The Southbound interface between the switch and the controller was key to significant SDN possibilities.  However, the Northbound interface between orchestration and management systems with the controller will determine the success OpenFlow achieves in the network.

It is important to recognize that the OpenFlow controller is not necessarily a management platform.  It provides a standard and controlled interface to the data plane.  Without it, the idea of SDN requires communication directly to a network device.  The leading network vendors are currently promoting this approach when they talk about their SDN solutions.  While that meets the definition of “Software Defined Networking”, it is a short-sighted vision of the future network.  It would require every agent that wished to interact with the network to know about every device in the network (or at least those it wishes to control – and how does it know that?).

A controller, that has visibility into the entire network is the central control point.  But, the controller should not become a monolithic everything application.  Instead, it should interface with separate systems providing both updates about network performance and an interface for systems to provide network and security orchestration. These orchestration and management applications are the true brain of an OpenFlow network.  The controller is a middle-man.  And this is why the Northbound API is so critical.

This Northbound API is not defined today.  The ONF has taking the initial steps of creating a discussion group.  While this is short of a full working group to define an API, it is a pragmatic step that is likely to result in active projects such as OpenStack that are moving rapidly to influence the resulting API.  This is likely going to result in the continuation of proprietary API’s for sometime to come, but also result in rapid expansion of the Northbound API.  It may be chaotic, but I think this has the potential of creating a robust API in a short amount of time, which will be critical to the success of OpenFlow.

The possibilities of SDN, in particular OpenFlow, are exciting.  But to see the possible become reality will depend on how quickly a capable and usable Northbound API is developed.