This is one of the protocols that I’m kind of surprised isn’t in more use, by virtue of its simplicity in implementation. RFC 865 is a layer 7 debugging RFC where a socket listens for a TCP or UDP connection, and then sends a short, arbitrary message.

The first IETF standard was an informational affair, a housekeeping standardization-process snapshot that summed up the current ‘best practice’ RFCs that’s led up to it.