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Network Working GroupRequest for Comments: 393                         Joel M. WinettNIC 11585                                         Lincoln LaboratoryCategories: TELNET                                LL-67References:RFC 109, 139, 158,318, and 328        3 October 1972Comments on TELNET Protocol Changes    Through this RFC, I am registering my objection to two of thethree suggestions for changing the TELNET protocol as described in RFC328 and am adding my suggestion for the interpretation of the TELNETReverse Break Control Code.1.  Hide-your-input    This code was originally put in the TELNET protocol to give the    virtual terminal the ability to simulate a real terminal which has the    print suppress capability. If the terminals being used at some    installations do not have the ability to disable the printing    mechanism, the TELNET being used can either ignore this code or    attempt to simulate the function using other means (e. g., blacking    out a number of character positions and returning to the first    character position).  Every attempt should be made to allow a network    user of a time-sharing system to have the same facilities as a local    user of the time-sharing system. The specification of TELNET protocol    should not limit the function of users if a function cannot be    supported by all users.    The "Hide-your-input" and "Echo" TELNET control codes provide for the    support of two functions available in some time-sharing systems.  The    "Hide-your-input" function is really a special case of the "Echo" mode    of operation where the server tells the user that the server will echo    but the server does not. A separate code is used for this func- tion    since some servers may support this function but may not support the    full "Echo" mode of operation.   ] This material has not been reviewed for public release and is [   ] intended only for use with the ARPA network. It should not be [   ] quoted or cited in any publication not related to the ARPA    [   ] network.                                                      [                                                                [Page 1]

    The "Hide-your-input" and "Echo" modes of operation are disabled with    the "No-echo" control. ASCII control codes could have been chosen for    these functions but it was decided that the NVT ASCII control codes    should only be specified for commonly used functions.    To indicate the number of characters for which the printing should be    suppressed, the "Hide-your-input" TELNET control could be rede- fined    to include a byte following the "Hide-your-input" control to indicate    the number of characters for which the printing should be concealed.    The "No-echo" control would still be sent so that systems with the    print suppress feature would not have to count characters.2.  Data Types    The protocol should allow a server to support users with character    codes other than ASCII, e. g., EBCDIC. The definition of an alter-    nate character code should include the definition of the TELNET    control codes. An EBCDIC code has been proposed in RFC # 109 and has    been implemented on the Lincoln Laboratory 360/67. If it is desired to    allow one to return to the network standard ASCII code, the non-ASCII    code should contain a code to indicate return to ASCII.3.  Reverse Break    The code for Break is defined as a 129th ASCII data code. It is    usually transmitted from a user's network virtual terminal to a server    when a corresponding key (break key or attention key) is typed on the    TELNET terminal and is interpreted by serving systems as that special    key. Since a common function of this key is to interrupt a running    process the server must be alerted to the fact that this code has been    transmitted no matter when it is sent.  Thus, the TELNET SYNC (TELNET    data mark together with a network interrupt on the TELNET send socket)    must also be trans- mitted to cause the serving process to examine the    received charac- ters. The ASCII control code EOT (Octal 4) could have    been chosen for the break function but his code is not interpreted by    all systems.  Thus, it was decided that an NVT TELNET control code    should be used for this purpose.    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                                                                [Page 2]

    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                                                                [Page 3]

    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 ofmessage 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 beinterpreted as an end of message code when sent from server to user.          [ This RFC was put into machine readable form for entry ]          [ into the online RFC archives by BBN Corp. under the   ]          [ direction of Alex McKenzie.                      1/97 ]                                                                [Page 4]