ional) UE inactivity monitoring in the gNB-DU. RRC-Container with the DL-DCCH-Message IE, defined in (3GPP TS 38.331). Serving PLMN. gNB-DU UE Aggregate Maximum Bit Rate Uplink. All the F1AP UE context management-related messages are specified in 3GPP TS 38.473, clause 9.2.2, and the detailed definition of the F1 UE Context Setup Request message can be found in 3GPP TS 48.473, clause 9.2.2.1. Multiple RRC containers referenced in 3GPP TS 38.473 are defined in 3GPP TS 38.331. 4.2.3.1.3 RRC Message Transfer, SI and Paging Procedures Since the gNB-CU hosts the RRC protocol functionality, RRC and SI transfer procedures need to be defined, to carry the information to the gNB-DU, which then delivers it to a UE. An example of this has been already shown as part of the mobility procedure above (see Figure 4.2.5), where the procedure was used to transfer the measurement results. In theory, most RRC messages carried on SRBs could be transferred over the F1 interface in 4 NG-RAN Architecture exactly the same manner as the user-plane messages on DRBs. However, since the F1 user plane uses unreliable transport (i.e. GTP-U), 3GPP decided to specify RRC message trans- fer over the F1 control plane, which uses SCTP. This was done for increased reliability control-plane information which is considered more important than the user plane. To this end, the following F1AP messages have been defined: Initial UL RRC Message Transfer DL RRC Message Transfer UL RRC Message Transfer. They carry the RRC-Container IE with an actual RRC message, defined in the RRC speci- fication (3GPP TS 38.331). These are largely transparent to gNB-DU, which simply transmits over the air to the UE upon reception from gNB-CU; however, they are typically carried over the F1 interface along with some supplementary information, for example, SRB ID in the case of DL RRC Message Transfer to indicate to a gNB-DU which SRB shall be used for that RRC message. Despite the fact that according to the functional split architecture RRC resides in the gNB-CU, certain fun