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This paper describes a novel technique for the specification and implementation of layered communication software. The technique is called Parallel Activity Specification Scheme (PASS) and is based on an extended-state machine model of protocol automata. It allows a convenient description of the communication behavior of concurrent systems and semiautomatic generation of programming language code from the specification. The first large-scale experience gained with this technique was in the specification and implementation of an ISO session layer. The code generation process and the embedding of the session code into a portable OSI operating system environment are described in detail.
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