The testing of encoding and decoding (aka codec) functions is a fault-prone and time-consuming task. But these functions offer a special opportunity: As the input equals the output, if you encode and decode it successively, it is possible to generate test cases for these functions automatically. These can be used for a systematic regression testing. In this book Dirk Jebing shows an approach to develop a test case generator, which tests codecs for internet protocols in the testing language TTCN-3 (used in the telecommunications industry). This approach can be adopted easily for other languages and codecs. After introducing TTCN-3 the situation for codec testing is analyzed and different strategies, how the test cases should look like, are developed, and which problems may occur by designing them. To minimize the time for discovering faults, ideas for optimizing the generated test suite are presented.The developed strategies are compared empirically by testing a codec for the DHCP protocol. This book is adressed to software developers and testers who want to improve the quality of their software. Furthermore it provides a deep insight in automated regression testing methods.