The Power of QDDs (Extended Abstract)

Queue-content Decision Diagrams (QDDs) are finite-automaton based data structures for representing (possibly infinite) sets of contents of a finite collection of unbounded FIFO queues. Their intended use is to serve as a symbolic representation of the possible queue contents that can occur in the state space of a protocol modeled by finite-state machines communicating through unbounded queues. This is done with the help of a loop-first search, a state-space exploration technique that attempts whenever possible to compute symbolically the effect of repeatedly executing a loop any number of times, making it possible to analyze protocols with infinite state spaces though without the guarantee of termination. This paper first solves a key problem concerning the use of QDDs in this context: it precisely characterizes when, and shows how, the operations required by a loop-first search can be applied to QDDs. Then, it addresses the problem of exploiting QDDs and loop-first searches to broaden the range of properties that can be checked from simple state reachability to temporal logic. Finally, a sufficient criterion for the termination of a loop-first search using QDDs is given.

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