High-speed wide-area networks enable innovative applications that integrate geographically distributed computing, database, graphics, and networking resources. The Message Passing Interface (MPI) can be used as a portable, high-performance programming model for such systems. However, the wide-area environment introduces challenging problems for the MPI implementor, because of the heterogeneity of both the underlying physical infrastructure and the authentication and software environment at

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... rent sites. In this article, we describe an MPI implementation that incorporates solutions to these problems. This implementation, which was developed for the I-WAY distributed-computing experiment, was constructed by layering MPICH on the Nexus multithreaded runtime system. Nexus provides automatic configuration mechanisms that can be used to select and configure authentication, process creation, and communication mechanisms in heterogeneous systems.