Provided is a method of controlling traffic transmitted over a network path from a transmitter to a receiver via a router, the traffic comprising a plurality of packets, and the method comprising: at one of said transmitter and receiver, estimating a maximum queue length at a buffer of the router based on a maximum observed delay for traffic to travel from the transmitter to the receiver, and estimating a current queue length at the buffer of the router based on an observed delay for one or more specified packets to travel from the transmitter to the receiver; and based on said estimates of current queue length and maximum queue length, controlling traffic between the transmitter and receiver over said network path.