Ask HN: Which RFCs illumined you and why?

[invertedlambda]

For web folks, RFC 2616 could/should be considered required reading. I'm wondering which RFCs gave you the most "a-ha" moments and why?

And, which RFCs do you think everyone should read and why?

[sagacity]

RFC 2229 - A Dictionary Server Protocol

http://tools.ietf.org/html/rfc2229

It even inspired and enabled us to create:

http://www.TheEnglishDictionary.org