In the SDN architecture we are talking about a controller that communicate with applications using the northbound API and with the infrastructure layer with southbound API.
what is the difference between southbound API , southbound interfaces or southbound protocols, which is more correct
“Southbound API” is the interface between the control and data planes. So they should be Vendor-specific interfaces, the most commonly Southbound API is Open-flow.
“Northbound API” is the interface between the control and application planes. In fact, the Northbound interface enables applications to access control plane functions and services without needing to know the details of the underlying network switches. Northbound interface is more typically viewed as a software API rather than a protocol. At last, unlike Southbound API there is no widely accepted standard.
Recommend to refer - “Fundamental of Modern Networking” by William Stallings.