Refers to the general SND controller architecture described in figure 2.1, what types of messages flow across an SDN controllers northbound and southbound APIs? Who is the recipient of these messages sent from the controller across the southbound interface, and who sends messages to the controller across the northbound interface?

Network-control Applications Routing Access Control Load Balancer Control plane Northbound API SDN Controller (network operating system) Southbound API Data plane SDN-Controlled Switches Figure 2.1 General SDN controller architecture