Content for  TS 23.548  Word version:  18.6.0

The untrusted AF may subscribe via the H-NEF to the H-PCF or the trusted AF may directly subscribe to the H-PCF, then the H-PCF subscribe to the H-SMF for the event related with a mobility of the PDU Session towards a serving PLMN where local traffic offload is possible, i.e. mobility of the PDU Session either towards HPLMN or towards a VPLMN where HR-SBO is possible i.e. supported and allowed.

Step 1 in Figure 4.23.7.3.2-1 of TS 23.502 (initiation of the N2-based handover). For an intra VPLMN HO, the source AMF provides the target AMF with whether HR-SBO is allowed for each PDU Session of the UE. For an inter PLMN HO, the target T-AMF uses local policies related to the SLA with the HPLMN of the UE to determine whether HR-SBO allowed indication is sent to the V-SMF(s) for the PDU Sessions of the UE.

Step 2 in Figure 4.23.7.3.2-1 of TS 23.502 takes place. If the T-AMF considers that HR-SBO is possible, the T-AMF selects a V-SMF supporting HR-SBO.

(V-SMF insertion or V-SMF change) step 2 in Figure 4.23.7.3.2-1 of TS 23.502 takes place. For V-SMF insertion and inter VPLMN V-SMF change case, the T-AMF sends an HR-SBO allowed indication to the target V-SMF in Nsmf_PDUSession_CreateSMContext Request as part of the step 3 of the procedure in Figure 4.23.7.3.2-1 of TS 23.502.

(V-SMF removal) steps 9 to 12b in Figure 4.23.7.3.2-1 of TS 23.502 take place. The target AMF sends to H-SMF a Nsmf_PDUSession_CreateSMContext request as in step 9 in Figure 4.23.7.3.2-1 of TS 23.502. For EAS discovery with EASDF, the H-SMF selects an EASDF in HPLMN and configures the DNS context in this EASDF (Neasdf_DNSContext_Create) and may select and configure UPF(s) in HPLMN. For EAS discovery with Local DNS Server, the H-SMF selects a Local DNS Server.

(V-SMF insertion or V-SMF change) step 4 or step 5 in Figure 4.23.7.3.2-1 of TS 23.502.

(V-SMF insertion case) The (target) V-SMF retrieves SM context from H-SMF using Nsmf_PDUSession_Context Request, followed by the Nsmf_PDUSession_Context Response.

(V-SMF change case) The target V-SMF retrieves SM context from the source V-SMF using Nsmf_PDUSession_Context Request/Response. In step 3b, if source and target V-SMFs belong to same VPLMN, the SM context includes Authorization Result for HR-SBO, VPLMN Specific Offloading Information and/or corresponding Offload Identifier(s), the SM context also includes (if previously received by source V-SMF from H-SMF in the case of V-SMF change) the HPLMN address information, and the DNS Server address provided by the HPLMN. The SM context may also include EAS information to be refreshed for EAS re-discovery, i.e. FQDN(s) corresponding to the old target DNAI selected by the source V-SMF (V-SMF change case). If source and target V-SMFs belong to same VPLMN, the SM context may also include EAS IP replacement information (i.e. source EAS IP address and port number, target EAS IP address and port number) and the target DNAI.

(V-SMF insertion or V-SMF change) The (target) V-SMF selects a new V-EASDF e.g. based on the target UE location. The (target) V-SMF invokes Neasdf_DNSContext_Create including the DNN, S-NSSAI and HPLMN ID to obtain the new V-EASDF address. In the case of V-SMF insertion, the UE IP address is set to the unspecified address in Neasdf_DNSContext_Create as specified in clause 7.1.2.2. In the case of V-SMF change with inter PLMN mobility or intra PLMN mobility, the target V-SMF shall select a new V-EASDF if it is configured to use an EASDF for the PDU Session. To support EAS discovery with local DNS server as described in clause 6.7.2.4, the (target) V-SMF (re-)selects a local DNS server, e.g. based on the target UE location.

(V-SMF insertion or V-SMF change) The (target) V-SMF may perform UL CL/BP and local PSA insertion/change/removal as described in step 2 of Figure 6.7.2.2-1.

The H-SMF (V-SMF removal) or the target V-SMF (V-SMF insertion or V-SMF change) responses to the target AMF with a Nsmf_PDUSession_CreateSMContext Response as in step 8 (V-SMF insertion or V-SMF change) or step 13 in Figure 4.23.7.3.2-1 of TS 23.502 (V-SMF removal).

(V-SMF insertion or V-SMF change) when the T-AMF sends to the Target V-SMF an indication of Handover Complete within Nsmf_PDUSession_UpdateSMContext Request (at step 2 in Figure 4.23.7.3.3-1 of TS 23.502) the target V-SMF invokes Nsmf_PDUSession_Update Request to the H-SMF. If a V-EASDF or a local DNS server has been selected in step 4, the selected V-EASDF or local DNS server address is provided in the request. The target V-SMF needs not provide the selected V-EASDF or local DNS server address if the target V-SMF is configured to use EAS discovery with V-EASDF using IP replacement mechanism corresponding to clause 6.7.2.5. If the UE indicated support of refreshing stale EAS information, the V-SMF may also provide an EAS rediscovery indication and EAS information to be refreshed for EAS re-discovery if received in step 3 to the H-SMF. This Nsmf_PDUSession_UpdateSMContext Request triggers steps 8 to 11.

(V-SMF insertion or V-SMF change): If the UE has changed of serving VPLMN or has moved from HPLMN to a VPLMN, the H-SMF:

• invokes Nudm_SDM_Get service to get subscription data from UDM by providing serving PLMN ID to get subscription data specific to the Serving PLMN;
• authorizes the request for HR-SBO based on the SM subscription data (i.e., HR-SBO authorization indication); and
• If the HR-SBO is authorized, retrieves the VPLMN Specific Offloading Policy from H-PCF.

(V-SMF insertion or V-SMF change): As defined in step 8 in clause 4.23.7.3.3 of TS 23.502, the H-SMF invokes Nsmf_PDUSession_Update Response, providing the same HR-SBO related information as sent by the H-SMF in step 2 of Figure 6.7.2.2-1. The H-SMF furthermore provides V-SMF with PCO including EAS rediscovery indication and impact field based on the EAS information to be refreshed for EAS re-discovery if received at step 7. The target V-SMF stores the PCO including the V-EASDF address/local DNS server address, EAS rediscovery, impact field for the UE until it receives an indication of HO completion as described in step 2 in Figure 4.23.7.3.3-1 of TS 23.502 or in step 11. If the request for HR-SBO is not authorized and a V-EASDF context had been created, the V-SMF deletes the DNS context from the selected V-EASDF, and the subsequent steps related to the HR-SBO in this procedure are skipped.

(V-SMF insertion or V-SMF change): The (target) V-SMF configures the V-EASDF with the DNS handling rules as defined in step 3 of Figure 6.7.2.2-1. If the (target) V-SMF has interacted with the V-EASDF in step 4, the V-SMF invokes Neasdf_DNSContext_Update Request including UE IP address to complete the configuration of the context in the V-EASDF. The (target) V-SMF may configure the ULCL/L-PSA selected in the step 5 to forward DNS messages to V-EASDF. When the V-SMF is configured to use EAS discovery with V-EASDF using IP replacement mechanism as described in clause 6.7.2.5, the (target) V-SMF configures the UL CL/L-PSA in the VPLMN with the IP replacement information (i.e. IP address of the DNS server provided by the HPLMN, the selected V-EASDF IP address and port number and DNS traffic filtering rules related with when IP replacement applies).

(V-SMF insertion or V-SMF change): The target V-SMF sends the PCO for the UE (received at step 9) in a PDU Session Modification Command sent to the UE. (V-SMF removal): At step 10 in Figure 4.23.7.3.3-1 of TS 23.502, when receiving Nsmf_PDUSession_UpdateSMContext Request (Handover Complete indication), the H-SMF sends the PCO including the V-EASDF or Local DNS Server address selected in step 2b in a PDU Session Modification Command sent to the UE. If the UE indicated support of refreshing stale EAS information, the PCO may include EAS rediscovery indication.

(V-SMF change or removal): When the S-AMF invokes Nsmf_PDUSession_ReleaseSMContext Request to inform the Source V-SMF to release the SM context of the PDU Session, e.g. at step 3a or 11a in Figure 4.23.7.3.3-1 of TS 23.502, the DNS context in the old (V-)EASDF is removed by the (source) V-SMF using Neasdf_DNSContext_Delete service. (V-SMF insertion): When the target V-SMF sends Nsmf_PDUSession_Update Request with a Handover Complete Indication at step 6 in Figure 4.23.7.3.3-1 of TS 23.502, the DNS context in the old EASDF is removed by the H-SMF using Neasdf_DNSContext_Delete service.

If the AF had subscribed in step 0 and a serving PLMN change occurred towards a PLMN where local traffic offload is possible for the PDU Session, the H-SMF notifies the H-PCF then either via H-NEF or directly notify the AF indicating the target serving PLMN ID and DNN, S-NSSAI of the HPLMN. This may take place as soon as the H-SMF has received an indication of Handover Complete.

EAS discovery procedure by UE may take place: (V-SMF change or V-SMF insertion): EAS Discovery procedure with V-EASDF for HR-SBO (according to clause 6.7.2.3), with Local DNS for HR-SBO (according to clause 6.7.2.4), or with V-EASDF using IP replacement mechanism (according to clause 6.7.2.5); or (V-SMF removal): EAS Discovery procedure with EASDF or Local DNS Server (according to clause 6.2.3.2).

Same as step 0 of Figure 6.7.2.6-1.

The UE establishes a HR-SBO PDU Session as described in clause 6.7.2.2.

The UE initiates Mobility Registration Update procedure to the Target AMF as described in clauses 4.23.3 and 4.2.2.2.2 of TS 23.502. During the registration procedure, the target AMF (in roaming case) receives the SMF selection data including HR-SBO allowed indication per DNN/S-NSSAI from the UDM in the step 14b of the procedure in clause 4.2.2.2.2 of TS 23.502.

(V-SMF insertion or V-SMF change): at step 17 of clause 4.2.2.2.2 of TS 23.502, the Target AMF selects and inserts a V-SMF supporting HR-SBO and provides the received HR-SBO allowed indication for the established Session for DNN, S-NSSAI in the PDUSession_CreateSMContext Request sent to the target V-SMF.

(V-SMF removal) in case the Target AMF is an AMF of HPLMN (the UE moves from a VPLMN to HPLMN), in step 3b-1, the Target AMF sends a PDUSession_CreateSMContext Request sent to the H-SMF. In step 3b-2, the H-SMF retrieves SM Context from the V-SMF by invoking Nsmf_PDUSession_Context Request. The H-SMF may select an EASDF in HPLMN and configure the DNS context in this EASDF (Neasdf_DNSContext_Create) and may select and configure UPF(s) in HPLMN as in step 3b-3. For the V-SMF removal case, the steps 4 to 10 are skipped.

(V-SMF insertion case) The (target) V-SMF retrieves SM context from H-SMF using Nsmf_PDUSession_Context Request/Response, followed by the Nsmf_PDUSession_Context Request/Response.

(V-SMF change case) The target V-SMF retrieves SM context from the source V-SMF using Nsmf_PDUSession_Context Request/Response indicating HR-SBO support in the Request in the case of an intra-PLMN V-SMF case. In step 4b, if source and target V-SMFs belong to same VPLMN, the SM context from the source V-SMF includes Authorization Result for HR-SBO, VPLMN Specific Offloading Information and/or corresponding Offload Identifier(s) and (if previously received by source V-SMF from H-SMF) the HPLMN address information, and the DNS Server address provided by the HPLMN.

As in step 4 of Figure 6.7.2.6-1, the (target) V-SMF may invoke Neasdf_DNSContext_Create.

As in step 5 of Figure 6.7.2.6-1, the (target) V-SMF may perform UL CL/BP and local PSA insertion/change/removal.

The target V-SMF invokes Nsmf_PDUSession_Update Request to the H-SMF as in step 7 of Figure 6.7.2.6-1.

As in step 8 of Figure 6.7.2.6-1, the H-SMF invokes Nudm_SDM_Get service, authorizes the request for HR-SBO based on the SM subscription data, and retrieves the VPLMN Specific Offloading Policy from H-PCF in case the HR-SBO is authorized.

As in step 9 of Figure 6.7.2.6-1, once the HR-SBO is authorized, the H-SMF sends the VPLMN Specific Offloading Information in the Nsmf_PDUSession_Update Response message to the target V-SMF.

As in step 9 of Figure 6.7.2.6-1, the (target) V-SMF may configure the V-EASDF based on the received VPLMN Specific Offloading Information.

The (target) V-SMF may also update the configuration of the V-UPF based on the received VPLMN Specific Offloading Information.

As in step 6 of Figure 6.7.2.6-1, the target V-SMF (in the case of V-SMF removal, the H-SMF) answers to the target AMF with PDUSession_CreateSMContext Response.

The registration procedure continues as defined in clause 4.2.2.2.2 of TS 23.502 from step 18 on.

In the case of V-SMF removal or change, when, at step 17a of clause 4.23.4.3 of TS 23.502, receiving Nsmf_PDUSession_ReleaseSMContext Request from the source AMF, the source V-SMF initiates a Neasdf_DNSContext_Delete if a V-EASDF was used for the PDU Session. In the case of V-SMF insertion, when at step 9 the H-SMF sends to the V-SMF VPLMN Specific Offloading Information in Nsmf_PDUSession_Update Response, the DNS context in the old EASDF may be removed by the H-SMF using Neasdf_DNSContext_Delete service.

(V-SMF insertion or V-SMF change): The target V-SMF sends the DNS information for the UE (received at step 9) and EAS rediscovery indication via PCO in a PDU Session Modification Command sent to the UE. (In the case of V-SMF removal): the H-SMF sends the DNS information (received at step 3) for the UE and EAS rediscovery indication via PCO in a PDU Session Modification Command sent to the UE.

As in step 13 of Figure 6.7.2.6-1, notification from H-SMF to the AF either via H-NEF or directly to the AF.

As in step 14 of Figure 6.7.2.6-1: EAS discovery procedure by UE may take place.