[Docs] [txt|pdf] [Tracker] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02 03 04 RFC 2217

INTERNET-DRAFT




Network Working Group                                         Glen Clark
Request for Comments: nnnn                           Cisco Systems, Inc.
Category: Standards Track                                     July  1997
Revision: 0006

                       Telnet Com Port Control Option
                     <draft-clark-telnet-control-04.txt>

Preamble:
  This document is an Internet-Draft.  Internet-Drafts are working
  documents of the Internet Engineering Task Force (IETF), its
  areas, and its working groups.  Note that other groups may also
  distribute working documents as Internet-Drafts.

  Internet-Drafts are draft documents valid for a maximum of six
  months and may be updated, replaced, or obsoleted by other
  documents at any time.  It is inappropriate to use Internet-
  Drafts as reference material or to cite them other than as
  "work in progress."

  To learn the current status of any Internet-Draft, please check
  the "1id-abstracts.txt" listing containing in the Internet-
  Drafts Shadow Directories on ftp.is.co.za (Africa),
  nic.nordu.net (Europe), munnari.oz.au (Pacific Rim),
  ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast).

Status Section:
  This document specifies an Internet standards track protocol for the
  Internet community, and requests discussion and suggestions for
  improvements. Please refer to the current edition of the
  "Internet Official Protocol Standards" (STD 1) for the
  standardization state and status of this protocol. Distribution of
  this memo is unlimited.

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

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                                                           Page [1]

RFC: NNNN              Telnet Com Port Control Option         July, 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:
    -  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.







Clark                                                           Page [2]

RFC: NNNN              Telnet Com Port Control Option         July, 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:
    -  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.





















Clark                                                           Page [3]

RFC: NNNN              Telnet Com Port Control Option         July, 1997

Illustration:

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














Clark                                                           Page [4]

RFC: NNNN              Telnet Com Port Control Option         July, 1997

Table of Contents
    1. Negotiation of the Com Port
       Control Option Protocol          ..................   6
    2. Com Port Configuration Commands  ..................   6
         Version
         Baud Rate
         Data Bit Size
         Parity
         Stop Bit size
    3. Special Com Port Control Commands .................   9
         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          ..................    14

  Command Names and Codes:
       COM-PORT-OPTION       40

                   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.







Clark                                                           Page [5]

RFC: NNNN              Telnet Com Port Control Option         July  1997

 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




Clark                                                           Page [6]

RFC: NNNN              Telnet Com Port Control Option         July  1997

  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.























Clark                                                           Page [7]

RFC: NNNN              Telnet Com Port Control Option         July  1997

  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.









Clark                                                           Page [8]

RFC: NNNN              Telnet Com Port Control Option         July  1997

  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)

                      (Table continues on next page)
Clark                                                           Page [9]

RFC: NNNN              Telnet Com Port Control Option         July  1997

         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









Clark                                                          Page [10]

RFC: NNNN              Telnet Com Port Control Option         July  1997

    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











Clark                                                          Page [11]

RFC: NNNN              Telnet Com Port Control Option         July  1997

    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
Clark                                                          Page [12]

RFC: NNNN              Telnet Com Port Control Option         July  1997

    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.










Clark                                                          Page [13]

RFC: NNNN              Telnet Com Port Control Option         July  1997

    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 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.























Clark                                                          Page [14]

RFC: NNNN              Telnet Com Port Control Option         July  1997

Author Address:

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

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

Reference Section:

[1]  Joe Campbell. C Programmer's Guide to Serial Communications,
     Second Edition. Indianapolis: SAMS Publishing, 1993. 213-224.
[2]  Internet Engineering Task Force, Telnet Working Group,
     C. Hedrick and D. Borman, "Telnet Remote Flow Control Option",
     RFC 1372, Cray Research, Inc., October 1992.
[3]  Internet Engineering Task Force, Telnet Working Group,
     D. Borman, "Telnet Authentication: Kerberos Version 4",
     RFC 1411, Cray Research, Inc., January 1993.
[4]  Internet Engineering Task Force, Telnet Working Group,
     K. Alagappan, "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                                                          Page [15]


Html markup produced by rfcmarkup 1.122, available from https://tools.ietf.org/tools/rfcmarkup/