Today's Internet hosts are threatened by IP spoofing attacks and large scale Distributed Denial-of-Service (DDoS) attacks. We propose a new defense mechanism, StackPi, which unlike previous approaches, allows the host being attacked, or its upstream ISP, to filter out attack packets and to detect spoofed source IP addresses, on a per-packet basis. In StackPi, a packet is marked deterministically by routers along its path towards the destination. Packets traveling along the same path will have the same marking so that an attack victim need only identify the StackPi marks of attack packets to filter out all further attack packets with the same marking. In addition, the victim can associate StackPi marks with source IP addresses to detect source IP address spoofing by changes in the corresponding StackPi mark. StackPi filtering can thus defend against not only DDoS attacks, but also many IP spoofing attacks - such as TCP hijacking, and multicast source spoofing attacks. Because each complete mark fits within a single packet, the StackPi defense responds quickly to attacks and can be effective after the first attack packet in a IP spoofing attack, or after a small number of attack packets in the case of a DDoS attack. StackPi also supports incremental deployment, such that significant benefits are realized even if only one third of Internet routers implement StackPi marking. We show these results through analysis and simulations based on several real Internet topologies.

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