Flow in a data network is policed by determining a measure of greediness of a flow through a node, comparing the measure of greediness with a measure indicative of acceptable greediness dependent on the expected greediness of a compliant flow experiencing substantially similar path conditions, and designating the flow for possible sanction in the event that the greediness is not in accordance with the acceptable greediness. Such allows for the policing of a data network when one or more fields in a data packet that carry information relating to a characterization of an end-to-end path and/or a downstream path of the data packet are used in order to police the data network.