MUST and SHOULD figured out

Wed Nov 12, 2014

Those of us trying to figure out the meaning of deontic modals, especially the distinction between weak and strong necessity, should just pack in and go on vacation. There’s an official RFC1 that settles the issue.

Scott Bradner, all around internet wizard at Harvard, wrote RFC 2119 “Key words for use in RFCs to Indicate Requirement Levels” in 1997. It has this definitive pronouncement on the difference between strong and weak necessity expressions:

Exercise for the reader: do any current semantic proposals mesh with this official pronouncement on how must and should differ?


  1. “A Request for Comments (RFC) is a publication of the Internet Engineering Task Force (IETF) and the Internet Society, the principal technical development and standards-setting bodies for the Internet.” (Wikipedia) [return]