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Telnet Communications Model and the Network Virtual Terminal (NVT)
(Page 2 of 4)
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.
Figure 320: Telnet Communication and the Network Virtual Terminal (NVT)
Telnet uses the Network Virtual Terminal (NVT) representation to allow a user terminal and remote host that use different internal formats to communicate.
Key Concept: The Telnet Network Virtual Terminal (NVT) is a uniform data representation that ensures the compatibility of communication between terminals and hosts that may use very different hardware, software and data formats. The Telnet client translates user input from the terminals native form to NVT for transport to the Telnet server, where it is converted to the hosts internal format. The process is reversed for output from the host to the user.
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
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Version 3.0 - Version Date: September 20, 2005
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