Fast Routing Table Construction Using Small Messages

We describe a distributed randomized algorithm computing approximate distances and routes that approximate shortest paths. Let n denote the number of nodes in the graph, and let HD denote the hop diameter of the graph, i.e., the diameter of the graph when all edges are considered to have unit weight. Given 0 < eps <= 1/2, our algorithm runs in weak-O(n^{1/2 + eps} + HD) communication rounds using messages of O(log n) bits and guarantees a stretch of O(eps^-1 log eps^-1) with high probability. This is the first distributed algorithm approximating weighted shortest paths that uses small messages and runs in weak-o(n) time (in graphs where HD in weak-o(n)). The time complexity nearly matches the lower bounds of weak-Omega(sqrt(n) + HD) in the small-messages model that hold for stateless routing (where routing decisions do not depend on the traversed path) as well as approximation of the weigthed diameter. Our scheme replaces the original identifiers of the nodes by labels of size O(log eps^-1 log n). We show that no algorithm that keeps the original identifiers and runs for weak-o(n) rounds can achieve a polylogarithmic approximation ratio. Variations of our techniques yield a number of fast distributed approximation algorithms solving related problems using small messages. Specifically, we present algorithms that run in weak-O(n^{1/2 + eps} + HD) rounds for a given 0 < eps <= 1/2, and solve, with high probability, the following problems: - O(eps^-1)-approximation for the Generalized Steiner Forest (the running time in this case has an additive weak-O(t^{1 + 2eps}) term, where t is the number of terminals); - O(eps^-2)-approximation of weighted distances, using node labels of size O(eps^-1 log n) and weak-O(n^eps) bits of memory per node; - O(eps^-1)-approximation of the weighted diameter; - O(eps^-3)-approximate shortest paths using the labels 1,...,n.