<pre>Network Working Group                                         D. Crocker
Request for Comments: 560                                      J. Postel
Category: Protocols, TELNET                               20 August 1973
NIC: 18492


         Remote Controlled Transmission &amp; Echoing TELNET Option

   Currently, a terminal in character-at-a-time transmission and
   foreign-host echo causes four Network Messages for each character
   struck.  (The character sent from local to foreign host; its RFNM;
   the echoed character sent from the foreign to the local host; and its
   RFNM.)

   By eliminating most echoing (1/2 as many messages) and packaging the
   characters into useful units (assuming an average of five character
   per unit; therefore another 80 per cent reduction), it is believed
   that almost a 90 per cent reduction in character-mode interactive
   Network terminal traffic can be attained.

   The packaging of characters and elimination of foreign echoing should
   also lessen the load placed on the foreign hosts.

<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>. Command name and code:</span>

   RCTE

<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>. Command meanings:</span>

   IAC WILL RCTE

      The sender of this command REQUESTS or AGREES to use the RCTE
      option, and will send instructions for controlling the other
      side's terminal printer.

   IAC WON'T RCTE

      The sender of this option REFUSES to send instructions for
      controlling the other side's terminal printer.

   IAC DO RCTE

      The sender REQUEST or AGREES to have the other side (sender of
      WILL RCTE) issue commands which will control his (sender of the
      DO) output to the terminal printer.






<span class="grey">Crocker, et. al.                                                [Page 1]</span></pre>
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<span class="grey"><a href="./rfc560">RFC 560</a>               RCT &amp; Echoing TELNET Option            August 1973</span>


   IAC DON'T RCTE

      The sender of this command REFUSES to allow the other side to
      control his (sender of DON'T) terminal printer.

   IAC SB RCTE &lt;cmd&gt; [BC1 BC2] [TC1 TC2]

      where:

         &lt;cmd&gt; is one 8-bit byte having the following flags (bits are
         counted from the right):

         Bit   Meaning

         0      0 = Ignore all other bits in this byte and repeat the
                    last &lt;cmd&gt; that was sent.  Equals a 'continue what
                    you have been doing'.
                1 = Perform actions as indicated by other bits in this
                    byte.

         1      0 = Print (echo) Break character
                1 = Skip (don't echo) Break character

         2      0 = Print (echo) text up to Break character
                1 = Skip (don't echo) text up to Break character

         3      0 = Continue using same classes of Break characters.
                1 = The two 8-bit bytes following this byte contain
                    flags for the new Break classes.

         4      0 = Continue using same classes of Transmit characters.
                1 = Reset Transmit classes according to the two bytes
                    following 1) the Break classes bytes, if the Break
                    classes are also being reset, or 2) this byte, if
                    the Break classes are NOT also being reset.

      Value (decimal) of the &lt;cmd&gt; byte and its meaning:

                0 = Continue what you have been doing

                1 = Print (echo) up to AND INCLUDING Break character

                3 = Print up to Break character and SKIP (don't echo)
                    Break character

                5 = Skip text (don't echo) up to Break character, but
                    PRINT Break character




<span class="grey">Crocker, et. al.                                                [Page 2]</span></pre>
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<span class="grey"><a href="./rfc560">RFC 560</a>               RCT &amp; Echoing TELNET Option            August 1973</span>


                7 = Skip up to and including Break character

                Add one of the previous non-zero values to one of the
                    following values, to get the total decimal value for
                    the byte (Note that Classes may not be reset without
                    also resetting the printing action; so an odd number
                    is guaranteed):

                8 = Set Break classes (using the next two bytes [BC1
                    BC2])

                16 = Set Transmission classes (using the next two bytes
                    [TC1 TC2])

                24 = Set Break classes (using the next two bytes [BC1
                    BC2]) and the Transmission classes (using the two
                    bytes after that [TC1 TC2]).

      Sub-commands (IAC SB RCTE...) are only sent by the Controlling
      Host and, in addition to other functions, functionally replace the
      Go-Ahead (IAC GA) Telnet Command.

<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>. Default:</span>

   WON'T RCTE -- DON'T RCTE

      Neither host asserts special control over the other host's
      terminal printer.

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>. Motivation for the option:</span>

   RFC's 1, 5 and 51 discuss Network and process efficiency and
   smoothness.

   <a href="./rfc357">RFC 357</a>, by John Davidson, introduces the problem of echoing delay
   that occurs when a remote user accesses a full-duplex host, thru a
   satellite link.  In order to save the many thousands of miles of
   transit time for each echoed character, while still permitting full
   server responsiveness and clean terminal output, an echo control
   similar to that used by some Time-sharing systems is suggested for
   the entire Network.

      In effect, the proposed option involves making a user host
      carefully regulate the local terminal printer according to
      explicit instructions from the foreign (serving) host.






<span class="grey">Crocker, et. al.                                                [Page 3]</span></pre>
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<span class="grey"><a href="./rfc560">RFC 560</a>               RCT &amp; Echoing TELNET Option            August 1973</span>


   An important additional issue is efficient Network transmission.
   Implementation of the Davidson Echoing Scheme will eliminate almost
   all server-to-user echoing.

      The proposed option also requests using hosts to buffer a
      terminal's input to the foreign host until it forms a useful unit
      (with "useful unit" delimited by Break or Transmission characters
      as described below).  Therefore, fewer messages are sent on the
      user-to-server path.

   N.B.: This option is only intended for use with full-duplex hosts.
   The Go-Ahead Telnet feature is completely adequate for HALF-duplex
   server hosts.

<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>. Explicit description of control mechanism:</span>

   A. Overview of Interaction

      (1) Agree to use RCTE option
      (2) User holds echo printing until instructed by server to do
          otherwise
      (3) Server may send output to terminal printer.
      (4) Network output is printed up to an RCTE command
      (5) Server sends IAC SB RCTE &lt;cmd&gt;
      (6) User acts upon the command up to a Break character or until
          receipt of output from the server host.
      (7) Go to (2)

   Note: Output from the server host may occur at any time, in which
         case, the flow of control switches to (2) and then proceeds to
         (3), (4), etc.

   B. Explanation:

      (1) Both Hosts agree to use the RCTE option.  After that, the
          using host (IAC DO RCTE) merely acts upon the Controlling
          (serving) host's commands and does not issue any RCTE commands
          unless and until it (using host) decides to stop allowing use
          of the option (by sending IAC DON'T RCTE).

      (2) User host begins synchronization between the serving host and
          itself by suspending terminal echo printing until directed to
          do otherwise by the controlling host, thru an IAC SB RCTE
          &lt;cmd&gt;.







<span class="grey">Crocker, et. al.                                                [Page 4]</span></pre>
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<span class="grey"><a href="./rfc560">RFC 560</a>               RCT &amp; Echoing TELNET Option            August 1973</span>


      (3) The server may send output to the terminal printer, either in
          response to input from the user (in which case it is already
          synchronized with the terminal input) or spontaneously.  In
          the latter case, flow of control automatically switches to (2)
          and continues from there.  Output from the server is defined
          as completed when step (5) occurs.  That is, text from the
          Server to the terminal printer MUST end with an RCTE command.

      (4) Any output from the server is printed on the terminal
          IMMEDIATELY.  Again note that the end of such output is
          defined to be the occurrence of an IAC SB RCTE &lt;cmd&gt; command.

      (5) Server sends an RCTE command.  The command may redefine Break
          and Transmission classes, Action to be performed on Break
          characters, and action to be performed on text.  Each of these
          independent functions is controlled by separate bits in the
          &lt;cmd&gt; byte.

          a. A Transmission character is one which REQUIRES the User
             Host to transmit all text accumulated up to and including
             its occurrence.  (For Net efficiency, User hosts are
             DISCOURAGED from sending before the occurrence of a
             Transmission character).

             If the Transmission Classes bit (Bit 4) is on, the two
                bytes following the two Break Classes bytes (or
                immediately following the &lt;cmd&gt; byte, if the Break
                Classes bit is not on) will indicate what classes are to
                be enabled.

             If the Bit is OFF, the Transmission classes remain
                unchanged.  When the RCTE option is first initiated, NO
                CLASSES are in effect.  That is, no character will be
                considered a Transmission character.  (As if both TC1
                and TC2 are zero.)

          b. A Break character has the effect of a Transmission
             character, but also causes the User host to stop its
             print/discard action upon the User's input text, until
             directed to do otherwise by another IAC SB RCTE &lt;cmd&gt;
             command from the Serving host.  Break characters therefore
             define printing units.  "Break character" as used in this
             document does NOT mean Telnet Break character.

             If the Break Classes bit (Bit 3) is on, the two bytes
                following &lt;cmd&gt; will indicate what classes are to be
                enabled.  There are currently nine (9) classes defined,
                with room for expansion.



<span class="grey">Crocker, et. al.                                                [Page 5]</span></pre>
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<span class="grey"><a href="./rfc560">RFC 560</a>               RCT &amp; Echoing TELNET Option            August 1973</span>


             If the bit is OFF, the Break classes remain unchanged.
                When the RCTE option is initiated, CLASSES 4, 5, and 9
                are to be in effect.  That is, Format Effectors, Non-
                format effector Control Characters and DEL, and
                Punctuation characters are to be Break characters.

          c. The list of the character classes, used to define Break and
             Transmission classes are listed at the end of this
             document, in the "Tables" Section.

          d. Because Break characters are special, the print/discard
             action that should be performed upon them is not always the
             same as should be performed upon the rest of the input
             text.

             For example, while typing a filename to TENEX, I want the
                text of the filename to be printed (echoed); but I do
                not want the &lt;escape&gt; (if I use the name completion
                feature) to be printed.

             If Bit 1 is ON The Break character is NOT to be printed.

          e. A separate bit (Bit 2) signals whether or not the text
             itself should be printed (echoed) to the terminal.  If Bit
             2 = 0, then the text IS to be printed.

          f. Yet another bit (Bit 0 - right-most bit) signals whether or
             not any of the other bits of the command should be checked.
             If this bit is OFF, then the command should be interpreted
             to mean "continue whatever echoing strategy you have been
             following, using the same Break and Transmission classes."

             This is particularly useful for the &lt;cmd&gt; command that
                follows spontaneously generated output from the Serving
                host (such as "System Going Down") which needs to signal
                End-of-Message, but does not usually want to reset any
                other conditions.

             The server may, however, alter user action after a
                spontaneous message, but it is possible that text will
                be lost, or printed when it should not be, since there
                is no guarantee that the RCTE &lt;cmd&gt; from the serving
                host will be properly synchronized with the terminal
                input.







<span class="grey">Crocker, et. al.                                                [Page 6]</span></pre>
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<span class="grey"><a href="./rfc560">RFC 560</a>               RCT &amp; Echoing TELNET Option            August 1973</span>


      (6) Input from the terminal is (hopefully) buffered up to the
          occurrence of a Transmission or Break character; and the input
          text is echoed or not echoed, up to the occurrence of a Break
          Character.  The most recent RCTE command determines the echo,
          Transmission and Break actions.

      (7) When a Break character is typed, the cycle of control is
          complete and action re-commences at (2).  Action also
          automatically switches to (2) upon receipt of any text from
          the Server host.

   C. Notes, Comments, Etc.:

      (1) Even-Numbered Commands, greater than zero, are in error, since
          they will have the low-order bit off.  The command should be
          interpreted as equal to zero, which means that any Classes
          Reset bytes ([TC1 TC2] [BC1 BC2]) will be in error.

      (2) Servers will generally instruct Users NOT to echo Break
          Characters, even though it might be alright to echo most Break
          characters.  For example, &lt;cr&gt; is usually a safe character to
          echo but &lt;esc&gt; is not.  TENEX Exec is willing to accept
          either, during filename specification.  Therefore, the user
          must be instructed NOT to echo ANY Break Characters.

          This is generally a tolerable problem, since the server has to
          send an RCTE command at this point, anyhow.  Adding the Break
          character to the message (so that it appears to be echoed)
          will not cause any extra Network traffic.

      (3) The RCTE Option entails a rather large overhead.  In a true
          character-at-a-time situation, this overhead is not justified.
          But on the average, it should result in significant savings,
          both in Network traffic and Host wake-ups.

      (4) A severe (User) site-dependent problem will be buffering
          type-ahead input from the terminal.  It is possible,
          especially in the case of TIPS, that the input buffer will
          overflow often.  If the receiving (serving) host will permit,
          the accumulated text should be transmitted at this point.  If
          the text cannot be transmitted and further typing by the user
          will result in lost text, the user should be notified.









<span class="grey">Crocker, et. al.                                                [Page 7]</span></pre>
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<span class="grey"><a href="./rfc560">RFC 560</a>               RCT &amp; Echoing TELNET Option            August 1973</span>


   D. Sample Interaction:

      "S:" is sent from serving (WILL RCTE) host to Using host.
      "U:" is sent from Using (DO RCTE) host to Serving host.
      "T:" is entered by the terminal user.
      "P:" is printed on the terminal.

      Text surrounded by square brackets ([]) is commentary.
      Text surrounded by angle brackets (&lt;&gt;) is to be taken as a single
         unit.  E.G., carriage return is &lt;cr&gt;, and the decimal value 27
         is represented &lt;27&gt;.

      The following interaction shows a Logon to a Tenex, initiation of
      the DED editor, insertion of some text and return to the Exec
      level.  A Telnet connection has already been opened, but the TENEX
      prompt has not yet been issued.  The hosts first discuss using the
      RCTE option:

      S: &lt;IAC&gt;&lt;WILL&gt;&lt;RCTE&gt;

      U: &lt;IAC&gt;&lt;DO&gt;&lt;RCTE&gt;

      S: TENEX 1.31.18, TENEX EXEC 1.50.2 &lt;cr&gt;&lt;lf&gt;@
         &lt;IAC&gt;&lt;SB&gt;&lt;RCTE&gt;&lt;11&gt;&lt;1&gt;&lt;24&gt;

         [Print the Herald and echo input text upto a Break character,
         but do not echo the Break Character.  Classes 4 (Format
         Effectors), 5 (Non-format effector Controls and &lt;DEL&gt;), and 9
         (&lt;space&gt;) act as Break Characters.]

      P: TENEX 1.31.18, TENEX EXEC 1.50.2 &lt;cr&gt;&lt;lf&gt;@

      T: LOGIN ARPA &lt;cr&gt;

      P: LOGIN

      U: LOGIN &lt;space&gt;

      S: &lt;space&gt;&lt;IAC&gt;&lt;SB&gt;&lt;RCTE&gt;&lt;0&gt;

      P: &lt;space&gt;ARPA

      U: ARPA &lt;cr&gt;

      S: &lt;cr&gt;&lt;lf&gt; (PASSWORD) : &lt;IAC&gt;&lt;SB&gt;&lt;RCTE&gt;&lt;7&gt;

      P: &lt;cr&gt;&lt;lf&gt; (PASSWORD) :




<span class="grey">Crocker, et. al.                                                [Page 8]</span></pre>
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<span class="grey"><a href="./rfc560">RFC 560</a>               RCT &amp; Echoing TELNET Option            August 1973</span>


      T: WASHINGTON 1000&lt;cr&gt;

         [The password "WASHINGTON" is not echoed.  Action on "1000&lt;cr&gt;"
         is withheld]

      U: WASHINGTON &lt;space&gt;

      S: &lt;space&gt;&lt;IAC&gt;&lt;SB&gt;&lt;RCTE&gt;&lt;3&gt;

      P: &lt;space&gt; 1000

      U: 1000&lt;cr&gt;

      S: &lt;cr&gt;&lt;lf&gt; JOB 17 ON TTY41 7-JUN-73 14:13 &lt;cr&gt;&lt;lf&gt;@
         &lt;IAC&gt;&lt;SB&gt;&lt;RCTE&gt;&lt;0&gt;

      P: &lt;cr&gt;&lt;lf&gt; JOB 17 ON TTY41 7-JUN-73 14:13 &lt;cr&gt;&lt;lf&gt;@

      T: DED &lt;esc&gt;&lt;cr&gt;

      P: DED

      U: DED&lt;esc&gt;

      S: .SAV;1 &lt;IAC&gt;&lt;SB&gt;&lt;RCTE&gt;&lt;0&gt;

      P: .SAV;1

      U: &lt;cr&gt;

      S: &lt;cr&gt;&lt;lf&gt;&lt;lf&gt; Ded   3/14/73 DRO,KRK &lt;cr&gt;&lt;lf&gt;:
         &lt;IAC&gt;&lt;SB&gt;&lt;RCTE&gt;&lt;15&gt;&lt;1&gt;&lt;255&gt;

         [The program is started and the DED prompt ":" is sent.  At the
         command level, DED responds to every character.]

      P: &lt;cr&gt;&lt;lf&gt;&lt;lf&gt; DED   3/14/73 DRP,KRK &lt;cr&gt;&lt;lf&gt;:

      T: IThis is a text line.&lt;cr&gt; This is another test line.&lt;^Z&gt; Q

         ["I" means Insert Text.  The text follows, terminated by a
         Control-Z.  The "Q" instructs DED to Quit.]

      U: I







<span class="grey">Crocker, et. al.                                                [Page 9]</span></pre>
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<span class="grey"><a href="./rfc560">RFC 560</a>               RCT &amp; Echoing TELNET Option            August 1973</span>


      S: I&lt;cr&gt;&lt;lf&gt;* &lt;IAC&gt;&lt;SB&gt;&lt;RCTE&gt;&lt;11&gt;&lt;0&gt;&lt;24&gt;

         [DED prompts the user, during text input, with an asterisk at
         the beginning of every line.]

      P: I&lt;cr&gt;&lt;lf&gt; *This is a test line.

      U: This is a test line.&lt;cr&gt;

      S: &lt;cr&gt;&lt;lf&gt;* &lt;IAC&gt;&lt;SB&gt;&lt;RCTE&gt;&lt;O&gt;

      P: &lt;cr&gt;&lt;lf&gt;* This is another test line.

      U: This is another test line.&lt;^Z&gt;

      S: ^Z&lt;cr&gt;lf&gt;: &lt;IAC&gt;&lt;SB&gt;&lt;RCTE&gt;&lt;15&gt;&lt;1&gt;&lt;255&gt;

         [The returned "^Z" is two characters, not the ASCII Control-Z.]

      U: Q

         [Note that the "Q" is not yet printed on the terminal, since it
         is a Break character.]

      S: Q&lt;cr&gt;&lt;lf&gt;@ &lt;IAC&gt;&lt;SB&gt;&lt;RCTE&gt;&lt;11&gt;&lt;1&gt;&lt;24&gt;

      P: Q&lt;cr&gt;&lt;lf&gt; @

      And the user is returned to the Exec level.

   E. Tables:

      (1) &lt;cmd&gt; is one 8-bit byte having the following flags (bits are
          counted from the right):

          Bit Meaning

          0   0 = Ignore all other bits in this byte and repeat the last
                  &lt;cmd&gt; that was sent.  Equals a 'continue what you have
                  been doing'.

                  1 = Perform actions as indicated by other bits in this
                  byte.

          1   0 = Print (echo) Break character
              1 = Skip (don't echo) Break character





<span class="grey">Crocker, et. al.                                               [Page 10]</span></pre>
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<span class="grey"><a href="./rfc560">RFC 560</a>               RCT &amp; Echoing TELNET Option            August 1973</span>


          2   0 = Print (echo) text up to Break character
              1 = Skip (don't echo) text up to Break character

          3   0 = Continue using same classes of Break characters.
              1 = The two 8-bit bytes following this byte contain flags
                  for the new Break classes.

          4   0 = Continue using same classes of Transmit characters

              1 = Reset Transmit classes according two the two bytes
                  following 1) the Break classes bytes, if the Break
                  classes are also being reset, or 2) this byte, if the
                  Break classes are NOT also being reset.

      Byte Value (decimal) and its meaning:

         0 = Continue what you have been doing

         Even numbers greater than zero (i.e., numbers with the right-
             most bit off) are in error and should be interpreted as
             equal to zero.  When the &lt;cmd&gt; is an even number greater
             than zero, Classes bytes TC1 &amp; TC2 and/or BC1 &amp; BC2 must
             not be sent.

         1 = Print (echo) up to AND INCLUDING Break character

         3 = Print up to Break character and SKIP (don't echo) Break
             character

         5 = Skip text (don't echo) up to Break character, but PRINT
             Break character

         7 = Skip up to and including Break character

         Add one of the previous non-zero values to one of the following
             values, to get the total decimal value for the byte (Note
             that Classes may not be reset, without also resetting the
             printing action; so an odd number is guaranteed):

         8 = Set Break classes (using the next two bytes [BC1 BC2])

         16 = Set Transmission classes (using the next two bytes [TC1
             TC2])

         24 = Set Break classes (using the next two bytes [BC1 BC2]) and
             the Transmission classes (using the two bytes after that
             [TC1 TC2]).




<span class="grey">Crocker, et. al.                                               [Page 11]</span></pre>
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<span class="grey"><a href="./rfc560">RFC 560</a>               RCT &amp; Echoing TELNET Option            August 1973</span>


      (2) Classes for Break and Transmission (The right-most bit of the
          second byte (TC2 or BC2) represents Class 1; the left-most bit
          of the first byte (TC1 or BC1) represents the currently
          undefined Class 16):

          1: Upper-Case Letter (A-Z)

          2: Lower-case letters (a-z)

          3: Numbers (0-9)

          4: Format Effectors (&lt;BS&gt; &lt;CR&gt; &lt;LF&gt; &lt;FF&gt; &lt;HT&gt; &lt;VT&gt;)

          5: Non-format effectors Control Characters, &lt;DEL&gt; and &lt;ESC&gt;

          6: . , ; : ? !

          7; - [ ( &lt; &gt; ) ] |

          8: ' "  / \ % @ $ # + - * = ^ &lt;- _ (square box symbol)

          9: &lt;space&gt;

          And Telnet commands (IAC...) are ALWAYS to have the effect of
          a Break character.


       [ This RFC was put into machine readable form for entry ]
             [ into the online RFC archives by Via Genie ]






















Crocker, et. al.                                               [Page 12]
</pre>