5.15.15.2 UE Configuration of network-controlled Slice Usage Policy

The UE during the Registration procedure may indicate in UE MM Core Network Capability that it supports UE configuration of network-controlled Slice Usage Policy. If so, the AMF determines Slice Usage Policy for one (or more) Network Slice(s) for the UE and configures the UE with this information together with Configured NSSAI to control the usage of this (or these) Network Slice(s). The AMF may be locally configured with network Slice Usage Policy, or receive the policy from the AM-PCF, or per the information received from UDM for AF managed timer values (see clause 5.15.15.3 for more details).

The network-controlled Slice Usage Policy is provided to the UE in the Registration Accept or the UE Configuration Update Command and may include:

• An indication, for one or more of S-NSSAI(s) of the HPLMN in the Configured NSSAI, whether the UE only registers with the Network Slice with the network when applications in the UE require data transmission in the Network Slice (i.e. the UE can only register the Network Slice only on demand and consider the Network Slice as on demand S-NSSAI).

NOTE: All Other Network Slices in the Configured NSSAI are handled by the UE using UE specific policies (e.g. they may be registered irrespective of applications need).

• Optionally, for on demand S-NSSAI(s) of the HPLMN in the Configured NSSAI, a slice deregistration inactivity timer per access type that causes the UE to deregister the Network Slice. Regarding the slice deregistration inactivity timer, following applies:

a. The UE and AMF start the timer when one of the following conditions occur:

• the Network Slice is added to the Allowed NSSAI and there are no PDU Sessions associated with the Network Slice over the corresponding access type; and
• the last PDU Session associated with the Network Slice over the same access type is released.

b. The UE and AMF stop the timer and reset when at least one PDU Session associated with the Network Slice is successfully established or the Network Slice is removed from the Allowed NSSAI.

c. When the slice deregistration inactivity timer for a Network Slice over an access type expires, the UE and AMF locally remove the S-NSSAI from the Allowed NSSAI.

If the UE and network state became misaligned, the UE may, for example, request connectivity in a Network Slice which is no longer allowed. In this case, the AMF shall provide the updated Allowed NSSAI in a UE Configuration Update Command after rejecting the PDU Session establishment. The UE may then re-register with the Network Slice if needed.

The UE stores the received Slice Usage Policy with the Configured NSSAI for the serving PLMN and this is kept stored for as long as a Configured NSSAI remains stored for the PLMN. When the Configured NSSAI is updated, the AMF may also provide a new Slice Usage Policy to the UE.

The AMF receives slice deregistration inactivity timer values, i.e. the configured slice deregistration inactivity timer, as described in clause 5.15.15.3.

At mobility between AMFs, the old AMF provides both the ongoing slice deregistration inactivity timer information, if it is started, to enable the new AMF know when the timer expires and the configured slice deregistration inactivity timer value(s) in the UE context to the new AMF if the slice deregistration inactivity timer is received from the AM-PCF. If the configured slice deregistration inactivity timer value(s) have been updated at the old AMF but not notified to the UE, the old AMF provides the new configured slice deregistration inactivity timer value(s) as the configured slice deregistration inactivity timer value(s) to the new AMF. The new AMF includes the slice deregistration inactivity timer in Registration Accept Message sent to the UE.

If the slice deregistration inactivity timer value is updated and the AMF determines to provide the timer to the UE, the AMF provides the updated value to the UE, if the UE supports UE configuration of network-controlled Slice Usage Policy, during the registration procedure or any subsequent registration if there is no ongoing registration procedure. The AMF and the UE, if the update been provided to the UE successfully, use the updated slice deregistration inactivity timer value next time the slice deregistration inactivity timer starts.