1. A method of calculating data traffic flow in a communications network comprising a plurality of source nodes, a plurality of destination nodes, and a plurality of intermediate nodes, each of said plurality of intermediate nodes including at least one elbow comprising one ingress interface and one egress interface of the intermediate node, the method comprising obtaining local data traffic measurements at each of said elbows, wherein said local data traffic measurements comprise data traffic arriving at the intermediate node via said ingress interface and leaving the intermediate node via said egress interface, and utilizing said local data traffic measurements in said data traffic flow calculations.