5.15.6 Network Slicing Support for Roaming

For roaming scenarios:

• If the UE only uses standard S-NSSAI values, then the same S-NSSAI values can be used in VPLMN as in the HPLMN.
• If the VPLMN and HPLMN have an SLA to support non-standard S-NSSAI values in the VPLMN, the NSSF of the VPLMN maps the Subscribed S-NSSAIs values to the respective S-NSSAI values to be used in the VPLMN. The S-NSSAI values to be used in the VPLMN are determined by the NSSF of the VPLMN based on the SLA. The NSSF of the VPLMN need not inform the HPLMN of which values are used in the VPLMN.

Depending on operator's policy and the configuration in the AMF, the AMF may decide the S-NSSAI values to be used in the VPLMN and the mapping to the Subscribed S-NSSAIs.

For the home routed case, the AMF or NSSF may select an S-NSSAI (if the UE does not provide an S-NSSAI for the PDU session establishment) and a Network Slice instance, based on load level and/or Observe Service Experience and/or Dispersion analytics of the VPLMN and/or that of the HPLMN from NWDAF as described in TS 23.288 [86].

• The UE constructs Requested NSSAI and provides the mapping of S-NSSAIs of the Requested NSSAI to HPLMN S-NSSAIs if the mapping is stored in the UE, as described in clause 5.15.5.2.1.
• The NSSF in the VPLMN determines the Allowed NSSAI without interacting with the HPLMN.
• the HPLMN may provide NSSRG Information as part of the Subscription information as described in clause 5.15.12.
• The Allowed NSSAI in the Registration Accept includes S-NSSAI values used in the VPLMN. The mapping information described above is also provided to the UE with the Allowed NSSAI as described in clause 5.15.4.
• If the S-NSSAI values are subject to NSAC, depending on operator's policy, a roaming agreement or an SLA between VPLMN and HPLMN, the AMF or SMF in VPLMN triggers a request for NSAC for these S-NSSAI values as described in clause 5.15.11.3.
• In PDU Session Establishment procedure, the UE includes both:

(a) the S-NSSAI that matches the application (that is triggering the PDU Session Request) within the NSSP in the URSP rules or within the UE Local Configuration as defined in clause 6.1.2.2.1 of TS 23.503 [45]; the value of this S NSSAI is used in the HPLMN; and

(b) an S-NSSAI belonging to the Allowed NSSAI that maps to (a) using the mapping of the Allowed NSSAI to HPLMN S-NSSAIs; the value of this S-NSSAI is used in the VPLMN.

For the home routed case, the AMF may select the V-SMF and the H-SMF based on network data analytics (NF load, etc.) of the VPLMN and that of the HPLMN from the NWDAF as described in TS 23.288 [86]. The V-SMF sends the PDU Session Establishment Request message to the H-SMF along with the S-NSSAI with the value used in the HPLMN (a). If the S-NSSAI values are subject to NSAC, the V-SMF or H-SMF triggers a request for NSAC for these S-NSSAI values as described in clause 5.15.11.3.

• When a PDU Session is established, the CN provides to the AN the S-NSSAI with the value from the VPLMN corresponding to this PDU Session, as described in clause 5.15.5.3.
• The Network Slice instance specific network functions in the VPLMN are selected by the VPLMN by using the S-NSSAI with the value used in the VPLMN and querying an NRF that has either been pre-configured, or provided by the NSSF in the VPLMN. The Network Slice specific functions of the HPLMN (if applicable) are selected by the VPLMN by using the related S-NSSAI with the value used in the HPLMN via the support from an appropriate NRF in the HPLMN, identified as specified in clause 4.17.5 of TS 23.502 [3] and, for SMF in clause 4.3.2.2.3.3 of TS 23.502 [3].
• If the serving AMF supports the Network Slice Replacement feature and is configured to use the NSSF for Network Slice Replacement triggering, the AMF subscribes with the NSSF of the VPLMN for notifications when an HPLMN S-NSSAI needs to be replaced with an Alternative S-NSSAI, in addition to notifications for the Serving PLMN S-NSSAIs. The NSSF of the VPLMN shall subscribe with the NSSF of the HPLMN for notifications when an HPLMN S-NSSAI needs to be replaced with an Alternative S-NSSAI.
• If the serving AMF support the Network Slice Instance Replacement and configured to use Network Slice Instance Replacement, the AMF subscribes with the NSSF of the VPLMN for notifications when a Network Slice instance is congested or no longer available as described in clause 5.15.20. The NSSF of the VPLMN shall subscribe with the NSSF of the HPLMN for notifications when the Network Slice instance is congested or no longer available.