Multidimensional approximate agreement in Byzantine asynchronous systems

  • Authors:

  • Hammurabi Mendes;Maurice Herlihy

  • Affiliations:

  • Brown University, Providence, RI, USA;Brown University, Providence, RI, USA

  • Venue:
  • Proceedings of the forty-fifth annual ACM symposium on Theory of computing
  • Year:
  • 2013

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Abstract

The problem of ε-approximate agreement in Byzantine asynchronous systems is well-understood when all values lie on the real line. In this paper, we generalize the problem to consider values that lie in Rm, for m ≥ 1, and present an optimal protocol in regard to fault tolerance. Our scenario is the following. Processes start with values in Rm, for m ≥ 1, and communicate via message-passing. The system is asynchronous: there is no upper bound on processes' relative speeds or on message delay. Some faulty processes can display arbitrarily malicious (i.e. Byzantine) behavior. Non-faulty processes must decide on values that are: (1) in Rm; (2) within distance ε of each other; and (3) in the convex hull of the non-faulty processes' inputs. We give an algorithm with a matching lower bound on fault tolerance: we require n t(m+2), where n is the number of processes, t is the number of Byzantine processes, and input and output values reside in Rm. Non-faulty processes send O(n2 d log(m/ε max{δ(d): 1 ≤ d ≤ m})) messages in total, where δ(d) is the range of non-faulty inputs projected at coordinate d. The Byzantine processes do not affect the algorithm's running time.