A system and method for detecting and diagnosing routing problems in a network in real-time by recording TCP flow information from at least one server to at least one prefix, and observing retransmission packets communicated from the at least one server to the at least one prefix. When a predetermined threshold for TCP flows to a prefix is reached, traceroutes may be triggered to a destination in the prefix, and the traceroutes analyzed to determine whether to issue an alarm for a routing failure. The system includes a real-time data collection engine for recording unidirectional TCP flow information, a real-time detection engine for observing the retransmission packets and issuing a warning upon a retransmission counter exceeding a predetermined threshold, and a real-time diagnosis engine for triggering at least one traceroute to a destination in the prefix that is randomly selected from TCP flows in retransmission states.