A method of characterizing a communications channel between two communications nodes, the method comprising: determining at least one feature of a communications path between a first communications node and a second communications node; determining at least one feature of a communications path between the first communications node and a third communications node; transmitting at least one packet from the first node to propagate through the second and third nodes and return to the first node; and determining at least one feature of the communications channel between the second and third nodes responsive to receiving the packet at the first node, the at least one determined feature of the first node and the second node, and/or information comprised in the packet.