with the list of available P-CSCF addresses otherwise, upon reception of Delete Bearer Request the MME sends Delete EPS Bearer Context Request to the UE with NAS cause code "reactivation requested". When the PDN connection is released, the UE re-activates the IMS PDN connection and selects an available P-CSCF. If the UE has received Modify EPS Bearer Context Request, the UE as per PCO based P-CSCF Restoration procedures, selects an available P-CSCF from the list for IMS registration. The UE performs a new initial IMS registration.