RFC 1073 (rfc1073) - Page 2 of 4
Telnet window size option
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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. Waitzman
