The Ident protocol is a simple protocol where a remote machine can request the username or a cookie uniquely identifying a local user associated with creating a TCP connect for use in reporting abuse. It's used a lot on IRC Networks, but is mostly deprecated now that users tend to have one machine per user. The Ident protocol also doesn't transverse NAT very well due to the fact that ports are rewritten, and it's difficult to tell which host the ident request should go to.

The Ident protocol uses port 113, and is sometimes called "auth".