Telnet window size option (RFC1073)
Original Publication Date: 1988-Oct-01
Included in the Prior Art Database: 2019-Feb-15
Internet Society Requests For Comment (RFCs)
This RFC describes a proposed Telnet option to allow a client to convey window size to a Telnet server.
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 WIDTH HEIGHT HEIGHT 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
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.
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...