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Telnet Protocol Commands
(Page 2 of 3)
Escaping Commands and the Interpret As Command (IAC) Character
All Telnet commands are sent in the
same communication stream as regular data; they are represented using
special byte values in the range from 240 to 254. To differentiate between
data bytes of these values and Telnet commands, every command is preceded
by a special escape character, given the name Interpret As
Command (IAC). IAC has a value of 255; when the recipient sees this
character, it knows the next byte is a command and not data. So, since
the Telnet Interrupt Process command has the value 244, to send
this command the Telnet client would transmit the byte 255 and then
244. If the actual data byte value 255 needs to be sent, it is transmitted
as two 255 bytes. Some Telnet commands also include additional bytes
data, which are sent after the command code itself; a good example is
the use of parameters in Telnet
You may be wondering at this point
why the IAC character is needed at all. After all, Telnet uses US ASCII,
which is 7-bit data in the byte range of 0 to 127, and the Telnet commands
have values higher than 127. One general rationale for using the IAC
escape is to be explicit that a command is being sent. A more specific
reason is to accommodate the optional sending of 8-bit binary data over
Telnet, which the client and server can negotiate. If this mode were
enabled and commands were not preceded by the IAC character, this would
require all data bytes with values from 240 to 255 to be somehow marked
so they would be interpreted as data and not commands. It is more efficient
to include an extra byte for commands than data, since they are sent
less frequently. By escaping commands, only data byte value 255 requires
two bytes to be sent.
Key Concept: The Telnet protocol defines a set of protocol commands that are used for two purposes: first, to represent standard control functions that need to be sent between a terminal and host, such as the command to interrupt a process; and second, to enable protocol communication between the Telnet client and server software. Protocol commands are sent in the normal data communication stream over the Telnet sessions TCP connection. Each is represented by a byte value from 240 to 254, and is preceded by the Interpret As Command (IAC) command, byte value 255, which tells the recipient that the next byte in the stream is a command.
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Version 3.0 - Version Date: September 20, 2005
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