Investigating Optimal Progress Measures for Verification of the WebSocket Protocol

The sweep-line method is a state space reduction technique for memory-efficient on-the-fly verification of concurrent systems. The method relies on a progress measure capturing inherent progress in the system under verification to store only fragments of the state space in memory at a time and thereby reduce peak memory usage. The sweepline method has been applied to many concurrent systems, but the optimality of progress measures in terms of the peak number of states stored has not been investigated. Assessing the optimality of a progress measure is important since memory in most cases is the limiting factor in verification using state spaces. We derive lower bounds for the peak number states and present initial experimental results on near optimal progress measures for verification of the IETF WebSocket protocol.