On the placement of controllers in software-defined networks

The software-defined network (SDN) approach assumes a logically centralized control plane, which is physically decoupled from the data plane. Since this approach simplifies network management and speeds up network innovations, these benefits have led not just to production-intent prototypes, but real SDN deployments. For wide-area SDN deployments, multiple controllers are often required, and the placement of these controllers influences every aspect of an SDN. Since optimizing every metric is generally an non-deterministic polynomial (NP)-hard problem, it is of great importance to find an efficient controller placement algorithm. In this paper, we develop several placement algorithms to make informed placement decisions, which can be used to maximize the reliability of SDN, since network failures could easily cause disconnections between the control and forwarding planes. These algorithms are evaluated using real topologies. Simulation results show that the controller placement strategies are crucial to SDN performance, and a greedy algorithm provides placement solutions that are close to optimal.