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.
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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
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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
character.
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
IAC WILL 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
References
[1] Internet Engineering Task Force, Telnet Working Group,
D. Borman, Editor, "Telnet Linemode Option", RFC 1184,
Cray Research, Inc., October 1990.
Acknowledgments
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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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
Email: hedrick@cs.rutgers.edu
Mailing List: telnet-ietf@CRAY.COM
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