Section: 4.3.3.3 UE or network requested PDU Session Modification (home-routed roaming)
Content: The UE or network requested PDU Session Modification procedure (home-routed roaming scenario) is depicted in figure 4.3.3.3-1. Figure 4.3.3.3-1: UE or network requested PDU Session Modification (for home-routed roaming scenario) 1. The procedure is triggered by one of the following events: 1a. (UE or serving network requested) As in step 1a of clause 4.3.3.2 with the addition that: - The V-SMF checks whether it can accept the request from the UE; - The V-SMF invokes an Nsmf_PDUSession_Update Request (SM Context ID, UE request for PDU Session Modification or the QoS modification request from the VPLMN, UE location information, Time Zone, the current Access Type, PCO, [Always-on PDU Session Requested]) service operation to inform the H-SMF to update the PDU Session. The H-SMF responds to the request immediately. If the AMF notified the V-SMF that the access type of the PDU session can be changed, as described in the UE Triggered Service Request procedure in clause 4.2.3.2, the V-SMF shall also indicate that the access type can be changed. The PS Data Off status, if changed, shall be included in PCO (Protocol Configuration Option) in the PDU Session Modification Request message. When PCF is deployed, the SMF shall further report the PS Data Off status to PCF if the PS Data Off event trigger is provisioned, the additional behaviour of SMF and PCF for 3GPP PS Data Off is defined in TS 23.503[ Policy and charging control framework for the 5G System (5GS); Stage 2 ]  [20]. 1b. (HPLMN requested) This step is the same as step 1b in clause 4.3.3.2. If the H-SMF received the indication that the access type of the PDU session can be changed, the H-SMF shall indicate the target access type to the PCF in the Access Type information of the Npcf_SMPolicyControl_Update Request. 1c. (HPLMN requested) This step is the same as step 1c in clause 4.3.3.2. 1d. (HPLMN requested) This step is the same as step 1d in clause 4.3.3.2. 1e. As in step 1e of clause 4.3.3.2 with addition that: - The AMF invokes Nsmf_PDUSession_UpdateSMContext (SM context ID, N2 SM information) and sends it to the V-SMF; - The V-SMF invokes an Nsmf_PDUSession_Update Request (SM context ID, ULI, AN type, QoS Flow to be released) service operation to inform the H-SMF to update the PDU Session. The H-SMF responds to the request immediately. For AN initiated notification control in step 1e of clause 4.3.3.2, the V-SMF includes also QoS Flow notification information as specified in clause 5.7 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. NOTE 1: SM Context ID between AMF and V-SMF and between V-SMF and H-SMF are different. SM Context ID has local significance per SMF instance. 1f. (Slice Replacement) As in step 1h of clause 4.3.3.2 with addition: - When the AMF determines as described in clause 5.15.19 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2] that VPLMN S-NSSAI is to be replaced by an Alternative VPLMN S-NSSAI, and/or HPLMN S-NSSAI is to be replaced by an Alternative HPLMN S-NSSAI, the AMF invokes Nsmf_PDUSession_UpdateSMContext (SM context ID, VPLMN S-NSSAI, Alternative VPLMN S-NSSAI, HPLMN S-NSSAI, Alternative HPLMN S-NSSAI) and sends it to the V-SMF. If the AMF determines that the current PDU Session cannot be retained (e.g. the current V-SMF cannot support the Alternative VPLMN S-NSSAI), the AMF includes a release indication. - If Alternative HPLMN S-NSSAI is received from the AMF, the V-SMF invokes an Nsmf_PDUSession_Update Request (SM context ID, HPLMN S-NSSAI, Alternative HPLMN S-NSSAI) service operation to inform the H-SMF. The H-SMF responds to the request immediately. The V-SMF forwards the release indication if it is received from the AMF. 2. This step is the same as steps 2 in clause 4.3.3.2 with the SMF is H-SMF. 3. (UE or serving network requested or HPLMN requested) The H-SMF invokes the Nsmf_PDUSession_Update Request (SM Context ID, QoS profiles, [Alternative QoS profile(s)], Session-AMBR, information needed to build the SM PDU Session Modification Command message towards the UE including the QoS rule(s) and QoS Flow level QoS parameters if needed for the QoS Flow(s) associated with the QoS rule(s) and QoS rule operation and the QoS Flow level QoS parameters operation) service operation to the V-SMF. Based on operator policies and roaming agreements, the V-SMF may decide to fully accept or reject the QoS information provided by the H-SMF. The V-SMF shall also be able to accept a subset of the QoS flows requested to be created or modified within a single H-SMF request i.e. V-SMF can accept some QoS flows and reject other QoS flows in same response to H-SMF. If an Always-on PDU Session Granted indication was provided by the H-SMF to indicate that the PDU Session is to be changed to an always-on PDU Session, the V-SMF decides whether to accept or reject the request from the H-SMF based on local policies. For network slice replacement, based on the information received from the V-SMF, the H-SMF determines whether to retain the PDU Session as described in clause 5.15.19 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. If the H-SMF decides to retain the PDU Session, the H-SMF includes the Alternative HPLMN S-NSSAI and a cause value indicating that the PDU Session is to be retained in the Nsmf_PDUSession_Update Request message. For SSC mode 3 PDU Session if the H-SMF decides to establish a new PDU Session on the Alternative HPLMN S-NSSAI, 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 in the Nsmf_PDUSession_Update Request message. For SSC mode 1 and SSC mode 2 PDU session if the H-SMF decides to establish a new PDU Session on the HPLMN Alternative S-NSSAI, the H-SMF initiates release of the PDU Session as described in clause 4.3.4.3. 3a-3b (HPLMN requested) These steps are executed if new QoS Flow(s) are to be created. The SMF updates the UPF with UL Packet Detection Rules of the new QoS Flow. These steps are executed if the HPLMN S-NSSAI is replaced by an Alternative HPLMN S-NSSAI and the PDU Session is retained. The SMF updates the UPF with Alternative S-NSSAI. NOTE 2: This allows the UL packets with the QFI of the new QoS Flow to be transferred. 4a-4b. These steps are the same as step 3a-3b in clause 4.3.3.2 but controlled from the V-SMF. The V-SMF uses the information received in step 3 to generate any N1 and/or N2 signalling to be sent towards the UE and/or the (R)AN. For network slice replacement in step 1f, the following applies: - If the PDU Session is to be retained and only the VPLMN S-NSSAI is to be replaced by an Alternative VPLMN S-NSSAI, the V-SMF includes Alternative VPLMN S-NSSAI in the N1 PDU Session Modification Command message and a cause value to notify the UE that the VPLMN S-NSSAI of the PDU Session is replaced with the Alternative VPLMN S-NSSAI. The V-SMF also set the S-NSSAI in N2 SM information to Alternative VPLMN S-NSSAI. - If the PDU Session is to be retained and only the HPLMN S-NSSAI is to be replaced by an Alternative HPLMN S-NSSAI, the V-SMF includes Alternative HPLMN S-NSSAI in the N1 PDU Session Modification Command message and a cause value to notify the UE that the HPLMN S-NSSAI is replaced with the Alternative HPLMN S-NSSAI - If the PDU Session is to be retained and both VPLMN S-NSSAI and HPLMN S-NSSAI are to be replaced, the V-SMF includes Alternative HPLMN S-NSSAI and VPLMN Alternative S-NSSAI in the N1 PDU Session Modification Command message and a cause value to notify the UE that the HPLMN S-NSSAI is replaced with the Alternative HPLMN S-NSSAI and the VPLMN S-NSSAI of the PDU Session is replaced with Alternative VPLMN S-NSSAI. The V-SMF sets the S-NSSAI in N2 SM information to Alternative VPLMN S-NSSAI. - If a new PDU Session establishment is required, the V-SMF includes Alternative HPLMN S-NSSAI and/or Alternative VPLMN S-NSSAI in the N1 PDU Session Modification Command message and a cause value to notify the UE that a new PDU Session establishment on the HPLMN Alternative S-NSSAI and/or Alternative VPLMN S-NSSAI is required. 5-7. These steps are the same as step 4-6 in clause 4.3.3.2. 8. This step is the same as step 7a in clause 4.3.3.2 with the difference that the SMF is V-SMF. If the N2 SM information indicates modification failure and the V-SMF rejected the PDU session modification as described in step 7 in clause 4.3.3.2, step 9 is skipped. 9a-9b are the same as step 8a-8b in clause 4.3.3.2 but executed in Visited PLMN 10. This step is the same as step 7b in clause 4.3.3.2 with the difference that the SMF is V-SMF. 11-12. These steps are the same as steps 8-9 in 4.3.3.2. 13-14. These steps are the same as step 11a-11b in clause 4.3.3.2 but executed in Visited PLMN. 15. V-SMF responds to the H-SMF with an Nsmf_PDUSession_Update response carrying the information like PCO provided by the UE in the SM PDU Session Modification Command Ack message from the UE to the V-SMF, Secondary RAT usage data. The H-SMF shall modify the PDU Session context. If the V-SMF has rejected QFI(s) (step3) or the (R)AN has rejected QFI(s) in step 6 of Figure 4.3.3.2-1, the H-SMF is responsible of later updating the QoS rules and QoS Flow level QoS parameters if needed for the QoS Flow(s) associated with the QoS rule(s) in the UE. 16-17. These steps are the same as steps 12-13 in clause 4.3.3.2 with the difference that the SMF is H-SMF.