Methods, systems, and apparatus, including computer programs encoded on computer storage media, for detecting sources of computer network failures. One of the methods includes identifying a network flow in a computer network between a source and a destination; performing a first probe to determine whether there is end-to-end connectivity between the source and the destination; in response to determining that there is no end-to-end connectivity between the host and the destination, performing one or more additional probes including a second probe to determine whether each hop in the path of the network flow between the source and the destination is operational including requesting that the source transmit a respective first trace diagnostic packet to each hop in the path of the network flow; and determining whether at least one link of the computer network that is part of the path of the network flow has failed based on the results.