A Network of Workstations (NOW) is emerging as a costeffective solution to high performance computing. However, we need to lower the cost of communicating between the workstations to make this platform viable. With the advent of high-performance networks such as ATM and Myrinet, the physical network is no longer the communication bottleneck. Rather, the major overhead can now be attributed to software. This overhead is a direct result of the cost that a message incurs as it travels through

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... rent protection domains. We can alleviate this problem by allowing protected user-level access directly to the network, thereby eliminating the kernel from the critical path. This paper presents a description of the design, implementation and performance of a protected user-level messaging system over Myrinet, called MU-Net, that can handle multiple application processes concurrently. MU-Net has been implemented on the SUN Solaris 2.5 operating system.