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.