A simple population protocol for fast robust approximate majority

  • Authors:
  • Dana Angluin;James Aspnes;David Eisenstat

  • Affiliations:
  • Yale University, Department of Computer Science;Yale University, Department of Computer Science;Princeton University, Department of Computer Science

  • Venue:
  • DISC'07 Proceedings of the 21st international conference on Distributed Computing
  • Year:
  • 2007

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Abstract

We describe and analyze a 3-state one-way population protocol for approximate majority in the model in which pairs of agents are drawn uniformly at random to interact. Given an initial configuration of x's, y's and blanks that contains at least one non-blank, the goal is for the agents to reach consensus on one of the values x or y. Additionally, the value chosen should be the majority non-blank initial value, provided it exceeds the minority by a sufficient margin. We prove that with high probability n agents reach consensus in O(n log n) interactions and the value chosen is the majority provided that its initial margin is at least ω(√n log n). This protocol has the additional property of tolerating Byzantine behavior in o(√n) of the agents, making it the first known population protocol that tolerates Byzantine agents. Turning to the register machine construction from [2], we apply the 3-state approximate majority protocol and other techniques to speed up the per-step parallel time overhead of the simulation from O(log4 n) to O(log2 n). To increase the robustness of the phase clock at the heart of the register machine, we describe a consensus version of the phase clock and present encouraging simulation results; its analysis remains an open problem.