What types of messages flow across an SDN controller’s northbound and southbound APIs? Who is the recipient of these messages sent from the controller across the southbound interface, and who sends messages to the controller across the northbound interface?

Answer and Explanation:

Messages flow across an SDN controller's:  

Northbound APIs:  

• 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.  

Southbound APIs:  

• 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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