TOP/PDT, a toolkit for the development and the simulation of communication protocols, is presented. A protocol is described by a high-level language, derived from SDL, and named PDL, short for protocol description language. A compiler translates PDL programs into C programs; these are then compiled and linked using the utilities of the host operating system. TOP/PDT is an open environment: to add functionalities or to optimize a protocol, users can add their own functions, usually written in C, to the existing function library. The validation of the protocol is done by simulation. PDL allows the characterization of the data link(s) used to transmit and provides a set of measurement functions to evaluate the protocol performance. The modular structure of TOP/PDT and the use of the C programming language make it easily portable. The executable program generated using TOP/PDT can also run without a host operating system, since the PDT kernel provides a minimal operating system able to handle an unlimited number of processes and ensures their mutual exclusion in the access to shared data.>