Telnet Com Port Control Option
RFC 2217

Document Type RFC - Experimental (October 1997; Errata)
Author Glen Clark 
Last updated 2015-07-01
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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.


   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
            Baud Rate
            Data Bit Size
            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


   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

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:

     -  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                         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:

     -  serial printers
     -  plotters
     -  monitoring devices such as pipe line monitors or medical
     -  general office equipment such as photo-copiers and cash

Definition of Terms

   Access Server - Any network device which accepts Telnet sessions
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