1. A method of calculating traffic values in a communications network, the communications network comprising a plurality of nodes, the nodes being connected to one another by links, the method comprising: (a) obtaining at a management system, traffic data measurements through said nodes or links in an initial scenario as input data; (b) deriving at the management system, a traffic flow model for a modified scenario using a plurality of constraints describing the interdependency of said initial to said modified scenario and derived from the network topology and network behavior of the initial network; and (c) calculating at the management system, upper and lower bounds of traffic values for said modified scenario from said traffic flow model using said input data, wherein calculating comprises solving a linear programming problem by computing the upper and lower bounds of the traffic values; wherein said modified scenario comprises one or more of: a modified network topology, modified routing algorithm parameters, modified traffic engineering constraints, or modified traffic load compared to the initial scenario.