Network Working Group
Request For Comments: 718
NIC #35874
Jon Postel (SRI-ARC)
Jun 1976

The following memo was a page of a document describing changes in version 1.34 of the Tenex system. I believe that the author is Ray Tomlinson or someone else in the BBN-RCC Tenex group. In any case Ray has agreed that these comments should be circulated to the to the network community, rather than to only the Tenex community.

Comments on RCTE from the TENEX Implementation Experience

The code to implement the RCTE option of the new TELNET protocol for TENEX has been completed. The RCTE option permits a reduction in network traffic by deferring the transmission of characters which will not cause the receiving user program to be activated until a character which will cause the user program to be activated. A further reduction is achieved by minimizing the flow of echo characters back to the user TELNET program. This is done by having the server instruct the user TELNET to echo the group of characters up through the next wakeup character. By sending this command as the user program is about to read the first character of that group, the echo is guaranteed to follow any response to the preceding group of characters.

Significant problems with the RCTE protocol were encountered. The handling of spontaneous output was specified in a way that made the implementation extremely difficult to do correctly (if, indeed, a correct implementation is possible). The solution here was to completely isolate the control of input transmission and echoing from the characters flowing in the output stream. Synchronization of input and output then occurs directly by virtue of the embedding of control information in the output stream. This approach permits a simplified coding of both the user TELNET and server TELNET.

A second problem was the handling of interrupt characters. The RCTE protocol fails to provide an explicit mechanism for interrupt characters thus necessitating the handling of interrupt characters as program wakeup characters. Since the interrupt characters are not actually handled as program wakeup characters and, in fact, bypass the terminal input buffer, a special provision had to be made to get the command sent back to the user TELNET to indicate that the character stream should be echoed beyond the point where the interrupt character was typed. The transmission must be synchronized with the processing of the terminal input buffer so that it will be sent at the proper time. This was achieved by putting a marker in the input buffer at the point where the interrupt character was. This marker is never given to the user's program and must not be counted as an input character. A new counter was installed indicating the number of such markers in the input buffer and the SIBE JSYS modified to indicate "empty" only if the difference of the total characters in the buffer and the number of markers in the buffer is greater than 0.

A third problem is handling the case where the input buffer is cleared. Since the buffer may contain various wakeup characters and special markers, when the buffer is cleared, the user TELNET and SERVER may get out of sync. It is infeasible to scan the buffer and send a RCTE command for each such wakeup character or special marker. instead, a command is sent to the user TELNET meaning "clear your input buffer and reset your counters". This command is implemented by sending "WILL RCTE". This is the reverse case from a normal RCTE (i.e. DO RCTE) and thus cannot be confused with the normal use of the RCTE option. This saves adding a new option.