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Unformatted text preview: ary device from which both ends of the connection, the client and server, map their
real terminal to and from. That is, the client operating system must map whatever type of
terminal the user is on to the NVT. The server must then map the NVT into whatever
terminal type the server supports.
The NVT is a character device with a keyboard and printer. Data typed by the user on the
keyboard is sent to the server, and data received from the server is output to the printer. By
default the client echoes what the user types to the printer, but we'll see that options are
normally supported to change this.
The term NVT ASCII refers to the 7-bit U.S. variant of the ASCII character set used
throughout the Internet protocol suite. Each 7-bit character is sent as an 8-bit byte, with the
high-order bit set to 0.
An end-of-line is transmitted as the 2-character sequence CR (carriage return) followed by
an LF (linefeed). We show this as \r\n. A carriage return is transmitted as the 2-character
sequence CR followed by a NUL (byte of 0). We show this as \r\0.
In the following chapters we'll see that FTP, SMTP, Finger, and Whois all use NVT ASCII
for client commands and server responses.
Telnet uses in-band signaling in both directions. The byte 0xff (255 decimal) is called
IAC, for "interpret as command." The next byte is the command byte. To send the data byte
255, two consecutive bytes of 255 are sent. (In the previous paragraph we said that the data
stream is NVT ASCII, which are 7-bit values, implying a data byte of 255 cannot be sent
by Telnet. There is a binary option for Telnet, RFC 856 [Postel and Reynolds 1983b], file:///D|/Documents%20and%20Settings/bigini/Docum...ti/homenet2run/tcpip/tcp-ip-illustrated/telnet.htm (13 of 26) [12/09/2001 14.47.45] Chapter 26. Telnet and Rlogin: Remote Login which we don't discuss, that allows 8-bit data transmission.) Figure 26.8 lists all the Telnet
commands. Name Code
(decimal) Description EOF 236 end-of-file SUSP 237 suspend current process (job control) ABORT 238 abort process EOR 239 end of record SE 240 suboption end NOP 241 no operation DM 242...
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