📄 rfc2217.txt
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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].
Clark Experimental [Page 10]
RFC 2217 Telnet Com Port Control Option October 1997
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
Clark Experimental [Page 11]
RFC 2217 Telnet Com Port Control Option October 1997
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
Clark Experimental [Page 12]
RFC 2217 Telnet Com Port Control Option October 1997
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 RFC 1372:
Telnet Remote Flow Control Option [2]. RFC 1372 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 RFC 1411 [3] or the SPX authentication protocol
established in RFC 1412 [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
Clark Experimental [Page 13]
RFC 2217 Telnet Com Port Control Option October 1997
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",
RFC 1372, Cray Research, Inc., October 1992.
[3] Borman, D., "Telnet Authentication: Kerberos Version 4",
RFC 1411, Cray Research, Inc., January 1993.
[4] Alagappan, K., "Telnet Authentication: SPX",
RFC 1412, 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.
Clark Experimental [Page 14]
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