Network Virtual Terminal (NVT)

Telnet is designed for terminal to terminal communication and distributed computer processing. Each host sets up a Network Virtual Terminal (NVT) and a host at one end assumes that an NVT has been set up at the other end. The NVT defines a set of rules for how information is formatted and sent, such as character set, line termination, and how information about the Telnet session itself is sent.There is the mechanism to negotiate options so that the hosts can operate a more elaborate interface at each end using different fonts etc. than the NVT. The User Host is the one that initiates a conversation whilst the Server Host is the one that is providing services.

The Network Virtual Terminal (NVT) is a bi-directional character device. The NVT 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 and, if echoes are desired, to the NVT's printer as well. Any code conversion and timing considerations are local problems and do not affect the NVT.

Brief NVT description

- NVT comands are inserted to the data stream via TCP/IP before sending to the TCP/IP connection.
- Every NVT command is prefixed by character "0xFF".
- There are some basic commands with 2 byte interpretation only (EOF, ABORT, BRK, AYT, NOP, EC), and others with defined start ( = 0xFF 0xFA) and defined end ( = 0xFF 0xF0) commands.
- The TCP/IP device separates NVT commands and processes them without delay, while the data stream stores to the output stack.
- The NVT commands can't be found in the serial port data, if the device is Serial / TCP/IP converter.
- If you are sending character "0xFF" (255), the PC will just double it, because in NVT "0xFFFF" means send character "0xFF" to the output.
- NVT uses a negotiation process. It's a way of testing if terminals on the opposite side use ECHO or not ar if there are specific terminals etc.

The TELNET (Terminal Network) Protocol

TELNET (TErminaL NETwork) is a network protocol used on the Internet or local area networks to provide a bidirectional interactive communications facility.
- Telnet offers users the capability of running programs remotely and facilitates remote administration.
- Telnet is available for practically all operating systems and eases integration in heterogeneous networking environments.
- The Telnet protocol is applied on a TCP connection to send data in ASCII format coded over 8 bits between which the Telnet check sequences come.

Communication is established using TCP/IP and is based on a Network Virtual Terminal (NVT). On the client, the Telnet program is responsible for translating incoming NVT codes to codes understood by the client's display device as well as for translating client-generated keyboard codes into outgoing NVT codes.

Commands

The Telnet protocol uses various commands to control the client-server connection. These commands are transmitted within the data stream. The commands are distinguished from the data by setting the most significant bit to 1.

Telnet Options

Options give the client and server a common view of the connection. They can be negotiated at any time during the connection by the use of commands. They are described in separate RFCs.

Use of TELNET

The use of Telnet for remote logins should be discontinued under all normal circumstances, for the following reasons :
- Telnet, by default, does not encrypt any data sent over the connection (including passwords).
- Telnet have no authentication that would ensure communication is carried out between the two desired hosts and not intercepted in the middle.

Telnet is popular in various application areas:

- Enterprise networks to access host applications, e.g., on IBM Mainframes.
- Administration of network elements, e.g., in commissioning, integration and maintenance of core network elements in mobile communication networks, and many industrial control systems.
- MUD games played over the Internet.
- Internet game clubs.
- Embedded systems.
- Mobile data collection applications.