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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

Waitzman [Page 1]  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.

Waitzman [Page 2]  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]

Waitzman [Page 3]  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.

Waitzman [Page 4]