Counterexamples for Timed Probabilistic Reachability

Husain Aljazzar, Holger Hermanns, Stefan Leue

Abstract: The inability to provide counterexamples for the violation of timed probabilistic reachability properties constrains the practical use of CSL model checking for continuous time Markov chains (CTMCs). Counterexamples are essential tools in determining the causes of property violations and are required during debugging. We propose the use of explicit state model checking to determine runs leading into property offending states. Since we are interested in finding paths that carry large amounts of probability mass we employ directed explicit state model checking technology to find such runs using a variety of heuristics guided search algorithms, such as Best First search and Z*. The estimates used in computing the heuristics rely on a uniformisation of the CTMC. We apply our approach to a probabilistic model of the SCSI-2 protocol.