5.19.7.3 DNN based congestion control

DNN based congestion control is designed for the purpose of avoiding and handling of NAS SM signalling congestion for the UEs with a back-off timer associated with or without a DNN regardless of the presence of an S-NSSAI. Both UE and 5GC shall support the functionality to enable DNN based congestion control.

SMFs may apply DNN based congestion control towards the UE by rejecting PDU Session Establishment Request message, or PDU Session Modification Request message except for those sent for the purpose of reporting 3GPP PS Data Off status change for a specific DNN with a running back-off timer. The SMF may release PDU Sessions belonging to a congested DNN by sending a PDU Session Release Command message towards the UE with a DNN back-off timer. If a DNN back-off time is set in the PDU Session Release Command message, the cause value of "reactivation requested" shall not be set.

If NWDAF is deployed, the SMF may make use of Session Management Congestion Control Experience analytics provided by NWDAF, as defined in clause 6.12 of TS 23.288 [86], to determine back-off timer provided to UEs.

NOTE: For example, the SMF can apply a short back-off timer to the UEs in the list of high-experienced UEs while the SMF can apply a long back-off timer to the UEs in the list of low-experienced UEs.

When DNN based congestion control is activated at AMF e.g. configured by OAM, the AMF provides a NAS Transport Error message for the NAS Transport message carrying an SM message and in the NAS Transport Error message it includes a DNN back-off timer.

The UE associates the received back-off time with the DNN (i.e. no DNN, DNN only) which the UE included in the uplink NAS MM message carrying the corresponding NAS SM request message.

The UE associates the received back-off time with the DNN (i.e. no DNN, DNN only) in any PLMN unless the DNN associated with the back-off timer is an LADN DNN in which case the UE only associates it to the PLMN in which the back-off time was received.

The UE behaves as follows when the DNN back-off timer is running:

• If a DNN is associated with the back-off timer, the UE shall not initiate any Session Management procedures for the congested DNN. The UE may initiate Session Management procedures for other DNNs. The UE shall not initiate any Session Management procedure for the corresponding APN when UE moves to EPS. The UE may initiate Session Management procedures for other APNs when the UE moves to EPS;
• If no DNN is associated with the back-off timer, the UE may only initiate Session Management requests of any PDU Session Type for a specific DNN;
• Upon Cell/TA/PLMN/RAT change, change of untrusted non-3GPP access network or change of Access Type, the UE shall not stop the back-off timer;
• The UE is allowed to initiate the Session Management procedures for high priority access and emergency services;
• The UE is allowed to initiate the Session Management procedure for reporting Data Off status change to the network;
• If the UE receives a network initiated Session Management message other than PDU Session Release Command for the congested DNN associated to a running back-off timer, the UE shall stop the back-off timer and respond to the 5GC;
• If the UE receives a PDU Session Release Command message for the congested DNN, it shall stop the back-off timer unless it receives a new back-off time from SMF;
• The UE is allowed to initiate PDU Session Release procedure (i.e. sending PDU Session Release Request message). The UE shall not stop the back-off timer when the related PDU Session is released;
• The list above is not an exhaustive list, i.e. more details of the above actions and further conditions, if any, are specified in TS 24.501 [47].

If UE initiates one of the Session Management procedures that are exempted from NAS congestion control, the UE indicates that the carried NAS SM message is exempted from NAS congestion control in the UL NAS Transport message as described in TS 24.501 [47]. When the DNN based congestion control is activated at AMF, if the UE indicates that the NAS SM message in the UL NAS Transport message is exempted from NAS congestion control, the AMF shall not apply DNN based congestion control on the UL NAS Transport message and shall forward the NAS SM message to the corresponding SMF with an indication that the message was received with exemption indication. The SMF evaluates whether the NAS SM message is allowed to be exempted from DNN based congestion control. If it is not, 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 UE shall maintain a separate back-off timer for each DNN that the UE may use.

To avoid that large amounts of UEs initiate deferred requests (almost) simultaneously, the 5GC should select the back-off timer value so that deferred requests are not synchronized.

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

The DNN based Session Management congestion control is applicable to the NAS SM signalling initiated from the UE in the Control Plane. The Session Management congestion control does not prevent the UE from sending and receiving data or initiating Service Request procedures for activating User Plane connection towards the DNN(s) that are under Session Management congestion control.