This paper deals with testing techniques for communication protocols. Several protocol testing support systems designed for testing protocol layers of the ISO/OSI (International Standards Organisation/Open Systems Interconnection) Reference Model are introduced. These systems are mostly designed for the conformance testing of protocols produced by third parties, which is intended not only to provide test services for both suppliers and users but also to meet the requirements of standards. Several test case generation methodologies are described, and the authors' experience in using one of the methods is presented.