A survey: Control plane scalability issues and approaches in Software-Defined Networking (SDN)

Software-Defined Networking (SDN) architecture has emerged in response to limitations of traditional networking architectures in satisfying today's complex networking needs. In particular, SDN allows network administrators to manage network services through abstraction of lower-level functionality. However, SDN is a logically centralized technology. Therefore, scalability, and especially the control plane (i.e. controller) scalability in SDN is one of the problems that needs more attention. In this survey paper, we first discuss the scalability problems of controller(s) in an SDN architecture. We then comprehensively survey and summarize the characterizations and taxonomy of state-of-the-art studies in SDN control plane scalability. We organize the discussion on control plane scalability into two broad approaches: Topology-related approaches and Mechanisms-related approaches. In Topology-related approaches, we study the relation between topology of architectures and scalability issues. It has sub-categories of Centralized (Single) Controller Designs and Distributed approaches. Distributed approaches, in turn, have also sub-categories: Distributed (Flat) Controller Designs, Hierarchical Controller Designs, and Hybrid Designs. In Mechanisms-related approaches, we review the relation between various mechanisms used to optimize controllers and scalability issues. It has sub-categories of Parallelism-based Optimization and Control Plane Routing Scheme-based Optimization. Furthermore, we outline the potential challenges and open problems that need to be addressed further for more scalable SDN control planes.