Methods and apparatus for determining measures indicative of downstream path congestion in respect of a node via which a plurality of data units are traversing an end-to-end path from a sender to a receiver, each data unit having a first indication settable in dependence on information from the receiver to each of at least two states whereby to indicate different levels of congestion experienced on the end-to-end path by data units previously received by the receiver; and a second indication settable to an initial state and to at least one updated state indicative of congestion experienced by data units on their path from the sender to said node; the method comprising receiving a plurality of said data units at said node, and determining said downstream path congestion measure in dependence on an inflation factor itself dependent on the proportion of a set of said data units whose second indication on receipt at said node is in said initial state.