When an application communicates with an SDN controller, which mechanism does it use to communicate?

The northbound interface (NBI) is responsible for allowing communication between applications and the core of the controller. Applications therefore directly communicate with the core through the northbound interface. The southbound interface (SBI) directly communicates with the SDN devices. The core of the controller is the mechanism that connects the NBI to the SBI. The Simple Network Management Protocol (SNMP) is used for the monitoring and collection of device metrics.