This chapter illustrates the use of Z in capturing certain aspects of the state information relevant to a network protocol. Its aim is both to illustrate the applicability of formal specification to realistic computer systems and to demonstrate the use of a formal framework for design. The description covers an abstract design of a network protocol, along the lines of the seven layer network protocol model proposed by the International Standards Organisation (ISO) for Open System Interconnection (OSI). The design is conducted by a kind of refinement, starting from a high level specification. An alternative way of looking at each refinement is as a different abstraction of the protocol. The chapter also shows the kind of proofs that may be required in such refinements and how these may be accomplished.
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