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.

Supporting end-to-end Quality of Service (QoS) in existing network architectures is an ongoing problem. Although researchers from both academia and industry have proposed many solutions to solve the QoS limitations of the current networking, many of them either failed or were not implemented. Software Defined Networking (SDN) paradigm has emerged in response to limitations of traditional networking architectures. Its main advantages are the centralized global network view, programmability, and separation of the data plane and control plane. These features have got attention of researchers to improve the QoS provisioning of today's various network applications. In this survey paper, we aim at making a picture of QoS-motivated literature in OpenFlow-enabled SDN networks by comprehensively surveying relevant research studies. We organize the related studies according to the categories that are the most prominent ways in which QoS can benefit from the concept of SDN: Multimedia flows routing mechanisms, inter-domain routing mechanisms, resource reservation mechanisms, queue management and scheduling mechanisms, Quality of Experience (QoE)-aware mechanisms, network monitoring mechanisms, and other QoS-centric mechanisms such as virtualization-based QoS provisioning and QoS policy management etc. In addition, we discuss QoS capabilities of OpenFlow protocol by reviewing its versions along with some well-known, open-source, and community-driven controller projects. Furthermore, we outline the potential challenges and open problems that need to be addressed further for better and complete QoS abilities in SDN/OpenFlow networks and lessons we have learned during preparation of this survey paper.

All new networking architectures come with their own problems. Software Defined Networking (SDN) has its own challenges which are needed to be addressed by researchers as well. One of the crucial problems with SDN is the control plane scalability since it is a bottleneck for its evolution. As the network grows, the number of messages a controller receives also increases. This increase puts the controller scalability in the heart of problems of SDN. In this paper, we propose a hierarchy-based network architecture along with an inter-AS routing approach with QoS. We exploit idea of levels in which networks with controllers reside and a main controller, which works like a broker, is on top of them to keep the global network state and view. Our experiment results indicate that a controller in a hierarchic setting handles 50% less number of traffic than a controller in a non-hierarchic environment.