Scalability and Resilience of Software-Defined Networking: An Overview
  Software-Defined Networking (SDN) allows to control the available network resources by an intelligent and centralized authority in order to optimize traffic flows in a flexible manner. However, centralized control may face scalability issues when the network size or the number of traffic flows increases. Also, a centralized controller may form a single point of failure, thereby affecting the network resilience.   This article provides an overview of SDN that focuses on (1) scalability concerning the increased control overhead faced by a central controller, and (2) resiliency in terms of protection against controller failure, network topology failure and security in terms of malicious attacks. 
Benjamin J. van Asten, Niels L. M. van Adrichem and Fernando A.   Kuipers
cs.NI