Structuring and Verifying Distributed Algorithms

We present a structuring and verification method for distributed algorithms. The basic idea is that an algorithm to be verified is stepwise transformed into a high level specification through a number of steps, so-called coarsenings. At each step some mechanism of the algorithm is identified and removed while the basic computation of the original algorithm is preserved. Only the essential parts of the algorithm are then left for verification. The method is based on a program development technique called superposition and it is formalized within the refinement calculus. We will show the usefulness of the method by verifying a complex distributed algorithm for minimum-hop route maintenance due to Chu.