Content for TS 23.380 Word version: 18.1.0

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5.8.4 UDM/HSS Based P-CSCF Restoration 5.8.4.1 Overview and principles 5.8.4.2 Trigger P-CSCF Restoration Procedure via SMF 5.8.4.3 Trigger P-CSCF Restoration Procedure via AMF
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5.8.4 UDM/HSS Based P-CSCF Restoration p. 48

5.8.4.1 Overview and principles p. 48

The UDM/HSS based P-CSCF restoration is an optional mechanism.

As the IMS system interfaces an HSS, the UDM/HSS in this clause shall be interpreted as TS 23.228, Annex Y.0.

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 UDM shall, either:

send a P-CSCF restoration indication to the SMF serving IMS PDU session, and the SMF thus initiates IMS PDU session update procedure or the IMS PDU session re-establishment, so as to trigger the UE re-selects P-CSCF and registers again to IMS, or
send a P-CSCF restoration indication to the AMF serving the UE, and the AMF initiates IMS PDU session re-establishment, and the UE re-selects P-CSCF and registers again to IMS.

5.8.4.2 Trigger P-CSCF Restoration Procedure via SMF p. 48

During establishing PDU Session for IMS service, the SMF performs registration to the UDM and provides sufficient information for triggering P-CSCF restoration procedure (e.g. DNN="IMS", callback URI for P-CSCF restoration…), as specified in TS 29.503.

When the UDM determines that a P-CSCF restoration needs to be triggered, the UDM selects the SMF serving IMS, according to the SMF registration information in the UDM, to trigger the P-CSCF restoration procedure.

The following Figure 5.8.4.2-1 illustrates how the SMF supports P-CSCF restoration procedure in 5G network.

Copy of original 3GPP image for 3GPP TS 23.380, Fig. 5.8.4.2-1: Trigger P-CSCF Restoration Procedure via SMF

Figure 5.8.4.2-1: Trigger P-CSCF Restoration Procedure via SMF
(⇒ copy of original 3GPP image)

Step 0.

The SMF serving the IMS PDU session registers at the UDM. If the SMF supports the UDM based P-CSCF restoration mechanism, it provides a callback URI for P-CSCF restoration notifications.

Step 1-5.

The S-CSCF receives incoming SIP message. If the S-CSCF detects the previous P-CSCF is failed (e.g. due to return SIP error or lack of response), the S-CSCF sends Cx SAR message to the HSS, including P-CSCF restoration indication and, optionally, the address of the failed P-CSCF.

After receiving the Cx SAR message, the HSS forwards the P-CSCF restoration indication and the address of the failed P-CSCF to the UDM by means of the Nudm_UECM P-CSCF-RestorationTrigger service operation (see TS 23.632).

Step 6.

The UDM sends Nudm_UECM_PCscfRestoration notification to the SMF serving IMS PDU session, using the received callback URI for P-CSCF restoration notifications. The notification may include the address of the failed P-CSCF. The SMF accepts the Nudm message and sends HTTP response message to the UDM.

Step 7.

The S-CSCF sends a SIP response back to the originating side.

Step 8.a.

If the requirements listed at clause 5.8.2.2 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 5.8.2.2; if the SMF received the address of the failed P-CSCF in the notification from UDM, it may take it into account to build the list of available P-CSCFs that should be sent to the UE.

Step 8.b.

If the requirements listed at clause 5.8.2.3 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 5.8.2.3.

Step 8.c.

Otherwise the SMF initiates the PDU Session Release with Reactivation procedure to the UE, to trigger the UE re-establish IMS PDUS session, as specified in clause 5.8.2.4.

Step 9.

Subsequent to step 8, the UE performs an initial IMS registration towards the new P-CSCF, as specified in clause 5.8.2.

Step 10.

The S-CSCF sends the suspended terminating SIP message to a newly selected P-CSCF after the successful SIP registration for the UE.

5.8.4.3 Trigger P-CSCF Restoration Procedure via AMF p. 50

During establishing PDU Session for IMS service, the AMF performs registration to the UDM and provides sufficient information for triggering P-CSCF restoration procedure, as specified in TS 29.503.

When the UDM determines P-CSCF restoration need to be triggered, the UDM selects the AMF serving the UE according to the AMF registration information in the UDM, to trigger the P-CSCF restoration procedure.

The following Figure 5.8.4.3-1 illustrates how the AMF supports P-CSCF restoration procedure in 5G network.

Copy of original 3GPP image for 3GPP TS 23.380, Fig. 5.8.4.3-1: Trigger P-CSCF Restoration Procedure via AMF

Figure 5.8.4.3-1: Trigger P-CSCF Restoration Procedure via AMF
(⇒ copy of original 3GPP image)

Step 0.

The AMF serving the UE registers at the UDM. If the AMF supports the UDM based P-CSCF restoration mechanism, it provides a callback URI for P-CSCF restoration notifications.

Step 1-5.

After receiving the Cx SAR message, the HSS forwards the P-CSCF restoration indication to the UDM by means of the Nudm_UECM P-CSCF-RestorationTrigger service operation (see TS 23.632).

Step 6.

The UDM sends Nudm_UECM_PCscfRestoration notification to the AMF serving the UE, using the received callback URI for P-CSCF restoration notifications. The AMF accepts the Nudm message and sends HTTP response message to the UDM.