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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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:

     -  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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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:

     -  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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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









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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