Southbound APIs are used to communicate between the SDN Controller and the switches and routers of the network. They can be open-source or proprietary. They facilitate control over the network and enable the SDN Controller to dynamically make changes according to real-time demands and needs. OpenFlow and OpFlex are two examples of southbound APIs. OpenFlow, which was developed by the Open Networking Foundation (ONF), is the first and probably most well-known southbound interface. OpenFlow defines the way the SDN Controller should interact with the forwarding plane to make adjustments to the network, so it can better adapt to changing business requirements. With OpenFlow, entries can be added and removed to the internal flow-table of switches and routers to make the network more responsive to real-time traffic demands. OpFlex, which was proposed by Cisco, is another southbound interface that uses a declarative model to communicate policies and state between the controller and the network devices. OpFlex allows the network devices to retain some intelligence and autonomy, while still being managed by the controller. OpFlex is designed to be flexible and extensible, and can support different types of network devices and services. Services running over the network, external application APIs, and applications running over the network are not southbound APIs, as they do not directly communicate between the controller and the network devices. Therefore, the correct answer is B and E. References := Some possible references are:
* What are SDN Southbound APIs? - Where they are used. - SDxCentral
* SDN North-bound and South-bound APIs and Interfaces
* Cisco Application Centric Infrastructure Fundamentals