While the IETF document BCP 38 on ingress filtering does not specify a method of implementing source address filtering, some router vendors have implemented a mechanism which employs lookups in the router's tables to perform this check. (See also "reverse path forwarding.") This is often implemented as a lookup in the FIB of the "source" address of the packet. If the interface has no route to the source address, the packet is assumed to be part of a denial of service attack, using a spoofed source address, and the router discards the packet.