6.2.7 Handling of DNN based congestion control

The network may detect and start performing DNN based congestion control when one or more DNN congestion criteria as specified in 3GPP TS 23.501 [8] are met. If the UE does not provide a DNN for a non-emergency PDU session, then the network uses the selected DNN.

In the UE, 5GS session management timers T3396 for DNN based congestion control are started and stopped on a per DNN basis except for an LADN DNN in case of PLMN. For an LADN DNN, 5GS session management timers T3396 for DNN based congestion control is applied to the registered PLMN and its equivalent PLMNs. In case of SNPN, if the UE does not support access to an SNPN using credentials from a credentials holder and equivalent SNPNs, in the UE, 5GS session management timers T3396 for DNN based congestion control are started and stopped on a per DNN and SNPN basis. If the UE supports access to an SNPN using credentials from a credentials holder, equivalent SNPNs or both, in the UE 5GS session management timers T3396 for DNN based congestion control are started and stopped on a per DNN, SNPN and selected entry of the "list of subscriber data" or selected PLMN subscription basis. Upon receipt of a 5GMM message or 5GSM message from the network for which the UE needs to stop the running timers T3396 associated with an LADN DNN as specified in clause 6.3.2.3, 6.3.3.3, 6.4.1.4.2 and 6.4.2.4.2, only the running timer T3396 which is associated with the PLMN and equivalent PLMNs where the timer was started is stopped.

In an SNPN, if the UE supports equivalent SNPNs, the UE applies the timer T3396 for all the equivalent SNPNs, otherwise the UE applies the timer T3396 for the registered SNPN.

The DNN associated with T3396 is the DNN provided by the UE during the PDU session establishment. If no DNN is provided by the UE along the PDU SESSION ESTABLISHMENT REQUEST, then T3396 is associated with no DNN. For this purpose, the UE shall memorize the DNN provided to the network during the PDU session establishment. The timer T3396 associated with no DNN will never be started due to any 5GSM procedure related to an emergency PDU session. If the timer T3396 associated with no DNN is running, it does not affect the ability of the UE to request an emergency PDU session.

In a PLMN, if T3396 is running or is deactivated, then the UE is not allowed to initiate the:

• PDU session establishment procedure;
• PDU session modification procedure; or
• NAS transport procedure for sending CIoT user data;

for the respective DNN or without a DNN unless the UE is a UE configured for high priority access in selected PLMN or to report a change of 3GPP PS data off UE status.

In an SNPN, if T3396 is running or is deactivated for the registered SNPN, is associated with a DNN or with no DNN, with the RSNPN and, if the UE supports access to an SNPN using credentials from a credentials holder, with the selected entry of the "list of subscriber data" or the selected PLMN subscription, then the UE is not allowed to initiate the

• PDU session establishment procedure; or
• PDU session modification procedure;

for the respective DNN or without a DNN unless the UE is a UE configured for high priority access in the RSNPN or to report a change of 3GPP PS data off UE status.

In an SNPN, if the UE supports equivalent SNPNs, the timer T3396 is running or is deactivated for all the equivalent SNPNs, is associated with a DNN or no DNN, with the RSNPN or an equivalent SNPN, and with the selected entry of the "list of subscriber data" or the selected PLMN subscription, then the UE is not allowed to initiate the:

• PDU session establishment procedure; or
• PDU session modification procedure;

for the respective DNN or without a DNN unless the UE is a UE configured for high priority access in selected SNPN or to report a change of 3GPP PS data off UE status.

If the network does not include timer T3396 with 5GSM cause #26 "insufficient resources", the UE may use a local back-off timer that has exponential value or a default value, which is provisioned using implementation specific means, to:

• prevent sending any 5GSM procedure for the respective DNN till its expiry; and
• allow sending any 5GSM procedure for the respective DNN after its expiry.