 
Summary: OntheFly Analysis of Systems with Unbounded, Lossy
FIFO Channels
Parosh Aziz Abdulla 1 , Ahmed Bouajjani 2 , and Bengt Jonsson 1
1 Dept. of Computer Systems, P.O. Box 325, S751 05 Uppsala, Sweden,
fparosh,bengtg@docs.uu.se
2 VERIMAG, Centre Equation, 2 av. de Vignate 38610 Gieres, France,
Ahmed.Bouajjani@imag.fr
Abstract. We consider symbolic onthefly verification methods for systems of finite
state machines that communicate by exchanging messages via unbounded and lossy FIFO
queues. We propose a novel representation formalism, called simple regular expressions
(SREs), for representing sets of states of protocols with lossy FIFO channels. We show
that the class of languages representable by SREs is exactly the class of downward closed
languages that arise in the analysis of such protocols. We give methods for (i) computing
inclusion between SREs, (ii) an SRE representing the set of states reachable by executing
a single transition in a system, and (iii) an SRE representing the set of states reachable
by an arbitrary number of executions of a control loop of a program. All these operations
are rather simple and can be carried out in polynomial time.
With these techniques, one can straightforwardly construct an algorithm which explores
the set of reachable states of a protocol, in order to check various safety properties. We also
show how one can perform modelchecking of LTL properties, using a standard automata
