I've only seen this used with IRC clients/servers, but it's a pretty simple protocol.
When the client connects to the IRC server, a random port is (usually) picked on the client side, so your local port may be 5000, and the remote port is what you connected to, usually 6667. The server then initates a separate connection to the client on port 113. When it connects, the server sends CLIENTPORT : SERVERPORT followed by a vbCrLf, for example:
5000 , 6667
The client is expected to reply with CLIENTPORT,SERVERPORT : USERID : SYSTEMTYPE : USERNAME, followed by a vbCrLf, for example:
5000,6667 : USERID : UNIX : Lycaon
The string USERID is literally the word "USERID". I'm assuming that the SYSTEMTYPE can be WIN, MAC, UNIX, etc, but I've only ever seen UNIX. USERNAME is supposed to be the name of the currently logged in user.
The examples I've given are from mIRC, and nah, I'm not sure why it sends UNIX as the system type when it's on a Windows machine I'm assuming the spaces around the comma don't matter, but I'm not 100% sure.