Abstract: An estimate of a portion of network traffic that is nonconforming to a communication transmission control protocol is used to signal that a distributed denial of service attack may be occurring. Traffic flows are aggregated and packets are intentionally dropped from the flow aggregate in accordance with an assigned perturbation signature. The flow aggregates are observed to determine if the rate of arrival of packets that have a one-to-one transmission correspondence with the dropped packets are similarly responsive to the perturbation signature. By assigning orthogonal perturbation signatures to different routers, multiple routers may perform the test on the aggregate and the results of the test will be correctly ascertained at each router. Nonconforming aggregates may be redefined to finer granularity to determine the node on the network that is under attack, which may then take mitigating action.