The Complete April Fools RFCs (edited by myself and Peter H. Salus) includes one RFC that, it turns out, was not a joke. The book reprints all the April Fools and various "funny" RFCs and includes commentary not available online. And, err, umm... we recently learned that it includes on RFC that was not meant to be funny at all. We apologize if this has created any confusion.
RFC 2410: The NULL Encryption Algorithm and Its Use With IPsec by Rob Glenn and Stephen Kent, it turns out, is an actual part of the IPSec standard. (Thanks to Merike Kaeo for reporting this to us.) If you want to use IPSec to encode your packets without any of that messy "encryption" getting in the way of reading the payload, I'm sure RFC2410 is a great technique.
The first clue that this RFC was real was that it wasn't published on April 1st like other funny RFCs. However, in our defense, neither was RFC 527: ARPAWOCKY or other documents we reprinted in Part III: Poetry in Motion.
Ironically we had thought of picking a random non-funny RFC and publishing just to see if anyone noticed. Originally all RFCs were published on the first of the month, so many many many RFCs were published on April 1st of each year. Sorting through all of them to find which were fake was a difficult task. The RPC mechanism that resulted in NFS have so little security was published on April 1st. Was it a joke all along? That would explain a lot.