Network Working Group D. Waitzman
Request For Comments: 1073 BBN STC
October 1988
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 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.
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]
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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