This paper describes an implementation of generic MPI-2 single sided communications for SUN-MPI. Our implementation is layered on top of point-to-point MPI communications and therefore can be adapted to other MPI implementations. The code is designed to co-exist with other MPI-2 single sided implementations (for example direct use of shared memory) providing a generic fall-back implementation for those communication paths where an optimised single-sided implementation is not available. MPI-2 single sided communications require the transfer of data-type information as well as user data. We describe a type packing and caching mechanism used to optimise the transfer of data-type information. The performance of this implementation is measured in comparison to equivalent point to point operations and the shared memory implementation provided by SUN.