Single-source unsplittable flow

The max-flow min-cut theorem of Ford and Fulkerson is based on an even more foundational result, namely Menger´s theorem on graph connectivity Menger´s theorem provides a good characterization for the following single-source disjoint paths problem: given a graph G, with a source vertex s and terminals t₁,...,t_k, decide whether there exist edge-disjoint s-t_i paths for i=1,...,k. We consider a natural, NP-hard generalization of this problem, which we call the single-source unsplittable flow problem. We are given a source and terminals as before; but now each terminal t_i has a demand p_i⩽1, and each edge e of G has a capacity c_e ⩾1. The problem is to decide whether one can choose a single s-t_i path for each i, so that the resulting set of paths respects the capacity constraints-the total amount of demand routed across any edge e must be bounded by the capacity c_e. The main results of this paper are constant-factor approximation algorithms for three natural optimization versions of this problem, in arbitrary directed and undirected graphs. The development of these algorithms requires a number of new techniques for rounding fractional solutions to network flow problems; for two of the three problems we consider, there were no previous techniques capable of providing an approximation in the general case, and for the third, the randomized rounding algorithm of Raghavan and Thompson provides a logarithmic approximation. Our techniques are also of interest from the perspective of a family of NP-hard load balancing and machine scheduling problems that can be reduced to the single-source unsplittable flow problem