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+Network Working Group                                         D. Crocker
+Request for Comments: 560                                      J. Postel
+Category: Protocols, TELNET                               20 August 1973
+NIC: 18492
+
+
+         Remote Controlled Transmission & 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.
+
+1. Command name and code:
+
+   RCTE
+
+2. Command meanings:
+
+   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.
+
+
+
+
+
+
+Crocker, et. al.                                                [Page 1]
+
+RFC 560               RCT & Echoing TELNET Option            August 1973
+
+
+   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 <cmd> [BC1 BC2] [TC1 TC2]
+
+      where:
+
+         <cmd> 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 <cmd> 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 <cmd> 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
+
+
+
+
+Crocker, et. al.                                                [Page 2]
+
+RFC 560               RCT & Echoing TELNET Option            August 1973
+
+
+                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.
+
+3. Default:
+
+   WON'T RCTE -- DON'T RCTE
+
+      Neither host asserts special control over the other host's
+      terminal printer.
+
+4. Motivation for the option:
+
+   RFC's 1, 5 and 51 discuss Network and process efficiency and
+   smoothness.
+
+   RFC 357, 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.
+
+
+
+
+
+
+Crocker, et. al.                                                [Page 3]
+
+RFC 560               RCT & Echoing TELNET Option            August 1973
+
+
+   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.
+
+5. Explicit description of control mechanism:
+
+   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 <cmd>
+      (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
+          <cmd>.
+
+
+
+
+
+
+
+Crocker, et. al.                                                [Page 4]
+
+RFC 560               RCT & Echoing TELNET Option            August 1973
+
+
+      (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 <cmd> 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
+          <cmd> 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 <cmd> 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 <cmd>
+             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 <cmd> will indicate what classes are to be
+                enabled.  There are currently nine (9) classes defined,
+                with room for expansion.
+
+
+
+Crocker, et. al.                                                [Page 5]
+
+RFC 560               RCT & Echoing TELNET Option            August 1973
+
+
+             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 <escape> (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 <cmd> 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 <cmd> from the serving
+                host will be properly synchronized with the terminal
+                input.
+
+
+
+
+
+
+
+Crocker, et. al.                                                [Page 6]
+
+RFC 560               RCT & Echoing TELNET Option            August 1973
+
+
+      (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, <cr> is usually a safe character to
+          echo but <esc> 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.
+
+
+
+
+
+
+
+
+
+Crocker, et. al.                                                [Page 7]
+
+RFC 560               RCT & Echoing TELNET Option            August 1973
+
+
+   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 (<>) is to be taken as a single
+         unit.  E.G., carriage return is <cr>, and the decimal value 27
+         is represented <27>.
+
+      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: <IAC><WILL><RCTE>
+
+      U: <IAC><DO><RCTE>
+
+      S: TENEX 1.31.18, TENEX EXEC 1.50.2 <cr><lf>@
+         <IAC><SB><RCTE><11><1><24>
+
+         [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 <DEL>), and 9
+         (<space>) act as Break Characters.]
+
+      P: TENEX 1.31.18, TENEX EXEC 1.50.2 <cr><lf>@
+
+      T: LOGIN ARPA <cr>
+
+      P: LOGIN
+
+      U: LOGIN <space>
+
+      S: <space><IAC><SB><RCTE><0>
+
+      P: <space>ARPA
+
+      U: ARPA <cr>
+
+      S: <cr><lf> (PASSWORD) : <IAC><SB><RCTE><7>
+
+      P: <cr><lf> (PASSWORD) :
+
+
+
+
+Crocker, et. al.                                                [Page 8]
+
+RFC 560               RCT & Echoing TELNET Option            August 1973
+
+
+      T: WASHINGTON 1000<cr>
+
+         [The password "WASHINGTON" is not echoed.  Action on "1000<cr>"
+         is withheld]
+
+      U: WASHINGTON <space>
+
+      S: <space><IAC><SB><RCTE><3>
+
+      P: <space> 1000
+
+      U: 1000<cr>
+
+      S: <cr><lf> JOB 17 ON TTY41 7-JUN-73 14:13 <cr><lf>@
+         <IAC><SB><RCTE><0>
+
+      P: <cr><lf> JOB 17 ON TTY41 7-JUN-73 14:13 <cr><lf>@
+
+      T: DED <esc><cr>
+
+      P: DED
+
+      U: DED<esc>
+
+      S: .SAV;1 <IAC><SB><RCTE><0>
+
+      P: .SAV;1
+
+      U: <cr>
+
+      S: <cr><lf><lf> Ded   3/14/73 DRO,KRK <cr><lf>:
+         <IAC><SB><RCTE><15><1><255>
+
+         [The program is started and the DED prompt ":" is sent.  At the
+         command level, DED responds to every character.]
+
+      P: <cr><lf><lf> DED   3/14/73 DRP,KRK <cr><lf>:
+
+      T: IThis is a text line.<cr> This is another test line.<^Z> Q
+
+         ["I" means Insert Text.  The text follows, terminated by a
+         Control-Z.  The "Q" instructs DED to Quit.]
+
+      U: I
+
+
+
+
+
+
+
+Crocker, et. al.                                                [Page 9]
+
+RFC 560               RCT & Echoing TELNET Option            August 1973
+
+
+      S: I<cr><lf>* <IAC><SB><RCTE><11><0><24>
+
+         [DED prompts the user, during text input, with an asterisk at
+         the beginning of every line.]
+
+      P: I<cr><lf> *This is a test line.
+
+      U: This is a test line.<cr>
+
+      S: <cr><lf>* <IAC><SB><RCTE><O>
+
+      P: <cr><lf>* This is another test line.
+
+      U: This is another test line.<^Z>
+
+      S: ^Z<cr>lf>: <IAC><SB><RCTE><15><1><255>
+
+         [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<cr><lf>@ <IAC><SB><RCTE><11><1><24>
+
+      P: Q<cr><lf> @
+
+      And the user is returned to the Exec level.
+
+   E. Tables:
+
+      (1) <cmd> 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
+                  <cmd> 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
+
+
+
+
+
+Crocker, et. al.                                               [Page 10]
+
+RFC 560               RCT & Echoing TELNET Option            August 1973
+
+
+          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 <cmd> is an even number greater
+             than zero, Classes bytes TC1 & TC2 and/or BC1 & 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]).
+
+
+
+
+Crocker, et. al.                                               [Page 11]
+
+RFC 560               RCT & Echoing TELNET Option            August 1973
+
+
+      (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 (<BS> <CR> <LF> <FF> <HT> <VT>)
+
+          5: Non-format effectors Control Characters, <DEL> and <ESC>
+
+          6: . , ; : ? !
+
+          7; - [ ( < > ) ] |
+
+          8: ' "  / \ % @ $ # + - * = ^ <- _ (square box symbol)
+
+          9: <space>
+
+          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]
+