Hierarchical clustering for an efficient controllers' placement in software defined networks

Controllers' placement is one of the most important concerns that emerged recently to solve scalability and reliability issues of SDN networks. The placement efficiency is influenced by both network operator strategy and the supported services' requirements, which makes more complex the decision-making process. In particular, the need to support QoS-constrained services may guide the controllers' placement in a way to ensure services' efficiency while optimizing the underlying infrastructure. In this paper, we propose a model for the placement of network controllers, and we formulate the optimization problem, which is shown to be NP hard. To provide more flexibility and to avoid time-prohibitive calculations, varied heuristics approaches, considering several metrics, need to be proposed. To that purpose, we propose a hierarchical clustering strategy for the controllers' placement allowing to minimize the number of network controllers while reducing the potential disparity of burden between the different controllers. Besides, the algorithm's structure makes it easy to act on other network parameters to improve the reliability of the SDN network. The results show that the proposed algorithm, despite the great diversity of network topologies, achieves better performance than conventional techniques without increasing significantly the number of controllers.