RFC 1073 (rfc1073) - Page 2 of 4

Telnet window size option

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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