RFC 793, "Transmission Control Protocol", September 1981Source of RFC: Legacy
Area Assignment: tsv
Errata ID: 3972
Reported By: Richard Scheffenegger
Date Reported: 2014-04-23
Rejected by: Martin Stiemerling
Date Rejected: 2014-04-24
Section 3.1 says:
Reserved: 6 bits Reserved for future use. Must be zero.
It should say:
Reserved: 6 bits Reserved for future use. SHOULD be zero.
RFC3168 specifies 2 additional bits (ECE and CWR), RFC3540 specifies one additional bit (NS). Even though RFC793 predates RFC2119, and section 2.10 states
" be conservative in what you do, be liberal in what you accept from others.", any
update to this RFC should adjust the wording apropriately.
The text is correct, as is. For implementations implementing RFC 793 purely the have to set this to zero.