Answer;
-Northbound APIs
Communication between SDN controllers and routers is governed by Northbound APIs.
Step-by-step explanation;
-The northbound application programming interface (API) on an SDN controller enables applications and orchestration systems to program the network and request services from it.
-Northbound APIs can enable network functions like path computation, loop avoidance, routing and security. The northbound APIs can also used to facilitate innovation and enable efficient orchestration and automation of the network to align with the needs of different applications via SDN network programmability.