Network Virtual Terminal NVT

At its simplest, Telnet is just a keyboard and screen interface to the TCP stack: whatever is typed on the keyboard is sent as TCP data (generally one character at a time), and whatever is received by the TCP stack is put on the display. The character set is restricted to seven bits, sent as a byte value with the most significant bit cleared. The end-of-line sequence is CR LF (carriage return, line feed). If a carriage return is to be sent on its own, it is followed by a null (byte value of zero) to avoid confusion with the end-of-line marker.

This imaginary terminal in known as a Network Virtual Terminal (NVT), and the character set is known as NVT-ASCII. A lot of TCP applications assume the existence of this baseline terminal; for example, the simple mail transfer protocol (SMTP) specifies its transactions using NVT-ASCII.

Although the NVT is sufficient to drive many protocols manually, it is inadequate for remote login to a multi-user system. The system and the NVT need to negotiate about...