5.19 Control Plane Load Control, Congestion and Overload Control

23.5013GPPRelease 18System architecture for the 5G System (5GS)TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

5.19.1 General

In order to ensure that the network functions within 5G System are operating under nominal capacity for providing connectivity and necessary services to the UE. Thus, it supports various measures to guard itself under various operating conditions (e.g. peak operating hour, extreme situations). It includes support for load (re-)balancing, overload control and NAS level congestion control. A 5GC NF is considered to be in overload when it is operating over its nominal capacity resulting in diminished performance (including impacts to handling of incoming and outgoing traffic).

5.19.2 TNLA Load Balancing and TNLA Load Re-Balancing

AMF can support load balancing and re-balancing of TNL associations between 5G-AN and AMF by using mechanisms specified in clause 5.21.1.

5.19.3 AMF Load Balancing

The AMF Load Balancing functionality permits UEs that are entering into an AMF Region/AMF Set to be directed to an appropriate AMF in a manner that achieves load balancing between AMFs. This is achieved by setting a Weight Factor for each AMF, such that the probability of the 5G-AN selecting an AMF is proportional to Weight Factor of the AMF. The Weight Factor is typically set according to the capacity of an AMF node relative to other AMF nodes. The Weight Factor is sent from the AMF to the 5G-AN via NGAP messages (see TS 38.413 [34]).

NOTE 1: An operator may decide to change the Weight Factor after the establishment of NGAP connectivity as a result of changes in the AMF capacities. e.g. a newly installed AMF may be given a very much higher Weight Factor for an initial period of time making it faster to increase its load.

NOTE 2: It is intended that the Weight Factor is NOT changed frequently. e.g. in a mature network, changes on a monthly basis could be anticipated, e.g. due to the addition of 5G-AN or 5GC nodes.

NOTE 3: Weight Factors for AMF Load Balancing are associated with AMF Names.

Load balancing by 5G-AN node is only performed between AMFs that belong to the same AMF set, i.e. AMFs with the same PLMN, AMF Region ID and AMF Set ID value.

The 5G-AN node may have their Load Balancing parameters adjusted (e.g. the Weight Factor is set to zero if all subscribers are to be removed from the AMF, which will route new entrants to other AMFs within an AMF Set).

5.19.4 AMF Load Re-Balancing

The AMF load re-balancing functionality permits cross-section of its subscribers that are registered on an AMF (within an AMF Set) to be moved to another AMF within the same AMF set with minimal impacts on the network and end users. AMF may request some or all of the 5G-AN node(s) to redirect a cross-section of UE(s) returning from CM-IDLE state to be redirected to another AMF within the same AMF set, if the 5G-AN is configured to support this. The AMF may request some or all of the 5G-AN node(s) to redirect the UEs served by one of its GUAMI(s) to a specific target AMF within the same AMF set or to any different AMF within the same AMF set.

When indicating a specific target AMF, the AMF should ensure that the load re-balancing will not cause overload in the target AMF.

NOTE: This requirement can be fulfilled by the AMF itself or by the OAM.

For UE(s) in CM-IDLE state, when UE subsequently returns from CM-IDLE state and the 5G-AN receives an initial NAS message with a 5G S-TMSI or GUAMI pointing to an AMF that requested for redirection, the 5G-AN should select the specific target AMF (provided by the original AMF) or a different AMF from the same AMF set and forward the initial NAS message.

For UE(s) in CONNECTED mode, similar mechanisms for AMF Management can be used to move the UE to another AMF in the same AMF set as described in clause 5.21.2, except that the old AMF deregisters itself from NRF.

The newly selected/target AMF (which is now the serving AMF) will re-assign the GUTI (using its own GUAMI(s)) to the UE(s). It is not expected that the 5G-AN node rejects any request or enables access control restriction when it receives a request for redirection for load control from the connected AMF(s).

When the AMF wants to stop redirection, the AMF can indicate that it can serve all UE(s) in CM-IDLE state to stop the redirection.

NOTE 1: An example use for the AMF load re-balancing functionality is for the AMF to pro-actively re-balance its load prior to reaching overload i.e. to prevent overload situation.

NOTE 2: Typically, AMF Load Re-Balancing is not needed when the AMF becomes overloaded because the Load Balancing function should have ensured that the other AMFs within the AMF Set are similarly overloaded.

5.19.5 AMF Control Of Overload

5.19.5.1 General

The AMF shall contain mechanisms for avoiding and handling overload situations. This includes the following measures:

– N2 overload control that could result in RRC reject, RRC Connection Release and unified access barring.

– NAS congestion control.

5.19.5.2 AMF Overload Control

Under unusual circumstances, if AMF has reached overload situation, the AMF activates NAS level congestion control as specified in Clause 5.19.7 and AMF restricts the load that the 5G-AN node(s) are generating, if the 5G-AN is configured to support overload control. N2 overload control can be achieved by the AMF invoking the N2 overload procedure (see TS 38.300 [27] and TS 38.413 [34]) to all or to a proportion of the 5G-AN nodes with which the AMF has N2 connections. The AMF may include the S-NSSAI(s) in NGAP OVERLOAD START message sent to 5G-AN node(s) to indicate the Network Slice(s) with which NAS signalling is to be restricted. To reflect the amount of load that the AMF wishes to reduce, the AMF can adjust the proportion of 5G-AN nodes which are sent NGAP OVERLOAD START message, and the content of the overload start procedure.

When NGAP OVERLOAD START is sent by multiple AMFs or from the same AMF set in the same PLMN towards the 5G-AN, it should be ensured that the signalling load is evenly distributed within the PLMN and within each AMF set.

A 5G-AN node supports restricting of 5G-AN signalling connection when a signalling connection establishment are attempted by certain UEs (which are registered or attempting to register with the 5GC), as specified in TS 38.331 [28] and TS 36.331 [51]. Additionally, a 5G-AN node provides support for the barring of UEs as described in TS 22.261 [2]. These mechanisms are further specified in TS 38.331 [28] and TS 36.331 [51]. For 3GPP Access Type, the signalling connection establishment attempt includes a RRC Connection Resume procedure from RRC-Inactive.

By sending the NGAP OVERLOAD START message, the AMF can request the 5G-AN node to apply the following behaviour for UEs that the AMF is serving:

a) Restrict 5G-AN signalling connection requests that are not for emergency, not for exception reporting and not for high priority mobile originated services; or

b) Restrict 5G-AN signalling connection requests for uplink NAS signalling transmission to that AMF;

c) Restrict 5G-AN signalling connection requests where the Requested NSSAI at AS layer only includes the indicated S-NSSAI(s) in the NGAP OVERLOAD START message. This applies also to RRC-Inactive Connection Resume procedure where the Allowed NSSAI in the stored UE context in the RAN only includes S-NSSAIs included in the NGAP OVERLOAD START.

d) only permit 5G-AN signalling connection requests for emergency sessions and mobile terminated services for that AMF; or

e) only permit 5G-AN signalling connection requests for high priority sessions, exception reporting and mobile terminated services for that AMF;

The above applies for RRC Connection Establishment procedure and RRC Connection Resume procedures over 3GPP access, as well as for the UE-N3IWF connection establishment over untrusted Non-3GPP access and for the UE-TNGF connection establishment over trusted Non-3GPP access.

The AMF can provide a value that indicates the percentage of connection requests to be restricted in the NGAP OVERLOAD START, and the 5G-AN node may consider this value for congestion control.

When restricting a 5G-AN signalling connection, the 5G-AN indicates to the UE an appropriate wait timer that limits further 5G-AN signalling connection requests until the wait timer expires.

During an overload situation, the AMF should attempt to maintain support for emergency services and for MPS.

When the AMF is recovering, the AMF can either:

– send a NGAP OVERLOAD START message with a new percentage value that permits more connection requests to be successful, or

– send a NGAP OVERLOAD STOP message.

to the same 5G-AN node(s) the NGAP OVERLOAD START was previously sent.

5.19.6 SMF Overload Control

The SMF shall contain mechanisms for avoiding and handling overload situations. This can include the following measures:

– SMF overload control that could result in rejections of NAS requests.

The SMF overload control may be activated by SMF due to congestion situation at SMF e.g. configuration, by a restart or recovery condition of a UPF, or by a partial failure or recovery of a UPF for a particular UPF(s).

Under unusual circumstances, if the SMF has reached overload situation, the SMF activates NAS level congestion control as specified in clause 5.19.7. The SMF may restrict the load that the AMF(s) are generating, if the AMF is configured to enable the overload restriction.

5.19.7 NAS level congestion control

5.19.7.1 General

NAS level congestion control may be applied in general (i.e. for all NAS messages), per DNN, per S-NSSAI, per DNN and S-NSSAI, or for a specific group of UEs.

NAS level congestion control is achieved by providing the UE a back-off time. To avoid that large amounts of UEs initiate deferred requests (almost) simultaneously, the 5GC should select each back-off time value so that the deferred requests are not synchronized. When the UE receives a back-off time, the UE shall not initiate any NAS signalling with regards to the applied congestion control until the back-off timer expires or the UE receives a mobile terminated request from the network, or the UE initiates signalling for emergency services or high priority access.

AMFs and SMFs may apply NAS level congestion control, but should not apply NAS level congestion control for procedures not subject to congestion control.

5.19.7.2 General NAS level congestion control

This clause only applies to NAS Mobility Management congestion control.

Under general overload conditions the AMF may reject NAS messages from UEs using any 5G-AN. When a NAS request is rejected, a Mobility Management back-off time may be sent by the AMF to the UE. While the Mobility Management back-off timer is running, the UE shall not initiate any NAS request except for Deregistration procedure and procedures not subject to congestion control (e.g. high priority access, emergency services) and mobile terminated services. After any such Deregistration procedure, the back-off timer continues to run. While the Mobility Management back-off timer is running, the UE is allowed to perform Mobility Registration Update if the UE is already in CM-CONNECTED state. If the UE receives a paging request or a NAS notification message from the AMF 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 Mobility Registration Update procedure over 3GPP access and/or non-3GPP access as applicable. Over non-3GPP access, if the UE is in CM-IDLE state when the back-off timer is stopped, it shall initiate the UE-triggered Service Request procedure as soon as it switches back to CM-CONNECTED state.

In order to allow the UE to report the PS Data Off status change in PDU Session Modification Request message, the UE behaves as follows while keeping the NAS MM back-off timer running in the UE:

– When the UE is in CM-IDLE state and has not moved out of the Registration Area, the UE is allowed to send a Service Request message with an indication that the message is exempted from NAS congestion control. When the UE is in CM-IDLE mode and has moved out of the Registration Area, the UE is allowed to send a Mobility Registration Update request message, with a Follow-on request, and with an indication that the message is exempted from NAS congestion control.

– When the UE is in CM-CONNECTED state, the UE sends a PDU Session Modification Request with PS Data Off status change carried in UL NAS Transport message with an indication that the message is exempted from NAS congestion control.

When the NAS MM congestion control is activated at AMF, if the UE indicates that the NAS MM message is exempted from NAS congestion control, the AMF shall not reject the NAS MM message and shall forward the NAS SM message to the corresponding SMF with an indication that the NAS SM message was indicated to be exempted from NAS congestion control. The SMF ensures that the NAS SM message is not subject to congestion control otherwise 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 Mobility Management back-off timer shall not impact Cell/RAT/Access Type and PLMN change. Cell/RAT/TA/Access Type change does not stop the Mobility Management back-off timer. The Mobility Management back-off timer shall not be a trigger for PLMN reselection or SNPN reselection. The back-off timer is stopped as defined in TS 24.501 [47] when a new PLMN or SNPN, that is not an equivalent PLMN or is not an equivalent SNPN, is accessed.

To avoid that large amounts of UEs initiate deferred requests (almost) simultaneously, the AMF should select the Mobility Management back-off timer value so that the 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 the NAS MM congestion control timer was running and was unable to initiate MM signalling, the UE defers the related MM signalling until the MM congestion control timer expires and initiates after the expiry of the timer.

In the case of a UE with scheduled communication pattern, the AMF may consider the UE’s communication pattern while selecting a value for the Mobility Management back-off timer so that the UE does not miss its only scheduled communication window.

The AMF should not reject Registration Request message for Mobility Registration Update that are performed when the UE is already in CM-CONNECTED state.

The AMF may reject the Service Request message and a UL NAS Transfer with a Mobility Management back-off time when the UE is already in CM-CONNECTED state. If UE receives a DL NAS Transfer message from the AMF while the Mobility Management back off timer is running, the UE shall stop the Mobility Management back-off timer.

For CM-IDLE state mobility, the AMF may reject Registration Request messages for Mobility Registration Update by including a Mobility Management back off time value in the Registration Reject message.

If UE registered in the same PLMN for 3GPP access and non-3GPP access and receives a Mobility Management back-off time from the AMF, the back-off time (and corresponding start and stop) is applied equally to both 3GPP access and non-3GPP access. If UE registered in different PLMNs for 3GPP access and non-3GPP access respectively and receives a Mobility Management back-off time, the back-off time is only applied to the PLMN that provides the time to the UE.

If the AMF rejects Registration Request messages or Service Request with a Mobility Management back-off time which is larger than the sum of the UE’s Periodic Registration Update timer and the Implicit Deregistration timer, the AMF should adjust the mobile reachable timer and/or Implicit Deregistration timer such that the AMF does not implicitly deregister the UE while the Mobility Management back-off timer is running.

NOTE: This is to minimize signalling after the Mobility Management back-off timer expires.

If the AMF deregisters the UE with an indication of re-registration required, the UE behaviour for handling the back-off timer(s) is as specified in TS 24.501 [47].

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.

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.

5.19.7.5 Group specific NAS level congestion control

The group specific NAS level congestion control applies to a specific group of UEs. Group specific NAS level congestion control is performed at the 5GC only, and it is transparent to UE. The AMF or SMF or both may apply NAS level congestion control for a UE associated to an Internal-Group Identifier (see clause 5.9.7).

NOTE: 5GC logic for Group specific NAS level congestion control is not described in this Release of the specification.

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 reponse 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.