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This paper describes the methods used to specify and implement a camplex communicatioas protacd that prwides reliable delivery of data in multicastcapable. packet-switching telecrmmuniatim networks. The protocol. called the Reliable Multicasting Protocol @IMP). was developed inmenta l ly by two ccrnplementary teams using a cmbiiatim of f m a l and infama1 techniques in an attempt to ensure the ca-rectnt~ of the pro tad implementaticn. The &st team. called tbe Design team. initially specified protocol requirement., dsing a variant of S(7R requirements tables and implemented a prototype solutiai. The secmd team. called the VBtV team. devclopeci a state model based on the requirements tabla and derived test cases fran these tables to exercise the implementatim. In a series of iterative steps. be Design team added new functimality to the implementation whife the V&V team kept the state model in lidelity with the impiementatiaa thrcugh testing. Test cases derived from state uansiticm paths in the formal model form& the dialogue between teams during development and served as the vehicles f a keepiog the model and implmentatim in fidelity with cach other. Ttris papu desc-ibes our experiences in developing our process model. details of cur approach. and sane example problems f w d during the development of RMP.