4.6.3.4 Session management based network slice replacement

If:

• the UE and network support network slice replacement;
• the UE is provided with the mapping information between the S-NSSAI to be replaced and the alternative S-NSSAI; and
• the UE decides to establish a new PDU session with the S-NSSAI to be replaced,

the UE provides both the S-NSSAI to be replaced and the alternative S-NSSAI during PDU session establishment procedure.

If:

• the UE and network support network slice replacement;
• the UE initiates UE-requested PDU session establishment procedure with an S-NSSAI; and
• the AMF determines that the S-NSSAI needs to be replaced with an alternative S-NSSAI;

the AMF shall initiate the generic UE configuration update procedure as part of the UE-requested PDU session establishment procedure to provide to the UE:

• the allowed NSSAI including the alternative S-NSSAI, if not included yet;
• the configured NSSAI including the alternative S-NSSAI, if not included yet; and
• the alternative NSSAI including the mapping information between the S-NSSAI to be replaced and the corresponding alternative S-NSSAI.

If the timer T3584 or timer T3585 is running for the S-NSSAI to be replaced, the UE should not stop the timer during PDU session establishment procedure. If the SMF receives both the S-NSSAI to be replaced and the alternative S-NSSAI during PDU session establishment procedure, the SMF proceeds with the PDU session establishment procedure with the alternative S-NSSAI. If the PDU session establishment request is accepted, the SMF includes the alternative S-NSSAI in the PDU SESSION ESTABLISHMENT ACCEPT message. The S-NSSAI for the established PDU session is the S-NSSAI to be replaced and the alternative S-NSSAI on the UE side.

If the UE is provided with the mapping of the VPLMN S-NSSAI to a VPLMN alternative S-NSSAI, the UE provides both the VPLMN alternative S-NSSAI and the VPLMN S-NSSAI during PDU session establishment procedure. The AMF sends both VPLMN alternative S-NSSAI and VPLMN S-NSSAI to the SMF. If the UE is provided with the mapping of the HPLMN S-NSSAI to an HPLMN Alternative S-NSSAI, the UE provides both the HPLMN alternative S-NSSAI and the HPLMN S-NSSAI during PDU session establishment procedure. The AMF sends both HPLMN alternative S-NSSAI and HPLMN S-NSSAI to the SMF.

If the SMF receives from the AMF an alternative S-NSSAI for the existing PDU session and:

• if the SMF decides to retain the existing PDU session (i.e. the SMF can serve both the alternative S-NSSAI and the S-NSSAI to be replaced), the SMF sends the alternative S-NSSAI to the UE during network-requested PDU session modification procedure; or
• if the SMF decides to re-activate the existing PDU session and:
• if the SSC mode of the PDU session is SSC mode 3, the SMF initiates PDU session modification procedure to trigger PDU session reactivation by the UE; or
• if the SSC mode of the PDU session is SSC mode 1 or SSC mode 2, the SMF initiates PDU session release procedure to trigger PDU session reactivation by the UE, and the UE provides both the S-NSSAI to be replaced and the alternative S-NSSAI during PDU session establishment procedure.

When the replaced S-NSSAI becomes available again, if the SMF receives from the AMF the replaced S-NSSAI for the existing PDU session and:

• if the SMF decides to retain the existing PDU session (i.e. the SMF can serve the replaced S-NSSAI), the SMF sends the replaced S-NSSAI to the UE during network-requested PDU session modification procedure; or
• if the SMF decides to re-activate the existing PDU session and:
• if the SSC mode of PDU session is SSC mode 3, the SMF sends the replaced S-NSSAI to the UE during PDU session modification procedure to trigger PDU session reactivation by the UE; or
• if the SSC mode of the PDU session is SSC mode 1 or SSC mode 2, the SMF sends the replaced S-NSSAI to the UE during PDU session release procedure to trigger PDU session reactivation by the UE; and the UE provides the replaced S-NSSAI during PDU establishment procedure.

If the timer T3584 or timer T3585 is running for the replaced S-NSSAI and the replaced S-NSSAI in alternative NSSAI is available and the AMF provides the updated allowed NSSAI, partially allowed NSSAI and configured NSSAI to the UE, the UE should stop the timer.