INTERNET-DRAFT
Network Working Group Glen Clark
Request for Comments: nnnn ESDL, Inc.
Category: Standards Track May 1997
Revision: 0005
Telnet Comport Control Option
Preamble:
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its
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Status Section:
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the
"Internet Official Protocol Standards" (STD 1) for the
standardization state and status of this protocol. Distribution of
this memo is unlimited.
Introduction Section:
This memo proposes a protocol to allow greater use of modems
attached to a network. Increased needs for "off network"
communications has increased the need for modems. Increasing the
functionality of Telnet increases the functionality of network
attached modems. For example, the ability to send a FAX via a
network attached modem. This memo addresses configuration of the
comport to which the modem is attached. It does not address the
internal configuration of the modem itself.
Clark Page [1]
RFC: NNNN Telnet Comport Control Option May 1997
Table of Contents
1. Negotiation of the Comport
Control Option Protocol .................. 3
2. Comport Configuration Commands .................. 3
Version
Baud Rate
Data Bit Size
Parity
Stop Bit size
3. Special Comport Control Commands .................. 6
XON/XOFF Flow Control
HARDWARE Flow Control
BREAK Signal
DTR Signal
RTS Signal
4. Notification of Comport and .................. 9
Modem Line Changes
5. Flow Control .................. 10
6. Security Considerations .................. 11
Command Names and Codes:
COMPORT-OPTION 40
Client to Host Host to Client
SIGNATURE text text
SET-BAUDRATE 1 101
SET-DATASIZE 2 102
SET-PARITY 3 103
SET-STOPSIZE 4 104
SET-CONTROL 5 105
NOTIFY-LINESTATE 6 106
NOTIFY-MODEMSTATE 7 107
FLOWCONTROL-SUSPEND 8 108
FLOWCONTROL-RESUME 9 109
SET-LINESTATE-MASK 10 110
SET-MODEMSTATE-MASK 11 111
PURGE-DATA 12 112
Discussion: As initially proposed, comport configuration commands
are only sent from the client to the host. There is no
current vision that the host would initiate the use a
comport configuration command, only the notify
commands. However, to allow for host initiated
comport configuration different command values
have been established.
Clark Page [2]
RFC: NNNN Telnet Comport Control Option May 1997
1. Negotiation of the Comport Control Option Protocol
The negotiation of the comport control option protocol uses the
standard Telnet negotiation protocol mechanism:
IAC WILL COMPORT-OPTION
The sender of this command is willing to send comport
control option commands.
IAC WONT COMPORT-OPTION
The sender refuses to send comport control option commands.
IAC DO COMPORT-OPTION
The sender of this command is willing to accept comport
control option commands.
IAC DONT COMPORT-OPTION
The sender of this command refuses to accept comport control
options commands.
Typically a client will use WILL and WONT, while a host will use
DO and DONT.
2. Comport Configuration Commands
Once DO and WILL have been negotiated, the client may send any of
the following commands. The client can send these commands at any
time and multiple times throughout the Telnet session. Each
command transmitted from the client to the host should be
acknowledged with a confirmation of the command and the actual
value set. It is at the host's discretion to acknowledge commands
received. If an acknowledgment is generated, the client should
expect it within a reasonable time (such as twice the delay
acknowledgment timer). The client may wish to resend any command
which is not acknowledged, however the client may also ignore
the failure to receive an acknowledgment.
Though the commands may be sent from the client to the host in any
sequence, there are sequences which may result in invalid
configurations for the comport (for example: EVEN parity is only
valid if the data size is set to less than 8 bits). Thus it is
recommended that commands be issued in the following sequence:
1. SET-BAUDRATE
2. SET-DATASIZE
3. SET-PARITY
4. SET-STOPSIZE
Discussion: RFC 1696 [5] refers to "baud rate" as "modulation
speed". "Baud" and "baud rate" are used in this
document for clarity.
Discussion: The prompt response mechanism suggested here allows
the protocol to be used to query the values at the host
as well as set the values. The use of this mechanism
provides more functionality than a simple ACK/NAK
protocol.
Clark Page [3]
RFC: NNNN Telnet Comport Control Option May 1997
IAC SB COMPORT-OPTION SIGNATURE <text> IAC SE
This command may be sent by either the client or the host
to exchange signature information. If the command is sent
without <text> it is a request from the sender to receive
the signature text of the receiver. The text may be a
combination of any characters. If IAC appears in the text
it must be translated to IAC-IAC to avoid conflict with
the IAC which terminates the command.
IAC SB COMPORT-OPTION SET-BAUD <value(4)> IAC SE
This command is sent by the client to the host to set the baud
rate of the comport. The value is four octets (4 bytes). This value
is represented in network standard format. The value is the baud
rate being requested. A special case is the value 0. If the baud
rate is zero the client is questing the current baud rate of the
comport.
Discussion: Though baud rates used today form a very sparse space,
and the initial version of the option used an index
based baud rate table, after much discussion with a
number of groups it has been determined that the
actual baud rate should be used. There are two main
reasons. 1) It limits the number of updates to the
option as faster baud rates come into use,
2) It provides the greatest amount of flexibility
in the selection of the baud rate.
Clark Page [4]
RFC: NNNN Telnet Comport Control Option May 1997
IAC SB COMPORT-OPTION SET-DATASIZE <value> IAC SE
This command is sent by the client to the host to set the data bit
size. The command can also be sent to query the current data bit
size. The value is one octet (byte). The value is an index into
the following value table:
Value Data Bit Size
0 Request Current Data Bit Size
1 Available for Future Use
2 Available for Future Use
3 Available for Future Use
4 Available for Future Use
5 5
6 6
7 7
8 8
9-127 Available for Future Use
Discussion: There are only eight possible values for the data bit
size, only four have ever been used historically and
only two are commonly used today. The use of the
command-value format is recommended to preserve
consistency with other commands. It also reduces the
number of commands defined in the protocol, and allows
for future expansion.
IAC SB COMPORT-OPTION SET-PARITY <value> IAC SE
This command is sent by the client to the host to set the parity.
The command can also be sent to query the current parity. The
value is one octet (byte). The value is an index into the
following value table:
Value Parity [1]
0 Request Current Data Size
1 NONE
2 ODD
3 EVEN
4 MARK
5 SPACE
6-127 Available for Future Use
Discussion: There are only five possible values for parity
commonly used today. The use of the command-value
format is recommended to preserve consistency with
other commands.
Clark Page [5]
RFC: NNNN Telnet Comport Control Option May 1997
IAC SB COMPORT-OPTION SET-STOPSIZE <value> IAC SE
This command is sent by the client to the host to set the number
of stop bits. The command can also be sent to query the current
stop bit size. The value is one octet (byte). The value is an
index into the following value table:
Value Stop Bit Size
0 Request Current Data Size
1 1
2 2
3 1.5
4-127 Available for Future Use
Discussion: Stop bit 1.5 is supported by most comport hardware
only if data size is set to 5 bits. It is not
commonly used.
3. Special Comport Control Commands
The client can send this command to the host at any time and
multiple times throughout the Telnet session. Each command
transmitted from the client to the host is acknowledged with a
confirmation of the command and the actual value set. The client
should expect a response within a reasonable time (such as twice
the delay acknowledgment timer). The client may wish to resend any
command which is not acknowledged, however the client may also
ignore the failure to receive an acknowledgment.
IAC SB COMPORT-OPTION SET-CONTROL <value> IAC SE
This command is sent by the client to the host to set special
comport options. The command can also be sent to query the current
option value. The value is one octet (byte). The value is an
index into the following value table:
Value Control Commands
0 Request Comport Flow Control Setting
(outbound/both)
1 Use No Flow Control (outbound/both)
2 Use XON/XOFF Flow Control (outbound/both)
3 Use HARDWARE Flow Control (outbound/both)
4 Request BREAK State
5 Set BREAK State ON
6 Set BREAK State OFF
7 Request DTR Signal State
8 Set DTR Signal State ON
9 Set DTR Signal State OFF
10 Request RTS Signal State
11 Set RTS Signal State ON
12 Set RTS Signal State OFF
13 Request Comport Flow Control Setting (inbound)
14 Use No Flow Control (inbound)
15 Use XON/XOFF Flow Control (inbound)
16 Use HARDWARE Flow Control (inbound)
(Table continues on next page)
Clark Page [6]
RFC: NNNN Telnet Comport Control Option May 1997
17 Use DCD Flow Control (outbound/both)
18 Use DTR Flow Control (inbound)
19 Use DSR Flow Control (outbound/both)
20-127 Available for Future Use
Discussion: Outbound is defined as flow of data from the modem
to the dialed up host.
Inbound is defined as flow of data from the dialed up
host to the modem.
Discussion: Flow control options were divided into inbound and
outbound to take full advantage of existing programming
interfaces and modem sharing devices (Telnet host).
Discussion: The outbound values should set flow control for both
outbound and inbound. If inbound is to be, or can be,
set separately it should be done after the setting of
the outbound value.
Discussion: If the Telnet host is not able to set inbound flow
control differently from the outbound flow control, it
should ignore the inbound flow control commands and
set the flow control option based on the outbound flow
control commands only.
IAC SB COMPORT-OPTION SET-LINESTATE-MASK <value> IAC SE
This command is sent by the client to the host to set a
bit mask for the sending of the NOTIFY-LINESTATE option (see
section 4). When the LINESTATE changes on the host, the host
will "AND" the new LINESTATE with the LINESTATE-MASK. If the
result is not zero, the host will send the result of the
"AND" as the value in a NOTIFY-LINESTATE comport option. If
more than one bit satisfies the LINESTATE-MASK, only one
NOTIFY-LINESTATE, with all the satisfying bits, will be
sent to the client. The SET-LINESTATE-MASK may be any
combination of bits as listed below. These are the same
bit values used in the NOTIFY-LINESTATE option. The
SET-LINESTATE-MASK values are based on the most popular
UART (comport control chip) in use. [1]
Bit Position Value Meaning
7 128 Time-out Error
6 64 Transfer Shift Register Empty
5 32 Transfer Holding Register Empty
4 16 Break-detect Error
3 8 Framing Error
2 4 Parity Error
1 2 Overrun Error
0 1 Data Ready
Clark Page [7]
RFC: NNNN Telnet Comport Control Option May 1997
Discussion: The SET-LINESTATE-MASK value of 0 will prevent the
host from sending NOTIFY-LINESTATE options to the
client.
Discussion: The SET-LINESTATE-MASK value of 255 will allow the
host to send a NOTIFY-LINESTATE option to the
client each time the LINESTATE changes on the host.
Discussion: The initial LINESTATE-MASK at the host is 0.
Discussion: The client does not have to send a new
SET-LINESTATE-MASK after receiving a
NOTIFY-LINESTATE. The LINESTATE-MASK on the host
is retained until set by the client or reset at
the start of a new Telnet session.
IAC SB COMPORT-OPTION SET-MODEMSTATE-MASK <value> IAC SE
This command is sent by the client to the host to set a
bit mask for the sending of the NOTIFY-MODEMSTATE option (see
section 4). When the MODEMSTATE changes on the host, the host
will "AND" the new MODEMSTATE with the MODEMSTATE-MASK. If the
result is not zero, the host will send the result of the
"AND" as the value in a NOTIFY-MODEMSTATE comport option. If
more than one bit satisfies the MODEMSTATE-MASK, only one
NOTIFY-MODEMSTATE, with all the satisfying bits, will be
sent to the client. The SET-MODEMSTATE-MASK may be any
combination of bits as listed below. These are the same
bit values used in the NOTIFY-MODEMSTATE option. The
SET-MODEMSTATE-MASK values are based on the most popular
UART (comport control chip) in use. [1]
Bit Position Value Meaning
7 128 Receive Line Signal Detect
(also known as Carrier Detect)
6 64 Ring Indicator
5 32 Data-Set-Ready Signal State
4 16 Clear-To-Send Signal State
3 8 Delta Receive Line Signal Detect
2 4 Trailing-edge Ring Detector
1 2 Delta Data-Set-Ready
0 1 Delta Clear-To-Send
Clark Page [8]
RFC: NNNN Telnet Comport Control Option May 1997
Discussion: The SET-MODEMSTATE-MASK value of 0 will prevent the
host from sending NOTIFY-MODEMSTATE options to the
client.
Discussion: The SET-MODEMSTATE-MASK value of 255 will allow the
host to send a NOTIFY-MODEMSTATE option to the
client each time the MODEMSTATE changes on the host.
Discussion: The initial MODEMSTATE-MASK at the host is 255.
Discussion: The client does not have to send a new
SET-MODEMSTATE-MASK after receiving a
NOTIFY-MODEMSTATE. The MODEMSTATE-MASK on the host
is retained until set by the client or reset at
the start of a new Telnet session.
IAC SB COMPORT-OPTION PURGE-DATA <value> IAC SE
This command is sent by the client to the host to instruct
the host to immediately clear all data from the buffer or
buffers referenced by the value. The value is one octet
(byte). The value is an index into the following value table:
Value Purge Data Buffer
0 Available for Future Use
1 Purge host receive data buffer
2 Purge host transmit data buffer
3 Purge both the host receive data buffer
and the host transmit data buffer
4-127 Available for Future Use
4. Notification of Comport and Modem Line Changes
The host can send these commands to the client any time and
multiple times throughout the Telnet session. The host should
send the appropriate command to the client as soon as the
comport or modem line changes occurs. The client does not issue
a response to these commands.
IAC SB COMPORT-OPTION NOTIFY-LINESTATE <value> IAC SE
The value is one octet (byte). The value is a bit level
composition made up from the value table below. Multiple bit
values may be set in a single transmission. The values are based
on the most popular UART (comport control chip) in use. [1]
Bit Position Value Meaning
7 128 Time-out Error
6 64 Transfer Shift Register Empty
5 32 Transfer Holding Register Empty
4 16 Break-detect Error
3 8 Framing Error
2 4 Parity Error
1 2 Overrun Error
0 1 Data Ready
Clark Page [9]
RFC: NNNN Telnet Comport Control Option May 1997
Discussion: The LINESTATE is the line state of the UART on
the host.
IAC SB COMPORT-OPTION NOTIFY-MODEMSTATE <value> IAC SE
The value is one octet (byte). The value is a bit level
composition made up from the value table below. Multiple bit
values may be set in a single transmission. The values are based
on the most popular UART (comport control chip) in use. [1]
Bit Position Value Meaning
7 128 Receive Line Signal Detect
(also known as Carrier Detect)
6 64 Ring Indicator
5 32 Data-Set-Ready Signal State
4 16 Clear-To-Send Signal State
3 8 Delta Receive Line Signal Detect
2 4 Trailing-edge Ring Detector
1 2 Delta Data-Set-Ready
0 1 Delta Clear-To-Send
5. Flow Control
The client and/or host can send these commands any time and
multiple times throughout the Telnet session.
IAC SB COMPORT-OPTION FLOWCONTROL-SUSPEND IAC SE
Sender of this command is requesting that the receiver suspend
transmission of both data and commands until the
FLOWCONTROL-RESUME is transmitted by the sender.
IAC SB COMPORT-OPTION FLOWCONTROL-RESUME IAC SE
Sender of this command is requesting that the receiver resume
transmission of both data and commands.
Discussion: Established Telnet sessions are initially in a
resume state between the client and Telnet host and
the Telnet host and the client. There is no need
to send the resume command during session
initialization.
Discussion: Multiple concurrent suspend commands may be sent.
Secondary suspend commands can be ignored.
Transmission will resume with the sending of a
single resume command.
Clark Page [10]
RFC: NNNN Telnet Comport Control Option May 1997
Discussion: The flow control option is designed to handle client
to Telnet host flow control for the Telnet session.
This option has been added in deference to
RFC 1372: Telnet Remote Flow Control Option [2].
RFC 1372 uses simple character XON/XOFF technology
to implement flow control. This can lead to two
problems. First, the flow control characters may
be valid data values. Second, the flow control
characters may be used for end to end flow control
(client application to remote dialup host).
6. Security Considerations
There are two security issues to discuss; authentication
and resetting resources.
Authentication can follow either the Kerberos authentication
protocol established in RFC 1411 [3] or the SPX authentication
protocol established in RFC 1412 [4].
The Telnet host should ensure the connection to the dialed up
host is disconnected and the com port geometry (baud rate,
data size, stop bits, parity, and flow control) is reset to a
factory or administrator defined configuration.
Discussion: Resetting resources is needed to ensure that each
session begins with the same configuration.
This will make operation more predicable and
avoid problems which might occur from starting
with different configurations.
Clark Page [11]
RFC: NNNN Telnet Comport Control Option May 1997
Author Address:
Glen Clark, Chief Architect
Enterprise Solutions Development Laboratories, Inc.
15127 NE 24th Street, Suite 157
Redmond, WA 98052
Internet: glenc@esdl.com
WEB: www.esdl.com
Reference Section:
[1] Joe Campbell. C Programmer's Guide to Serial Communications,
Second Edition. Indianapolis: SAMS Publishing, 1993. 213-224.
[2] Internet Engineering Task Force, Telnet Working Group,
C. Hedrick and D. Borman, "Telnet Remote Flow Control Option",
RFC 1372, Cray Research, Inc., October 1992.
[3] Internet Engineering Task Force, Telnet Working Group,
D. Borman, "Telnet Authentication: Kerberos Version 4",
RFC 1411, Cray Research, Inc., January 1993.
[4] Internet Engineering Task Force, Telnet Working Group,
K. Alagappan, "Telnet Authentication: SPX",
RFC 1412, Digital Equipment Corporation, January 1993.
[5] Internet Engineering Task Force, Telnet Working Group,
J. Barnes, et. al., "Modem Management Information Base (MIB)
using SMIv2", RFC 1696, Xylogics, Inc., August 1994.
Clark Page [12]