Communication complexity of consensus in anonymous message passing systems

  • Authors:
  • Emanuele G. Fusco

  • Affiliations:
  • Computer Science Department, Sapienza University of Rome, Rome, Italy

  • Venue:
  • OPODIS'11 Proceedings of the 15th international conference on Principles of Distributed Systems
  • Year:
  • 2011

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Abstract

We consider the message complexity of achieving consensus in synchronous anonymous message passing systems. Unlabeled processors (nodes) communicate through links of a network. In each round every processor can exchange messages with all neighbors and the duration of each transmission is one round. An adversary wakes up some subset of processors at possibly different times and assigns them arbitrary numerical input values. All other processors are dormant and do not have input values. Any message wakes up a dormant processor. The goal of consensus is to wake up all processors and have them agree on one of the input values. We seek deterministic consensus algorithms using as few messages as possible. As opposed to most of the literature on consensus, the difficulty of our scenario are not faults (we assume that the network is fault-free) but the arbitrary network topology combined with the anonymity of nodes. For unknown n-node networks we show a consensus algorithm using O(n2) messages; this complexity is optimal for this class. We show that if the network is known, then the complexity of consensus decreases significantly. Our main contribution is an algorithm that uses O(n3/2log2n) messages on any n-node network and we show that some networks require Ω(nlogn) messages to achieve consensus. We also observe that availability of distinct labels of nodes helps to improve complexity of consensus for known networks but has no effect for the class of unknown networks. Indeed, even with labeled nodes, Ω(n2) messages are sometimes necessary if the network is unknown but for known labeled networks consensus can be always achieved with O(n) messages.