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+Network Working Group
+Request for Comments: 393                         Joel M. Winett
+NIC 11585                                         Lincoln Laboratory
+Categories: TELNET                                LL-67
+References: RFC 109, 139, 158,318, and 328        3 October 1972
+
+
+                  Comments on TELNET Protocol Changes
+
+
+
+    Through this RFC, I am registering my objection to two of the
+three suggestions for changing the TELNET protocol as described in RFC
+328 and am adding my suggestion for the interpretation of the TELNET
+Reverse 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.                                                      [
+
+
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+                                                                [Page 1]
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+    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 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 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 ]
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+                                                                [Page 4]
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