Section: 5.15.15.3 Network-based per UE Network Slice usage behaviour control
Content: The 5GC performs Network Slice usage monitoring to be able to enforce the release of inactive PDU Sessions, and deregistering of UEs from Network Slices with no PDU Sessions on them according to its own policies. In order to support usage monitoring for a Network Slice: - the AMF runs a slice deregistration inactivity timer per S-NSSAI and access type to deregister the Network Slice which is started when the Network Slice is not used by any PDU Session over the corresponding access type. The slice deregistration inactivity timer is stopped and reset when at least a PDU Session associated with the Network Slice is successfully established or the Network Slice is removed form the Allowed NSSAI. When the slice deregistration inactivity timer for a Network Slice over an access type expires, the AMF removes the Network Slice from the Allowed NSSAI over the access type by sending the UE Configuration Update Command to impacted UE(s). - the SMFs provide to UPFs that handle the PDU sessions in the Network Slice a PDU Session inactivity timer. The PDU Session inactivity timer is started after no data packet is transmitted or received and runs until the next data packet is transmitted or received which restarts the timer again. If the PDU Session inactivity timer expires before any packet is received or transmitted, the UPF reports this PDU Session inactivity event to the SMF to cause the SMF to release the PDU Session. While releasing the PDU session the SMF may indicate the release cause because of slice inactivity. When the AMF receives the notification of PDU Session release and it includes the release cause of slice inactivity and if the Network Slice of the released PDU Session is not used by other PDU Sessions (i.e. the last PDU Session using the Network Slice is released) over the corresponding access type, the AMF may trigger the UE Configuration Update procedure to remove the Network Slice from the Allowed NSSAI over that corresponding access type or start slice deregistration inactivity timer for the Network Slice. If the PDU Session inactivity timer value is updated, the SMF provides the updated PDU Session inactivity timer value to the UPF. The UPF uses the updated PDU Session inactivity timer value immediately or next time the PDU Session inactivity timer starts. NOTE: For MA PDU Session, the PDU Session inactivity timer is independent of Access Type. If an S-NSSAI is dedicated for a single AF, and if authorized by operator policy to provide deregistration inactivity/PDU Session inactivity timer values for the S-NSSAI, the AF uses external parameter provisioning procedure to provide deregistration inactivity and PDU session inactivity timer values as described in clause 4.15.6.3g of TS 23.502[ Procedures for the 5G System (5GS) ]  [3]. In this case, the AF provided timer values are stored in the UDM and provided to the AMF/SMF as part of subscription data for the corresponding S-NSSAI. If no AF is authorized to provide deregistration inactivity/PDU Session inactivity timer values for the S-NSSAI, i.e. no timer value received from UDM, the slice deregistration inactivity timer value and PDU Session inactivity timer value are either pre-configured in the AMF/SMF or received by the AMF/SMF during the AM Policy Association / SM Policy Association procedure respectively. To enable a serving network to direct UEs to a preferred Network Slice, the AMF may request the UE to transfer a PDU Session from one S-NSSAI to another S-NSSAI as described in clause 5.15.19.