5.15.5.2.2a AF Requested modification of the Set of Network Slice(s) for a UE

In non-roaming cases, an authorized AF or an AF subject to authorization via the NEF, may request to replace a certain S-NSSAI (Replaced S-NSSAI) for a UE with another S-NSSAI (Alternative S-NSSAI) which is part of the UE subscription. The AF may also indicate a policy duration. The request in either case is sent to the PCF for the UE handling the impacted subscriber. Similarly, an authorized AF or an AF subject to authorization via the NEF, may request to terminate the replacement then request to use the Replaced S-NSSAI instead of the Alternative S-NSSAI for a UE again. The PCF notifies the AF may subscribe to Notification on the outcome of the AF requested Network Slice Replacement. The network slice associated with the Alternative S-NSSAI is assumed in this specification to have NS-AoS to be covering at least the NS-AoS of the network slice associated with the Replaced S-NSSAI.

Only a single AF can request the modification of the Set of Network Slice(s) for one replaced S-NSSAI.

NOTE 1: It is assumed that in the area where the AF requested Network Slice Replacement feature described in this clause is expected to operate, the AMFs supports AF requested Network Slice Replacement notifications handling from the PCF. How the AF obtains the S-NSSAI to be replaced and the Alternative S-NSSAI is out of scope of this specification.

NOTE 2: Application of this feature over non-3GPP access is not further specified.

The PCF for the UE based on operator policies considers the AF request. Consequently the PCF for the UE triggers the Network Slice Replacement at the AMF as defined in TS 23.503 [45].The PCF sends Slice replacement management in the access and mobility management policies to the AMF per implicitly subscription, as described in TS 23.503 [45]. The Slice replacement management policy includes the Replaced S-NSSAI and the corresponding Alternative S-NSSAI as provided in the AF request, PCRT to request Notification on outcome of Network Slice Replacement request, together with a Network Slice Replacement Type. The Network Slice Replacement Type indicates that the Network Slice Replacement is requested by an AF. The AMF handles the procedure as following:

• The AMF shall not handle the Network Slice Replacement procedure as described in clause 5.15.19.
• If the Alternative S-NSSAI is not within the UE subscription, or is not supported in the UE Registration Area, the AMF shall notify the PCF that the Network Slice Replacement is not triggered with a cause value. The PCF further notifies the AF.
• When the Alternative S-NSSAI is within the UE subscription and is supported or partially supported in the UE Registration Area:
• If the Alternative S-NSSAI can be indicated as uniformly supported in the UE Registration Area, e.g., AMF can restrict UE Registration Area to TAs where the Alternative S-NSSAI is uniformly supported or Alternative S-NSSAI is uniformly supported in the UE Registration Area, the AMF includes the Alternative S-NSSAI in the Allowed NSSAI (if not present) and the replacement is successful.
• Otherwise:
• for a UE supporting the feature in clause 5.15.17 (Partial Network Slice support in a Registration Area), the AMF includes the Alternative S-NSSAI in the Partially Allowed NSSAI (if not present) and the replacement is successful;
• for a UE not supporting the feature in clause 5.15.17 (Partial Network Slice support in a Registration Area), the AMF notifies the PCF that the Network Slice Replacement is not triggered with a cause value. The PCF further notifies the AF.

In addition, if the replacement is successful:

• The AMF removes the Replaced S-NSSAI from the Allowed NSSAI or Partial Allowed NSSAI and add the Replaced S-NSSAI into the Rejected S-NSSAI by a UE Configuration Update procedure.
• The AMF and UE release the PDU sessions that were using the Replaced S-NSSAI.
• The AMF stores in the UE context the access and mobility management policy information as received from PCF and transfers to new AMF during inter AMF mobility.
• After performing the above steps a) to c), the AMF reports to the PCF the outcome of the network slice replacement (e.g. the S-NSSAI has been replaced by an Alternative S-NSSAI) as defined in TS 23.503 [45]. The PCF further notifies the AF.

NOTE 3: Operator policies in the PCF ensures that the UE is configured with a URSP rule that contains the Alternative S-NSSAI to establish a new PDU Session. These policies should not cause the selection of the RSDs with the Alternative S-NSSAI with priority over the RSDs with the Replaced S-NSSAI for the same traffic (i.e. for traffic matching the same TD).

If AF provides policy duration the PCF starts a timer based on the policy duration to terminate the network slice replacement. The trusted AF or a NEF may also send request to PCF for the UE to terminate the Network Slice Replacement. In this case the PCF for the UE sends access and mobility management policies to the AMF, with the Slice replacement management information described in TS 23.503 [45] indicating that the Replaced S-NSSAI is available again. The AMF updates the Allowed NSSAI or Partially Allowed NSSAI accordingly to include the Replaced S-NSSAI and removes the Replaced S-NSSAI from the Rejected S-NSSAI by a UE Configuration Update procedure. After the UE is successfully updated, the AMF removes the related information which was stored at bullet c) above. The UE may request to register the Replaced S-NSSAIs again next time the UE sends a Requested NSSAI. The AMF reports the outcome of the network slice replacement and the PCF further notifies the AF.