Content for TS 23.380 Word version: 19.0.0

0… 4… 5… 5.2… 5.4… 5.5… 5.6… 5.8… 5.8.4… 5.8.5… 6… 7… 8…

7.1 General 7.2 UE Terminating Procedure 7.2.1 Introduction 7.2.2 P-CSCF triggering SMF/PGW-C failure checking 7.2.3 P-CSCF receiving EPC/5GC NF failure notification from PCRF/PCF

7 Detection of EPC/5GC NF Failures and IMS Restoration Procedures |R19| p. 55

7.1 General p. 55

The following clauses show the requirements and flows of detection of EPC/5GC NF failures and IMS restoration procedures.

7.2 UE Terminating Procedure p. 56

When the failure of EPC/5GC NF, e.g. PGW-C, PGW-U SMF or UPF is detected during the IMS terminating procedure, the following clauses specify the procedures to speed up the restoration of the IMS terminating session.

Failure may happen to EPC/5GC NF during the IMS terminating procedure.In such case, the P-CSCF may detect such EPC/5GC NF failure as follows:

P-CSCF requires PCRF/PCF to perform PGW-C/SMF failure detection;
P-CSCF receives EPC/5GC NF failure notification from PCRF/PCF via Rx Abort-Session-Request (ASR) or Npcf_PolicyAuthorization_Notify message indicating service unavailability;
P-CSCF detects PGW-U/UPF failure by detecting transmission failure of IMS message to terminating UE.

7.2.2 P-CSCF triggering SMF/PGW-C failure checking p. 56

The P-CSCF triggering SMF/PGW-C failure checking is an optional mechanism. If supported, the mechanism shall be executed when the terminating UE does not respond to the initial INVITE request due to an EPC/5GC NF failure.

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

P-CSCF supports to check whether the SMF/PGW-C is normal when timeout by sending an Rx Authorization-Authentication-Request (AAR) with the Specific-Action AVP set to ACCESS_NETWORK_INFO_REPORT (12) specified in TS 29.214 or invoke the Npcf_PolicyAuthorization service to PCF/PCRF.
P-CSCF supports to send a 503 Service Unavailable response with the P-CSCF Restoration indication to the I-/S-CSCF after receiving the Rx Authorization-Authentication-Answer (AAA) with the Result-Code AVP set to DIAMETER_UNKNOWN_SESSION_ID (5002) as specified in RFC 6733 or the Rx Experimental-Result-Code AVP set to IP-CAN_SESSION_NOT_AVAILABLE (5065) as specified in clause 5.5.1 of TS 29.214.
S-CSCF supports to start the HSS based P-CSCF Restoration procedure specified in clause 5.4.2.1 in EPC network or the UDM/HSS based P-CSCF Restoration procedure specified in clause 5.8.4 in 5GC network, after receiving a 503 Service Unavailable response with the P-CSCF Restoration indication.
PCF/PCRF support to send a Gx Re-Auth-Request (RAR) with Event-Trigger AVP set to ACCESS_NETWORK_INFO_REPORT (45) as specified in TS 29.212 to the SMF/PGW-C after receiving an Rx AAR with the Specific-Action AVP set as ACCESS_NETWORK_INFO_REPORT (12) from the P-CSCF.
PCF/PCRF support to return a Rx AAA with the Result-Code AVP set to DIAMETER_UNKNOWN_SESSION_ID (5002) specified in RFC 6733 or the Rx Experimental-Result-Code AVP set to IP-CAN_SESSION_NOT_AVAILABLE (5065) specified in TS 29.214 to the P-CSCF when a response with the Result-Code AVP set to DIAMETER_UNKNOWN_SESSION_ID (5002) is returned by the SMF/PGW-C.
SMF/PGW-C support to return a Gx RAA with the Result-Code AVP set to DIAMETER_UNKNOWN_SESSION_ID (5002) specified in RFC 6733 to the PCF/PCRF when the SMF/PGW-C is not reachable.

Figure 7.2.2-1 illustrates the quick restoration procedure of IMS terminating session in the EPC/5GC network below.

Copy of original 3GPP image for 3GPP TS 23.380, Fig. 7.2.2-1: Quick restoration procedure of IMS terminating session

Figure 7.2.2-1: Quick restoration procedure of IMS terminating session
(⇒ copy of original 3GPP image)

Step 1-3.

The I-/S-CSCF in the terminating network receives an incoming INVITE request and routes it to the P-CSCF serving UE B. Then the P-CSCF routes the INVITE request to UE B.

Step 4-5.

The P-CSCF does not receive any response from UE B after some time out, so the P-CSCF determines that the SIP signalling has already attempted the corresponding re-transmissions or retrials. Therefore, the P-CSCF sends a Rx AAR with the Specific-Action AVP set to ACCESS_NETWORK_INFO_REPORT (12) to PCRF or invokes the Npcf_PolicyAuthorization_Create service operation to the PCF. This action serves as a test to determine whether a failure exists in the EPC/5GC network.

Step 6-7.

Upon receiving the AAR or Npcf_PolicyAuthorization_Create request, if the PCF/PCRF has identified that the SMF/PGW-C is reachable, then it sends a Gx RAR with Event-Trigger AVP set to ACCESS_NETWORK_INFO_REPORT (45) or invokes the Npcf_SMPolicyControl_UpdateNotify service operation to the SMF/PGW-C.

Step 8-9.

If the SMF/PGW-C finds that there is no PDU session context of User B, it returns an Gx RAA with the Result-Code AVP set to DIAMETER_UNKNOWN_SESSION_ID (5002) or the Npcf_SMPolicyControl_UpdateNotify service response to the PCF/PCRF.

Step 10.

The PCF/PCRF returns either an Rx AAA with the Result-Code AVP set to DIAMETER_UNKNOWN_SESSION_ID (5002) or the Rx Experimental-Result-Code AVP set to IP-CAN_SESSION_NOT_AVAILABLE (5065), or the Npcf_PolicyAuthorization_Create service response to the P-CSCF.

Step 11-12.

The P-CSCF sends a 503 Service Unavailable response with the P-CSCF Restoration indication to the I-/S-CSCF. Then the I-/S-CSCF starts the HSS based P-CSCF Restoration procedure in EPC network specified in clause 5.4.2.1, or the UDM/HSS based P-CSCF Restoration procedure in 5GC network specified in clause 5.8.4 by sending a Cx SAR to the HSS, including a P-CSCF Restoration indication.

UE B will re-attach/re-registered to the EPC/5GC and register into the IMS quickly as specified in the HSS based or UDM/HSS based P-CSCF restoration procedure. Then UE B will receive the next incoming session for it has re-connected to the EPC/5GC network successfully. After the EPS bearer/QoS flow for IMS signalling is established, UE A initiates the initial registration procedure which specified in TS 23.228.

7.2.3 P-CSCF receiving EPC/5GC NF failure notification from PCRF/PCF p. 58

7.2.3.1 Introduction p. 58

The PCRF-based restoration is optionally extended by reusing part of the "HSS-based P-CSCF restoration for 3GPP access" mechanism described in clause 5.4 when the P-CSCF receives EPC/5GC NF failure notification from PCRF/PCF.

The extension is based on the possibility for the PCRF/PCF detecting whether the PGW-C/SMF fails.

7.2.3.2 Description p. 58

Copy of original 3GPP image for 3GPP TS 23.380, Fig. 7.2.3.2-1: P-CSCF receiving EPC/5GC NF failure notification from PCRF/PCF

Figure 7.2.3.2-1: P-CSCF receiving EPC/5GC NF failure notification from PCRF/PCF
(⇒ copy of original 3GPP image)

Step 1.

The terminating I/S-CSCF receives a SIP message for a destination UE and forwards the SIP message to this called UE's terminating P-CSCF.

Step 2.

When the P-CSCF (acting as the AF) determines that session related media requires PCC supervision, or the access network information (i.e. user location and/or user timezone information) is required, the P-CSCF in terminating network sends a Rx Authorization-Authentication-Request (AAR) to the PCRF as specified in TS 29.214 or invokes the Npcf_PolicyAuthorization_Create service operation to the PCF as specified in TS 29.513.

Step 3.

The PCRF/PCF returns the Rx Authorization-Authentication-Answer (AAA) or the Npcf_PolicyAuthorization_Create response to the P-CSCF.

Step 4a.

The PCRF/PCF decides to monitor the status of CN nodes. If the PCRF/PCF has previously stored the failure status of the CN failure for the same PGW-C/SMF and the status is still valid, step 4a-4d are skipped.

Step 4b.

PCRF/PCF sends a Gx Re-Auth-Request (RAR) or invokes the Npcf_SMPolicyControl_UpdateNotify service operation to the PGW-C/SMF.

Step 4c.

If the PCRF/PCF receives no response from the PGW-C/SMF, it may determine the service unavailability of the PGW-C/SMF.

If the PGW-C/SMF detects some errors in the user plane, it may send a response to the PCRF/PCF with service unavailability indication which is considered as the case of EPC/5GC NF failure.

Step 4d.

If the EPC/5GC NF failure is detected in 4a, 4b or 4c, the PCRF/PCF may store the failure status of the CN failure of the same PGW-C/SMF for future usage e.g. to avoid triggering Gx RAR message frequently. If any request or response with success code is received from the same PGW-C/SMF, the PCRF/PCF will update the status of the PGW-C/SMF.

Step 5.

If the EPC/5GC NF failure is detected in steps 4a-4d, the PCRF/PCF sends Rx Abort-Session-Request (ASR) or invokes the Npcf_PolicyAuthorization_Notify service operation to the P-CSCF including the service unavailability indication.

Step 6.

Based on this indication, P-CSCF decides to send a response with an error code (i.e. the UE registration data is not present) to the S-CSCF or not to send any response.

When the S-CSCF receives the error code (i.e. the UE registration data is not present) or no response, it considers the case as the called UE's terminating P-CSCF is not able to process this request. Then the steps 4-8 of the HSS based P-CSCF restoration procedure as described in clause 5.4.2.1 are executed.