Verification of Distributed Protocols: Decidable Modeling and Invariant Inference

Verification of distributed protocols and systems, where both the number of nodes in the systems and the state-space of each node are unbounded, is a long-standing research goal. In recent years, efforts around the Ivy verification tool [1]-[4] have pushed a strategy of modeling distributed protocols and systems in a new way that enables decidable deductive verification [5]-[8], i.e., given a candidate inductive invariant, it is possible to automatically check if it is inductive, and to produce a finite counterexample to induction in case it is not inductive. Complex protocols require quantifiers in both models and their invariants, including forall-exists quantifier alternations. Still, it is possible to obtain decidability by enforcing a stratification structure on quantifier alternations, often achieved using modular decomposition techniques, which are supported by Ivy. Stratified quantifiers lead not only to theoretical decidability, but to reliably good solver performance in practice, which is in contrast to the typical instability of SMT solvers over formulas with complex quantification.