path: root/doc/rfc/rfc2217.txt

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].



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   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 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.





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   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.



















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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









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                   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.








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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.



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       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



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              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)



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             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].




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           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].










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           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







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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







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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



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   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.














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