Section: 5.3.19 Handling of congestion control for transport of user data via the control plane
Content: The network may activate congestion control for transport of user data via the control plane, as specified in 3GPP TS 23.501[ System architecture for the 5G System (5GS) ]  [8]. If the UE has indicated support for the control plane CIoT 5GS optimizations and the network decides to activate the congestion control for transport of user data via the control plane, the network may include a value for the control plane data back-off timer T3448 in REGISTRATION ACCEPT, SERVICE ACCEPT or SERVICE REJECT message, and shall store an control plane data back-off time on a per UE basis. The UE starts the timer T3448 with the value informed in the message. To avoid that large numbers of UEs simultaneously initiate deferred requests, the network should select the value for the timer T3448 for the informed UEs so that timeouts are not synchronised. The network sends REGISTRATION ACCEPT message or SERVICE ACCEPT message without T3448 value IE to stop the timer T3448 running in the UE as specified in subclause 5.5.1.3.4 and subclause 5.6.1.4. Based on the stored control plane data back-off time for the UE, the network may reject the transfer of user data via the control plane initiated by the UE. While the timer T3448 is running, the UE in 5GMM-IDLE mode does not initiate the transport of user data via the control plane procedure, except if the UE is allowed to use exception data reporting (see the ExceptionDataReportingAllowed leaf of the NAS configuration MO in 3GPP TS 24.368[ Non-Access Stratum (NAS) configuration Management Object (MO) ]  [17] or the USIM file EFNASCONFIG in 3GPP TS 31.102[ Characteristics of the Universal Subscriber Identity Module (USIM) application ]  [22]) and the user data is related to an exceptional event. The UE is allowed: a) to respond to paging with CONTROL PLANE SERVICE REQUEST message without uplink data; or b) to send a CONTROL PLANE SERVICE REQUEST message for emergency services or for emergency services fallback; even if the timer T3448 is running. Upon entering the state 5GMM-DEREGISTERED or a new PLMN which is not equivalent to the PLMN where the UE started the timer T3448, or upon being switched off while the timer T3448 is running, the UE stops the timer T3448.