TELNET in Computer Networks

In this tutorial, we will be covering the application program TELNET in Computer Networks.

TELNET is basically the short form for TErminal NETwork.It is basically a TCP/IP protocol that is used for virtual terminal services and was mainly proposed by International Organization for Standards(ISO).

  • It is a general-purpose client/server application program.

  • This program enables the establishment of the connection to the remote system in such a way that the local system starts to appear as a terminal at the remote system.

  • It is a standard TCP/IP protocol that is used for virtual terminal service.

  • In simple words, we can say that the telnet allows the user to log on to a remote computer. After logging on the user can use the services of the remote computer and then can transfer the results back to the local computer.

  • The TELNET was mainly designed at the time when most operating systems operate in the time-sharing environment. And in this type of environment, a large computer can support multiple users. Usually, the interaction between the computer and user occurs via terminal(It is a combination of keyboard, mouse, and monitor).

  • TELNET makes the use of only one TCP/IP connection.

Logging

As in the timesharing environment, the users are a part of the system having some rights in order to access the resources. It is important to note that each user has an identification along with the password. This user identification mainly indicates that the user is a part of the system.

In order to access the system the user logs in to the system by the user-id.The system also includes password checking in order to prevent unauthorized access to the resources of a system.

The logging process can be further categorized into two:

  1. Local log-in

  2. Remote log-in

Local Log-in

Whenever a user logs in into a local timesharing system then it referred to as local log-in.

The figure indicates Local log-in

  1. Once the user types at the terminal, then the keystrokes are accepted by the terminal driver.

  2. The characters are then passed on to the Operating system by the terminal driver.

  3. After that, the operating system interprets the combination of the characters and then invokes the desired application program.

Remote Log-in

Whenever a user wants to access those application programs or utilities that locate on to the remote machine then it is referred to as remote log-in. In this type of log-in, the TELNET client/server program mainly comes into use.

The figure shows the concept of Remote Log-in

  • In this, the user mainly sends the keystroke to the terminal driver where the Operating system only accepts the characters and does not interpret them.

  • After that, the characters are sent to the TELNET client that mainly translates these characters into the universal character set that is commonly known as Network Virtual Terminal (NVT) characters.

  • After converting the TELNET client transfers them to the TCP/IP protocol stack.

  • The text that is in the form of NVT then travels through the internet and then arrives at the TCP/IP protocol stack present at the remote machine.

  • After that, the characters are delivered to the operating system which then passes these on to the TELNET Server,

  • The TELNET Server then changes these NVT characters to the corresponding characters that can be understood by the Remote computer.

  • The characters cannot be directly passed to the Operating system of the remote computer because the remote operating system is not designed in such a way so as to receive the characters from the TELNET server.

  • It is designed in such a way so as to receive the characters from the Terminal Driver.

  • There is a software program commonly known as Pseudoterminal driver that is added and mainly pretends as the characters are coming from the Terminal.

Network Virtual Terminal(NVT)

Mainly the client TELNET translates the characters that come from the Local terminal into the form of NVT and then delivers them to the Network. On the other hand, the server TELNET translates the data and commands that are in NVT form into the form that is mainly acceptable by the Remote Computers.

NVT Character Sets

The Network Virtual terminal mainly uses two sets of characters, one set is for data and the other set is for control.

  • For the data, the NVT is an 8-bit character set where the 7 lowest-order bits are the same as ASCII and the Highest bit is 0.

  • In order to send the control characters between the computers, the NVT uses an 8-bit bit character set where the highest-order bit is set to 1.

In order to send both data as well as control characters, the TELNET makes use of the same connection. And this is accomplished just by embedding the control characters into the data stream.

In order to distinguish the data characters from the control characters, Each control character is just preceded by the Special Control character that is commonly known as Interpret as control(IAC).

Options

These are the extra features that are available to a user with a more sophisticated terminal.

Some of the common options are as follows:

Code Option Name Meaning of the option
0 Binary This option interprets as 8-bit Binary transmission.
1 Echo This option echoes the data received from one side to the other.
3 Suppress Go ahead The Suppress go-ahead signals after the data
5 Status It is used to request the status of the TELNET.
6 Timing mark It is used to define the timing mark.
24 Terminal type It is used to set the terminal type.
32 Terminal Speed It is used to set the terminal speed
34 Line Mode It is used to change the line mode.

Option Negotiation

The TELNET program allows the client and the server to negotiate the options before or during the services.

Given below are the four control characters that are used for this purpose:

Character Decimal Binary Meaning
WILL 251 11111011 This control character:
1.Offers to enable
2.Accepts the request to enable
WONT 252 11111100 This control character:
1.Rejects a request to enable
2.Offers to disable.
3.Accepts the request to disable
DO 253 11111101 This control character:
1.Approves an Offer to enable.
2.Requesting to enable.
DONT 254 11111110 This control character:
1.Disapproves an offer to enable.
2.Approves an offer to disable.
3.Requesting to Disable.

Advantages of TELNET

Given below are some of the benefits of using TELNET:

  • One of the major benefits of TELNET is that it allows remote access to someone else's computer system.

  • As TELNET makes the use of Plain text. Thus this allows the user for more access with fewer problems in data transmission.

  • TELNET saves a lot of time.

  • TELNET is universal as it can be used on any computer, Even the oldest system can easily connect to newer machines while both having different versions of the Operating System.

Disadvantages of TELNET

Now it's time to take a look at the drawbacks of TELNET:

  • It can be very difficult for beginners.

  • As there is no concept of encryption because data is sent in the form of plain text. Thus it is not a secure method.

  • Also, remote devices do not know about the capabilities of the local device because of the reason that many capabilities are disabled.

Modes of Operation

The Implementation of the TELNET is mainly operating in one of the three given modes:

  1. Default Mode

  2. Character Mode

  3. Line Mode

Default Mode

  • This mode is used if no other modes are invoked through the option negotiation.

  • Echoing in this mode is done by the client.

  • The user types the character and the client echoes the character on the screen but does not send it until the completion of whole line.

Character Mode

  • In this mode, each character that is typed is sent by the client to the server.

  • After that, the server echoes the character back in order to be displayed on the client screen.

  • The Echoing of the character is done only if the time of transmission is too long.

Line Mode

  • This mode is proposed in order to compensate for the deficiencies of the default mode and the character mode.

  • Line echoing in this mode is done by the client.

  • After that, the whole line is sent by the client to the server.