Section: 4.6.2.7 Mobility management based network slice replacement
Content: 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 needs to be replaced with an alternative S-NSSAI, the AMF provides: a) the alternative S-NSSAI in the allowed NSSAI, if not included yet; b) the alternative S-NSSAI in the configured NSSAI, if not included yet; c) the alternative S-NSSAI in the NSAG information, if not included yet and the UE supports NSAG; and d) the mapping information between the S-NSSAI to be replaced and the alternative S-NSSAI, to the UE during the UE configuration update procedure or during the registration procedure as follows: a) for non-roaming UE, the AMF provides the mapping information between the S-NSSAI included in the allowed NSSAI and the alternative S-NSSAI to the UE; and NOTE 1: In non-roaming scenarios, the alternative S-NSSAI does not have to be part of the subscribed S-NSSAI(s) in the UE subscription. b) for roaming UE: 1) if the S-NSSAI included in the 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 and the alternative S-NSSAI to the UE; and 2) if the S-NSSAI included in the mapped S-NSSAI(s) for the 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 and the alternative S-NSSAI to the UE. NOTE 1A: In roaming scenarios, the alternative S-NSSAI part of the HPLMN S-NSSAIs does not have to be part of the subscribed S-NSSAI(s) in the UE subscription. NOTE 2: It is up to AMF local policy to determine when to provide the mapping information between the S-NSSAI to be replaced and the alternative S-NSSAI to the UE, e.g. when the alternative S-NSSAI is available and there is no PDU session associated with the S-NSSAI to be replaced, or wait until the UE establishes the first PDU session associated with the S-NSSAI to be replaced. If the AMF determines that the S-NSSAI which has been replaced is available, the AMF provides the updated alternative NSSAI excluding the S-NSSAI which has been replaced and the corresponding alternative S-NSSAI to the UE during the UE configuration update procedure or during the registration procedure. If all the S-NSSAI(s) that were replaced in alternative NSSAI are available, the AMF provides the alternative NSSAI with Length of Alternative NSSAI contents set to 0 in the UE configuration update procedure or registration procedure. The AMF also provides the updated allowed NSSAI and configured NSSAI to the UE. 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 NS-AoS of the alternative S-NSSAI is larger than the NS-AoS of the S-NSSAI to be replaced, the UE can request the establishment of user plane resources of PDU session(s) associated with the alternative S-NSSAI in the cell(s) outside the NS-AoS of the S-NSSAI to be replaced but within the NS-AoS of the alternative S-NSSAI.