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rfc:rfc393

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.                                                      [
                                                              [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 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 ]
                                                              [Page 4]
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