A method is described of probing a network route using a pairs of probe packets in which the first packets are typically smaller than the second. A first pair of probe packets with size ratio [L1/L2] and a second pair of probe packets with different size ratio [M1/M2] are transmitted onto a network route. An indicator of time taken for the second probe packet in the packet pairs to traverse the network route is derived and it is identified whether the values of the derived indicators vary or are substantially the same. The method, which can be used to probe home or domestic networks, indicates whether the first link in the route is or is not a bottleneck link, thereby providing further information about the route. A test apparatus, gateway device and computer to perform the method are also disclosed.