Para ${}^2$ : parameterized path reduction, acceleration, and SMT for reachability in threshold-guarded distributed algorithms

Automatic verification of threshold-based fault-tolerant distributed algorithms (FTDA) is challenging: FTDAs have multiple parameters that are restricted by arithmetic conditions, the number of processes and faults is parameterized, and the algorithm code is parameterized due to conditions counting the number of received messages. Recently, we introduced a technique that first applies data and counter abstraction and then runs bounded model checking (BMC). Given an FTDA, our technique computes an upper bound on the diameter of the system. This makes BMC complete for reachability properties: it always finds a counterexample, if there is an actual error. To verify state-of-the-art FTDAs, further improvement is needed. In contrast to encoding bounded executions of a counter system over an abstract finite domain in SAT, in this paper, we encode bounded executions over integer counters in SMT. In addition, we introduce a new form of reduction that exploits acceleration and the structure of the FTDAs. This aggressively prunes the execution space to be explored by the solver. In this way, we verified safety of seven FTDAs that were out of reach before.