Bowman, H. and Gomez, R.S. and Su, Li
How to stop time stopping (preliminary version).
University of Kent, Canterbury, Kent, CT2 7NF, UK
(Full text available)
Timed automata are a very successful notation for specifying and verifying real-time systems. One problem of the approach though is that timelocks can freely arise. These are counter-intuitive situations in which a specifier's description of a component automaton can inadvertently prevent time from passing beyond a certain point. This means, in fact, that the entire system stops. We identify a number of different types of timelocks and argue that each type should be treated differently. We distinguish between time-actionlocks and zeno-timelocks and argue that a constructive approach should be applied to preventing the former of these, while an analytical approach should be used to prevent the latter. In accordance with this position, we present a revision of the interpretation of parallel composition in timed automata in order to prevent time-actionlocks. With respect to zeno-timelocks, we present an analytical method to ensure absence of zeno-timelocks which builds upon the notion of strong non-zenoness introduced by Tripakis. We show how Tripakis' results can be extended, broadening the class of timed automata specifications which can be guaranteed to be free from zeno-timelocks. Moreover, we present a tool that we have developed which implements this syntactic verification on UPPAAL-like timed automata specifications. Also, new syntactic properties, in the spirit of strong non-zenoness, are presented which also ensure zeno-timelock freedom. Finally, we illustrate the use of the tool on a real-life case study, the CSMA/CD protocol.
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