The Role of the Inter-Controller Consensus in the Placement of Distributed SDN Controllers

We consider a distributed Software Defined Networking (SDN) architecture adopting a cluster of controllers to improve network scalability and reliability. Different from previous works that focused solely on the control traffic exchanged between controllers and switches, we additionally consider the control traffic exchanged among the controllers (e.g., the control traffic to keep the shared data structures synchronized). We advocate a careful placement of the controllers, which should take into account both the two kinds of control traffic. We evaluate, for some real ISP network topologies, the delay tradeoffs for the controller placement problem and we propose a novel evolutionary algorithm to find the corresponding Pareto frontier. Furthermore, we develop a simple model to estimate the reaction time perceived by the switches, which is accurately validated in an operational Software Defined WAN (SDWAN). We also formalize the optimization problem to minimize the reaction time and devise a novel approximation algorithm, whose performance is assessed against the optimal solver in real ISP network topologies. Our work provides novel quantitative tools to optimize the planning and the design of the network supporting the control plane of SDN networks, especially when the network is very large and in-band control plane is adopted. We also show that for operationally distributed controllers (e.g., OpenDaylight and ONOS), the location of the controller that acts as leader in the consensus algorithm has a strong impact on the reactivity perceived by the switches.