A Perception-based Fault Model for Single-Round Agreement Algorithms

We propose a new hybrid fault model for clock synchronization and single-round (approximate) agreement in synchronous distributed systems1, which accurately captures both node and link faults. Unlike conventional “global” fault models, which rest upon the total number of faulty nodes in the system, it solely relies upon the number of faults in any two non-faulty nodes’ “perceptions” —conveyed by the messages from all other nodes— of the system. This way, arbitrary node and communication faults, including receiver-caused omission and time/value faults, can be modeled properly. We show that both the consistent broadcast primitive of Srikanth & Toueg and our Fault-Tolerant Interval convergence function (emulating Lundelius & Lynch’s Fault-Tolerant Midpoint algorithm as well) can be analyzed under this model. As far as link faults are concerned, our analysis reveals that both algorithms require 4f`a+2f`s+ 2f`o+1 nodes for tolerating at most f`a, f`s, and f`o asymmetric, symmetric, and omission faults at each receiving node.