The decoupled control and data plane architecture of Software-Defined Networking (SDN), as depicted in the following diagram, and in particular OpenFlow can be compared with an operating system and computer hardware. The OpenFlow controller (similar to the operating system) provides a programmatic interface to the OpenFlow switches (similar to computer hardware). Using this programmatic interface, network applications, referred to as Net Apps, can be written to perform control and management tasks and offer new functionalities. The control plane in SDN and OpenFlow in particular is logically centralized, and Net Apps are written as if the network is a single system.
With a reactive control model, the OpenFlow switches must consult an OpenFlow controller each time a decision must be made, such as when a new packet flow reaches an OpenFlow switch (that is, packet_in event). In the case of flow-based control granularity, there will be a small performance...