Checking Timed Bisimulation with Bounded Zone-History Graphs -- Technical Report

Abstract

Timed automata (TA) are a well-established formalism for discrete-state/continuous-time behaviors of time-critical reactive systems. Concerning the fundamental analysis problem of comparing a candidate implementation against a specification, both given as TA, it has been shown that timed trace equivalence is undecidable, whereas timed bisimulation equivalence is decidable. The corresponding proof utilizes region graphs, a finite, but very space-consuming characterization of TA semantics. In practice, most TA tools use zone graphs instead, a symbolic and generally more efficient representation of TA semantics, to automate analysis tasks. However, zone graphs only produce sound results for analysis tasks being reducible to plain reachability problems thus being too imprecise for checking timed bisimilarity. To the best of our knowledge, no practical tool is currently available for automated timed bisimilarity-checking. In this paper, we propose bounded zone-history graphs, a novel characterization of TA semantics facilitating an adjustable trade-off between precision and scalability of timed-bisimilarity checking. Our approach supports non-deterministic timed automata with silent moves. We further present experimental results gained from applying our tool TIMBRCHECK to a collection of community benchmarks, providing insights into trade-offs between precision and efficiency.