Model checking of systems with many identical timed processes

Over the last years there has been an increasing research effort directed towards the automatic verification of infinite state systems, such as timed automata, hybrid automata, data-independent systems, relational automata, Petri nets, lossy channel systems, context-free and push-down processes. We present a method for deciding reachability properties of networks of timed processes. Such a network consists of an arbitrary set of identical timed automata, each with a single real-valued clock. Using a standard reduction from safety properties to reachability properties, we can use our algorithm to decide general safety properties of timed networks. To our knowledge, this is the first decidability result concerning verification of systems that are infinite-state in "two dimensions": they contain an arbitrary set of (identical) processes, and they use infinite data-structures, viz real-valued clocks. We illustrate our method by showing how it can be used to automatically verify Fischer's protocol, a timer-based protocol for enforcing mutual exclusion among an arbitrary number of processes.Finally, we show undecidability of the recurrent state problem: given a state in a timed network, check whether there is a computation of the network visiting the state infinitely often. This implies undecidability of model checking for any temporal logic which is sufficiently expressive to encode the recurrent state problem, such as PTL, CTL, etc.