A Partial Order Reduction Algorithm without the Proviso

This paper presents a partial order reduction algorithm, called Two phase, that preserves stutter free LTL properties. Two phase dramatically reduces the number of states visited compared to previous partial order reduction algorithms on most practical protocols. The reason can be traced to a step of the previous algorithms, called the proviso step, that specifies a condition on how a state that closes a loop is expanded. Two phase avoids this step, and uses a new execution approach to obtain the reductions. Two phase can be easily combined with an on-the-fly model-checking algorithm to reduce the memory requirements further. Furthermore a simple but powerful selective-caching scheme can also be added to Two phase. Two phase has been implemented in a model-checker called PV (Protocol Verifier) and is in routine use on large problems.