. This is the general approach taken in most protocol stack. 3GPP network interface specifications - they are abstract of the transport layer. On the one hand, this abstraction model simplifies standardization and development; however, on the other hand, it makes it easy to overlook transport-related issues when discussing NG-RAN network interfaces. This problem is discussed further in the book in Section 6.6, which is dedicated to the transport network. In contrast to LTE, which also uses the same protocol stack for control-plane network interfaces, NG-RAN supports dynamic addition and removal of multiple SCTP endpoints, which is useful for deployments in virtualized environments. Every control-plane network interface has its own Application Protocol on top of SCTP, which is described in the respective section for each interface further down in the book. 3.3.2.2 User-Plane Protocol Stack All NG-RAN network interfaces (with the exception of E1, described GTP-U in Section 4.4) have a user plane, which uses the same protocol stack, shown in Figure 3.3.3. IPv6 (RFC 2460) The user-plane protocol stack used on NG, Xn, and F1 inter- and/or faces uses IP transport and GPRS Tunneling Protocol User Plane IPv4 (RFC 791) (GTP-U) on top of User Datagram Protocol (UDP)/IP to carry the Data link layer user-plane PDUs between the NG-RAN node and the UPF, or between NG-RAN network nodes. Physical layer GTP-U uses the notion of bearer identified by source GTP-U Tunnel Endpoint Identifier (TEID), destination GTP-U TEID, Figure 3.3.3 NG-U protocol stack. source IP address, and destination IP address. GTP-U bearers (Source: Reproduced can be mapped to NG-RAN bearers or to PDU sessions, depending by permission of © on the interface it is used on. 3GPP). 3.3 NG Radio Access Network GTP-U is used extensively in the core network; however, its usage in RAN requires some additional functionality. For example, since RAN network interfaces may be considered somewhat less reliable (compared with the CN interfaces) and RAN network nodes may h