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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).
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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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Version 3.0 - Version Date: September 20, 2005
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