Disclosed is a method of selecting a data source node from a plurality of data source nodes, the selected data source node being used to send data to a data requesting node over a packet data network. The method includes, at an intermediate node of the network, intercepting at least one message sent to or from the data requesting node, the message(s) relating to data source node suitability, determining a cost of communication between the data requesting node and each of the data source nodes to which the message(s) relate, and influencing data source node suitability information sent to the data requesting node to take account of the cost to enable the data requesting node to use the information to select a data source node.