RFC1073

Network Working Group D. Waitzman Request For Comments: 1073 BBN STC

                                                           October 1988

                      Telnet Window Size Option

Status of this Memo

  This RFC describes a proposed Telnet option to allow a client to
  convey window size to a Telnet server.  Distribution of this memo is
  unlimited.

1. Command Name and Option Code

     Name = NAWS (Negotiate About Window Size)

     Code = 31

2. Command Meanings

     IAC WILL NAWS

        Sent by the Telnet client to suggest that NAWS be used.

     IAC WON'T NAWS

        Sent by the Telnet client to refuse to use NAWS.

     IAC DO NAWS

        Sent by the Telnet server to suggest that NAWS be used.

     IAC DON'T NAWS

        Sent by the Telnet server to refuse to use NAWS.

     IAC SB NAWS <16-bit value> <16-bit value> IAC SE

        Sent by the Telnet client to inform the Telnet server of the
        window width and height.

  The window size information is conveyed via this option from the
  Telnet client to the Telnet server.  The information is advisory.
  The server may accept the option, but not use the information that is
  sent.

  The client and server negotiate sending the window size information
  using the standard Telnet WILL/DO/DON'T/WON'T mechanism.  If the

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RFC 1073 Telnet Window Size Option October 1988

  client and server agree, the client may then send a subnegotiation to
  convey the window size.  If the client's window size is later changed
  (for instance, the window size is altered by the user), the client
  may again send the subnegotiation.  Because certain operating
  systems, on which a server may be executing, may not allow the window
  size information to be updated, the server may send a DON'T NAWS to
  the client to forbid further subnegotiation after it was initially
  accepted.  A negotiation loop will not form following these rules.

  The subnegotiation consists of two values, the width and the height
  of the window in characters.  The values are each sent as two bytes,
  in the Internet standard byte and bit order.  This allows a maximum
  window width or height of 65535 characters.  A value equal to zero is
  acceptable for the width (or height), and means that no character
  width (or height) is being sent.  In this case, the width (or height)
  that will be assumed by the Telnet server is operating system
  specific (it will probably be based upon the terminal type
  information that may have been sent using the TERMINAL TYPE Telnet
  option).

  The syntax for the subnegotiation is:

     IAC SB NAWS WIDTH[1] WIDTH[0] HEIGHT[1] HEIGHT[0] IAC SE

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

3. Default Specification

     WON'T NAWS

     DON'T NAWS

  This option does not assume any default window size information.
  Often the terminal type, passed with the TERMINAL TYPE Telnet option,
  may imply a window size, but that is not necessary for this option.

4. Motivation

  With the increasing popularity of windowing systems, a Telnet client
  is often run inside a variable-sized window, and the Telnet server
  needs to know the window size for proper cursor control.  The window
  may also have its size changed during the Telnet session and the
  updated window size needs to be conveyed to the server.  This memo
  specifies an option to send the window height and width in characters
  from a client to a server.

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RFC 1073 Telnet Window Size Option October 1988

  The Telnet options Negotiate Output Line Width (NAOL) and Negotiate
  Output Page Size (NAOP) do not have the correct semantics for this
  purpose, and they are not in common use [see RFC-1011 "Official
  Internet Protocols", and the "Defense Protocol Handbook"].  The NAOL
  and NAOP options are bidirectional (i.e., the server might control
  the client's line width or page size), and are limited to 253
  characters in each axis.

  This option is a better model of the normal window negotiation
  process.  The client has total control over the size of its window
  and simply tells the server what the current window size is.
  Furthermore, the 253 character height and width limitation is too low
  so the new option has a limit of 65535 characters.  Finally, this
  option sends the window height and width concurrently because they
  are typically changed simultaneously and many operating systems and
  windowing applications prefer to think in terms of simultaneous
  changes in height and width.

5. Description and Implementation Notes

  A typical user of this option might be a Telnet client running under
  X.  After a user resizes the client's window, this must be
  communicated to the Telnet client.  In 4.3 BSD Unix, the signal
  SIGWINCH (window changed) might be caught by the Telnet process and a
  new NAWS subnegotiation sent to the server.  Upon receipt of a NAWS
  subnegotiation, the server might do the appropriate ioctl to handle
  the new information, and then could send a SIGWINCH to its child,
  probably a shell.

6. Examples

  In the following examples all numbers in the data stream are in
  decimal.

     1. Server suggest and client agrees to use NAWS.

        (server sends)  IAC DO NAWS
        (client sends)  IAC WILL NAWS
        (client sends)  IAC SB NAWS 0 80 0 24 IAC SE

           [A window 80 characters wide, 24 characters high]
           [some time occurs and the user changes the window size]

        (client sends)  IAC SB NAWS 0 80 0 64 IAC SE

           [A window 80 characters wide, 64 characters high]

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RFC 1073 Telnet Window Size Option October 1988

     In all numeric form:

        (server sends)  255 253 31
        (client sends)  255 251 31
        (client sends)  255 250 31 0 80 0 24 255 240

        (client sends)  255 250 31 0 80 0 64 255 240

      2.  Client suggests and server agrees to used NAWS.

        (client sends)  IAC WILL NAWS
        (server sends)  IAC DO NAWS
        (client sends)  IAC SB NAWS 1 44 0 24 IAC SE

           [A window 300 characters wide, 24 characters high]

      3.  Client suggest and server refuses to use NAWS.

        (client sends)  IAC WILL NAWS
        (server sends)  IAC DON'T NAWS

      4.  Server suggests and client refuses to use NAWS.

        (server sends)  IAC DO NAWS
        (client sends)  IAC WON'T NAWS

7. Acknowledgments

  A more elaborate, X window system specific, version of this option
  has been implemented at Carnegie-Mellon University by Glenn Marcy and
  the author.  It is widely used in the Carnegie-Mellon University
  Computer Science Department.  Mr. Marcy helped write an early draft
  of this memo documenting the more elaborate option.

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