Distributed Denial of Service (DDoS) attacks continue to plague the Internet. Defense against these attacks is complicated by spoofed source IP addresses, which make it difficult to determine a packet's true origin. We propose Pi (short for Path Identifier), a new packet marking approach in which a path fingerprint is embedded in each packet, enabling a victim to identify packets traversing the same paths through the Internet on a per packet basis, regardless of source IP address spoofing.