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Telnet Options and Option Negotiation
(Page 3 of 4)
Enabling Options Using Negotiation
The first stage in Telnet option
negotiation is for the client and server to decide whether or not they
want to enable a particular option. One of the aspects of Telnets
symmetry of operation is that either device may choose to initiate the
use of an option. The initiating device may either specify that it wants
to start using an option, or that it wants the other device to start
using it. The responding device may either agree or disagree in either
case; an option can only be enabled if both devices agree to its use.
This negotiation is performed using
four Telnet protocol commands, as follows:
- WILL: Sent by the initiator to
indicate that it wants to start using a particular option. There are
two possible replies by the responding device:
- DO: Sent to indicate agreement
that the initiator should use the option; it is then considered enabled.
- DONT: Sent to specify that the
initiator must not use the option.
- DO: Sent by the initiator to request
that the other device start using an option. That device may respond
in two ways:
- WILL: Sent to specify that the
responding device will agree to use the option; the option is enabled.
- WONT: Sent to tell the initiator
that the responder will not use the option requested.
The symmetry of Telnet, combined
with the fact that both DO and WILL can be used either
to initiate a negotiation or respond to one, make Telnets option
negotiation potentially complicated. Since either device can initiate
negotiation of an option at any time, acknowledgment loops could potentially
arise if both devices were to try to enable an option simultaneously,
or get into a situation where each kept responding to the others
For this reason, the Telnet standard
specifies restrictions on when the WILL and DO commands
are used. One is that a device may only send a negotiation command to
request a change in the status of an option; it cannot send DO
or WILL just to confirm or reinforce the current state of the
option. Another is that a device receiving a request to start using
an option it is already using should not acknowledge it using DO
Since an option may only be activated
if both devices agree to use it, either may disable the use of an option
at any time by sending either of these commands:
- WONT: Sent by a device to indicate
that it is going to stop using an option. The other device must respond
with DONT as a confirmation.
- DONT: Sent by a device to indicate
that it wants the other device to stop using an option. The other device
must respond with WONT.
Key Concept: Either device may choose to negotiate the use of a Telnet option. The initiator uses the WILL command to specify that it wants to start using a particular option; if the other device agrees, it responds with DO; otherwise, it sends DONT. Alternately, the initiator can use the DO command to indicate that it wants the other device to start using an option; that device responds with WILL if it agrees to do so, or WONT if it does not. Either device may disable the use of an option at any time by sending the other a WONT or DONT command.
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Version 3.0 - Version Date: September 20, 2005
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