The emerging trend towards all-IP networks provides network-layer interoperability for the Internet of Things (IoT). Service oriented architecture is expected to provide the application-layer interoperability and hence achieve IoT objectives. The first component into realizing this vision is efficient discovery of things and their services. In this paper, we present a fully distributed opportunistic approach to optimize the discovery of constrained-node services. We also discuss the impact of radio duty cycling on the performance of such a multicast-based approach. Simulation and testbed results have shown the capacity of such an approach to cope with network dynamics and provide high discovery rates, fast response times and low energy consumption.