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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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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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    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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       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: HEDRICK@ARAMIS.RUTGERS.EDU

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