ICMP Security Failure Message?

["Hu, Shicai" <shicai@cryptek.com>]

RFC 2521: ICMP Security Failure Message
Category: Experimental     
(What does a category of  "experimental" mean? Does IETF assign somebody to
do some experiment on this RFC document?)

I am wondering that is this "ICMP Security Failure Message" may be sent
after IPSec (Either IKE message or ESP message) message failed?


Thanks a lot.