4.6.2.7 Mobility management-based network slice replacement

The support for network slice replacement by a UE or network is optional. If the UE and network support network slice replacement, and the AMF determines that an S-NSSAI included in the allowed NSSAI or the partially allowed NSSAI needs to be replaced with an alternative S-NSSAI, the AMF provides:

• the alternative S-NSSAI in the allowed NSSAI or the partially allowed NSSAI, if not included yet;
• the alternative S-NSSAI in the configured NSSAI, if not included yet;
• the alternative S-NSSAI in the NSAG information, if not included yet and the UE supports NSAG; and
• the alternative NSSAI including the mapping information between the S-NSSAI to be replaced and the corresponding alternative S-NSSAI,

to the UE, during the generic UE configuration update procedure or during the registration procedure as follows:

• if mapped S-NSSAIs are not provided to the UE (as specified in clause 4.6.1, the AMF provides the mapping information between the S-NSSAI included in the allowed NSSAI and the alternative S-NSSAI or the partially allowed NSSAI to the UE; or

NOTE 1: For case a), the alternative S-NSSAI does not have to be part of the subscribed S-NSSAI(s) in the UE subscription.

• if mapped S-NSSAIs are provided to the UE (as specified in clause 4.6.1):
• if the S-NSSAI included in the allowed NSSAI or the partially allowed NSSAI needs to be replaced (i.e. the S-NSSAI to be replaced is part of the VPLMN S-NSSAIs), the AMF provides the mapping information between the S-NSSAI included in the allowed NSSAI or the partially allowed NSSAI and the alternative S-NSSAI to the UE; and
• if the S-NSSAI included in the mapped S-NSSAI(s) for the allowed NSSAI or the partially allowed NSSAI needs to be replaced (i.e. the S-NSSAI to be replaced is part of the HPLMN S-NSSAIs), the AMF provides the mapping information between the S-NSSAI to be replaced and the alternative S-NSSAI to the UE.

NOTE 1A: For case b.2, the mapped S-NSSAI for the alternative S-NSSAI, being part of the HPLMN S-NSSAIs does not have to be part of the subscribed S-NSSAI(s) in the UE subscription.

If the requested NSSAI contains alternative S-NSSAI(s) that are not subscribed S-NSSAI(s), the AMF shall verify the alternative S-NSSAI(s) based on the stored alternative NSSAI.

If the alternative S-NSSAI is subject to NSSAA, the alternative S-NSSAI provided by AMF shall be part of the subscribed S-NSSAI(s) in the UE subscription. The AMF shall perform NSSAA procedure for such alternative S-NSSAI and perform network slice replacement as specified above after the NSSAA procedure for the alternative S-NSSAI is completed as success.

If re-NSSAA procedure is ongoing for the S-NSSAI to be replaced, the AMF shall continue with the re-NSSAA procedure and perform network slice replacement as specified above after the re-NSSAA procedure for the S-NSSAI to be replaced is completed as success.

NOTE 2: When configuring the alternative S-NSSAI, the maximum number of S-NSSAIs defined for the allowed NSSAI, partially allowed NSSAI and configured NSSAI in clause 4.6.2.2 are applicable.

If the AMF determines that the replaced S-NSSAI is available again, the AMF provides the updated alternative NSSAI excluding the replaced S-NSSAI and the corresponding alternative S-NSSAI to the UE during the UE configuration update procedure or during the registration procedure.

If all the replaced S-NSSAI(s) in alternative NSSAI are available again, the AMF provides the alternative NSSAI with Length of Alternative NSSAI contents set to 0 in the UE configuration update procedure or registration procedure.

NOTE 3: If there is S-NSSAI location validity information for both the S-NSSAI to be replaced and the alternative S-NSSAI, the NS-AoS of the alternative S-NSSAI is the same as or larger than the NS-AoS of the S-NSSAI to be replaced.

If the UE is in a cell outside the NS-AoS of the replaced S-NSSAI but within the NS-AoS of the alternative S-NSSAI, the AMF provides the updated allowed NSSAI and partially allowed NSSAI excluding the replaced S-NSSAI, if included, in the allowed NSSAI or partially allowed NSSAI during the UE configuration update procedure or during the registration procedure.

If the alternative S-NSSAI is not part of the subscribed S-NSSAI(s), and:

• the replaced S-NSSAI is removed from the allowed NSSAI or the partially allowed NSSAI; or
• the replaced S-NSSAI is available again,

then the AMF shall provide updated allowed NSSAI or partially allowed NSSAI excluding the alternative S-NSSAI to the UE over the current access type during the UE configuration update procedure or during the registration procedure. Additionally, if the alternative S-NSSAI is not included in allowed NSSAI or partially allowed NSSAI over the other access type:

• for case a), the AMF may also provide updated configured NSSAI excluding the alternative S-NSSAI to the UE during the UE configuration update procedure or during the registration procedure; or
• for case b), the AMF shall also provide updated configured NSSAI excluding the alternative S-NSSAI to the UE during the UE configuration update procedure or during the registration procedure.

Based on SMF local configuration, if the replaced S-NSSAI is subject to NSAC or the alternative S-NSSAI is subject to NSAC or both, the SMF may perform NSAC for the replaced S-NSSAI or the alternative S-NSSAI or both.

If the alternative S-NSSAI is not available and the replaced S-NSSAI is not yet available, the AMF provides the updated alternative NSSAI excluding the replaced S-NSSAI and the alternative S-NSSAI to the UE during the UE configuration update procedure or during the registration procedure.