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Telnet Options and Option Negotiation
(Page 4 of 4)
All of the DO/DONT/WILL/WONT
stuff above only serves to enable or disable an option. Some options
are only either off or on, in which case the negotiation above is sufficient.
An example would be the binary transmission option, TRANSMIT-BINARY.
Others require that after they are enabled, the client and server exchange
parameters to control how the option works. For example, the TERMINAL-TYPE
option requires some way for the client to send the server the name
of the terminal.
Telnet allows the client and server
to send an arbitrary amount of data related to the option using a process
called option subnegotiation. A device begins this process by
sending a special sequence of Telnet protocol commands and data. First
the command SB is sent, followed by the option number and parameters
as defined by the particular option; the end of the subnegotiation data
is marked by the protocol command SE. Of course, both SB
and SE must be preceded by the Interpret As Command (IAC)
So, lets take the terminal
type option again as an example. Suppose the server supports this option
and would like the client to use it. It will send:
IAC DO TERMINAL-TYPE
Note: I am showing the text mnemonics for convenience; this would really be sent as the three decimal bytes 255 253 24.
Assuming the client agrees, it will
IAC WILL TERMINAL-TYPE
Now the terminal type option is in
effect, but the server still doesnt know what terminal the client
is using. It can prompt the client to provide that information by sending
IAC SB TERMINAL-TYPE SEND
The client, receiving this subnegotiation
command, will respond with:
IAC SB TERMINAL-TYPE IS
<some_terminal_type> IAC SE
Key Concept: The WILL and DO commands only turn on a Telnet option that a client and server agree to use. In some cases, an option requires additional information to be sent between the client and server device for it to function properly. This is accomplished through a process of option subnegotiation. Either device sends the other a set of data relevant to the option, bracketed by the SB (subnegotiation) and SE (end subnegotiation) Telnet protocol commands.
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Version 3.0 - Version Date: September 20, 2005
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