Section: 4.3.7.4.2 NAS level congestion control
Content: 4.3.7.4.2.1 General NAS level congestion control contains the functions: "APN based congestion control" and "General NAS level Mobility Management control". The use of the APN based congestion control is for avoiding and handling of EMM and ESM signalling congestion associated with UEs with a particular APN. Both UEs and network shall support the functions to provide APN based EMM and ESM congestion control. The MME may detect the NAS signalling congestion associated with the APN and start and stop performing the APN based congestion control based on criteria such as: - Maximum number of active EPS bearers per APN; - Maximum rate of EPS Bearer activations per APN; - One or multiple PDN GWs of an APN are not reachable or indicated congestion to the MME; - Maximum rate of MM signalling requests associated with the devices with a particular subscribed APN; and/or - Setting in network management. The MME may detect the NAS signalling congestion associated with the UEs belonging to a particular group. The MME may start and stop performing the group specific NAS level congestion control based on criteria such as: - Maximum rate of MM and SM signalling requests associated with the devices of a particular group; and/or - Setting in network management. The MME may detect the NAS signalling congestion associated with the UEs that belong to a particular group and are subscribed to a particular APN. The MME may start and stop performing the APN and group specific NAS level congestion control based on criteria such as: - Maximum number of active EPS bearers per group and APN; - Maximum rate of MM and SM signalling requests associated with the devices of a particular group and a particular subscribed APN; and/or - Setting in network management. The MME should not apply NAS level congestion control for high priority access and emergency services. With General NAS level Mobility Management control, the MME may also use the reject of NAS level Mobility Management signalling requests under general congestion conditions such as detecting congestion of one or several DCNs in an MME serving multiple DCNs. In addition, for UEs that don't support the Service Gap Control feature (see clause 4.3.17.9), the MME may return a Mobility Management back-off timer to the UE in responses to requests where the intention is to send MO data or re-attach with PDN connectivity when the Service gap timer for the UE is running at the MME. 4.3.7.4.2.2 APN based Session Management congestion control The APN based Session Management congestion control may be activated by MME due to e.g. congestion situation at MME, or by OAM at MME, or by a restart or recovery condition of a PDN GW, or by a partial failure or recovery of a PDN GW for a particular APN(s). The MME may reject the EPS Session Management (ESM) requests from the UE (e.g. PDN Connectivity, or Bearer Resource Allocation Requests) with a Session Management back-off timer when ESM congestion associated with the APN is detected. If the UE provides no APN, then the MME uses the APN which is used in PDN GW selection procedure. The MME may deactivate PDN connections belonging to a congested APN by sending the NAS Deactivate EPS Bearer Context Request message to the UE with a Session Management back-off timer. If Session Management back-off timer is set in the NAS Deactivate EPS Bearer Context Request message then the cause "reactivation requested" should not be set. NOTE 1: UEs that don't support the Session Management back-off timer (including earlier release of UE) might contribute to increasing the signalling load in the MME by reattempting Session Management procedure. The MME may store a Session Management back-off time per UE and APN when congestion control is active for an APN if a request without the low access priority indicator is rejected by the MME. The MME may immediately reject any subsequent request from the UE targeting to the APN before the stored Session Management back-off time is expired. If the MME stores the Session Management back-off time per UE and APN and the MME decides to send a Session Management Request message to a UE connected to the congested APN (e.g. due to decreased congestion situation), the MME shall clear the Session Management back-off time prior to sending any Session Management Request message to the UE. NOTE 2: The above functionality is to diminish the performance advantage for UEs that do not support the NAS level back-off timer (e.g. pre-Rel-10 UEs) compared to UEs that do support it. Upon reception of the Session Management back-off timer in the EPS Session Management reject message or in the NAS Deactivate EPS Bearer Context Request message, the UE shall take the following actions until the timer expires: - If APN is provided in the rejected EPS Session Management Request message or if the Session Management back-off timer is received in the NAS Deactivate EPS Bearer Context Request message, the UE shall not initiate any Session Management procedures for the congested APN. The UE may initiate Session Management procedures for other APNs. - If APN is not provided in the rejected EPS Session Management Request message, the UE shall not initiate any Session Management requests of any PDN type without APN. The UE may initiate Session Management procedures for specific APN. - Cell/TA/PLMN/RAT change do not stop the Session Management back-off timer. - The UE is allowed to initiate the Session Management procedures for high priority access and emergency services even when the Session Management back-off timer is running. - The UE is allowed to initiate Bearer Resource Modification procedure to report 3GPP PS Data Off status change when the EPS Session Management back off timer is running. - If the UE receives a network initiated EPS Session Management Request message for the congested APN while the Session Management back-off timer is running, the UE shall stop the Session Management back-off timer associated with this APN and respond to the MME. - If the UE is configured with a permission for overriding low access priority and the Session Management back-off timer is running due to a reject message received in response to a request with low access priority, the upper layers in the UE may request the initiation of Session Management procedures without low access priority. The UE is allowed to initiate PDN disconnection procedure (e.g. sending PDN Disconnection Request) when the EPS Session Management back off timer is running. NOTE 3: The UE does not delete the related Session Management back-off timer when disconnecting a PDN connection. The UE shall support a separate Session Management back-off timer for every APN that the UE may activate. To avoid that large amounts of UEs initiate deferred requests (almost) simultaneously, the MME should select the Session Management back-off timer value so that deferred requests are not synchronized. The APN based Session Management congestion control is applicable to the NAS ESM signalling initiated from the UE in the control plane. The Session Management congestion control does not prevent the UE to send and receive data or initiate Service Request procedures for activating user plane bearers towards the APN(s) that are under ESM congestion control. 4.3.7.4.2.3 APN based Mobility Management congestion control The MME may perform the APN based congestion control for UEs with a particular subscribed APN by rejecting Attach procedures with a Mobility Management back-off timer. When congestion control is active for UEs with a particular subscribed APN, a Mobility Management back-off timer may be sent by the MME to UE. If MME maintains the UE context, the MME may store the back-off time per UE if a request without the low access priority indicator is rejected by the MME. The MME may immediately reject any subsequent request from the UE before the stored back-off time is expired. NOTE 1: The above functionality is to diminish the performance advantage for UEs that do not support the NAS level back-off timer (e.g. pre-Rel-10 UEs) compared to UEs that do support it. After rejecting Attach Requests, the MME should keep the Subscriber Data for some time. This allows for rejection of subsequent requests without HSS signalling when the congestion situation resulting from UEs with a particular subscribed APN persists. NOTE 2: Prior to the reject of attach messages of a UE by the MME, Subscriber Data for a UE may be present at the MME because it was not deleted after the UE's detach. In this case when APN based congestion control is active for a particular APN in the MME, the first reject of an attach message by the MME for this UE, may be done without HSS signalling as well. While the Mobility Management back-off timer is running, the UE shall not initiate any NAS request for Mobility Management procedures. However, the UE is allowed to initiate the Mobility Management procedures for high priority access and emergency services even when the Mobility Management back-off timer is running. While the Mobility Management back-off timer is running, the UE is allowed to perform Tracking Area Update if it is already in connected mode. While the Mobility Management back-off timer is running, the UE configured with a permission for overriding low access priority is allowed to initiate the Mobility Management procedures without low access priority if the Mobility Management back-off timer was started due to a reject message received in response to a request with low access priority and the upper layers in the UE request to activate a PDN connection without low access priority or the UE has an activated PDN connection that is not with low access priority. To avoid that large amounts of UEs initiate deferred requests (almost) simultaneously, the MME should select the Mobility Management back-off timer value so that deferred requests are not synchronized. NOTE 3: When receiving the Mobility Management back-off timer the UE behaviour is not APN specific. 4.3.7.4.2.4 General NAS level Mobility Management congestion control Under general overload conditions the MME may reject Mobility Management signalling requests from UEs. When a NAS request is rejected, a Mobility Management back-off timer may be sent by the MME and MME may store the back-off time per UE if a request without the low access priority indicator is rejected by the MME and if MME maintains the UE context. The MME may immediately reject any subsequent request from the UE before the stored back-off time is expired. While the Mobility Management back-off timer is running, the UE shall not initiate any NAS request for Mobility Management procedures except for Detach procedure and except for high priority access, emergency services and mobile terminated services. After any such Detach procedure, the back-off timer continues to run. While the Mobility Management back-off timer is running, the UE is allowed to perform Tracking Area Update if it is already in connected mode. If the UE receives a paging request from the MME 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 Tracking Area Update procedure as described in clause 5.3.3.0. While the Mobility Management back-off timer is running, the UE configured with a permission for overriding low access priority is allowed to initiate the Mobility Management procedures without low access priority if the Mobility Management back-off timer was started due to a reject message received in response to a request with low access priority and the upper layers in UE request to establish a PDN connection without low access priority or the UE has an established PDN connection that is without low access priority. While the Mobility Management back-off timer is running, the UE configured with permission for sending exception reporting is allowed to initiate the Control Plane Service Request procedure for exception reporting. If the Mobility Management back-off timer was started due to a reject message received in response to a request for exception reporting, the UE shall not initiate the Control Plane Service Request procedure for exception reporting while the Mobility Management back-off timer is running. The Mobility Management back-off timer shall not impact Cell/RAT and PLMN change. Cell/RAT and TA change do not stop the Mobility Management back-off timer. The Mobility Management back-off timer shall not be a trigger for PLMN reselection. The back-off timer is stopped as defined in TS 24.301[ Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS); Stage 3 ]  [46] when a new PLMN that is not an equivalent PLMN is accessed. To avoid that large amounts of UEs initiate deferred requests (almost) simultaneously, the MME should select the Mobility Management back-off timer value so that the deferred requests are not synchronized. When the UE receives a handover command, the UE shall proceed with the handover procedure regardless of whether Mobility Management back-off timer is running. The MME should not reject Tracking Area Update procedures that are performed when the UE is already in connected mode. For idle mode inter CN node mobility, the MME may reject Tracking Area Update procedures and include a Mobility Management back off timer value in the Tracking Area Reject message. If the MME rejects Tracking Area Update request or Service request with a Mobility Management back-off timer which is larger than the sum of the UE's periodic TAU timer plus the Implicit Detach timer, the MME should adjust the mobile reachable timer and/or Implicit Detach timer such that the MME does not implicitly detach the UE while the Mobility Management back-off timer is running. NOTE: This is to minimize unneeded signalling after the Mobility Management back-off timer expires. 4.3.7.4.2.5 Group specific NAS level congestion control The group specific NAS level congestion control applies to a specific group of UEs. Each group has a group identifier assigned. A UE belongs to a group, if the corresponding group identifier is stored in the UE's subscription data in the HSS. A UE may belong to multiple groups and the MME may perform the Group specific NAS level congestion control to an UE as described below independent of whether Group specific NAS level congestion control is activated for one, multiple, or all groups the UE belongs to. The group identifier shall be stored per UE in the HSS and obtained by the MME as part of normal HSS signalling. A UE is not aware of a group subscription. The group specific NAS level congestion control may be activated for Session Management signalling, or for Mobility Management signalling, or both. The group specific NAS level congestion control is activated based on operator policies. When the group specific NAS level congestion control for Session Management signalling is active for a particular group, the MME's behaviour is similar to that in clause 4.3.7.4.2.2, with the following modifications: - MME may apply ESM congestion control to all subscribed APNs for UEs that belong to this particular group. NOTE: How the MME applies ESM congestion control to all subscribed APNs is left to Stage 3. - The MME rejects the EPS Session Management (ESM) request(s) from the UE belonging to this particular group (e.g. PDN Connectivity, or Bearer Resource Allocation Requests) with a Session Management back-off timer. When group specific NAS level congestion control for Mobility Management signalling is active for a particular group, the MME's behaviour is similar to that in clause 4.3.7.4.2.3, but applied to UEs subscribed to this particular group rather that subscribed to a particular APN. Group specific NAS level congestion control is performed at the MME based on the UE's subscription information provided by the HSS. There is no impact on the UE, and hence, UE's behaviour as described in clauses 4.3.7.4.2.2 and 4.3.7.4.2.3 does not change. 4.3.7.4.2.6 APN and group specific NAS level congestion control The APN and group specific NAS level congestion control is the intersection of APN specific NAS level congestion control and Group specific NAS level congestion control, i.e. it applies to a specific group of UEs with a particular subscribed APN. Each group of UEs has a group identifier assigned and stored in the HSS. A UE may belong to multiple groups and the MME may perform the APN and group specific NAS level congestion control to an UE as described below independent of whether the APN and group specific NAS level congestion control is activated for one, multiple or all groups the UE belongs to. The group identifier(s) shall be stored per UE in the HSS and obtained by the MME as part of normal HSS signalling. A UE is not aware of the group identifier(s) that the UE belongs to. The APN and group specific NAS level congestion control may be activated for Session Management signalling, or for Mobility Management signalling, or both. The APN and group specific NAS level congestion control is activated based on operator policies. When the APN and group specific NAS level congestion control for Session Management signalling is activated for a UE belonging to a particular group and initiating signalling to a particular APN, the MME's behaviour is similar to that in clause 4.3.7.4.2.2, with the following modifications: - The EPS Session Management (ESM) congestion control is applied to this particular APN, and for UEs belonging to this particular group, - The MME may reject ESM requests from the UEs belonging to this particular group and attaching to this particular APN (e.g. PDN Connectivity, or Bearer Resource Allocation Requests) with a Session Management back-off timer. If the UE provides no APN, then the MME uses the APN which is used in PDN GW selection procedure. - The MME may deactivate PDN connections of the UEs, belonging to this particular group and attaching to this particular APN, by sending the NAS Deactivate EPS Bearer Context Request message to the UE with a Session Management back-off timer. When the APN and group specific NAS level congestion control for Mobility Management signalling is activated for a UE belonging to a particular group and with a particular subscribed APN, the MME's behaviour is similar to that in clause 4.3.7.4.2.3, but applied to UEs with this particular subscribed APN and belonging to this particular group. APN and group specific NAS level congestion control is performed at the MME based on the UE's subscription information provided by the HSS. There is no impact on the UE, and hence, UE's behaviour described in clauses 4.3.7.4.2.2 and 4.3.7.4.2.3 does not change. 4.3.7.4.2.7 Control Plane data specific NAS level congestion control Under overload conditions the MME may restrict requests from UEs for data transmission via Control Plane CIoT EPS Optimisation. A Control Plane data back-off timer may be returned by the MME (e.g.in Attach/TAU/RAU 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 EPS Optimisation, i.e. the UE shall not send any Control Plane Service Request with ESM Data Transport message as defined in TS 24.301[ Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS); Stage 3 ]  [46]. The MME shall store the Control Plane data back-off timer per UE and shall reject any further request (other than exception reporting and a response to paging) for data transmission via 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 initiate Control Plane Service Request for exception reporting even if Control Plane data back-off timer is running. The UE may respond with Control Plane Service Request without ESM Data Transport to a paging even if the Control Plane data back-off timer is running. If the MME receives a Control Plane Service Request in reponse to paging, and the MME has a Control Plane data back-off timer running for the UE, and the MME is not overloaded, and MME decides to accept the Control Plane Service Request, then the MME shall respond with Service Accept message without the Control Plane data back-off timer and stop the Control Plane data back-off timer. If the UE receives a Service Accept message without the Control Plane data back-off timer from the MME 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 MME is stopped at PLMN change. If the MME receives a Control Plane Service Request with ESM Data Transport message, and decides to send the UE a Control Plane data back-off timer, the MME may decide to process the Control Plane Service Request with ESM Data Transport message, i.e. decrypt and forward the data payload, or not based on the following: - If the UE has additionally indicated in a NAS Release Assistance Information in the NAS PDU that no further Uplink or Downlink Data transmissions are expected, then the MME may process (integrity check/decipher/forward) the received Control Plane data packet, and send 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 and starts the Control Plane data back-off timer. - For all other cases, the MME may decide to not process the received control plane data packet and sends 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. Then MME may take into consideration whether the PDN Connection is set to Control Plane only to make the decision whether to reject the packet and send SERVICE REJECT or move the PDN connection to user plane and process the data packet. - Alternatively, if UE has not provided in the in Control Plane service request the NAS Release Assistance Information, and the EPS bearer belongs to a PDN connection not set to Control Plane only, and UE supports User Plane CIoT Optimisation (or legacy S1-U), then the MME may initiate establishment of S1-U bearer during Data Transport in Control Plane CIoT EPS Optimisation (according to the procedure defined in clause 5.3.4B.4). In this case MME may also return a Control Plane data back-off timer within the NAS message. The MME only includes the Control Plane data back-off timer if the UE has indicated support for Control Plane data back-off timer in the Attach/TAU/RAU request. NOTE 3: If the MME is overloaded or close to overload, but the UE has not indicated support for Control Plane data back-off timer, the MME can use other overload control mechanisms, e.g. mobility management back-off timer or use user plane data communication.