cess control to mobility management can be viewed as embedded network applications. Applications offered to verticals, e.g. to customize the data plane via the north-bound interface are not there yet but worth further investigation. Having said that, despite the fact that E1 makes it possible to apply the SDN concept to NG-RAN, there are still certain obstacles that need to be overcome by implementations. The TCP/IP protocol suite is relatively simple compared with 3GPP protocols, and therefore the design of a south-bound interface such as OpenFlow is simpler compared with the E1 interface specified by 3GPP. For example, the E1-AP Bearer Context Modification Request message can consist of a large number of IEs, while an OpenFlow message typically has fewer fields. OpenFlow essentially creates a two-phase pipeline for each input packet, that is, match and action. Match specifies which fields of the header, for example, Ethernet, VLAN, or IP shall be examined, and action specifies what needs to be done, for example, output, push/pop a VLAN tag, or increase/decrease TTL once a rule or multiple rules have been matched. As of OpenFlow 1.5, roughly 40 fields and 30 actions are defined. In con- clusion, E1 does not directly map to OpenFlow, and therefore 3GPP control-user plane separation is not an implementation of SDN as such, but more of a concept alignment, which is illustrated by Figure 4.4.7. 4.4.3.4 Relation to 5GC As mentioned above, the principle of control- and user-plane separation in 3GPP originated in the core network domain. In the initial 4G EPC design, some control functions, including 4.4 Control-User Plane Separation Network Application(s) Open northbound API Controller Platform gNB-CU-CP Open southbound API gNB-CU-UP gNB-CU-UP Network Infrastructure NG-RAN w/ CP/UP separation Figure 4.4.7 Mapping of gNB-CU-CP and gNB-CU-UP to the elements of SDN. those related to connection management and bearer management, have been separated to form a control node, that is, MME. However, some other EPC user-plane ne