A method of estimating available capacity of a data transfer path that transfers data between a sending and a receiving data communication node of a data communication system is disclosed. The method includes: sending one or more setting packets wherein the one or more setting packets is/are used to set a traffic shaping node arranged in the data transfer path in a steady state; transmitting, from the traffic shaping node, probe packets received from the sending node, towards the receiving node at an average rate for the traffic shaping node; providing, in response to traversal of the data transfer path by the probe packets, and during the real-time operation of the data transfer path, measured data for use in estimating the available capacity of the data transfer path; and estimating the available capacity of the data transfer path using the measured data from the probe packets.