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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.
Hedrick [Page 1]
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. Hedrick [Page 2]
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.) Hedrick [Page 3]
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: HEDRICK@ARAMIS.RUTGERS.EDU Hedrick [Page 4]