Messages flow across an SDN controller’s:
• Messages which help in read/write state of the network and developing flow tables within the state management layer.
• Notifications for the state-change events.
• The interaction between the controller and network control applications is done through the northbound interface.
• Network control applications send messages to the controller.
• Messages which help for the up-to-date view of the network’s state like message for the attached link has gone up or down, new devices are joined the network, or indications of the device is up or down.
• Controller’s southbound interface is the communication among the controller and the controlled devices. Controlled devices are the recipients of the messages sent form the controller.
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