At its core, the OpenFlow protocol specifies how the switch and the controller should communicate with each other. When the switch initially boots up, it registers itself with the controller. In most cases, the controller IP is specified to the switch either during the bootup process or configured on the switch out of band via management. The switch will then initiate the channel to communicate with the controller:
OpenFlow switch to controller communication (source: http://archive.openflow.org/documents/openflow-spec-v1.1.0.pdf)
Following the bootup, the switch will communicate with the controller via several types of messages; ...
