Network Working Group                                         C. Hedrick
Request for Comments: 1080                            Rutgers University
                                                          November 1988


                  Telnet Remote Flow Control Option

Status of This Memo

  This RFC specifies a standard for the Internet community.  Hosts on
  the Internet that do remote flow control within the Telnet protocol
  are expected to adopt and implement this standard.  Distribution of
  this memo is unlimited.

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
  need to disable it.)

  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.  Thus this RFC defines a way to propagate flow control
  status from the host computer to the user telnet process.








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RFC 1080           Telnet Remote Flow Control Option       December 1988


1. Command Name and Code

     TOGGLE-FLOW-CONTROL

     Code = 33

2. Command Meanings

     IAC WILL TOGGLE-FLOW-CONTROL

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

     IAC WON'T TOGGLE-FLOW-CONTROL

        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
        protocol.

     IAC DO TOGGLE-FLOW-CONTROL

        Sender is willing to send commands to enable and disable flow
        control.

     IAC DON'T TOGGLE-FLOW-CONTROL

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

     IAC SB TOGGLE-FLOW-CONTROL OFF IAC SE

        Sender requests receiver to disable flow control.  The code for
        OFF is 0.

     IAC SB TOGGLE-FLOW-CONTROL ON IAC SE

        Sender requests receiver to enable flow control.  The code for
        ON is 1.

3. Default

     WON'T TOGGLE-FLOW-CONTROL

        Flow control information will not be exchanged.






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RFC 1080           Telnet Remote Flow Control Option       December 1988


     DON'T TOGGLE-FLOW-CONTROL

        Flow control information will not be exchanged.

4. 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 reenabled 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.  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.  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.

  Currently, only two command codes are defined for the
  subnegotiations: flow control off (code 0) and flow control on (code
  1).  Neither of these codes requires any additional data.  However it
  is possible that additional commands may be added.  Thus
  subnegotiations having command codes other than 0 and 1 should be
  ignored.

     Here is an example of use of this option:

        Host1: IAC DO TOGGLE-FLOW-CONTROL

        Host2: IAC WILL TOGGLE-FLOW-CONTROL

        (Host1 is now free to send commands to change flow control.
        Note that host2 must now have enabled flow control.)






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RFC 1080           Telnet Remote Flow Control Option       December 1988


        Host1: IAC SB TOGGLE-FLOW-CONTROL OFF IAC SE

        Host1: IAC SB TOGGLE-FLOW-CONTROL ON IAC SE

Author's Address:

  Charles Hedrick
  Rutgers University
  Center for Computer and Information Services
  Hill Center, Busch Campus
  P.O. Box 879
  Piscataway, NJ 08855-0879

  Phone: (201) 932-3088

  Email: [email protected]



































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