F or the past three years the National Bureau of Standards (NBS) has investigated Open SystemInterconnection (OSI) protocol performance issues. To support this investigation the NBS has developed several transport layer performance tools and has made many performance measurements. This article describes the function and design of a software system and the implementation of a testbed comprising the software and hardware required to evaluate transport layer performance over a local area network.* Further, some performance results are given to demonstrate the utility of the testbed. Other transport layer performance tools, developed by, and measurement projects, conducted at, the NBS are not recounted here, but the appropriate references are included [1,2,6,8,9].