The Network Virtual Terminal

Next: Telnet Commands Up: How Telnet Works Previous: How Telnet Works

The Network Virtual Terminal

Communication is established using the TCP/IP protocols and communication is based on a set of facilities known as a Network Virtual Terminal (NVT). At the user or client end the telnet client program is responsible for mapping incoming NVT codes to the actual codes needed to operate the user's display device and is also responsible for mapping user generated keyboard sequences into NVT sequences.

The NVT uses 7 bit codes for characters, the display device, referred to as a printer, is only required to display the "standard" printing ASCII characters represented by 7 bit codes and to recognise and process certain control codes. The 7 bit characters are transmitted as 8 bit bytes with most significant bit set to zero. An end-of-line is transmitted as the character sequence CR (carriage return) followed by LF (line feed). If it is desired to transmit an actual carriage return this is transmitted as a carriage return followed by a NUL (all bits zero) character.

Note that NVT ASCII is used by many other Internet protocols.

The following control codes are required to be understood by the Network Virtual Terminal.

Figure 1.1: Internet - a large collection of networks
Name code Decimal Value Function

NULL NUL No operation

Line Feed LF 10 Moves the printer to the next print line,

keeping the same horizontal position

Carriage Return CR 13 Moves the printer to the left margin

of the current line

**Figure 1.1:** Internet - a large collection of networks
Name	code	Decimal Value	Function
NULL	NUL		No operation
Line Feed	LF	10	Moves the printer to the next print line,
			keeping the same horizontal position
Carriage Return	CR	13	Moves the printer to the left margin
			of the current line

The following further control codes are optional but should have the indicated defined effect on the display.

Name code Decimal Value Function

BELL BEL 7 Produces an audible or visible signal

(which does NOT move the print head)

Back Space BS 8 Moves the print head one character position

towards the left margin.

On a printing devices this mechanism

was commonly used to form

composite characters by printing two

basic characters on top of each other.

Horizontal Tab HT 9 Moves the printer to the next horizontal

tab stop.

It remains unspecified how either party

determines or

establishes where such tab stops are located.

Vertical Tab VT 11 Moves the printer to the next vertical

tab stop.

It remains unspecified how either

party determines or

establishes where such tab

stops are located.

Form Feed FF 12 Moves the printer to the top of the

next page, keeping

the same horizontal position.

On visual displays

this commonly clears the screen and

moves the cursor to the top left corner.

The NVT keyboard is specified as being capable of generating all 128 ASCII codes by using keys, key combinations or key sequences.

Next: Telnet Commands Up: How Telnet Works Previous: How Telnet Works

Dave Marshall
9/28/2001

Name	code	Decimal Value	Function
BELL	BEL	7	Produces an audible or visible signal
			(which does NOT move the print head)
Back Space	BS	8	Moves the print head one character position
			towards the left margin.
			On a printing devices this mechanism
			was commonly used to form
			composite characters by printing two
			basic characters on top of each other.
Horizontal Tab	HT	9	Moves the printer to the next horizontal
			tab stop.
			It remains unspecified how either party
			determines or
			establishes where such tab stops are located.
Vertical Tab	VT	11	Moves the printer to the next vertical
			tab stop.
			It remains unspecified how either
			party determines or
			establishes where such tab
			stops are located.
Form Feed	FF	12	Moves the printer to the top of the
			next page, keeping
			the same horizontal position.
			On visual displays
			this commonly clears the screen and
			moves the cursor to the top left corner.