Software-defined networking (SDN) is a new way of building and operating communications networks that enable better end-to-end control, automation and service agility. SDN evolved from the classic wide area network (WAN) model, in which the control and data planes are integrated in a network element, but they are now physically separated in the current SDN (see Figure 1). The control plane function resides in a centralized platform called the SDN controller that is equipped with an open, northbound application programming interface (API). Hence, the network is programmable, allowing other applications to programmatically control and monitor the network using the controller.