Telnet Extended Options: List Option
also
- std-32
Extracted elements (13)
Option code 255 is reserved for EXOPL now, in anticipation of a future need for a 257th Telnet option, rather than waiting until the 256-option space is exhausted. Early reservation prevents future conflicts.
The subcommand codes for DO, DON'T, WILL, WON'T, and SB within EXOPL are assigned the same values as the corresponding Telnet command codes (250–254) for consistency and to simplify implementation.
Both parties must first agree to use EXOPL (via DO/WILL exchange) before any extended option negotiation can occur; the extended option subcommands mirror standard Telnet command semantics exactly for ease of implementation.
The default state is WON'T EXOPL / DON'T EXOPL: negotiation of options on the Extended Options List is not permitted unless explicitly agreed upon.
EXOPL defines five subcommand codes — WILL, WON'T, DO, DON'T, and SB — with the same semantics as the corresponding Telnet commands and the same numeric values (250–254).
The EXTENDED-OPTIONS-LIST (EXOPL) option is assigned Telnet option code 255. It provides a mechanism for extending the Telnet option space beyond the standard 256 options.
Telnet option code 255 is assigned to EXTENDED-OPTIONS-LIST (EXOPL), reserving it for the purpose of extending the Telnet option numbering space.
IAC DO EXOPL requests that the receiver begin negotiating Telnet options on the Extended Options List, or confirms that the receiver is expected to do so.
IAC SB EXOPL <subcommand> carries information required for negotiating an option from the Extended Options List; the subcommand format is defined in section 5.
IAC WILL EXOPL requests permission to begin negotiating, or confirms that the sender will begin negotiating, Telnet options on the Extended Options List.
IAC WON'T EXOPL indicates the sender refuses to negotiate or continue negotiating options on the Extended Options List. IAC DON'T EXOPL demands the receiver conduct no further negotiation of such options.
Once both parties agree to use EXOPL, a specific extended option is negotiated with the format: IAC SB EXOPL DO/DON'T/WILL/WON'T <option code> IAC SE.
Subnegotiation of a specific extended option uses the format: IAC SB EXOPL SB <option code> <parameters> SE IAC SE, after both sides have agreed to use that option.