3.2.4 Telnet

When it receives "urgent" TCP data, a User or Server Telnet MUST discard all data except Telnet commands until the DM (and end of urgent) is reached.

When it sends Telnet IP (Interrupt Process), a User Telnet SHOULD follow it by the Telnet "Synch" sequence, i.e., send as TCP urgent data the sequence "IAC IP IAC DM". The TCP urgent pointer points to the DM octet.

When it receives a Telnet IP command, a Server Telnet MAY send a Telnet "Synch" sequence back to the user, to flush the output stream. The choice ought to be consistent with the way the server operating system behaves when a local user interrupts a process.

When it receives a Telnet AO command, a Server Telnet MUST send a Telnet "Synch" sequence back to the user, to flush the output stream.

A User Telnet SHOULD have the capability of flushing output when it sends a Telnet IP; see also Section 3.4.5.

DISCUSSION:

There are three possible ways for a User Telnet to flush the stream of server output data:

1. Send AO after IP.

   This will cause the server host to send a "flush- buffered-output" signal to its operating system. However, the AO may not take effect locally, i.e., stop terminal output at the User Telnet end, until the Server Telnet has received and processed the AO and has sent back a "Synch".

2. Send DO TIMING-MARK [TELNET:7] after IP, and discard all output locally until a WILL/WONT TIMING-MARK is received from the Server Telnet.

   Since the DO TIMING-MARK will be processed after the IP at the server, the reply to it should be in the right place in the output data stream. However, the TIMING-MARK will not send a "flush buffered output" signal to the server operating system. Whether or not this is needed is dependent upon the server system.

3. Do both.

The best method is not entirely clear, since it must accommodate a number of existing server hosts that do not follow the Telnet standards in various ways. The safest approach is probably to provide a user-controllable option to select (1), (2), or (3).