RWAPI is a low-level communication interface designed for clusters of PCs. It has been developed to provide performance to higher applications on a wide variety of architectures. We implemented RWAPI on top of the modular software architecture called GRWA. RWAPI supports Ethernet, InfiniBand and Myrinet network interconnects. This paper introduces RWAPI and the design of its network component on top of both InfiniBand and Myrinet interconnects. We obtained a very low latency and high throughput compared to MPI results.