On the runtime and robustness of randomized broadcasting

  • Authors:
  • R. Elsässer;T. Sauerwald

  • Affiliations:
  • University of Paderborn, Inst. for Comp. Sci., 33102 Paderborn, Germany;University of Paderborn, Inst. for Comp. Sci., 33102 Paderborn, Germany

  • Venue:
  • Theoretical Computer Science
  • Year:
  • 2009

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Abstract

In this paper, we study the following randomized broadcasting protocol. At some time t an information r is placed at one of the nodes of a graph. In the succeeding steps, each informed node chooses one neighbor, independently and uniformly at random, and informs this neighbor by sending a copy of r to it. We begin by developing tight lower and upper bounds on the runtime of the algorithm described above. First, it is shown that on @D-regular graphs this algorithm requires at least log"2"-"1"@Dn+log"("@D"@D"-"1")"^"@Dn-o(logn)~1.69log"2n rounds to inform all n nodes. Together with a result of Pittel [B. Pittel, On spreading a rumor, SIAM Journal on Applied Mathematics, 47 (1) (1987) 213-223] this bound implies that the algorithm has the best performance on complete graphs among all regular graphs. For general graphs, we prove a slightly weaker lower bound of log"2"-"1"@Dn+log"4n-o(logn)~1.5log"2n, where @D denotes the maximum degree of G. We also prove two general upper bounds, (1+o(1))nlnn and O(n@D@d), respectively, where @d denotes the minimum degree. The second part of this paper is devoted to the analysis of fault-tolerance. We show that if the informed nodes are allowed to fail in some step with probability 1-p, then the broadcasting time increases by at most a factor 6/p. As a by-product, we determine the performance of agent based broadcasting in certain graphs and obtain bounds for the runtime of randomized broadcasting on Cartesian products of graphs.