SDN

Software-defined networking (SDN) is a network architecture that separates the control plane, which makes decisions about traffic forwarding, from the data plane, which actually forwards packets. This separation enables centralized, software-based control of the network and makes it programmable, adaptable, and easier to manage.

In SDN, the control plane is implemented in one or more software controllers that communicate with network devices through southbound interfaces such as OpenFlow, NETCONF, or RESTful APIs. The devices act as forwarding elements. Applications and orchestration layers use northbound APIs to request network behavior, define policies, and automate provisioning. The resulting architecture typically includes the data plane (switches and routers), the control plane (SDN controllers), and the application or management plane (network apps and orchestrators).

Common SDN components and projects include OpenDaylight, ONOS, and Ryu, with OpenFlow being a well-known southbound protocol. Open standards and open-source initiatives underpin interoperability, while vendors offer various controller ecosystems and campus, data center, and cloud-grade implementations. SDN is often deployed alongside related paradigms such as network functions virtualization (NFV) and software-defined WAN (SD-WAN) to enable centralized policy, automation, and scalable multi-vendor environments.

History and scope: the term and core concepts emerged from late-2000s research, notably around the OpenFlow protocol and the work of the Open Networking Foundation (ONF). SDN has become common in data centers, enterprise campuses, and modern WANs, where centralized control facilitates rapid provisioning and flexible traffic engineering.

Benefits and challenges: SDN can increase agility, improve resource utilization, and simplify network management. Challenges include ensuring controller scalability and reliability, securing the control plane, migrating from legacy networks, and navigating evolving standards to avoid vendor lock-in.