1. A method of calculating at a network device, 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 one of the intermediate nodes, the method comprising: obtaining local data traffic measurements at each of said elbows, wherein said local data traffic measurements comprise data traffic flowing through the elbows based on data traffic arriving at the intermediate node via said ingress interface and leaving the intermediate node via said egress interface; and performing at a processor at the network device, said data traffic flow calculations utilizing said local data traffic measurements; wherein obtaining said local data traffic measurements comprises classifying and measuring said data traffic based on how said data traffic passes across the intermediate node, said measuring comprising measuring a proportion of said data traffic routed over each of the elbows.