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Telnet Interrupt Handling Using Out-Of-Band Signaling: The Telnet Synch Function
(Page 2 of 2)
The Synch Function
When needed, as in the example just
described, the synch function is invoked by the client sending the special
Telnet Data Mark (DM) protocol command, while instructing its
TCP layer to mark that data “urgent”. The URG bit in
the TCP
segment carrying this command causes it
to bypass TCP’s normal flow control mechanism so it is sent over
to the remote host. The Telnet server software, seeing the synch in
the data stream, then searches through all of the data in its buffer
looking only for Telnet control commands such as Interrupt Process,
Abort Output and Are You There. These commands are then
executed immediately. The server continues to search for important commands
up to the point where the Data Mark command is seen. All intervening
data is discarded; it will need to be retransmitted. After the Data
Mark is processed, the server returns to normal operation.
It is also possible for the server
to use the synch function in communication with the user on the client
device. For example, if the user sends the Abort Output command
to the server, he or she is telling the server to discard all remaining
output from the current process. The server will stop sending that output,
and can also use the synch function to clear all outstanding data that
is waiting in buffers to be sent to the client machine (since it causes
data to be discarded as mentioned above).
 Key Concept: Telnet protocol commands are sent in the same stream with user data, which means a problem with the remote host that stops the flow of data might cause user commands to become backed up and never received by the host. Since this may include commands issued by the user to try to fix the problem on the host, this can be a serious problem. To alleviate this situation, Telnet includes the synch function, which uses TCP’s urgent data transmission feature to force the receipt of essential commands even when regular data is not being processed.
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