A method and apparatus for estimating available capacity of a data transfer path (16) that transfers data between data communication nodes (12, 14) of a data communication system (10). The method comprising: receiving (305) probe packets (15) that traverse the data transfer path (16) during real-time operation of the data transfer path (16); providing (310) measured data (zU, zE) indicating strain of the received probe packets for use in estimating the available capacity of the data transfer path (16); classifying (320) the measured data based on the strain of the received probe packets; filtering (325) the classified measured data into a discrete representation of a probability density function of available capacity; and estimating (330) the available capacity by using the discrete representation of the probability density function.