5.2.5 Unused IP Header Bits: RFC-791 Section 3.1

The IP header contains several reserved bits, in the Type of Service field and in the Flags field. Routers MUST NOT drop packets merely because one or more of these reserved bits has a non-zero value.

Routers MUST ignore and MUST pass through unchanged the values of these reserved bits. If a router fragments a packet, it MUST copy these bits into each fragment.

DISCUSSION

Future revisions to the IP protocol may make use of these unused bits. These rules are intended to ensure that these revisions can be deployed without having to simultaneously upgrade all routers in the Internet.