5.19.7.4 S-NSSAI based congestion control

S-NSSAI based congestion control is designed for the purpose of avoiding and handling of NAS signalling congestion for the UEs with back-off timer associated with or without an S-NSSAI regardless of the presence of a DNN.

The UE associates the received back-off time with the S-NSSAI and DNN (i.e. no S-NSSAI and no DNN, no S-NSSAI, S-NSSAI only, an S-NSSAI and a DNN) which was included in the uplink NAS MM message carrying the corresponding NAS SM request message for the PLMN which is under congestion.

S-NSSAI based congestion control is applied as follows:

• If an S-NSSAI is determined as congested, then the SMF may apply S-NSSAI based congestion control towards the UE for SM requests except for those sent for the purpose of reporting 3GPP PS Data Off status change for a specific S-NSSAI and provides a back-off time and an indication of HPLMN congestion;
• If the UE receives an S-NSSAI based back-off time without an indication of HPLMN congestion, the UE shall apply the S-NSSAI back-off timer only in the PLMN in which the back-off time was received. If the UE receives S-NSSAI based back-off time with an indication of HPLMN congestion, the UE shall apply the S-NSSAI based back-off timer in the PLMN in which the back-off time was received and in any other PLMN;
• The SMF may release PDU Sessions belonging to a congested S-NSSAI by sending a PDU Session Release Request message towards the UE with a back-off time associated either to the S-NSSAI only (i.e. with no specific DNN) or a combination of the S-NSSAI and a specific DNN. 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.

• If S-NSSAI based congestion control is activated at AMF e.g. configured by OAM and an S-NSSAI is determined as congested, then the AMF applies S-NSSAI based congestion control towards the UE for UE-initiated Session Management requests. In this case, the AMF provides a NAS Transport Error message for the NAS Transport message carrying the SM message and in the NAS Transport Error message it includes a back-off timer; If NWDAF is deployed, the AMF may determine that S-NSSAI is congested based on the network slice load level analytics defined in TS 23.288 [86].
• The UE behaves as follows in the PLMN where the S-NSSAI based congestion control applies when the back-off timer is running:
• If the back-off timer was associated with an S-NSSAI only (i.e. not associated with an S-NSSAI and a DNN), the UE shall not initiate any Session Management procedures for the congested S-NSSAI;
• If the back-off timer was associated with an S-NSSAI and a DNN, then the UE shall not initiate any Session Management procedures for that combination of S-NSSAI and DNN;
• If the UE receives a network-initiated Session Management message other than PDU Session Release Command for the congested S-NSSAI, the UE shall stop this back-off timer and respond to the 5GC;
• If the UE receives a PDU Session Release Command message for the congested S-NSSAI, it shall stop the back-off timer unless it receives a new back-off time from SMF;
• 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 for any S-NSSAI or any combination of S-NSSAI and DNN;
• The UE is allowed to initiate the Session Management procedures for high priority access and emergency services for the S-NSSAI;
• The UE is allowed to initiate the Session Management procedure for reporting Data Off status change for the S-NSSAI or the combination of S-NSSAI and DNN.
• If the back-off timer is not associated to any S-NSSAI, the UE may only initiate Session Management procedures for specific S-NSSAI;
• If the back-off timer is not associated to any S-NSSAI and DNN, the UE may only initiate Session Management procedures for specific S-NSSAI and DNN;
• The UE is allowed to initiate PDU Session Release procedure (e.g. 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].

The UE shall maintain a separate back-off timer for each S-NSSAI and for each combination of S-NSSAI and DNN that the UE may use.

If UE initiates one of the Session Management procedure that are exempt 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 S-NSSAI 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 S-NSSAI 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 that the NAS SM message is allowed to be exempted from S-NSSAI 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 back-off timer associated with an S-NSSAI or a combination of an S-NSSAI and a DNN shall only apply to congestion control for Session Management procedures when UE is in 5GS.

To avoid that large amounts of UEs initiate deferred requests (almost) simultaneously, the 5GC should select the value of the back-off timer for the S-NSSAI based congestion control 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 S-NSSAI based congestion control timer was running and was unable to initiate SM signalling, the UE defers the related SM signalling until the S-NSSAI based congestion control timer expires and initiates the necessary SM signalling after the expiry of the timer.

The S-NSSAI based congestion control does not prevent the UE from sending and receiving data or initiating Service Request procedure for activating User Plane connection for a PDU Session associated to the S-NSSAI that is under the congestion control.