Shibashis Guha

On decidability of prebisimulation for timed automata

In this paper, we propose an at least as fast as relation between two timed automata states and investigate its decidability. The proposed relation is a prebisimulation and we show that given two processes with rational clock valuations it is decidable whether such a prebisimulation relation exists between them. Though bisimulation relations have been widely studied with respect to timed systems and timed automata, prebisimulations in timed systems form a much lesser studied area and according to our knowledge, this is the first of the kind where we study the decidability of a timed prebisimulation. This prebisimulation has been termed timed performance prebisimulation since it compares the efficiency of two states in terms of their performances in performing actions. if s and t are time abstracted bisimilar and every possible delay by s and its successors is no more than the delays performed by t and its successors where the delays are real numbers. The prebisimilarity defined here falls in between timed and time abstracted bisimilarity.

Reducing Clocks in Timed Automata while Preserving Bisimulation

Model checking timed automata becomes increasingly complex with the increase in the number of clocks. Hence it is desirable that one constructs an automaton with the minimum number of clocks possible. The problem of checking whether there exists a timed automaton with a smaller number of clocks such that the timed language accepted by the original automaton is preserved is known to be undecidable. In this paper, we give a construction, which for any given timed automaton produces a timed bisimilar automaton with the least number of clocks. Further, we show that such an automaton with the minimum possible number of clocks can be constructed in time that is doubly exponential in the number of clocks of the original automaton.

A Unifying Approach to Decide Relations for Timed Automata and their Game Characterization

In this paper we present a unifying approach for deciding various bisimulations, simulation equivalences and preorders between two timed automata states. We propose a zone based method for deciding these relations in which we eliminate an explicit product construction of the region graphs or the zone graphs as in the classical methods. Our method is also generic and can be used to decide several timed relations. We also present a game characterization for these timed relations and show that the game hierarchy reflects the hierarchy of the timed relations. One can obtain an infinite game hierarchy and thus the game characterization further indicates the possibility of defining new timed relations which have not been studied yet. The game characterization also helps us to come up with a formula which encodes the separation between two states that are not timed bisimilar. Such distinguishing formulae can also be generated for many relations other than timed bisimilarity.

LogicFence: A Framework for Enforcing Global Integrity Constraints at Runtime

Large information systems (IS) comprise of several independent applications that share a common set of resources and data. Usually, there are implicit and subtle dependencies across these applications that are not specifically captured. This is especially so if the applications are bought off the shelf or are developed by independent third parties. Dependencies or global semantic constraints are difficult to discern and incorporate into the design of individual software components. Global constraints may change over time and it is usually expensive or infeasible to change individual application logic in every such situation. In order to address such an issue, we propose LogicFence, a framework that accepts a definition of global constraints and translates these constraints into primitives that are embedded into the run-time environments of application programs (currently, into the JVM of Java applications). LogicFence monitors the state of application programs and prevents the disparate instances to collectively form a globally inconsistent state

From Traces to Proofs: Proving Concurrent Programs Safe

Nondeterminism in scheduling is the cardinal reason for difficulty in proving correctness of concurrent programs. A powerful proof strategy was recently proposed [6] to show the correctness of such programs. The approach captured data-flow dependencies among the instructions of an interleaved and error-free execution of threads. These data-flow dependencies were represented by an inductive data-flow graph (iDFG), which, in a nutshell, denotes a set of executions of the concurrent program that gave rise to the discovered data-flow dependencies. The iDFGs were further transformed in to alternative finite automatons (AFAs) in order to utilize efficient automata-theoretic tools to solve the problem. In this paper, we give a novel and efficient algorithm to directly construct AFAs that capture the data-flow dependencies in a concurrent program execution. We implemented the algorithm in a tool called ProofTraPar to prove the correctness of finite state cyclic programs under the sequentially consistent memory model. Our results are encouraging and compare favorably to existing state-of-the-art tools.

Mean-Payoff Games on Timed Automata.

Mean-payoff games on timed automata are played on the infinite weighted graph of configurations of priced timed automata between two players, Player Min and Player Max, by moving a token along the states of the graph to form an infinite run. The goal of Player Min is to minimize the limit average weight of the run, while the goal of the Player Max is the opposite. Brenguier, Cassez, and Raskin recently studied a variation of these games and showed that mean-payoff games are undecidable for timed automata with five or more clocks. We refine this result by proving the undecidability of mean-payoff games with three clocks. On a positive side, we show the decidability of mean-payoff games on one-clock timed automata with binary price-rates. A key contribution of this paper is the application of dynamic programming based proof techniques applied in the context of average reward optimization on an uncountable state and action space.

Revisiting Robustness in Priced Timed Games

Priced timed games are optimal-cost reachability games played between two players---the controller and the environment---by moving a token along the edges of infinite graphs of configurations of priced timed automata. The goal of the controller is to reach a given set of target locations as cheaply as possible, while the goal of the environment is the opposite. Priced timed games are known to be undecidable for timed automata with

Inferring Fences in a Concurrent Program Using SC proof of Correctness

3

Timed Network Games with Clocks.

or more clocks, while they are known to be decidable for automata with

Timed Vacuity

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