a method for diagnosing network performance problems includes providing a graph representing a topology of a network including first nodes that are classified as providers of a resource , second nodes that are classified as consumers of the resource , and third nodes that are classified as mediators intervening between the providers and the consumers . responsively to a report of a performance problem encountered by a given consumer in the network , at least one input / output path is traced through the graph connecting the given consumer to one or more of the providers . one or more problematic nodes are identified along the at least one i / o path as possible causes of the performance problem .