Standardized data communication procedures facilitate the economical exchange of information between Computer systems interconnected by data networks. These procedures are necessarily complex to support a variety of application with a wide range of network characteristics. Specification of these standards using informal techniques can lead to ambiguous interpretation and consequently, incompatible or inefficient implementations. Formal languages are required for more effective specification of future data communication standards by increasing the degree of accuracy, readability, and comprehension. This paper gives an introduction to the revised Specification and Description Language which is a candidate language for protocol specification in CCTTT Recommendations. The relationships between the key concepts in the Specification and Description Language and the Open Systems Interconnection Reference Model are explained and the benefits of the application of the language to the specification of the class 0 transport protocol are illustrated.