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RFC2217 - Telnet Com Port Control Option

王朝other·作者佚名  2008-05-31
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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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