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The Telnet Protocol

INTRODUCTION

What is TELNET ?

Telnet, which stands for Telecommunications Network, is a protocol that provides a way for users (or clients) to connect to multiuser computers (or servers) on the Internet, whether in the next building or across the other side of the world. In most cases, users use Telnet to communicate with a remote login service.

Telnet is based on three main principles:

Telnet over TCP/IP

On the Internet, the ability to connect with another machine is made possible by the Transmission Control Protocol (TCP), which enables two machines to transmit data back and forth in a manner coherent to the operating systems of each device, and the Internet Protocol (IP), which provides a unique, 32-bit address for each machine connected to the network. The telecommunications application built over these capabilities provides the local terminal with the means to emulate a terminal compatible with the remote computer.

Connection Establishment :

The Telnet TCP connection is established between the userís port U and the serverís port L. The server listens on itís well-known port L for such connections. Since a TCP connection is full-duplex and indentified by the pair of ports, the server can engage in many simultaneous connections involving itís port L and different user ports U. Port Assignment - When used for remote user access to service hosts, this protocol is assigned server port 23, that is L = 23.

You can read more about the TCP / IP protocol suite at   Computer_and_Internet:Protocols:TCP/IP


REMOTE LOGIN

The telnet protocol gives you the ability to connect to a machine, by giving commands and instructions interactively to that machine, thus creating an interactive connection. In such a case, the local system becomes transparent to the user, who gets the feeling that he is connected directly to the remote computer. The commands typed by the user are transmitted directly to the remote machine and the response from the remote machine is displayed on the userís monitor screen. An interactive connection is also know as remote login.

In order to remote login the userís computer must have the ability to


NETWORK VIRTUAL TERMINAL

The Network Virtual Terminal is a device used by Telnet to enable a local computer to communicate with a remote machine.To make Telnet interoperate between as many systems as possible, it must accommodate the details of heterogeneous computers aand operating systems. To accommodate heterogeneity, Telnet defines how data and command sequences are sent across the Internet. The definition is known as the Network Virtual Terminal (NVT). The NVT defines how data and commands are sent across the internet. The NVT is a bi-directional character device that has a printer and a keyboard. The printer responds to incoming data and the keyboard produces outgoing data which is sent over the Telnet connection. The NVT is viewed as a half-duplex device.

The Network Virtual Terminal implements a client-server architecture. A Telnet client transfers characters between the userís terminal and a remote service. On one side, it uses the local operating system functions when it interacts with the userís terminal. On the other side, it uses a TCP connection when it communicates with the remote service.

The Telnet protocol defines the character set for the virtual terminal. Several of the keys correspond to conceptual operations instead of data values. For example, one key causes an Interrupt  or Abort.

The definition of NVT format is fairly straightforward. All communication involves 8-bit bytes. At startup, NVT uses the standard 7-bit USASCII representation for data and reserves bytes with the high order bit set for command sequences. The USASCII character set includes 95 characters that have "printable" graphics (letters, digits and punctuation marks) as well as 33 "control" codes. All printable characters are assigned the same meaning as in the standard USASCII character set. The NVT standard defines interpretations for control characters as shown in the table below.

ASCII Control Code

Decimal Value

Assigned Meaning

NUL

0

No operation (has no effect on output)

BEL

7

Sound audible/visible signal (no motion)

BS

8

Move left one character position

HT

9

Move right to the next horizontal tab stop

LF

10

Move down (vertically) to the next line

VT

11

Move down to the next vertical tab stop

FF

12

Move to the top of the next page

CR

13

Move to the left margin on the current line

other control

--

No operation (has no effect on output)

To provide a full-duplex connection between the userís terminal and a remote service, a Telnet client performs two tasks simultaneously :

The client must read characters that the user types on the keyboard and send them across a TCP connection to the remote service.

The client must read characters that arrive from the TCP connection and display them on the userís terminal screen.

Because the remote service can emit output at any time or the user can type at any time, a client cannot know which source of data will become available first. Thus, it cannot block indefinitely waiting for input from one of the two sources without also checking for input from the other. In short, the client must transfer data in both directions concurrently.

The cheif advantage of using a Network Vitrual Terminal is that it permits clients from a variety of computers to connect to a service.


TELNET COMMANDS

All Telnet commands consist of at least a two byte sequence: the "Interpret as Command" (IAC) escape character followed by the code for the command. The commands dealing with option negotiation are three byte sequences, the third byte being the code for the option referenced (see RFC 855). This format was chosen so that as more comprehensive use of the "data space" is made -- by negotiations from the basic NVT, of course -- collisions of data bytes with reserved command values will be minimized, all such collisions requiring the inconvenience, and inefficiency, of "escaping" the data bytes into the stream. With the current set-up, only the IAC need be doubled to be sent as data, and the other 255 codes may be passed transparently.

The following are the defined Telnet commands. Note that these codes and code sequences have the indicated meaning only when immediately preceded by an IAC.

NAME

CODE

MEANING

SE 240 End of subnegotiation parameters.
NOP 241 No operation
Data Mark 242 The data stream portion of a Synch.

This should always be accompanied by a TCP Urgent notification

Break 243 NVT character BRK
Interrupt Process 244 The function IP
Abort output 245 The function AO
Are You There 246 The function AYT
Erase character 247 The function EC.
Erase Line 248 The function EL.
Go ahead 249 The GA signal
SB 250 Indicates that what follows is subnegotiation of the indicated option.
WILL (option code) 251 Indicates the desire to begin performing, or confirmation that

you are now performing, the indicated option.

WON'T (option code) 252 Indicates the refusal to perform, or continue performing, the indicated option.
DO (option code) 253 Indicates the request that the other party perform, or confirmation

that you are expecting the other party to perform, the indicated option.

DON'T (option code) 254 Indicates the demand that the other party stop performing, or confirmation

that you are no longer expecting the other party to perform, the indicated option.

IAC 255 Data Byte 255

The syntax of the Telnet commands :

A Brief Summary :

The main objectives of the Network Virtual Terminal - NVT :


CONCEPT OF NEGOTIATIONS

The option negotiation machanism allows the user to set terminal parameters to values other than the default or to negotiate more sophisticated facilities. The Telnet protocol negotiation mechanism can be initiated by either side. Besides negotiating whether or not a particular option is to be in effect, the mechanism also allows one to specify, when appropriate, which side is to perform the function. For example, a user may negotiate the Echo option and specify whether echoing is to be done locally or remotely to the initiator of the negotiation.

Four commands support option negotiation ( DO, DONíT, WILL and WONíT).

For example, suppose the terminal wanted to use binary transmission. It would send a DO Binary Transmission to the remote host. The host could then respond with either a WILL Binary Transmission (a positive acknowledgment) or a WONíT Binaary Transmission (a negative acknowledgment). If the terminal does not want itís characters echoed across the Telnet connection, it would send WONíT Echo; If the remote host agrees that no characters will be echoed, it would return DONíT Echo.


A SYMMETRICAL VIEW

The third concept of Telnet is one of symmetry in the negotiation syntax. This symmetry allows either the client or server ends of the connection to request a particular option as required, thus optimizing the service provided by the other party. A terminal protocol should not only allow a terminal to interact with an application process on a host, but it should also allow process-process and terminal-terminal interactions. Although a majority of the connections established will be of the process-terminal form, the symmetrical forms can be very useful.


USING TELNET

In order to initiate a Telnet session, you need to know the address of the remote host computer.
The syntax for using Telnet is :

Every Telnet site has two addresses - one composed of words that are easier for people to remember ; The other numerical address better suited for computers.

Many remote hosts require you to have an acount to login (you must have a user-id and a password). However, there are some remote hosts that do not require users to have an acount. Users can login with the general user-id and password are usually not required (it is inserted automatically).

To see a user - friendly way to Telnet see Telnet Tutorial by Trumpet

Troubleshooting


REFERENCES TO TELNET SITES


BIBLIOGRAPHY


ABOUT THE AUTHORS

This Telnet Homepage was build by four students of the School of Exact Science / Tel-Aviv University  as the final project in the course
' Protocols and Computer Networks ' by Dr. Debby Koren from RAD .

For any questions / comments about this project , email :