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 k3(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.Comment: 20 pages, 1 figure, 28 reference