Message-passing algorithms (MPAs) are an algorithmic paradigm for the following generic problem: given a system consisting of several interacting components, compute a new version of each component representing its behaviour inside the system. MPAs avoid computing the full state space by propagating messages along the edges of the system interaction graph. We present an MPA for verifying local properties of distributed protocols with a tree communication structure. We report on an implementation, and validate it by means of two case studies, including an analysis of the PGM protocol.