Network Working Group J. Postel Request for Comments: 866 ISI May 1983
One active users service is defined as a connection based application on TCP. A server listens for TCP connections on TCP port 11. Once a connection is established a list of the currently active users is sent out the connection (and any data received is thrown away). The service closes the connection after sending the list.
Another active users service service is defined as a datagram based application on UDP. A server listens for UDP datagrams on UDP port 11. When a datagram is received, an answering datagram is sent containing a list of the currently active users (the data in the received datagram is ignored).
If the list does not fit in one datagram then send a sequence of datagrams but don't break the information for a user (a line) across a datagram. The user side should wait for a timeout for all datagrams to arrive. Note that they are not necessarily in order.
There is no specific syntax for the user list. It is recommended that it be limited to the ASCII printing characters, space, carriage return, and line feed. Each user should be listed on a separate line.