Network Working Group                                         C. Hedrick
Request for Comments: 1372                            Rutgers University
Obsoletes: RFC 1080                                            D. Borman
                                                     Cray Research, Inc.
                                                            October 1992

                   Telnet Remote Flow Control Option

Status of This Memo

   This RFC specifies an IAB standards track protocol for the Internet
   community, and requests discussion and suggestions for improvements.
   Please refer to the current edition of the "IAB Official Protocol
   Standards" for the standardization state and status of this protocol.
   Distribution of this memo is unlimited.


   This document specifies an extended version of the Telnet Remote Flow
   Control Option, RFC 1080, with the addition of the RESTART-ANY and
   RESTART-XON suboptions.

1.  Command Names and Codes

       OFF                    0
       ON                     1
       RESTART-ANY            2
       RESTART-XON            3

2.  Command Meanings


      Sender is willing to enable and disable flow control upon command.


      Sender refuses to enable and disable flow control.  Nothing is
      implied about whether sender does or does not use flow control.
      It is simply unwilling to enable and disable it using this


      Sender is willing to send commands to enable and disable flow

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RFC 1372           Telnet Remote Flow Control Option        October 1992


      Sender refuses to send command to enable and disable flow control.


      Sender requests receiver to disable flow control.


      Sender requests receiver to enable flow control.


      Sender requests that when flow control is enabled, the receiver
      allow any character (except another XOFF) to restart output.


      Sender requests that when flow control is enabled, the receiver
      allows only the XON character to restart output.

3.  Default Specification

   The default specification for this option is


   meaning flow control information will not be exchanged in either

4.  Motivation

   This memo describes a method of remotely toggling flow control
   between a user telnet process and the attached terminal.  Only flow
   control of data being transmitted from the telnet process to the
   terminal is considered.  Many systems will also allow flow control of
   data from the terminal to the telnet process, however there is seldom
   need to change this behavior repeatedly during the session.

   There are two common ways of doing flow control: hardware and
   software.  Hardware flow control uses signals on wires dedicated for
   this purpose.  Software flow control uses one or two specific
   characters sent along the same path as normal input data.  Most
   commonly, XOFF (control-S) and XON (control-Q) are used to stop and
   start output, respectively.  The option described herein is useful
   primarily where software flow control is being used.  (Since hardware
   flow control does not preempt any characters, there is normally no

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RFC 1372           Telnet Remote Flow Control Option        October 1992

   need to disable it.)  It should also be noted that flow control can
   either be generated automatically by the terminal when its buffers
   are close to overflowing, or manually by the user, when he/she cannot
   read the information as fast as it is being displayed, and unread
   information will start scrolling off the screen.

   The primary difficulty with software flow control is that it preempts
   one or two characters.  Host software often requires the user to be
   able to input every possible ASCII character.  (Certain editors are
   notorious for having XOFF and XON as commonly-used commands.)  For
   this reason, operating systems often allow programs to disable flow
   control.  While it is disabled, the characters that normally signal
   flow control may be read as normal input.  In a telnet environment,
   flow control is normally done by the user telnet process, not by the
   host computer.  In addition, many operating systems, when flow
   control is enabled, the user may specify whether the XOFF character
   is the only character that is allowed to re-enable the output of
   data, or whether any typed character should re-enable the flow of
   data.  Thus this RFC defines a way to propagate flow control status
   from the host computer to the user telnet process.

5.  Description of the Option

   Use of the option requires two phases.  In the first phase, the
   telnet processes agree that one of them will TOGGLE-FLOW-CONTROL.
   WILL and DO are used only in this first phase.  In general there will
   be only one exchange of WILL and DO for a session.  Subnegotiations
   must not be issued until DO and WILL have been exchanged.  It is
   permissible for either side to turn off the option by sending a WONT
   or DONT.  Should this happen, no more subnegotiations may be sent,
   unless the option is re-enabled by another exchange of DO and WILL.

   Once the hosts have exchanged a WILL and a DO, the sender of the DO
   TOGGLE-FLOW-CONTROL is free to send subnegotiations to enable and
   disable flow control in the other process, and to send
   subnegotiations for recommendations on when to restart output.
   Normally, the sender of the DO will be a host, and the other end will
   be a user telnet process, which is connected to a terminal.  Thus the
   protocol is normally asymmetric, however it may be used in both
   directions without confusion should need for this arise.

   As soon as the DO and WILL have been exchanged, the sender of the
   WILL must enable flow control.  This allows flow control to begin in
   a known state.  The decision of whether to restart output only when
   the XON character is received, or when any character received, starts
   in a system dependent state.  (This is to make it consistent with
   older implementations of the TOGGLE-FLOW-CONTROL option that do not
   understand the RESTART-ANY and RESTART-XON suboptions.)  The sender

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RFC 1372           Telnet Remote Flow Control Option        October 1992

   of the DO should send either a RESTART-ANY or RESTART-XON suboption
   to put the restart characteristics to a know state.  Some clients
   might not be able to support both of the RESTART-ANY and RESTART-XON
   modes due to system limitations, and would then choose to ignore
   these commands.  There is no way for the server to be notified of
   this condition, but a client should make every attempt to honor the
   state requested by the RESTART-ANY and RESTART-XON modes.  Should the
   option be disabled by exchange of DONT and WONT, flow control may
   revert to an implementation-defined default state.  It is not safe to
   assume that flow control will remain in the state requested by the
   most recent subnegotiation.

   In most implementations of software flow control, when enabled, the
   XOFF and XON characters are never propagated to the server; they are
   typically eaten by the terminal driver between the telnet client and
   the attached terminal.  In most implementations that support the
   RESTART-ANY functionality, the typed character that re-enables the
   output is not eaten by the terminal driver, unless it is the XON

   Currently, only four command codes are defined for the
   subnegotiations: flow control off (code 0), flow control on (code 1),
   restart output on any character (code 2) and restart output only on
   XON (code 3).  None of these codes requires any additional data,
   however it is possible that additional commands may be added.  Thus
   subnegotiations having command codes other than those defined in this
   document should be silently ignored.

   This option does not deal with the issue of allowing the DO side of
   the connection to inform the WILL side which characters should be
   used for XON and XOFF.  That functionality is already supplied by the
   LINEMODE [1] option.

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RFC 1372           Telnet Remote Flow Control Option        October 1992

6.  Example

   Here is an example of the use of this option:

       Client                           Server
                                        IAC DO TOGGLE-FLOW-CONTROL
       [ The server is now free to send commands to change flow control.
         Note that the client must now have enabled flow control, but
         that whether it is restart on XON only or on any character is
         client specific.  ]
                                        IAC SB TOGGLE-FLOW-CONTROL
                                        RESTART-ANY IAC SE

       [ The client should now switch to allowing output to restart when
         the user types any character, if the client is able to support
         that functionality.  ]
                                        IAC SB TOGGLE-FLOW-CONTROL OFF
                                        IAC SE
                                        IAC SB TOGGLE-FLOW-CONTROL ON
                                        IAC SE


   [1] Internet Engineering Task Force, Telnet Working Group,
       D. Borman, Editor, "Telnet Linemode Option", RFC 1184,
       Cray Research, Inc., October 1990.


   The original specification for this option was written by Charles
   Hedrick, and published as RFC 1080.  The RESTART-ANY and RESTART-XON
   suboptions were defined and added to this version of the document by
   David Borman.

Security Considerations

   Security issues are not discussed in this memo.

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RFC 1372           Telnet Remote Flow Control Option        October 1992

Authors' Addresses

   David Borman
   Cray Research, Inc.
   655F Lone Oak Drive
   Eagan, MN 55121

   Phone: (612) 452-6650
   Email: dab@CRAY.COM

   Charles Hedrick
   Director, LCSR Computing Facility
   Rutgers University
   227 CORE Building
   P.O. Box 879
   Piscataway, NJ  08855-0879

   Phone: (908) 932-3088

   Mailing List: telnet-ietf@CRAY.COM

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