The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Key Applications and Application Protocols            9  TCP/IP Interactive and Remote Application Protocols                 9  Telnet Protocol

Telnet Connections and Client/Server Operation 1 2 3 4 Telnet Protocol Commands

The Network Virtual Terminal

Telnet uses an approach similar to the analogy described above for dealing with its problem of hardware and software compatibility. Rather than having terminals and hosts communicate using their various native “languages”, all Telnet clients and servers agree to send data and commands that adhere to a fictional, “virtual” terminal type call the Network Virtual Terminal (NVT). 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.

Each Telnet client running on a terminal understands both its native language and NVT. When information is entered by the user on his or her local terminal, it is converted to NVT for transmission over the network in NVT form. When the Telnet server receives this information, it translates it from NVT to the format that the remote host expects to receive it. The identical process is performed for transmissions from the server to the client, in reverse. This is illustrated in Figure 320.

The NVT is defined to consist of a logical “keyboard” for input and a logical “printer” for output (the age of the protocol is reflected in these terms; decades ago there were no monitors, all output was on paper). NVT uses the 7-bit United States ASCII (USASCII) character set. Each character is encoded using one 8-bit byte. Note however that a client and server can use Telnet options to negotiate other data representations, including the transmission of either extended ASCII or even full 8-bit binary data.

Telnet Connections and Client/Server Operation 1 2 3 4 Telnet Protocol Commands