Xon/Xoff State Control for Telnet Com Port Control Option
draft-edwards-telnet-xon-xoff-state-control-00
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Grant Edwards
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2010-03-23
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Internet Draft Grant Edwards
<draft-edwards-telnet-xon-xoff-state-control-00.txt> Comtrol Corp
Intended status: Experimental March 23, 2010
Expires September 23, 2010
Xon/Xoff State Control for Telnet Com Port Control Option
<draft-edwards-telnet-xon-xoff-state-control-00.txt>
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Edwards Expires September 23, 2010 [Page 1]
Internet Draft Telnet Xon/Xoff Flow State Control March 23, 2010
Abstract
This document defines new values for use with the telnet com port
control option's SET-CONTROL sub-command defined in RFC2217. These
new values provide a mechanism for the telnet client to control and
query the outbound Xon/Xoff flow control state of the telnet server's
physical serial port. This capability is exposed in the serial port
API on some operating systems and is needed by telnet clients that
implement a port-redirector service which provides applications local
to the redirector/telnet-client with transparent access to the remote
serial port on the telnet server.
1. Introduction
When a telnet server acting as a serial device server (as shown in
the figure below and also described in [RFC2217]) is configured to
use Xon/Xoff outbound flow control, it maintains an internal state
variable that reflects whether or not it is currently allowed to
transmit data via the physical serial port. When the telnet server
receives an XOFF character via the physical serial port, the server
will stop transmitting data via the physical serial port until an XON
character is received via the physical serial port.
+-----------+ telnet/RFC2217
| telnet | connection physical
| client +----+ / serial port
| | | /
+-----------+ | /
| /
+----+----------+ /
| |/ RS-232 +----------+
| telnet ++ with | serial |
| server |+-----------------+ device |
| ++ Xon/Xoff | |
| | flow +----------+
+---------------+ control
This memo extends that behavior by defining new telnet com port
control option [RFC2217] SET-CONTROL subcommand values that allow the
telnet client to force the server to behave as if an XOFF or XON
character has been received via the physical serial port.
This extension is needed by telnet clients that implement a port-
redirection service in order to provide other local applications
access to the remote serial port (physically located on the telnet
server) via the same serial port API that is used to access local
serial ports. Since some operating systems provide in the serial
port API a mechanism to control the state of the outbound Xon/Xoff
flow control, the telnet com port control option is extended to
Edwards Expires September 23, 2010 [Page 2]
Internet Draft Telnet Xon/Xoff Flow State Control March 23, 2010
provide a corresponding mechanism.
This ability to control the internal state of the Xon/Xoff flow
control feature is not provided by [RFC1372] "Telnet Remote Flow
Control Option" which provides a way to enable and disable the
Xon/Xoff flow control feature itself rather than to control the state
of the feature after it has been enabled.
2. Sub-command Value Definition
New values are defined for use in the SET_CONTROL sub-command which
is defined in section 3 of RFC2217 and reproduced here for
convenience:
IAC SB COM-PORT-OPTION SET-CONTROL <value> IAC SE
This document defines new values for the <value> field as follows:
Value Command
20 REQUEST-Xon/Xoff-STATE
21 SET-XOFF-STATE
22 SET-XON-STATE
If Xon/Xoff outbound flow control is not enabled, then the sub-
commands defined above will not affect data transmission in any way,
and the flow state returned by the server is undefined.
3. General Discussion
The Xon/Xoff flow state is defined to have two states: "XON" and
"XOFF".
The server will only transmit data via the physical serial port when
the flow state is XON. Transmission of data via the physical serial
port by the server is disabled when in the XOFF state.
The flow state is initially XON when a session is initiated.
During normal operation the state is set to XOFF when the server
receives an XOFF character via the physical serial port (tx data is
halted). Conversely, when an XON character is received, the state is
set to XON (tx data resumes).
When the telnet client sends the SET-XOFF-STATE command, the telnet
server will set the state to XOFF and stop sending data via the
physical serial port until either an XON character is received via
the physical serial port or the SET-XON-STATE command is received
from the telnet client.
Likewise, if the server has received an XOFF character via the
Edwards Expires September 23, 2010 [Page 3]
Internet Draft Telnet Xon/Xoff Flow State Control March 23, 2010
physical serial port, data transmission via the physical serial port
will be halted until either an XON character is received via the
physical serial port or the SET-XON-STATE command is received from
the telnet client.
This is summarized in the table below:
Event Flow State Tx Data
-------------- ---------- ------------
rx XOFF char XOFF Halted
SET-XOFF-STATE cmd XOFF Halted
rx XON char XON Allowed
SET-XON-STATE cmd XON Allowed
This document describes only the server internal state that governs
transmission of data via the physical serial port by the telnet
server. It does not address the operation of the flow control
mechanism that governs the stream of data being received on the
physical serial port by the server.
The responses to all three of the newly defined sub-command values
will reflect the flow control state after execution of the command.
Examples are shown below (byte values are in decimal). "Cmd" is
client->server and "Rsp" is server->client:
Query Xon/Xoff State:
Cmd: IAC SB COM-PORT-OPTION SET-CONTROL REQUEST-Xon/Xoff-STATE IAC SE
255 250 44 5 20 255 240
Rsp: IAC SB COM-PORT-OPTION SET-CONTROL SET-X[ON|OFF]-STATE IAC SE
255 250 44 105 [21/22]* 255 240
*Value in response is 21 if state is XOFF (tx suspended)
and is 22 if state is XON (tx allowed).
Set XOFF state (stops tx data):
Cmd: IAC SB COM-PORT-OPTION SET-CONTROL SET-XOFF-STATE IAC SE
255 250 44 5 21 255 240
Rsp: IAC SB COM-PORT-OPTION SET-CONTROL SET-XOFF-STATE IAC SE
255 250 44 105 21 255 240
Set XON state (starts tx data):
Edwards Expires September 23, 2010 [Page 4]
Internet Draft Telnet Xon/Xoff Flow State Control March 23, 2010
Cmd: IAC SB COM-PORT-OPTION SET-CONTROL SET-XON-STATE IAC SE
255 250 44 5 22 255 240
Rsp: IAC SB COM-PORT-OPTION SET-CONTROL SET-XON-STATE IAC SE
255 250 44 105 22 255 240
4. Usage by Redirectors
It is intended that MS Windows com-port redirector software will
issue a SET-XOFF-STATE command to the telnet device server when the
user application calls EscapeCommFunction(handle,SETXOFF) on the
virtual COM port, and will issue a SET-XON-STATE command to the
server when the user application calls
EscapeCommFunction(handle,SETXON) on the virtual COM port.
The two EscapeCommFunction() calls shown above do not directly affect
flow of data between the redirector (telnet client) and the device
server (telnet server).
The control of data flow between the telnet client and telnet server
is defined by section 5 of RFC2217. Xon/Xoff flow control and the
corresponding EscapeCommFunction() calls affect flow of data between
physical serial port on the device server and the external serial
device to which it is attached.
However, the Xon/Xoff flow state may indirectly affect data flow
between client and server if Xon/Xoff flow state changes cause
internal buffers to fill/drain to a point that triggers flow control
actions as defined in RFC2217 section 5.
Likewise, the flow-control commands defined in section 5 of RFC2217
have no direct effect on flow of data between the device server's
physical serial port and the external serial device to which it is
connected.
5. Security Considerations
There are no security considerations relevant to this document.
6. IANA Considerations
No actions are required from IANA as result of the publication of
this document.
7. References
7.1. Normative References
[RFC2217] Clark, G., "Telnet Com Port Control Option", RFC 2217,
October 1997.
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Internet Draft Telnet Xon/Xoff Flow State Control March 23, 2010
7.2. Informative References
[RFC1372] Hedrick, C. and D. Borman, "Telnet Remote Flow Control
Option", RFC 1372, October 1992.
Author's Address
Grant Edwards
Comtrol Corporation
Minneapolis, Minnesota USA
Email: grant.edwards@comtrol.com
URL: www.comtrol.com
Edwards Expires September 23, 2010 [Page 6]