Which plane is centralized by an SDN controller?

The correct answer is the control plane. In Software-Defined Networking (SDN), the control plane, responsible for making forwarding decisions and managing network resources, is centralized and abstracted from the individual network devices. The SDN controller acts as the brain, providing a unified view and programmable interface for the entire network. This separation allows for greater flexibility, automation, and centralized management. The data plane (or forwarding plane) remains distributed across the network devices, handling the actual forwarding of packets based on instructions from the controller. The management plane deals with administrative tasks like monitoring and configuration, which can be integrated with or orchestrated by the SDN controller but isn't the plane that is centralized by the controller itself. There is no universally recognized "services plane" in the context of SDN architecture.