Defending against large, distributed Denial-of-Service attacks is challenging, with large changes to the network core or to end-hosts often suggested. To make matters worse, spoofing adds to the difficulty, since defenses must resist attempts to trigger filtering of other people’s traffic. Further, any solution has to provide incentives for deployment, or it will never see the light of day. We present a simple and effective architectural defense against distributed DoS attacks that requires no changes to the end-hosts, minimal changes to the network core, is robust to spoofing, provides incentives for initial deployment, and can be built with offthe-shelf hardware. Categories and Subject Descriptors C.2.1 [Computer Communication Networks]: [Network Architecture and Design] General Terms Design, Security Keywords Denial-of-service, Internet Architecture