Symbolic Verification of Communication Protocols with Infinite State Spaces Using QDDs (Extended Abstract)

We study the veriication of properties of communication protocols modeled by a nite set of nite-state machines that communicate by exchanging messages via unbounded FIFO queues. It is well-known that most interesting veriication problems, such as deadlock detection, are undecidable for this class of systems. However, in practice, these veriication problems may very well turn out to be decidable for a subclass containing most \real" protocols. Motivated by this optimistic (and, we claim, realistic) observation, we present an algorithm that may construct a nite and exact representation of the state space of a communication protocol, even if this state space is innnite. Our algorithm performs a loop-rst search in the state space of the protocol being analyzed. A loop-rst search is a search technique that attempts to explore rst the results of successive executions of loops in the protocol description (code). A new data structure named Queue-content Decision Diagram (QDD) is introduced for representing (possibly innnite) sets of queue-contents. Operations for manipulating QDDs during a loop-rst search are presented. A loop-rst search using QDDs has been implemented, and experiments on several communication protocols with innnite state spaces have been performed. For these examples, our tool completed its search, and produced a nite symbolic representation for these innnite state spaces.