5.8 P-CSCF Restoration for 5GC

23.3803GPPIMS Restoration ProceduresRelease 17TS

5.8.1 Introduction

The clause 5.8 describes solutions to support P-CSCF restoration in 5GC for UEs with an IMS PDU session.

5.8.2 Common Procedures for P-CSCF Restoration in 5GC

5.8.2.1 General

If IMS service is required through 5GC, the UE performs Registration procedure and requests PDU Session Establishment procedure for IMS service, as specified in 3GPP TS 23.501 [28] and 3GPP TS 23.502 [29]. The UE selects P-CSCF via PDU Session Establishment procedure or via DHCP procedure and performs initial IMS registration (see 3GPP TS 23.228 [27]).

P-CSCF failure may be detected during an originating SIP procedure or terminating SIP procedure. When P-CSCF failure is detected, various mechanisms for P-CSCF restoration can be invoked (by e.g. SMF, UDM or PCF), depending on the mechanism utilized.

Various mechanisms for P-CSCF restoration in 5GC have common procedures which are used to trigger the P-CSCF re-selection at the UE side, as specified in the following clauses.

NOTE: Amongst the three common procedures, the PDU Session Release with Reactivation Procedure described at clause 5.8.2.4 has the highest signalling impact on the radio access and core networks; on the other hand it doesn’t require the UE to support the optional P-CSCF Re-selection mechanism.

5.8.2.2 P-CSCF Address List Update Procedure

The following requirements shall be supported for the procedure specified in this clause:

1. P-CSCF discovery was performed by requesting and provisioning P-CSCF address(es) within Extended Protocol Configuration Options (ePCO), as specified in 3GPP TS 29.061 [9], clause 13a.2.1.

2. The UE has indicated the "P-CSCF Re-selection support" in the ePCO IE, as specified in 3GPP TS 24.008 [4], clause 10.5.6.3A.

Figure 5.8.2.2-1 describes the P-CSCF address list update procedure at detection of P-CSCF failure.

Figure 5.8.2.2-1: P-CSCF Address Update Procedure

When P-CSCF failure occurs, the SMF detects it or is informed of the event by another NF (e.g. AMF, UDM…) and it initiates the PDU Session Update procedure, in order to send the new P-CSCF list to the UE.

1. The SMF invokes Namf_Communication_N1N2MessageTransfer service operation, to trigger the PDU Session Modification procedure. Within the message, the SMF includes an ePCO IE as specified in 3GPP TS 24.008 [4], clause 10.5.6.3A with a new list of P-CSCF addresses.

2. The AMF triggers the PDU Session Modification procedure with the UE to deliver the new list of P-CSCF.

3. Upon receiving the new list of P-CSCFs, the UE selects a P-CSCF from the list to perform an initial SIP registration.

5.8.2.3 DHCP based P-CSCF Selection Triggering Procedure

The following requirements shall be supported for the procedure specified in this clause:

1. P-CSCF discovery was performed by requesting P-CSCF address(es) via DHCP method, as specified in 3GPP TS 29.061 [9], clause 13a.2.1

2. The UE has indicated the "P-CSCF Re-selection support" in the ePCO IE, as specified in 3GPP TS 24.008 [4], clause 10.5.6.3A.

Figure 5.8.2.3-1 describes the DHCP based P-CSCF selection triggering procedure.

Figure 5.8.2.3-1: P-CSCF Selection Triggering Procedure

When P-CSCF failure occurs, the SMF detects it or is informed of the event by another NF (e.g. AMF, UDM…) and it initiates the PDU Session Update procedure, in order to send a P-CSCF failure indication to the UE.

1. The SMF invokes Namf_Communication_N1N2MessageTransfer service operation, to trigger the PDU Session Modification procedure. Within the message, the SMF includes a P-CSCF failure indication in the ePCO IE as specified in 3GPP TS 24.008 [4], clause 10.5.6.3A.

2. The AMF triggers the PDU Session Modification procedure to the UE, to deliver the P-CSCF failure indication to the UE.

3. Upon receiving the P-CSCF failure indication, the UE discovers another P-CSCF via DHCP procedure to perform SIP registration.

5.8.2.4 PDU Session Release with Reactivation Procedure

The following requirements shall be supported for the procedure specified in this clause:

1. The UE has not indicated the "P-CSCF Re-selection support" in the ePCO IE, as specified in 3GPP TS 24.008 [4], clause 10.5.6.3.

Figure 5.8.2.4-1 describes the procedure of PDU session release with reactivation indication.

Figure 5.8.2.4-1: PDU Session Release with Reactivation Indication

When P-CSCF failure occurs, the SMF detects it or is informed of the event by another NF (e.g. AMF, UDM…) and it initiates the PDU Session Release procedure, with reactivation indication.

1. The network initiates a PDU session release with reactivation indication of the IMS PDU session according to 3GPP TS 23.502 [29], clause 4.3.4. The exact network function that triggers the procedure depends on the P-CSCF restoration method used.

2. The UE requests to re-establish IMS PDU session. The UE discovers P-CSCF during the IMS PDU session establishment or via DHCP procedure, to perform SIP registration.

5.8.3 P-CSCF Failure Detection at SMF/UPF

5.8.3.1 Overview and Principles

The P-CSCF failure detection at SMF/UPF is an optional mechanism.

This mechanism is executed when an originating request cannot be served due to a P-CSCF failure, as long as there are no other registration flows for this originating UE using an available P-CSCF.

Once the P-CSCF is reported as failed, the SMF initiates IMS PDU sesion update procedure, so as to trigger the UE to re-select a P-CSCF and to register again to IMS.

5.8.3.2 P-CSCF Monitoring and Failure Detection Procedure

The following figure 5.8.3.2-1 describes the SMF-instructed P-CSCF monitoring and failure detection. When P-CSCF failure is detected, the SMF triggers the related procedures, as specified in 5.8.2 for P-CSCF restoration.

Figure 5.8.3.2-1: P-CSCF Failure Detection at SMF/UPF

1. The UE sends a PDU Session Establishment Request towards the AMF, to establish a PDU session for IMS service.

2. The AMF invokes Nsmf_PDUSession_CreateSMContext service operation to the SMF, to request the SMF to create PDU session for IMS service.

3. The SMF invokes Npcf_SMFPolicyControl_Get service operation to the PCF, to retrieve session related policy.

4. The SMF selects appropriate UPF for IMS service and requests the UPF to establish N4 session.

5. When PDU session is created, the SMF invokes Namf_Communication_N1N2MessageTransfer service operation to the AMF, to inform the request of PDU session establishment. A list of P-CSCF address is also included in the ePCO as specified in 3GPP TS 24.008 [4], clause 10.5.6.3A.

6. The AMF sends PDU Establishenment Response message to the UE, with a list of P-CSCF included in the ePCO as specified in 3GPP TS 24.008 [4], clause 10.5.6.3A.

7. The UE performs an initial registration towards a P-CSCF from the received list.

8. The P-CSCF sends Rx Push (see 3GPP TS 29.214 [7]) to provide the PCF with the P-CSCF selected by the UE. The PCF sends Rx Push response.

9. The PCF sends P-CSCF address to the SMF, e.g. using Notification procedure.

Editor’s Note: It is FFS on which Npcf procedure/message is used for the PCF to send P-CSCF address to the SMF.

10. The P-CSCF sends 200 OK to the UE.

Upon P-CSCF failure detection by the SMF/UPF;

11.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

11.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.

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

12. Subsequent to step 11, the UE performs an initial IMS registration towards the new P-CSCF, as specified in clause 5.8.2.

5.8.4 UDM/HSS Based P-CSCF Restoration

5.8.4.1 Overview and principles

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 [27], annex Y, clause 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

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 3GPP TS 29.503 [31].

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.

Figure 5.8.4.2-1: Trigger P-CSCF Restoration Procedure via SMF

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.

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 3GPP TS 23.632 [33]).

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.

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

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.

NOTE 1: The address of the failed P-CSCF received by SMF from UDM corresponds to the Mw interface of the P-CSCF and the (updated) address list of P-CSCFs that the SMF sends to the UE over PCO corresponds to the Gm interface of the P-CSCF; the SMF derives the Gm IP address of the failed P-CSCF from the received Mw address (FQDN and/or IP address) (e.g., based on local mapping).

NOTE 2: The failed P-CSCF can be included again in the new P-CSCF list once it has recovered.

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.

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.

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

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

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 3GPP TS 29.503 [31].

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.

Figure 5.8.4.3-1: Trigger P-CSCF Restoration Procedure via AMF

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.

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.

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 3GPP TS 23.632 [33]).

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.

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

8. The AMF initiates a network-triggered PDU Session Release procedure of the IMS PDU session with Reactivation indication, as specified in clause 5.8.2.4.

9. Subsequent to step 8, the UE performs an initial IMS registration towards the new P-CSCF, as specified in clause 5.8.2.4.

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

5.8.5 PCF Based P-CSCF Restoration

5.8.5.1 Introduction

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.

5.8.5.2 Trigger P-CSCF Restoration Procedure via PCF

The following figure illustrates the details of PCF based P-CSCF restoration procedure in 5G network.

Figure 5.8.5.2-1: Trigger P-CSCF Restoration Procedure via PCF

The P-CSCF failure is detected by S-CSCF or IBCF/ATCF, as specified in clause 5.5.

1. The S-CSCF receives a terminating INVITE message.

2. 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.

3. 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 3GPP TS 23.503 [30]. 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.

4. The PCF sends P-CSCF restoration indication to the SMF serving IMS PDU session, as specified in 3GPP TS 29.512 [32] clause 4.2.3; the PCF may include, optionally, the address of the failed P-CSCF.

5.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 PCF, it may take it into account to build the list of available P-CSCFs that should be sent to the UE.

NOTE 1: The address of the failed P-CSCF received by SMF from PCF corresponds to the Mw interface of the P-CSCF and the (updated) address list of P-CSCFs that the SMF sends to the UE over PCO corresponds to the Gm interface of the P-CSCF; the SMF derives the Gm IP address of the failed P-CSCF from the received Mw address (FQDN and/or IP address) (e.g., based on local mapping).

NOTE 2: The failed P-CSCF can be included again in the new P-CSCF list once it has recovered.

5.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.

5.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 5.8.2.4.

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

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