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The definition of IBM's Systems Network Architecture (SNA) has evolved into a specification of a node in the form of a metaimplementation using formal, state-oriented descriptive techniques. This evolution is traced here, and the different formal techniques are described. The culmination of this process has been the development of a PL/I-based programming language, Format and Protocol Language (FAPL), as a descriptive tool. Using FAPL, the architects now define SNA by a programmed meta-implementation of a node. In this form, it is precise, readily accessible to the implementing product designers and programmers, and structurally close to the implementations. The essential features of the meta-implementation and of FAPL are described, along with the implications and advantages of describing the architecture in an executable form. One major benefit, already being realized, is the capability to test the logical consistency and completeness of the executable description itself. The current status of the validation of the executable description and sample results obtained are described.