Software Defined Networking (SDN) has been identified as a promising network paradigm for Wireless Sensor Networks (WSN) and the Internet of Things. It is a key tool for enabling Sensing as a Service, which provides infrastructure sharing thus reducing operational costs. While a few proposals on SDN southbound protocols designed for WSN are found in the literature, they lack adequate performance analysis. In this paper, we review ITSDN main features and present a performance evaluation with all the sensing nodes transmitting data periodically. We conducted a number of experiments varying the number of nodes and assessing the impact of flow table maximum capacity. We assessed the metrics of data delivery, data delay, control overhead and energy consumption in order to show the tradeoffs of... using IT-SDN in comparison to the IETF RPL routing protocol. We discuss the main challenges still faced by IT-SDN in larger WSN, and how they could be addressed to make IT-SDN use worthwhile.