5.19.7.6 Control Plane data specific NAS level congestion control

Under overload conditions the AMF may restrict requests from UEs for data transmission via Control Plane CIoT 5GS Optimisation. A Control Plane data back-off timer may be returned by the AMF (e.g. in Registration Accept messages, Service Reject message or Service Accept message). While the Control Plane data back-off timer is running, the UE shall not initiate any data transfer via Control Plane CIoT 5GS Optimisation, i.e. the UE shall not send any Control Plane Service Request with uplink data as defined in TS 24.501 [47]. The AMF shall store the Control Plane data back-off timer per UE and shall not process any further requests (other than exception reporting and a response to paging) for Data Transport via a Control Plane Service Request from that UE while the Control Plane data back-off timer is still running.

NOTE 1: The Control Plane data back-off timer does not affect any other mobility management or session management procedure.

NOTE 2: The Control Plane data back-off timer does not apply to user plane data communication.

If the UE is allowed to send exception reporting, the UE may send an initial NAS Message for exception reporting even if Control Plane data back-off timer is running.

The UE may respond to paging with an initial NAS Message without uplink data even if the Control Plane data back-off timer is running.

If the AMF receives an initial NAS Message in response to a paging and the AMF has a Control Plane data back-off timer running for the UE and the AMF is not overloaded and AMF decides to accept the Control Plane Service Request, then the AMF shall respond with Service Accept without the Control Plane data back-off timer and stop the Control Plane data back-off timer. If the UE receives a Service Accept without the Control Plane data back-off timer from the AMF while the Control Plane data back-off timer is running, the UE shall stop the Control Plane data back-off timer. The Control Plane data back-off timer in the UE and the AMF is stopped at PLMN change.

If the AMF receives a Control Plane Service Request with uplink data and decides to send the UE a Control Plane data back-off timer, the AMF may decide to process the Control Plane Service Request with uplink data, i.e. decrypt and forward the data payload, or not based on the following:

• If the UE has indicated Release Assistance Information that no further Uplink and Downlink Data transmissions are expected, then the AMF may process (integrity check/decipher/forward) the received Control Plane data packet and send a Service Accept to the UE with Control Plane data back-off timer. The UE interprets this as successful transmission of the Control Plane data packet starts the Control Plane data back-off timer.
• For all other cases, the AMF may decide to not process the received Control Plane data packet and send a Service Reject to the UE with Control Plane data back-off timer. The UE interprets this indication as unsuccessful delivery of the control plane data packet and starts the Control Plane data back-off timer. The AMF may take into consideration whether the PDU Session is set to Control Plane only to make the decision whether to reject the packet and send Service Reject or move the PDU Session to user plane and process the data packet as described in next bullet.
• Alternatively, if UE has not provided Release Assistance Information and the PDU Session not set to Control Plane only and UE supports N3 data transfer, then the AMF may initiate establishment of N3 bearer according to the procedure defined in clause 4.2.3 of TS 23.502 [3]. In this case the AMF may also return a Control Plane data back-off timer within the Service Accept.

The AMF only includes the Control Plane data back-off timer if the UE has indicated support for Control Plane CIoT 5GS optimizations in the Registration Request.