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Network Working Group                                        D. WaitzmanRequest For Comments: 1073                                       BBN STC                                                            October 1988Telnet Window Size OptionStatus 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 = 312. 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 theWaitzman                                                        [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 [seeRFC-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 NAWS7. 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]