Northbound and southbound APIs are two types of interfaces that enable communication between different layers of the SDN architecture. Northbound APIs relay information between the controller and the applications or policy engines, while southbound APIs relay information between the controller and the network devices.

Northbound APIs allow applications to request network services or resources from the controller, such as bandwidth, latency, security, or routing. The controller then translates these requests into network configurations and applies them to the network devices via the southbound APIs. Northbound APIs typically use RESTful API methods such as GET, POST, and DELETE to communicate with the controller.

Southbound APIs allow the controller to program the network devices to perform forwarding and other functions. The controller can use different protocols or standards to communicate with the network devices, depending on their capabilities and vendor-specific features. Some common examples of southbound APIs are CLI, SNMP, RESTCONF, NETCONF, OpenFlow, and OpFlex.