How flexible is dynamic SDN control plane?

In Software Defined Networking (SDN), for performance reasons the control plane consists of multiple SDN controllers that provide a logically centralized network control. To react to traffic changes, the control plane may adapt to improve the performance of the whole network. This adaptation includes the migration of controllers, i.e. the change of their placement, as well as the change of switch to controller assignments. We call such adaptive network control a dynamic SDN control plane. Whereas most state of the art focuses on an optimal placement of controllers considering static traffic demands, we draw the attention to the need for a dynamic control plane and the performance of the adaption process itself. The involved controller migration and switch re-assignments significantly affect the control plane performance. Moreover, in this work we evaluate the flexibility of a dynamic SDN control plane with respect to the number of controllers. In particular, we refer to a flexibility metric that takes the time into account that is needed for successful migrations to handle traffic changes. Our evaluations are based on detailed models of controller placement and migration time. The results confirm that the migration time is a critical parameter and reveal that in case only a short migration time is allowed, the number of controllers does not bring more flexibility. For relaxed migration times, a larger number of controllers leads to more flexibility as expected. The results also show that a dynamic adaptation to traffic changes, can provide an improvement of up to 60% over a non-dynamic SDN control plane.