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Content for  TS 23.548  Word version:  18.1.1

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6.7  Support of the local traffic routing in VPLMN for Home Routed PDU Session for roaming (HR-SBO) |R18|p. 49

6.7.1  Generalp. 49

When roaming, the UE establishes a Home Routed Session that is capable of supporting session breakout in V-PLMN based on the subscription. In this scenario, the Home PLMN and Visited PLMN have an agreement on the support of the local traffic routing (i.e. session breakout performed by V-SMF also called HR-SBO) in VPLMN for the home routed session.
After establishing the HR-SBO PDU Session, the UE can access EAS deployed in EHE in VPLMN while the UE can also access the data network in the Home PLMN.
The reference architecture supporting this scenario is depicted in Figure 4.2-5 in clause 4.2.
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6.7.2  Procedurep. 50

6.7.2.1  Generalp. 50

This procedure describes the abstract procedure for the authorization of the local traffic offloading using HR PDU Session.

6.7.2.2  PDU Session for supporting HR-SBO in VPLMNp. 50

Copy of original 3GPP image for 3GPP TS 23.548, Fig. 6.7.2.2-1: Procedure for PDU Session supporting HR-SBO in VPLMN
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Step 1.
During the registration procedure, the AMF receives the HR-SBO allowed indication per DNN/S-NSNAI from the UDM in the step 14b of the procedure in the clause 4.2.2.2.2 of TS 23.502.
Step 2.
During the PDU Session establishment procedure for Home-routed roaming as in clause 4.3.2.2.2 of TS 23.502, if the AMF had received in SMF selection data from UDM the HR-SBO allowed indication for the DNN/S-NSSAI in the step 1, the AMF selects a V-SMF supporting HR-SBO and sends an HR-SBO allowed indication to the V-SMF in the step 2 and the step3a of the procedure in Figure 4.3.2.2.2-1 in clause 4.3.2.2.2 of TS 23.502.
If the V-SMF supporting the HR-SBO receives the HR-SBO allowed indication from AMF, the V-SMF may:
  • select UL-CL and PSA functionalities based on UE location information and this indication in the step 4 of the Figure 4.3.2.2.2-1 of TS 23.502.
  • select a V-EASDF,
  • obtain the V-EASDF IP address based on local configuration, or invoke Neasdf_DNSContext_Create Request including the DNN, S-NSSAI, HPLMN ID and the UE IP address set to unspecified address as specified in clause 7.1.2.2 to obtain the V-EASDF IP address, and
  • send to the H-SMF the request for the establishment of the PDU Session supporting HR-SBO in VPLMN and the V-EASDF IP address to in the Nsmf_PDUSession_Create Request in the step 6 of the procedure in Figure 4.3.2.2.2-1 in clause 4.3.2.2.2 of TS 23.502.
The H-SMF authorizes the request for HR-SBO based on SM subscription data (i.e. HR-SBO authorization indication) in the step 7 of the procedure in the clause 4.3.2.2.2 of TS 23.502.
Once the HR-SBO is authorized, the H-SMF requests and retrieves the optional VPLMN Specific Offloading Policy from H-PCF.
If HR-SBO is authorized for the PDU session, the H-SMF provides in the Nsmf_PDUSession_Create Response in the step 13 of the procedure in Figure 4.3.2.2.2-1 in clause 4.3.2.2.2 of TS 23.502 with the following information:
  • optional VPLMN Specific Offloading Information
  • the V-EASDF IP address (corresponding to clause 6.7.2.3) or DNS server IP address of HPLMN (corresponding to clause 6.7.2.5) as DNS server address to be sent to the UE via PCO and
  • the DNS server address of HPLMN (to be used for DNS requests related with traffic not to be subject to HR-SBO)
  • the HR-SBO authorization result (i.e. whether HR-SBO request is authorized or not).
The H-SMF may indicate to the UE either that for the PDU Session the use of the EDC functionality is allowed or that for the PDU Session the use of the EDC functionality is required.
If the request for HR-SBO is not authorized and DNS context has been created, the V-SMF delete the DNS context from the selected V-EASDF, and the subsequent steps related to the EASDF in this procedure are skipped.
The detailed information of VPLMN Specific Offloading Policy is described in clause 6.4 of TS 23.503.
Step 3.
The V-SMF configures the V-EASDF with the DNS handling rules using the received VPLMN specific offloading policy and may configure the V-EASDF with the DNS server address of HPLMN as default DNS server, after the step 13 of the procedure in Figure 4.3.2.2.2-1 in clause 4.3.2.2.2 of TS 23.502 if they are received from H-SMF in the step 2.
If the V-SMF has interacted with the V-EASDF in step 2, then the V-SMF invokes Neasdf_DNSContext_Update Request including UE IP address to complete the configuration of the context in the V-EASDF.
The V-SMF configures the V-UPF selected in the step 2 to forward DNS messages to V-EASDF.
Step 4A.
EAS Discovery procedure with V-EASDF is performed as described in clause 6.7.2.3.
Step 4B.
EAS Discovery procedure with Local DNS Server/Resolver is performed as described in clause 6.7.2.4.
Step 4C.
EAS discovery procedure with V-EASDF using IP replacement mechanism as described in clause 6.7.2.5.
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6.7.2.3  EAS Discovery Procedure with V-EASDF for HR-SBOp. 52

Copy of original 3GPP image for 3GPP TS 23.548, Fig. 6.7.2.3-1: Procedure for EAS Discovery with V-EASDF for HR-SBO roaming scenario
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Step 1.
The DNS query sent by the UE reaches the V-EASDF via an UPF controlled by the V-SMF.
If the target FQDN of the DNS query is not part of the FQDN authorized by the H-SMF in step 2 of Figure 6.7.2.2-1, a) or b) will be performed:
  1. The V-EASDF proceeds to step 12 of Figure 6.2.3.2.2-1 where it sends the DNS request to the DNS server address of HPLMN received in step 2 of Figure 6.7.2.2-1 (to be used for DNS requests related with traffic not to be subject to HR-SBO) through N6. Upon receiving the DNS response, the procedure proceeds immediately to step 5.
  2. The UL CL/BP UPF sends the DNS request to the DNS server address of HPLMN via V-UPF (if exists) and H-UPF (through N9), by modifying the packet's destination IP address (corresponding to V-EASDF) to that of the DNS server of HPLMN
The rest of the procedure assumes the target FQDN of the DNS query is part of the FQDN authorized by the H-SMF in step 2 of Figure 6.7.2.2-1.
Step 2.
The step 8 to 15 of the procedure in the Figure 6.2.3.2.2-1 by replacing SMF and EASDF with V-SMF and V-EASDF respectively.
Step 3.
The V-SMF selects UL CL/BP and local PSA in VPLMN based on the V-EASDF notification and UE location. The V-SMF may perform insertion or change of UL CL/BP and local PSA in VPLMN .
If there is no other V-UPF between the selected UL CL/BP in this step and H-UPF, the V-SMF sets up user plane between this UL CL/BP and H-UPF via the interaction with H-SMF. Otherwise, the V-SMF sets up user plane between this ULCL/BP and the existing V-UPF.
The V-SMF sets up user plane between the selected UL CL/BP in this step and RAN (if no other V-UPF exists between RAN and this UL CL/BP) or the V-UPF (if exists between this UL CL/BP and RAN).
Step 4.
The steps 17 to 18 of the procedure in clause 6.2.3.2.2 by replacing SMF and EASDF with V-SMF and V-EASDF respectively.
Step 5.
V-EASDF sends the DNS Response to the UE.
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6.7.2.4  EAS Discovery Procedure with Local DNS for HR-SBOp. 53

Copy of original 3GPP image for 3GPP TS 23.548, Fig. 6.7.2.4-1: Procedure for EAS Discovery with local DNS for HR-SBO roaming scenario
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EAS Discovery procedure with local DNS are corresponding with the procedure Figure 6.2.3.2.3-1. The steps 1 to 5 are the same as the steps 1 to 6 of Figure 6.2.3.2.2-1 with following differences:
  • SMF is replaced with V-SMF.
  • UE, (R)AN, AMF, ULCL/BP UPF, L-PSA UPF, V-SMF, Local DNS Resolver/Server are located in VPLMN.
  • UPF, H-SMF, C-DNS are located in HPLMN.
Step 0.
The HR-SBO PDU Session is established. See the procedure in clause 6.7.2.2.
Step 1.
ULCL/BP insertion. See the step 1 of the procedure in Figure 6.2.3.2.3-1.
Step 2.
After ULCL/BP insertion is performed, the V-SMF sends new local DNS server address to the UE by performing PDU Session Modification procedure as in clause 4.3.3.3 of TS 23.502 with following additions:
Step 3.
See the step 4 of the procedure in Figure 6.2.3.2.3-1.
Step 4.
See the step 5 of the procedure in Figure 6.2.3.2.3-1.
Step 5.
See the step 6 of the procedure in Figure 6.2.3.2.3-1.
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6.7.2.5  EAS discovery procedure with V-EASDF using IP replacement mechanism for supporting HR-SBOp. 54

Based on the operator's configuration and local regulations, the IP replacement mechanism may be used for EAS discovery supporting HR-SBO:
  • For supporting HR-SBO, the H-SMF sends DNS server address of HPLMN included in PCO to UE via V-SMF during PDU Session Establishment/Modification procedure. The DNS query related to the edge computing (corresponding to FQDNs) can be routed to V-EASDF/Local DNS server in the VPLMN using IP replacement mechanism.
Copy of original 3GPP image for 3GPP TS 23.548, Fig. 6.7.2.5-1: EAS discovery procedure with V-EASDF using IP replacement mechanism for supporting HR-SBO
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Step 0.
The registration procedure is described in step 1 of clause 6.7.2.2.
Step 1.
The HR-SBO PDU Session establishment is described in the step 2 of clause 6.7.2.2 with the following differences:
After step 3 in clause 4.3.2.2.2 of TS 23.502, the V-SMF selects a UPF in VPLMN supporting UL-CL and PSA functionalities based on UE location information and supporting IP replacement mechanism.
The V-SMF sends the request for establishment of the PDU session supporting HR-SBO in VPLMN without the V-EASDF IP address in the step 6 of clause 4.3.2.2.2 of TS 23.502.
If the Nsmf_PDUSession_Create Request received by the H-SMF does not include the V-EASDF IP address, the H-SMF constructs PCO with DNS server address field set to DNS server address of HPLMN and sends the PCO to UE via V-SMF in the step 13 of clause 4.3.2.2.2 of TS 23.502.
Step 2.
The V-SMF configures the V-EASDF and the UPF in VPLMN as described in step 3 of clause 6.7.2.2 with the following differences:
Based on the FQDN range received from the VPLMN specific offloading policy, the V-SMF indicates the UPF in VPLMN to route DNS queries for the FQDN (range) query to V-EASDF. The V-SMF configures the UPF in VPLMN with IP replacement information (i.e. DNS server IP address and port number of HPLMN, V-EASDF IP address and port number). In uplink direction, UPF in VPLMN replaces the destination address of the DNS query targeting an FQDN eligible for HR-SBO related offload from DNS server IP address of HPLMN to V-EASDF IP address; In downlink direction, UPF in VPLMN replaces the source address of the DNS response targeting an FQDN eligible for HR-SBO related offload from V-EASDF IP address to DNS server IP address of HPLMN.
Step 3.
UE sends DNS query to DNS server of HPLMN.
Step 4a.
If the DNS query does not match the FQDN range eligible for HR-SBO related offload, UPF in VPLMN delivers the DNS query via H-PSA through N9 and H-PSA delivers the DNS query to the DNS server of HPLMN.
Step 4b.
If the DNS query matches the FQDN range eligible for HR-SBO related offload, the UPF in VPLMN delivers the DNS query to V-EASDF using IP replacement mechanism. The following EAS discovery procedure is based on step 4b.
Step 5.
The EAS discovery procedure described in step 8-18 of clause 6.2.3.2.2 applies with the following differences:
This EAS discovery procedure is implemented in the VPLMN.
In step 16, the V-SMF may perform insertion or change of UL CL/BP and local PSA in VPLMN as described in the step 3 of clause 6.7.2.5.
6. The V-EASDF sends the DNS response including FQDN to the UPF in VPLMN. The UPF in VPLMN replaces the source address from V-EASDF to DNS server of HPLMN in the DNS response based on the V-SMF instructions and sends this DNS response to the UE directly or via UL-CL of VPLMN if existing.
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6.7.3  EAS Re-discovery Procedurep. 55

6.7.3.1  Generalp. 55

The EAS re-discovery in VPLMN can be triggered due to UE mobility, AF interacting with HPLMN, or AF interacting with VPLMN.

6.7.3.2  EAS Re-discoveryp. 55

Figure 6.7.3.2-1 shows the procedure of EAS re-discovery in VPLMN for HR roaming.
Copy of original 3GPP image for 3GPP TS 23.548, Fig. 6.7.3.2-1: EAS rediscovery procedure
Figure 6.7.3.2-1: EAS rediscovery procedure
(⇒ copy of original 3GPP image)
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Step 1.
For UE mobility triggered EAS rediscovery, step 4 is skipped.
The procedure described in clause 4.23.7 (N2 based handover) or 4.23.11 (Xn based handover) of TS 23.502 is performed by replacing I-SMF with V-SMF and SMF with H-SMF.
Step 1a.
(V-SMF insertion case) The (target) V-SMF retrieves SM context from H-SMF using Nsmf_PDUSession_Context Request/Response.
Step 1b.
(V-SMF change case) The target V-SMF retrieves SM context from the source V-SMF using Nsmf_PDUSession_Context Request/Response.
In step 1b, the SM context includes Authorization Result for HR-SBO and VPLMN Specific Offloading Policy if source and target V-SMFs belongs to same VPLMN.
The SM context also includes EAS information to be refreshed for EAS re-discovery. The EAS information corresponds to the old target DNAI if it has been inserted by the H-SMF (V-SMF insertion case) or the source V-SMF (V-SMF change case).
In the case of V-SMF insertion or V-SMF change, when the AMF sends Nsmf_PDUSession_CreateSMContext Request to the target V-SMF, the AMF includes HR-SBO allowed indication if received from the UDM.
Step 1 is skipped if UE moves without V-SMF change.
Steps 1-6 are skipped if UE moves with V-SMF removal.
After the handover procedure, the (target) V-SMF may perform UL CL/BP and local PSA insertion/change/removal as described in clause 4.23.9 of TS 23.502 by replacing I-SMF with V-SMF and SMF and H-SMF. Differences are shown as steps 2 to 5.
Step 2.
The (target) V-SMF may (re-)select a new V-EASDF, e.g. based on the DNAI corresponding to the target UE location. The (target) V-SMF may invoke Neasdf_DNSContext_Create service to obtain the new V-EASDF address or the (target) V-SMF obtains the new V-EASDF address based on local configuration.
If a new V-EASDF is selected, the DNS context in the old (V-)EASDF is removed by the (source) V-SMF/H-SMF using Neasdf_DNSContext_Delete service.
In the case of V-SMF insertion, the H-SMF removes the DNS context in the EASDF in HPLMN after the H-SMF receives step 3 from V-SMF. In case of V-SMF change, the source V-SMF removes the DNS context in the old V-EASDF after the source V-SMF sends SM context to the target V-SMF as described in step 1b.
Step 3.
The (target) V-SMF invokes Nsmf_PDUSession_Update Request (EAS rediscovery indication, EAS information to be refreshed for EAS re-discovery (such as FQDN range and IP range corresponding to the old local part of DN) if available, the optional new V-EASDF address) to the H-SMF.
For the case of local PSA removal, the request message is sent to the H-SMF without including any DNAI.
The EAS information to be refreshed for EAS re-discovery may correspond to:
  • the new target DNAI in case of local PSA insertion.
  • the new target DNAI and old target DNAI in case of local PSA change.
  • the old target DNAI in case of local PSA removal or local PSA is unchanged.
Step 4a.
For AF triggered EAS rediscovery via interacting with HPLMN, the AF may indicate the EAS rediscovery for the impacted applications, which are identified by Application Identifier(s), to the H-SMF via the H-PCF using the AF influence on traffic routing procedure as described in clause 4.3.6 of TS 23.502.
Step 4b.
For AF triggered EAS rediscovery via interacting with VPLMN, the AF may indicate the EAS rediscovery for the impacted applications via the V-NEF using the procedure described in clause 4.3.6 of TS 23.502.
Step 5.
(V-SMF insertion or change case) The H-SMF initiates Nsmf_PDUSession_Update Request towards the (target) V-SMF including the PCO.
The PCO includes EAS rediscovery indication (optional), impact field (optional) and V-EASDF address (optional).
Step 6.
(V-SMF insertion or change case) The V-SMF initiates PDU Session Modification command including the PCO to the UE.
Step 7.
(V-SMF removal case) The (H-)SMF selects an EASDF in HPLMN and creates a context for the PDU session in that EASDF and then initiates PDU Session Modification command including PCO to the UE.
The PCO includes EAS rediscovery indication (optional) and the impact field (optional).
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6.7.4  AF request on PDU Sessions supporting HR-SBOp. 57

For HR-SBO PDU Sessions, the AF in VPLMN may send to V-NEF an AF request to influence traffic routing (e.g. for the purpose of subscription to UP path management events on HR-SBO Sessions in VPLMN). The AF request for the HR-SBO PDU Session from the AF is stored as Application Data (Data Subset = AF traffic influence request information) in the UDR of VPLMN as described in clause 4.3.6 of TS 23.502. To obtain the AF traffic influence request information, the V-SMF managing the PDU Session supporting HR-SBO subscribes to the NEF in VPLMN for notification of Application Data modification as specified in clause 4.3.6 of TS 23.502.
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