Overview
OpenFlow was the first standard southbound interface for SDN. It allows an SDN controller to directly interact with the forwarding plane of network devices, regardless of the manufacturer.
How it Works
OpenFlow allows the controller to define 'flows' in a switch's flow table. A flow consists of match fields (e.g., source IP, destination port), a priority, and actions (e.g., forward to port 3, drop, modify header).
Importance
OpenFlow enabled the decoupling of control and data planes, fostering innovation by allowing researchers and engineers to experiment with new protocols and routing algorithms without needing to change hardware.