Resilient Controller Placement in Hybrid SDN/Legacy Networks

Given the rate at which communication technologies and protocols evolve, network operators are often cautious of fully migrating to a new technology like Software Defined Networking (SDN) at one go, and prefer to do so in phases. Consequently, the number of SDN switches and in turn the amount of SDN control traffic in the network increases gradually. This control traffic processing occurs at the SDN controller(s), which are, hence, required incrementally in such hybrid SDN/legacy networks. The placement of these controllers significantly affects several aspects such as control latency, resiliency, and load balancing. All existing controller placement strategies place controller(s) in a pure SDN network at a point in time, which is contrary to the SDN predominant migration scenario described above. Even when suitably adapted for hybrid networks, the existing controller placement strategies lead to inefficient placements. In this paper, we introduce and formulate the controller placement problem for hybrid SDN/legacy networks over a period of time, with an aim to maximize the switch-controller control channel resilience. We consider an SDN migration trajectory and deduce a resilient controller placement schedule using an optimization approach. Based on 138 real network topologies, we comprehensively evaluate our approach against a well-known existing resilient controller placement strategy (suitably adapted herein for hybrid SDN/legacy networks for a fair comparison), and demonstrate that our approach is more effective with up to 77% higher resiliency, while requiring up to 33% fewer controllers.