A processor is configured to receive, from a first device, a plurality of first-direction packets belonging to a flow of communication, pass the first-direction packets to a second device, receive, from the second device, a plurality of second-direction packets belonging to the flow, pass the second-direction packets to the first device, calculate, after receiving each of at least some of the second-direction packets, an estimated in-flight number, by assuming that the second-direction packet acknowledges receipt of a number of the first-direction packets that is based on an estimated average number of received first-direction packets acknowledged by the second-direction packets, the estimated in-flight number being an estimated volume of payload data, contained at least partly in the first-direction packets, that has been passed by the processor to the second device but has not yet been received by the second device, and to regulate the flow, based on the estimated in-flight number.