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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)
      9  TCP/IP Key Applications and Application Protocols
           9  TCP/IP Interactive and Remote Application Protocols
                9  Telnet Protocol

Previous Topic/Section
Telnet Interrupt Handling Using Out-Of-Band Signaling: The Telnet Synch Function
Previous Page
Pages in Current Topic/Section
1
2
34
Next Page
Berkeley Remote ("r") Commands
Next Topic/Section

Telnet Options and Option Negotiation
(Page 2 of 4)

Common Telnet Options

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


Key Concept: The Telnet NVT specification ensures that all devices using Telnet can talk to each other, but accomplishes this by boiling down communication to the simplest of representations. To allow the use of more sophisticated formats and services, Telnet defines a number of options. If a client and server both implement a particular option, they can enable its use through a process of negotiation.



Previous Topic/Section
Telnet Interrupt Handling Using Out-Of-Band Signaling: The Telnet Synch Function
Previous Page
Pages in Current Topic/Section
1
2
34
Next Page
Berkeley Remote ("r") Commands
Next Topic/Section

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