Telnet control functions

The telnet protocol includes a number of control functions. These are initiated in response to conditions detected by the client (usually certain special keys or key combinations) or server. The detected condition causes a special character to be incorporated in the data stream.

Interrupt Process

-- This is used by the client to cause the suspension or termination of the server process. Typically the user types Ctrl-C on the keyboard. An IP (244) character is included in the data stream.

Abort Output

-- This is used to suppress the transmission of remote process output. An AO character is included in the data stream.

Are You There

--

This is used to trigger a visible response from the other end to confirm the operation of the link and the remote process. An AYT (246) character is incorporated in the data stream.

Erase character

--

Sent to the display to tell it to delete the immediately preceding character from the display. An EC character is incorporated in the data stream.

Erase line

--

Causes the deletion of the current line of input. An EL character is incorporated in the data stream.

Data Mark

--

Some control functions such as AO and IP require immediate action and this may cause difficulties if data is held in buffers awaiting input requests from a (possibly misbehaving) remote process. To overcome this problem a DM character is sent in a TCP Urgent segment, this tells the receiver to examine the data stream for "interesting" characters such as IP, AO and AYT. This is known as the telnet synch mechanism.

A DM not in a TCP Urgent segment has no effect.