4.2.2.3 Unused IP Header Bits: RFC 791 Section 3.1

The IP header contains two reserved bits: one in the Type of Service byte and the other in the Flags field. A router MUST NOT set either of these bits to one in datagrams originated by the router. A router MUST NOT drop (refuse to receive or forward) a packet merely because one or more of these reserved bits has a non-zero value; i.e., the router MUST NOT check the values of thes bits.

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.