INCEPT: INcremental ControllEr PlacemenT in Software Defined Networks

In almost a decade, Software Defined Networking (SDN) has transitioned from university laboratories to real-life networks. As networks evolve from legacy networks to hybrid OSPF/SDN networks to pure SDN networks, the associated challenge of appropriate controller placement - i.e. optimal count and location of controllers in an SDN network - need to be suitably addressed. Several studies investigated the controller placement problem at a point in time primarily from a latency minimization approach, in addition to various secondary objectives. However, considering long-term network growth, it is also important to evaluate the best timing to introduce a controller in an SDN network. In this paper, we introduce and formulate the incremental (multi- period) controller placement problem to add controllers in an SDN network in a phased manner over a finite planning horizon. We consider a practical scenario with increasing network traffic, as well as falling capital and operational expenditures of SDN controllers on account of vendor competition and technology maturity. We propose a multi-objective optimization model to derive the optimal multi- period controller roll-out plan. Using extensive simulations, we evaluate our solution in terms of various performance metrics and find that our multi-period placement schedule attains significant cost savings over single-period placement schedule at the cost of a small and acceptable increase in switch-controller latency.