Section: 4.3.4.3 UE or network requested PDU Session Release for Home-routed Roaming
Content: This procedure is used in the case of home-routed roaming scenarios. Figure 4.3.4.3-1: UE or network requested PDU Session Release for home-routed roaming 1. The procedure is triggered by one of the following events: 1a. (UE initiated release) As in step 1a of clause 4.3.4.2 with the addition that: - the V-SMF initiates N4 Session Modification to instruct the V-UPF to stop forwarding uplink traffic; and - the V-SMF invokes the Nsmf_PDUSession_Update Request (SM Context ID, information from the SM message from the UE e.g. PCO, "Trigger PDU Session Release" indication, Timezone, User Location Information) service operation to request the H-SMF to release the PDU Session. The H-SMF responds to the request immediately. 1b. (Serving network initiated release) The serving network initiates the PDU Session Release during UE or serving network initiated Deregistration procedure as specified in clause 4.2.2.3. There is no NAS SM message between the UE and the V-SMF in this case. This step is the same as step 1c in clause 4.3.4.2, with the addition that: - the V-SMF initiates N4 Session Modification to instruct the V-UPF to stop forwarding uplink traffic; and - the V-SMF initiates the release of the PDU Session at the H-SMF by invoking the Nsmf_PDUSession_Release request. The serving network also initiates the PDU Session Release where neither N1 nor N2 SM signalling is needed before releasing the SM context as in step 1c in clause 4.3.4.2 if e.g. due to the set of network slices for a UE changes where a network slice instance is no longer available (e.g. as described in clause 5.15.5.2.2 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2], or the AAA Server triggered Network Slice-Specific Re-authentication and Re-authorization procedure fails as specified in clause 4.2.9.2 or the AAA Server triggered Slice-Specific Authorization Revocation takes place as specified in clause 4.2.9.4). 1c. (HPLMN initiated release) This step is the same as step 1b in clause 4.3.4.2, with the addition that: - the H-SMF initiates N4 Session Modification to instruct the H-UPF to stop forwarding downlink traffic. 1d. This step is the same as step 1d in clause 4.3.4.2, with the addition that: - the V-SMF initiates N4 Session Modification to instruct the V-UPF to stop forwarding uplink traffic; and - the V-SMF invokes the Nsmf_PDUSession_Update Request towards H-SMF. 1e. (HPLMN initiated release) This step is the same as step 1e in clause 4.3.4.2, with the addition that: - if the H-SMF is notified by the V-SMF that the HPLMN S-NSSAI of the PDU Session with SSC mode 1 or SSC mode 2 is to be replaced with Alternative HPLMN S-NSSAI and if the H-SMF determines that a new PDU Session is to be established on the Alternative HPLMN S-NSSAI, the H-SMF initiates release of the PDU Session, as described in clause 5.15.19 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]; - the H-SMF initiates N4 Session Modification to instruct the H-UPF to stop forwarding downlink traffic. 1f. This step is the same as step 1f in clause 4.3.4.2, with the addition that: - the V-SMF initiates N4 Session Modification to instruct the V-UPF to stop forwarding uplink traffic; and - the V-SMF invokes the Nsmf_PDUSession_Update Request towards H-SMF. If the SMF receives one of the triggers in step 1a, 1c, 1e or 1f, the H-SMF starts PDU Session Release procedure. When a SMF above initiates N4 Session Modification to instruct the UPF to stop forwarding traffic this means also to stop taking into account the traffic for usage monitoring. 2a-2b. (UE-, (R)AN- or Serving network initiated) This step is performed in case the PDU Session Release is triggered by a message from V-SMF. These steps are the same as steps 2a-2b in clause 4.3.4.2. The SMF is the SMF in HPLMN. NOTE 1: This step 2a-2b can correspond to steps 1a, 1b, 1d, 1f. 3a. (UE or HPLMN initiated release) The H-SMF prepares the SM Release PDU Session Command message and initiates the PDU Session Release towards the UE by invoking the Nsmf_PDUSession_Update Request service operation towards the V-SMF. The Nsmf_PDUSession_Update Request contains necessary information to build the SM Release PDU Session Command by the V-SMF towards the UE (for example a Release Cause or PCO). For network slice replacement as in step 1e, the H-SMF includes Alternative HPLMN S-NSSAI and a cause value indicating that a new PDU Session re-establishment on the Alternative HPLMN S-NSSAI is required. 3b. (Serving network initiated release) The H-SMF responds to the PDU release request from the V-SMF with a Nsmf_PDUSession_Release response. If the UPF included Small Data Rate Control Status in step 2 then the SMF includes Small Data Rate Control Status in the request to the AMF. If the Control Plane CIoT 5GS Optimisation is enabled for this PDU Session, the steps 4a and 4b are skipped. 4a-4b. The V-SMF releases the corresponding User Plane resources. This includes the same procedure in step 2 but controlled from the SMF in VPLMN. 5-13. These steps are the same as steps 3-10 in clause 4.3.4.2, with the addition that: - In step 5c, for network slice replacement, based on information from H-SMF, the PDU Session Release Command message may include Alternative HPLMN S-NSSAI and/or Alternative VPLMN S-NSSAI and a cause value indicating that a PDU Session re-establishment on the Alternative HPLMN S-NSSAI and/or Alternative VPLMN S-NSSAI is required. 14. (UE or HPLMN initiated release) The V-SMF responds to the Nsmf_PDUSession_Update Request invoked at step 3a and confirms the PDU Session Release. The Nsmf_PDUSession_Update response may carry information such as PCO received from the UE in SM PDU Session Release Accept. as well as User Location Information, Time Zone and Secondary RAT Usage Data. 15a. (UE or HPLMN or Serving network initiated release) The H-SMF releases the SM policy control association with the PCF by invoking the SM Policy Association Termination procedure defined in clause 4.16.6. For serving network initiated PDU Session Release case, this step happens between step 1b and step 3b. 15b-15c. (HPLMN initiated release) In case the PDU Session Release is HPLMN-initiated (i.e. triggers in 1c, 1e), the H-SMF releases the corresponding User Plane resources. This includes the same procedure as in step 2. 15d. As in step 15 of clause 4.3.4.2, the SMF invokes the Nudm_UECM_Deregistration service operation. NOTE 2: Step 15d does not necessarily take place after step 15c. 16. (UE or HPLMN initiated release) The H-SMF shall remove all contexts associated with the PDU Session: 16a. The H-SMF requests the V-SMF to release all contexts associated with the PDU Session by invoking the Nsmf_PDUSession_StatusNotify (Release) operation. 16b. The V-SMF requests the AMF to release all contexts associated with the PDU Session by invoking the Nsmf_PDUSession_SMContexStatusNotify (Release). The AMF releases the association between the SMF ID and the PDU Session ID.