RFC 855 (rfc855) - Page 2 of 3


Telnet Option Specifications



Alternative Format: Original Text Document





RFC 855                                                         May 1983


      "subnegotiation" is required, there may be a larger number of
      possible commands.  The concept of "subnegotiation" is described
      in more detail below.

   Section 3 - Default Specification

      The default assumptions for hosts which do not implement, or use,
      the option must be described.

   Section 4 - Motivation

      A detailed explanation of the motivation for inventing a
      particular option, or for choosing a particular form for the
      option, is extremely helpful to those who are not faced (or don't
      realize that they are faced) by the problem that the option is
      designed to solve.

   Section 5 - Description (or Implementation Rules)

      Merely defining the command meanings and providing a statement of
      motivation are not always sufficient to insure that two
      implementations of an option will be able to communicate.
      Therefore, a more complete description should be furnished in most
      cases.  This description might take the form of text, a sample
      implementation, hints to implementers, etc.

A Note on "Subnegotiation"

   Some options will require more information to be passed between hosts
   than a single option code.  For example, any option which requires a
   parameter is such a case.  The strategy to be used consists of two
   steps:  first, both parties agree to "discuss" the parameter(s) and,
   second, the "discussion" takes place.

   The first step, agreeing to discuss the parameters, takes place in
   the normal manner; one party proposes use of the option by sending a
   DO (or WILL) followed by the option code, and the other party accepts
   by returning a WILL (or DO) followed by the option code.  Once both
   parties have agreed to use the option, subnegotiation takes place by
   using the command SB, followed by the option code, followed by the
   parameter(s), followed by the command SE.  Each party is presumed to
   be able to parse the parameter(s), since each has indicated that the
   option is supported (via the initial exchange of WILL and DO).  On
   the other hand, the receiver may locate the end of a parameter string
   by searching for the SE command (i.e., the string IAC SE), even if
   the receiver is unable to parse the parameters.  Of course, either
   party may refuse to pursue further subnegotiation at any time by
   sending a WON'T or DON'T to the other party.


Postel & Reynolds