Comments on Telnet Protocol Changes(2)

　　The use of the Break Code from server to user TELNET has not　previously been defined and, thus, could be used to solve the　following problems which occur with line at a time and half duplex　systems. Line at a time systems do not output characters to the　terminal a character at a time but, instead, wait until a line is ready　for output. If a CR-LF sequence (TELNET protocol for end of line)　is received it is interpreted as an end of line and the characters　received are output. If characters are received which do not end

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　　with CR-LF the user TELNET does not know whether or not other　characters will follow which are part of the current line. Thus, the　characters received thus far must be output, without a CR-LF (new　line). If an end of message code were transmitted, the user TELNET　would know whether or not other characters would be received for　output. The user TELNET would then print characters either when　the TELNET Break control is received or when the CR-LF newline　sequence is received.

　　If the user TELNET is being run from a half duplex terminal, the　terminal cannot receive input and type output at the same time. Thus,　if output is received while the terminal is being used for input the　TELNET program must either buffer the received characters or abort the　input mode of operation to write out the received charac- ters. If　characters received are written out as they are received, the terminal　operation would be very similar to a full duplex terminal. This mode　of operation requires that the terminal have a reverse break　capability to allow the input mode to be aborted by program control.

　　In some systems it is only desirable to abort the input mode of　operation when a complete line is ready for output. If a string of　characters received does not end with an end of line code, the　characters received will not be output until after the input line is　entered, i. e., the mode of operation changed from input to output.

　　If an end of message code were transmitted, the user TELNET could　abort the input mode of operation even though the end of line code was　not received.

　　In systems which do not support the reverse break feature or if the　terminal does not have this feature it is not possible to abort an　input mode of operation in order to output received characters. In　this case, the systems can operate in either of two modes, a) un-locked keyboard, or b) locked keyboard mode.

　　In an unlocked keyboard system, received characters are not output　until the user completes an input line. An input line is completed　when the end of line code is entered. This might be a CR, a LF, or　a NL code. After received characters, if any, are output, the input　modes is re-entered. To receive output the user must enter an input　line (possibly a null line). If the user is waiting for output, he must　repeatedly enter a line until the output has been received and typed.

　　Since an input line must be entered just to receive output, it is　desirable to define an input line which does not result in anything　being sent to the serving system. If a null line (a line consisting of　just the end of line code) is chosen for this purpose, some other input　line must be defined to cause a null line to be transmitted.

　　In a locked keyboard system, the input mode is not immediately

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　　re-entered after an input line is entered. It is re-entered only after　a defined prompt is received. The prompt can be defined to be the　reception of any character or can be defined to be a specific charac-ter code. If a specific code is chosen the serving site must send this　code whenever the terminal should be put into input mode. If an end of　message code were transmitted this code could be inter- preted to be　the input prompt code.

　　In summary, three situations have been described where an end of　message code would be desirable.

　　a) To indicate when a line which does not end with CR-LF should　be output for line at a time systems

　　b) To indicate that the input mode in half duplex operation should　be aborted so that received characters can be output

　　c) As a prompt character to cause the input mode to be entered　for locked keyboard half duplex systems

　　The ASCII TELNET control code for Break (Reverse Break) could be　interpreted as an end of message code when sent from server to user.

　　[ This RFCwas put into machine readable form for entry ]

　　[ into the online RFCarchives by BBN Corp. under the ]

　　[ direction of Alex McKenzie. 1/97 ]