The minimum cost multiple-source unsplittable flow problem is
studied in this paper. A simple necessary condition to get a
solution is proposed. It deals with capacities and demands and can
be seen as a generalization of the well-known semi-metric
condition for continuous multicommdity flows. A cutting plane
algorithm is derived using a superadditive approach. The
inequalities considered here are valid for single knapsack
constraints. They are based on nondecreasing superadditive
functions and can be used to strengthen the relaxation of any
integer program with knapsack constraints. Some numerical
experiments confirm the efficiency of the inequalities introduced
in the paper.