In distributed methods of spoofing defense, routers cooperate in order to discover
information for distinguishing valid and spoofing packets. The information
may be related to a key which only valid packets will carry, or to the
incoming direction for packets from a given source. First we discuss Spoofing
Prevention Method (SPM) and Passport, and then cover Distributed Packet
Filtering (DPF) and other path-based filtering works including Source Address
Validity Enforcement (SAVE), Inter-Domain Packet Filters (IDPF), and BGP
Anti-Spoofing Extension (BASE). Routers using SPM and Passport mark outgoing
packets with secret keys, while routers using DPF and other path-based
filtering systems need to learn the correct incoming direction of packets for a
given source.