RFC1097

Network Working Group B. Miller Request for Comments: 1097 CMU-NetDev

                                                           1 April 1989

                   TELNET SUBLIMINAL-MESSAGE Option

Status of this Memo

  This RFC specifies a standard for the Internet community.  Hosts on
  the Internet that display subliminal messages within the Telnet
  protocol are expected to adopt and implement this standard.
  Distribution of this memo is unlimited.

1. Command name and code.

  SUBLIMINAL-MESSAGE        257

2. Command meanings.

  IAC WILL SUBLIMINAL-MESSAGE

     The sender of this command REQUESTS permission to, or confirms
     that it will, display subliminal messages.

  IAC WONT SUBLIMINAL-MESSAGE

     The sender of this command REFUSES to display subliminal messages.

  IAC DO SUBLIMINAL-MESSAGE

     The sender of this command REQUESTS that the receiver, or grants
     the receiver permission to, display subliminal messages.

  IAC DONT SUBLIMINAL-MESSAGE

     The sender of this command DEMANDS that the receiver not display
     subliminal messages.

  IAC SB SUBLIMINAL-MESSAGE <16-bit value> <16-bit value> <string> IAC
     SE

     The sender specifies a message to be subliminaly displayed by the
     remote host.  If the client has agreed (via the standard WILL WONT
     DO DONT mechanism) to display subliminal messages, it must accept
     this subnegotiation and attempt to display the message string on
     the users console for the specified duration and continue to do so
     at fixed intervals until another SUBLIMINAL-MESSAGE subnegotiation
     is received.  The position and rendering of the message of

Miller [Page 1]

RFC 1097 SUBLIMINAL 1 April 1989

     implementation dependent.

     The first 16-bit value specifies the duration of the message in
     milliseconds.  It is sent MSB first.  The second 16-bit value
     specifies the frequency with which the message is displayed.  It
     represents the number of seconds between displays and is also sent
     MSB first.  The final parameter is the message itself.

     The syntax for this subnegotiation is:

               IAC SB SUBLIMINAL-MESSAGE
                   DURATION[1] DURATION[0]
                   FREQUENCY[1] FREQUENCY[0]
                   MESSAGE_STRING
               IAC SE

     As required by the Telnet protocol, any occurence of 255 in the
     subnegotiation must be doubled to destinguish it from the IAC
     character (which has a value of 255).

3. Default.

  WONT SUBLIMINAL-MESSAGE

  DONT SUBLIMINAL-MESSAGE

  i.e., subliminal messages will not be displayed.

4. Motivation for the option

  Frequently the use of "Message of the day" banners and newsletters is
  insufficient to convince stubborn users to upgrade to the latest
  version of telnet.  Some users will use the same outdated version for
  years.  I ran across this problem trying to convince people to use
  the REMOTE-FLOW-CONTROL Telnet option.  These users need to be gently
  "persuaded".

5. Description and implementation notes.

  The quality of the client implementation will depend on it's ability
  to display and erase text strings in a small amount of time.  The
  current implementation at CMU takes into acount terminal line speed,
  advanced video capabilities, and screen phospher persistance when
  calculating how long to wait before erasing a message.

  While it is permitted for the client to display the message text
  "in-line", best results at obtained by printing the message at the
  top or side of console screen where it will just catch the corner of

Miller [Page 2]

RFC 1097 SUBLIMINAL 1 April 1989

  the user's visual field.

  A version is currently under development at CMU to display the
  message using morse-code over the keyboard caps-lock LED.

6. Examples

  In the following example all numbers are in decimal notation.

  1.  Server suggests and client agrees to use SUBLIMINAL-MESSAGE.

     (Server sends) IAC DO SUBLIMINAL-MESSAGE
     (Client sends) IAC WILL SUBLIMINAL-MESSAGE
     (Server sends) IAC SB SUBLIMINAL-MESSAGE 0 5 0 20 "Use VMS" IAC SE

        [The server is "suggesting" that the user employ a stable
        operating system, not an unreasonable request...]

     The client should immediately begin displaying the message and
     should continue to do so at regular intervals.

  2.  Server preempts previous subliminal message.

     (Server sends) IAC SB SUBLIMINAL-MESSAGE 0 5 0 20 "Go home" IAC SE

     The client should now no longer display the previous message and
     should immediately begin displaying the new one.

  3.  Server has messed with user enough for one day.

     (Server sends) IAC SB SUBLIMINAL-MESSAGE 0 0 0 0 "" IAC SE

     The client must cease display of any subliminal messages.

7. Acknowledgements.

  We do things just a little sneakier here at CMU.

Miller [Page 3]