1-What is the purpose of the service abstraction layer in the OpenDaylight SDN controller?
2-Describe the purpose of two types of OpenFlow messages (of your choosing) that are sent from a controlled device to the controller. 
3- Describe the purpose of two types of Openflow messages (of your choosing) that are sent from the controller to a controlled device.
4- 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 the messages to the controller across the northbound interface?

5- Suppose you wanted to implement a new routing protocol in the SDN control plane. At which layer would you implement that protocol? Explain.
6-What is meant by a control plane that is based on logically centralized control? In such cases, are the data plane and the control plane implemented within the same device or in separate devices? Please explain.

7- when we say the network control and data planes are implemented "monolithically," what do we mean?