SDNController

An SDN controller is the software component in software-defined networking that provides the centralized control plane for the network. It communicates with forwarding devices (switches, routers) through southbound interfaces, translates network policies into forwarding rules, and exposes northbound APIs for applications and services to request network behavior. In typical deployments, the controller maintains a global view of the network topology, state, and policies, and uses it to optimize routing, perform traffic engineering, and enforce security.

Controllers can be centralized or distributed. In centralized deployments, a single controller instance makes decisions for all devices; in distributed deployments, state is replicated across multiple controller instances to improve scalability and resilience. Distributed architectures use consensus or data-store synchronization to ensure consistent network state. Controllers often provide additional features such as device management, topology discovery, flow rule installation, telemetry, and policy enforcement. They may also support virtualization, multi-tenancy, and integration with orchestration platforms.

Common southbound protocols and interfaces include OpenFlow, NETCONF, and RESTCONF, while northbound APIs typically include RESTful or gRPC interfaces used by applications and orchestration tools. Open-source controllers such as Ryu, OpenDaylight, and ONOS, as well as commercial offerings, exemplify the range of implementations. SDN controllers are central to many modern networks for dynamic provisioning, rapid reconfiguration, and centralized policy management.