Formalized Data Flow Diagrams (FDFD's) provide simple and natural abstractions for specifying network behavior. We have augmented FDFD's by providing (i) a notion of asynchronous timing of events, (ii) a convenient mechanism for describing "node classes", and (iii) features for specifying network architecture. These extensions will facilitate the modeling of a large computer network (using the langauge NET-SPECS) and the direct generation of a simulator of that network.