RFC2217 - Telnet Com Port Control Option

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　　Network Working Group G. Clark
Request for Comments: 2217 Cisco Systems, Inc.
Category: Experimental October 1997

Telnet Com Port Control Option

Status of this Memo

This memo defines an Experimental Protocol for the Internet
community. This memo does not specify an Internet standard of any
kind. Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.

Introduction

This memo proposes a protocol to allow greater use of modems attached
to a network for outbound dialing purposes.

Table of Contents
1. Negotiation of the Com Port
Control Option Protocol .................. 5
2. Com Port Configuration Commands .................. 6
Version
Baud Rate
Data Bit Size
Parity
Stop Bit size
3. Special Com Port Control Commands ................. 8
XON/XOFF Flow Control
HARDWARE Flow Control
BREAK Signal
DTR Signal
RTS Signal
4. Notification of Com Port and .................. 12
Modem Line Changes
5. Flow Control .................. 13
6. Security Considerations .................. 13
7. Author's Address .................. 14
8. Reference Section .................. 14

Discussion

The Telnet protocol defines an interactive, character-oriented
communications session. It was originally designed to establish a
session between a client and a remote login service running on a host
[5].

Many new business functions require a person to connect to remote
services to retrieve or deposit information. By in large, these
remote services are accessed via an async dial up connection. This
new class of functions include:

- dial up connections to the Internet
- connecting to bulletin boards
- connecting to internal and external databases
- sending and receiving faxes.

The general nature of this new class of function requires an
interactive, character-oriented communications session via an async
modem. This is typically known as outbound modem dialing.

To help defer the cost of installing and maintaining additional phone
lines which may be used very little per person, many equipment
manufacturers have added the ability to establish a Telnet session
directly to the outbound ports on many of the most popular access
servers and routers, here after referred to as access servers.

However, the current Telnet protocol definitions are not sufficient
to fully support this new use. There are three new areas of
functionality which need to be added to the Telnet protocol to
successfully support the needs of outbound modem dialing. These are:

- The ability for the client to send com port configuration
information to the access server which is connected to the
outbound modem. This is needed to ensure the data being
transmitted and received by the modem is formatted correctly
at the byte level.

- The ability for the access server to inform the client of any
modem line or signal changes such as RLSD changes (carrier
detect). This information is vital, since many client software
packages use this information to determine if a session with the
remote service has been established. RLSD changes are also
used for signaling in Class I faxing [6].

- The ability to manage flow control between the client and
the access server which does not interfere with the flow
control mechanisms used by the session between the client and
the remote service. Unfortunately RFC1372 "Telnet Remote
Flow Control Option" [2] can not be used for this purpose
because it relies on sending XON/XOFF style characters which
maybe transmitted or received as a normal course of the
client / remote service session.

Though this discussion has focused on outbound modem dialing as the
primary use of this protocol, the protocol can also be used for any
serial device attached to an access server. Such devices could be:

- serial printers
- plotters
- monitoring devices such as pipe line monitors or medical
monitors
- general office equipment such as photo-copiers and cash
registers

Definition of Terms

Access Server - Any network device which accepts Telnet sessions
and passes the data received to a com port, and
passes data received from the com port to the client
via the Telnet session.

Baud Rate - For the purposes of this document, baud rate will
mean the communications of data in bits per second.

Client - Any network device which initiates a Telnet session
to an access server.

Outbound - Transmission of data from the modem attached to the
access server to a remote service.

Inbound - Transmission of data from the remote service to the
modem attached to the access server.

Remove Service - Any service which accepts dial-up connections,
including fax machines.

Illustration

=====================
| |
| CLIENT |\
| | \ < ---- Local Area /
===================== \ Enterprise Network
\
\
=============================
| Telnet Interface |
| | |
| | |
| ACCESS SERVER | |
| | |
| | |
| Com Port Interface |
=============================
|
|
==================
| |
| MODEM |
| |
==================
|
Access to Remote Service |
most commonly Public Switched ----->|
Network |
|
|
======================
Could be Internet Service | |
Provider, Bulletin Board | |
or FAX machine | REMOTE SERVICE |
| |
| |
======================

Command Names and Codes:
COM-PORT-OPTION 44

Client to Access Server Access Server 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, com port configuration
commands are only sent from the client to the access
server. There is no current vision that the access
server would initiate the use of a com port configuration
command, only the notify commands. However, to allow for
access server initiated com port configurations different
command values have been established.

1. Negotiation of the Com Port Control Option Protocol

The negotiation of the com port control option protocol uses the
standard Telnet negotiation protocol mechanism:

IAC WILL COM-PORT-OPTION
The sender of this command is willing to send com port
control option commands.
IAC WONT COM-PORT-OPTION
The sender of this command refuses to send com port
control option commands.
IAC DO COM-PORT-OPTION
The sender of this command is willing to accept com port
control option commands.
IAC DONT COM-PORT-OPTION
The sender of this command refuses to accept com port control
options commands.

Typically a client will use WILL and WONT, while an access server
will use DO and DONT.

2. Com Port 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 access server must be acknowledged
once the command has been processed by the access server. This
confirmation informs the client of the value set at the access server
after the processing of the command. This acknowledgment is not used
to acknowledge the receipt of the command, which is handled at the
TCP protocol layer. Its purpose is to inform the client of the value
in use, which may be different than the value requested in the
client's command. For example, the client may request a baud rate
higher than the access service can provide. If an acknowledgment is
not received by the client within a reasonable time (such as twice
the delay acknowledgment timer), the client may wish to resend the
command or terminate the session.

Though the commands may be sent from the client to the access server
in any sequence, there are sequences which may result in invalid
configurations for the com port (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

IAC SB COM-PORT-OPTION SIGNATURE <text> IAC SE
This command may be sent by either the client or the access
server 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. There is no structure to the
<text> field. It may contain manufacturer information, version
number information, or any other information desired. If an
IAC character appears in the text it must be translated to
IAC-IAC to avoid conflict with the IAC which terminates
the command.

IAC SB COM-PORT-OPTION SET-BAUD <value(4)> IAC SE
This command is sent by the client to the access server to set
the baud rate of the com port. The value is four octets (4 bytes).
The value is represented in network standard format. The value
is the baud rate being requested. A special case is the value 0.
If the value is zero the client is requesting the current baud
rate of the com port on the access server.

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 rates.

IAC SB COM-PORT-OPTION SET-DATASIZE <value> IAC SE
This command is sent by the client to the access server 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 COM-PORT-OPTION SET-PARITY <value> IAC SE
This command is sent by the client to the access server 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.

IAC SB COM-PORT-OPTION SET-STOPSIZE <value> IAC SE
This command is sent by the client to the access server 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 com port hardware
only if data size is set to 5 bits. It is not
commonly used.

3. Special Com Port Control Commands

The client can send this command to the access server at any time
and multiple times throughout the Telnet session. Each command
transmitted from the client to the access server 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 or terminate the
session.

IAC SB COM-PORT-OPTION SET-CONTROL <value> IAC SE
This command is sent by the client to the access server to set
special com port 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 Com Port 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 Com Port Flow Control Setting (inbound)
14 Use No Flow Control (inbound)
15 Use XON/XOFF Flow Control (inbound)
16 Use HARDWARE Flow Control (inbound)
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: Flow control options were divided into inbound and
outbound to take full advantage of existing
programming interfaces and access server
capabilities.

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 access server 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 COM-PORT-OPTION SET-LINESTATE-MASK <value> IAC SE
This command is sent by the client to the access server to set a
bit mask for the sending of the NOTIFY-LINESTATE option (see
section 4). When the LINESTATE changes on the access server, the
access server will "AND" the new LINESTATE with the LINESTATE-
MASK. If the result is not zero, the access server will send the
result of the "AND" as the value in a NOTIFY-LINESTATE com port
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 (com port 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

Discussion: The SET-LINESTATE-MASK value of 0 will prevent the
access server from sending NOTIFY-LINESTATE options
to the client.

Discussion: The SET-LINESTATE-MASK value of 255 will allow the
access server to send a NOTIFY-LINESTATE option to
the client each time the LINESTATE changes on the
access server.

Discussion: The initial LINESTATE-MASK at the access server 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 access server
is retained until set by the client or reset at the
start of a new Telnet session.

IAC SB COM-PORT-OPTION SET-MODEMSTATE-MASK <value> IAC SE
This command is sent by the client to the access server to set a
bit mask for the sending of the NOTIFY-MODEMSTATE option (see
section 4). When the MODEMSTATE changes on the access server,
the access server will "AND" the new MODEMSTATE with the
MODEMSTATE-MASK. If the result is not zero, the access server
will send the result of the "AND" as the value in a NOTIFY-
MODEMSTATE com port 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 (com port 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

Discussion: The SET-MODEMSTATE-MASK value of 0 will prevent the
access server from sending NOTIFY-MODEMSTATE options
to the client.

Discussion: The SET-MODEMSTATE-MASK value of 255 will allow the
access server to send a NOTIFY-MODEMSTATE option to
the client each time the MODEMSTATE changes on the
access server.

Discussion: The initial MODEMSTATE-MASK at the access server
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 access server
is retained until set by the client or reset at the
start of a new Telnet session.

IAC SB COM-PORT-OPTION PURGE-DATA <value> IAC SE
This command is sent by the client to the access server to
instruct the access server 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 access server receive data buffer
2 Purge access server transmit data buffer
3 Purge both the access server receive data
buffer and the access server transmit data
buffer
4-127 Available for Future Use

4. Notification of Com port and Modem Line Changes

The access server can send these commands to the client any time
and multiple times throughout the Telnet session. The access
server should send the appropriate command to the client as soon
as the com port or modem line changes occurs. The client does
not issue a response to these commands.

IAC SB COM-PORT-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 (com port 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

Discussion: The LINESTATE is the line state of the UART on
the access server.

IAC SB COM-PORT-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 (com port 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 access server can send these commands any time and
multiple times throughout the Telnet session.

IAC SB COM-PORT-OPTION FLOWCONTROL-SUSPEND IAC SE
The 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 COM-PORT-OPTION FLOWCONTROL-RESUME IAC SE
The 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 the access server
and the access server 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.

Discussion: The flow control option is designed to handle client
to access server flow control for the Telnet session.
This option has been added in deference to RFC1372:
Telnet Remote Flow Control Option [2]. RFC1372 uses
a 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
dial up service).

6. Security Considerations

There are two security issues to discuss; authentication and
resetting resources.

Authentication can follow either the Kerberos authentication protocol
established in RFC1411 [3] or the SPX authentication protocol
established in RFC1412 [4].

Once the Telnet session between the client and the access server has
been terminated, the access server should ensure the connection to
the remote service 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. This ensures the com
port is in a known state and ready to receive the next client
session. This will make operations more predicable and avoid
problems which might occur from starting a new session with random
com port configurations.

7. Author's Address

Glen Clark, Software Architect
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 96134
USA

EMail: glenc@cisco.com
WEB: www.cisco.com

8. Reference Section

[1] Joe Campbell. C Programmer's Guide to Serial Communications,
Second Edition. Indianapolis: SAMS Publishing, 1993. 213-224.

[2] Hedrick, C., and D. Borman, "Telnet Remote Flow Control Option",
RFC1372, Cray Research, Inc., October 1992.

[3] Borman, D., "Telnet Authentication: Kerberos Version 4",
RFC1411, Cray Research, Inc., January 1993.

[4] Alagappan, K., "Telnet Authentication: SPX",
RFC1412, Digital Equipment Corporation, January 1993.

[5] D. E. Comer and David Stevens. Internetworking with TCP/IP,
Volume III. Prentice Hall, 1993.

[6] Andrew Margolis. The FAX Modem Sourcebook. John Wiley & Sons.
1995.

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