Content for TS 23.380 Word version: 18.1.0
5.8 P-CSCF Restoration for 5GC
5.8.1 Introduction
5.8.2 Common Procedures for P-CSCF Restoration in 5GC
5.8.3 P-CSCF Failure Detection at SMF/UPF
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5.8 P-CSCF Restoration for 5GC |R15| p. 43
5.8.1 Introduction p. 43
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 p. 43
5.8.2.1 General p. 43
If IMS service is required through 5GC, the UE performs Registration procedure and requests PDU Session Establishment procedure for IMS service, as specified in TS 23.501 and TS 23.502. The UE selects P-CSCF via PDU Session Establishment procedure or via DHCP procedure and performs initial IMS registration (see TS 23.228).
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.
5.8.2.2 P-CSCF Address List Update Procedure p. 44
The following requirements shall be supported for the procedure specified in this clause:
- P-CSCF discovery was performed by requesting and provisioning P-CSCF address(es) within Extended Protocol Configuration Options (ePCO), as specified in clause 13a.2.1 of TS 29.061.
- The UE has indicated the "P-CSCF Re-selection support" in the ePCO IE, as specified in clause 10.5.6.3A of TS 24.008.
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.
Step 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 clause 10.5.6.3A of TS 24.008 with a new list of P-CSCF addresses.
Step 2.
The AMF triggers the PDU Session Modification procedure with the UE to deliver the new list of P-CSCF.
Step 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 p. 44
The following requirements shall be supported for the procedure specified in this clause:
- P-CSCF discovery was performed by requesting P-CSCF address(es) via DHCP method, as specified in clause 13a.2.1 of TS 29.061.
- The UE has indicated the "P-CSCF Re-selection support" in the ePCO IE, as specified in clause 10.5.6.3A of TS 24.008.
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.
Step 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 clause 10.5.6.3A of TS 24.008.
Step 2.
The AMF triggers the PDU Session Modification procedure to the UE, to deliver the P-CSCF failure indication to the UE.
Step 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 p. 45
The following requirements shall be supported for the procedure specified in this clause:
- The UE has not indicated the "P-CSCF Re-selection support" in the ePCO IE, as specified in clause 10.5.6.3 of TS 24.008.
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.
Step 1.
The network initiates a PDU session release with reactivation indication of the IMS PDU session according to clause 4.3.4 of TS 23.502. The exact network function that triggers the procedure depends on the P-CSCF restoration method used.
Step 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 p. 46
5.8.3.1 Overview and Principles p. 46
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 p. 46
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 clause 5.8.2 for P-CSCF restoration.
Step 1.
The UE sends a PDU Session Establishment Request towards the AMF, to establish a PDU session for IMS service.
Step 2.
The AMF invokes Nsmf_PDUSession_CreateSMContext service operation to the SMF, to request the SMF to create PDU session for IMS service.
Step 3.
The SMF invokes Npcf_SMFPolicyControl_Get service operation to the PCF, to retrieve session related policy.
Step 4.
The SMF selects appropriate UPF for IMS service and requests the UPF to establish N4 session.
Step 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 clause 10.5.6.3A of TS 24.008.
Step 6.
The AMF sends PDU Establishenment Response message to the UE, with a list of P-CSCF included in the ePCO as specified in clause 10.5.6.3A of TS 24.008.
Step 7.
The UE performs an initial registration towards a P-CSCF from the received list.
Step 8.
The P-CSCF sends Rx Push (see TS 29.214) to provide the PCF with the P-CSCF selected by the UE. The PCF sends Rx Push response.
Step 9.
The PCF sends P-CSCF address to the SMF, e.g. using Notification procedure.
Step 10.
The P-CSCF sends 200 OK to the UE.
Upon P-CSCF failure detection by the SMF/UPF;
Step 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
Step 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.
Step 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.
Step 12.
Subsequent to step 11, the UE performs an initial IMS registration towards the new P-CSCF, as specified in clause 5.8.2.
