Controller role change

The following messages are related to controller role change:

OFPT_ROLE_REQUEST: Message from controller to the switch to change or query its role.
OFPT_ROLE_REPLY: Message sent in response to the OFPT_ROLE_REQUEST, returning the current role of the controller.
OFPT_SET_ASYNC: A controller, through this message can configure what asynchronous message notifications it wants to receive.
OFPT_GET_ASYNC: Controller uses this message to retrieve the asynchronous configuration set using the OFPT_SET_ASYNC message.

NOTE:

Whenever a connection is established between the switch and the controller, each controller starts in the role, OFPCR_ROLE_EQUAL. The controller can query and change its role if necessary.