Request for Comments: 866
This RFC specifies a standard for the ARPA Internet community. Hosts on the ARPA Internet that choose to implement an Active Users Protocol are expected to adopt and implement this standard.
A useful debugging and measurement tool is an active users service. An active users service simply sends a list of the currently active users on the host without regard to the input.
An active user is one logged in, such as listed in SYSTAT or WHO.
TCP Based Active Users Service
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.
UDP Based Active Users Service
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.
User List Syntax
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.