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rfc:rfc2217

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].

Clark Experimental [Page 1] RFC 2217 Telnet Com Port Control Option October 1997

 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:
  1. dial up connections to the Internet
  2. connecting to bulletin boards
  3. connecting to internal and external databases
  4. 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:
  1. 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.
  1. 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].
  1. 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 RFC 1372 "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.

Clark Experimental [Page 2] RFC 2217 Telnet Com Port Control Option October 1997

 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:
  1. serial printers
  2. plotters
  3. monitoring devices such as pipe line monitors or medical

monitors

  1. 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.

Clark Experimental [Page 3] RFC 2217 Telnet Com Port Control Option October 1997

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

Clark Experimental [Page 4] RFC 2217 Telnet Com Port Control Option October 1997

                 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.

Clark Experimental [Page 5] RFC 2217 Telnet Com Port Control Option October 1997

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.

Clark Experimental [Page 6] RFC 2217 Telnet Com Port Control Option October 1997

     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

Clark Experimental [Page 7] RFC 2217 Telnet Com Port Control Option October 1997

            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)

Clark Experimental [Page 8] RFC 2217 Telnet Com Port Control Option October 1997

           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].

Clark Experimental [Page 9] RFC 2217 Telnet Com Port Control Option October 1997

         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].

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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