In the combination of high-speed communication networks and layered protocol implementations, there is a large difference between the performance a user perceives at the application layer and the performance at the physical layer of the network. Still, there exists few general tools to aid a designer in tracing down the performance bottlenecks in the protocol implementations. In this paper we present such a tool which provides an environment where it is easy to develop, specify, and execute communication 'benchmarks', that can be used to compare the performance of different protocol stack implementations running on different hardware and operating systems. The tool measures user-oriented performance parameters such as throughput and response time without relying on special hardware or operating system support. The measurements are specified in a protocol independent specification language, thus different protocols can be compared by using the same measurement specification. The tool has been used to compare the performance of the ISODE OSI (FTAM) stack and the Arpa (FTP/TCP) protocols on Sun, MicroVAX and DS90 machines running UNIX.