Telnet Remote Flow Control Option
RFC 1372

Document Type RFC - Proposed Standard (October 1992; No errata)
Obsoletes RFC 1080
Last updated 2013-03-02
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IESG IESG state RFC 1372 (Proposed Standard)
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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.

Introduction

   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

   TOGGLE-FLOW-CONTROL   33
       OFF                    0
       ON                     1
       RESTART-ANY            2
       RESTART-XON            3

2.  Command Meanings

   IAC WILL TOGGLE-FLOW-CONTROL

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

   IAC WONT 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.

Hedrick & Borman                                                [Page 1]
RFC 1372           Telnet Remote Flow Control Option        October 1992

   IAC DONT 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.

   IAC SB TOGGLE-FLOW-CONTROL ON IAC SE

      Sender requests receiver to enable flow control.

   IAC SB TOGGLE-FLOW-CONTROL RESTART-ANY IAC SE

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

   IAC SB TOGGLE-FLOW-CONTROL RESTART-XON IAC SE

      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

      WONT TOGGLE-FLOW-CONTROL DONT TOGGLE-FLOW-CONTROL

   meaning flow control information will not be exchanged in either
   direction.

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

Hedrick & Borman                                                [Page 2]
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
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