Smoothed Analysis of Information Spreading in Dynamic Networks

Abstract

The best known solutions for $k$-message broadcast in dynamic networks of size $n$ require $Ω(nk)$ rounds. In this paper, we see if these bounds can be improved by smoothed analysis. We study perhaps the most natural randomized algorithm for disseminating tokens in this setting: at every time step, choose a token to broadcast randomly from the set of tokens you know. We show that with even a small amount of smoothing (one random edge added per round), this natural strategy solves $k$-message broadcast in $\tilde{O}(n+k^3)$ rounds, with high probability, beating the best known bounds for $k=o(\sqrt{n})$ and matching the $Ω(n+k)$ lower bound for static networks for $k=O(n^{1/3})$ (ignoring logarithmic factors). In fact, the main result we show is even stronger and more general: given $\ell$-smoothing (i.e., $\ell$ random edges added per round), this simple strategy terminates in $O(kn^{2/3}\log^{1/3}(n)\ell^{-1/3})$ rounds. We then prove this analysis close to tight with an almost-matching lower bound. To better understand the impact of smoothing on information spreading, we next turn our attention to static networks, proving a tight bound of $\tilde{O}(k\sqrt{n})$ rounds to solve $k$-message broadcast, which is better than what our strategy can achieve in the dynamic setting. This confirms that although smoothed analysis reduces the difficulties induced by changing graph structures, it does not eliminate them altogether. Finally, we apply our tools to prove an optimal result for $k$-message broadcast in so-called well-mixed networks in the absence of smoothing. By comparing this result to an existing lower bound for well-mixed networks, we establish a formal separation between oblivious and strongly adaptive adversaries with respect to well-mixed token spreading, partially resolving an open question on the impact of adversary strength on the $k$-message broadcast problem.