The TCP/IP Guide - Telnet Options and Option Negotiation

Each Telnet option is identified using a decimal byte code with a possible value of 0 to 254; the value 255 is reserved to extend the option list should more than 255 options ever be needed. Each option also has a text code string associated with it, which is often used as a symbol in place of the code number in both protocol discussions and diagnostic output. Table 282 lists some of the more interesting Telnet options and provides a brief description of each.

**Table 282: Common Telnet Options**
Option Number	Option Code	Option Name	Description	Defining RFC
0	*TRANSMIT-BINARY*	Binary Transmission	Allows devices to send data in 8-bit binary form instead of 7-bit ASCII.	856
1	*ECHO*	Echo	When you press a key on a terminal, you also expect to see the character you entered appear on the terminal screen as output; this is called echoing the input. Echoing is a significant issue in terminal applications because it can be implemented in a number of different ways. This option allows devices to negotiate any of a variety of different echo modes.	857
3	*SUPPRESS-GO-AHEAD*	Suppress Go Ahead	Allows devices not operating in half-duplex mode to no longer need to end transmissions using the Telnet Go Ahead command.	858
5	*STATUS*	Status	Lets a device request the status of a Telnet option.	859
6	*TIMING-MARK*	Timing Mark	Allows devices to negotiate the insertion of a special timing mark into the data stream, which is used for synchronization.	860
10	*NAOCRD*	Output Carriage-Return Disposition	Lets the devices negotiate how carriage returns will be handled.	652
11	*NAOHTS*	Output Horizontal Tab Stops	Allows the devices to determine what horizontal tab stop positions will be used for output display.	653
12	*NAOHTD*	Output Horizontal Tab Stop Disposition	Allows the devices to negotiation how horizontal tabs will be handled and by which end of the connection.	654
13	*NAOFFD*	Output Formfeed Disposition	Allows the devices to negotiation how form feed characters will be handled.	655
14	*NAOVTS*	Output Vertical Tabstops	Used to determine what vertical tab stop positions will be used for output display.	656
15	*NAOVTD*	Output Vertical Tab Disposition	Lets devices negotiation the disposition of vertical tab stops.	657
16	*NAOLFD*	Output Linefeed Disposition	Allows devices to decide how line feed characters should be handled.	658
17	*EXTEND-ASCII*	Extended ASCII	Lets devices agree to use extended ASCII for transmissions and negotiate how it will be used.	698
24	*TERMINAL-TYPE*	Terminal Type	Allows the client and server to negotiate the use of a specific terminal type. If they agree, this allows the output from the server to be ideally customized to the needs of the particular terminal the user is working on.	1091
31	*NAWS*	Negotiate About Window Size	Permits communication of the size of the terminal window.	1073
32	*TERMINAL-SPEED*	Terminal Speed	Allows devices to report on the current terminal speed.	1079
33	*TOGGLE-FLOW-CONTROL*	Remote Flow Control	Allows flow control between the client and the server to be enabled and disabled.	1372
34	*LINEMODE*	Linemode	Allows the client to send data one line at a time instead of one character at a time. This improves performance by replacing a large number of tiny TCP transmissions with a smaller number of larger ones.	1184
37	*AUTHENTICATION*	Authentication	Lets the client and server negotiate a method of authentication to secure connections.	1416

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