WSN have limitations that make them difficult to apply in critical and large-scale scenarios. The main identified barriers are the difficulty of building the management system, and their lack of flexibility to adapt to policy changes. Software defined networking (SDN) is a viable alternative to solve these limitations. Besides, SDN can bring several other benefits to WSN, allowing, for example, complex network applications to run on the controller. A new approach called software-defined wireless sensor network (SDWSN) is increasing in interest by the research community, and in this paper, we propose a new SDWSN control plane based on the constrained application protocol (CoAP). We provide a comprehensive specification for the control plane, including the communication infrastructure, the control plane protocol, and the basic networking functions in controllers (topology discovery and flow control). We show how the proposed control plan can be implemented in the Contiki operating system and present a preliminary performance evaluation of the overhead introduced by the control plane protocol.