OpenFlow

OpenFlow is a protocol used in software-defined networking to enable a controller to interact with the forwarding plane of switches and routers. It provides a standardized interface for programming network behavior, allowing the control plane to be centralized while the data plane handles forwarding. OpenFlow switches use flow tables to decide how packets are processed.

The project originated at Stanford University's Clean Slate Initiative. The Open Networking Foundation (ONF) formed later to promote the standard. Since its early versions, OpenFlow has evolved through several revisions, expanding features, security considerations, and interoperability. It has been influential in research networks and informed many commercial SDN implementations.

In an OpenFlow network, a central controller installs flow entries on switches. Each entry specifies match fields (such as Ethernet, IP, and ports) and actions (e.g., forward to a port, modify fields). If a packet doesn't match, the switch sends a Packet-In message to the controller, which may respond with new flow entries. Controllers may use meters and group tables for QoS and complex forwarding.

OpenFlow is a southbound interface within the SDN architecture, separating control and data planes. Features vary by version, including match fields, actions, and QoS controls. Limitations include version fragmentation, reliance on a central controller, scalability and reliability concerns, and security risks if the controller is compromised. Some vendors extend OpenFlow with proprietary features.

Today, OpenFlow remains foundational in SDN research and in some production environments. Many deployments also use alternative southbound interfaces or vendor-specific APIs. The ONF continues to promote interoperability and evolve SDN standards.