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Attack mitigation schemes actively throttle attack traffic generated in distributed denial-of-service (DDoS) attacks. This paper presents attack diagnosis (AD), a novel attack mitigation scheme that combines the concepts of Pushback and packet marking. AD's architecture is inline with the ideal DDoS attack countermeasure paradigm, in which attack detection is performed near the victim host and attack mitigation is executed close to the attack sources. AD is a reactive defense that is activated by a victim host after an attack has been detected. A victim activates AD by sending AD-related commands to its upstream routers. On receipt of such commands, the AD-enabled upstream routers deterministically mark each packet destined for the victim with the information of the input interface that processed that packet. By collecting the router interface information recorded in the packet markings, the victim can trace back the attack traffic to the attack sources. Once the traceback is complete, the victim issues messages that command AD-enabled routers to filter attack packets close to the source. The AD commands can be authenticated by the TTL field of the IP header without relying on any global key distribution infrastructure in Internet. Although AD can effectively filter traffic generated by a moderate number of attack sources, it is not effective against large-scale attacks. To address this problem, we propose an extension to AD called parallel attack diagnosis (PAD) that is capable of throttling traffic coming from a large number of attack sources simultaneously. AD and PAD are analyzed and evaluated using a realistic network topology based on the Skitter Internet map. Both schemes are shown to be robust against IP spoofing and incur low false positive ratios.