Detecting sources of computer network failures

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.