5.19.7.2 General NAS level congestion control

This clause only applies to NAS Mobility Management congestion control.

Under general overload conditions the AMF may reject NAS messages from UEs using any 5G-AN. When a NAS request is rejected, a Mobility Management back-off time may be sent by the AMF to the UE. While the Mobility Management back-off timer is running, the UE shall not initiate any NAS request except for Deregistration procedure and procedures not subject to congestion control (e.g. high priority access, emergency services, informing the network of UE unavailability period due to NR satellite access discontinuous coverage) and mobile terminated services. After any such Deregistration procedure, the back-off timer continues to run. While the Mobility Management back-off timer is running, the UE is allowed to perform Mobility Registration Update if the UE is already in CM-CONNECTED state. If the UE receives a paging request or a NAS notification message from the AMF while the Mobility Management back off timer is running, the UE shall stop the Mobility Management back-off timer and initiate the Service Request procedure or the Mobility Registration Update procedure over 3GPP access and/or non-3GPP access as applicable. Over non-3GPP access, if the UE is in CM-IDLE state when the back-off timer is stopped, it shall initiate the UE-triggered Service Request procedure as soon as it switches back to CM-CONNECTED state.

In order to allow the UE to report the PS Data Off status change in PDU Session Modification Request message, the UE behaves as follows while keeping the NAS MM back-off timer running in the UE:

• When the UE is in CM-IDLE state and has not moved out of the Registration Area, the UE is allowed to send a Service Request message with an indication that the message is exempted from NAS congestion control. When the UE is in CM-IDLE mode and has moved out of the Registration Area, the UE is allowed to send a Mobility Registration Update request message, with a Follow-on request and with an indication that the message is exempted from NAS congestion control.
• When the UE is in CM-CONNECTED state, the UE sends a PDU Session Modification Request with PS Data Off status change carried in UL NAS Transport message with an indication that the message is exempted from NAS congestion control.

When the NAS MM congestion control is activated at AMF, if the UE indicates that the NAS MM message is exempted from NAS congestion control, the AMF shall not reject the NAS MM message and shall forward the NAS SM message to the corresponding SMF with an indication that the NAS SM message was indicated to be exempted from NAS congestion control. The SMF ensures that the NAS SM message is not subject to congestion control otherwise the SMF rejects the message, e.g. the SMF shall reject PDU Session Modification received if it is not for Data Off status reporting.

The Mobility Management back-off timer shall not impact Cell/RAT/Access Type and PLMN change. Cell/RAT/TA/Access Type change does not stop the Mobility Management back-off timer. The Mobility Management back-off timer shall not be a trigger for PLMN reselection or SNPN reselection. The back-off timer is stopped as defined in TS 24.501 [47] when a new PLMN or SNPN, that is not an equivalent PLMN or is not an equivalent SNPN, is accessed.

To avoid that large amounts of UEs initiate deferred requests (almost) simultaneously, the AMF should select the Mobility Management back-off timer value so that the deferred requests are not synchronized. If NWDAF is deployed, the AMF may make use of UE signalling analytics provided by NWDAF, as defined in clause 6.22.3 of TS 23.288 [86], to determine the Mobility Management back-off timer value provided to UEs.

If the UE required to report 5GSM Core Network Capability change, or the Always-on PDU Session Requested indication while the NAS MM congestion control timer was running and was unable to initiate MM signalling, the UE defers the related MM signalling until the MM congestion control timer expires and initiates after the expiry of the timer.

In the case of a UE with scheduled communication pattern, the AMF may consider the UE's communication pattern while selecting a value for the Mobility Management back-off timer so that the UE does not miss its only scheduled communication window.

The AMF should not reject Registration Request message for Mobility Registration Update that are performed when the UE is already in CM-CONNECTED state or when UE is indicating unavailability period due to NR satellite access discontinuous coverage.

The AMF may reject the Service Request message and a UL NAS Transfer with a Mobility Management back-off time when the UE is already in CM-CONNECTED state. If UE receives a DL NAS Transfer message from the AMF while the Mobility Management back off timer is running, the UE shall stop the Mobility Management back-off timer.

For CM-IDLE state mobility, the AMF may reject Registration Request messages for Mobility Registration Update by including a Mobility Management back off time value in the Registration Reject message.

If UE registered in the same PLMN for 3GPP access and non-3GPP access and receives a Mobility Management back-off time from the AMF, the back-off time (and corresponding start and stop) is applied equally to both 3GPP access and non-3GPP access. If UE registered in different PLMNs for 3GPP access and non-3GPP access respectively and receives a Mobility Management back-off time, the back-off time is only applied to the PLMN that provides the time to the UE.

If the AMF rejects Registration Request messages or Service Request with a Mobility Management back-off time which is larger than the sum of the UE's Periodic Registration Update timer and the Implicit Deregistration timer, the AMF should adjust the mobile reachable timer and/or Implicit Deregistration timer such that the AMF does not implicitly deregister the UE while the Mobility Management back-off timer is running.

NOTE: This is to minimize signalling after the Mobility Management back-off timer expires.

If the AMF deregisters the UE with an indication of re-registration required, the UE behaviour for handling the back-off timer(s) is as specified in TS 24.501 [47].