Approximating minimum-cost k-node connected subgraphs via independence-free graphs

We present a 6-approximation algorithm for the minimum-cost k-node connected spanning subgraph problem, assuming that the number of nodes is at least k^3(k-1)+k. We apply a combinatorial preprocessing, based on the Frank-Tardos algorithm for k-outconnectivity, to transform any input into an instance such that the iterative rounding method gives a 2-approximation guarantee. This is the first constant-factor approximation algorithm even in the asymptotic setting of the problem, that is, the restriction to instances where the number of nodes is lower bounded by a function of k.