s UE context management procedures In the interest of clarity, this chapter only describes the most important procedures and functionalities. Some procedures, such as Warning Message Transmission procedures, are left out. 4.2 High-Level gNB-CU/DU Split Figure 4.2.3 F1-C protocol stack. (Source: Reproduced Control Plane by permission of © 3GPP). Radio Network Layer Transport Network Layer Data link layer Physical layer RRC message transfer procedures, system information procedures, and paging procedures. As for all other RAN interfaces, F1AP procedures can be class 1 and class 2. The class 1 procedures are the most common and they have a Request followed by a Response or an Error message call flow. Class 2 procedures do not require a response. The F1 interface has been specifically designed to allow gNB-CU deployments in the vir- tualized environment. This is different from LTE in which all the control-plane interfaces always use a single transport network layer (TNL) association (i.e. a single SCTP connection for a single interface instance). F1, on the other hand, can use multiple TNL associations on a single interface instance. This is helpful when gNB-CU is deployed as a virtual machine in, for example, a cloud, where computational resources can be added "on the fly" without disturbing the network operation, even if new resources need additional transport network capacity (e.g. additional transport network interfaces). Even though it is somewhat less likely that a gNB-DU would be virtualized, it can also use multiple SCTP endpoints for a single instance of an F1-C interface. This is not unique to the F1 interface, as a similar mechanism is also defined for NG and Xn interfaces. Full details about the F1AP control-plane protocol can be found in 3GPP TS 38.473. 4.2.3.1.1 Interface Management Procedures Interface management procedures are used to establish an F1 interface, to tear it down, to reset it (when and if needed), and to allow gNB-CU and gNB-DU to exchange and update configuration information (such as th