Telnet Options: Telnet Option Specifications

Network Working Group                                          J. Postel
Request for Comments: 855                                    J. Reynolds
                                                                     ISI
Obsoletes: NIC 18640                                            May 1983

                      TELNET OPTION SPECIFICATIONS


This RFC specifies a standard for the ARPA Internet community.  Hosts on
the ARPA Internet are expected to adopt and implement this standard.

The intent of providing for options in the TELNET Protocol is to permit
hosts to obtain more elegant solutions to the problems of communication
between dissimilar devices than is possible within the framework
provided by the Network Virtual Terminal (NVT).  It should be possible
for hosts to invent, test, or discard options at will.  Nevertheless, it
is envisioned that options which prove to be generally useful will
eventually be supported by many hosts; therefore it is desirable that
options should be carefully documented and well publicized.  In
addition, it is necessary to insure that a single option code is not
used for several different options.

This document specifies a method of option code assignment and standards
for documentation of options.  The individual responsible for assignment
of option codes may waive the requirement for complete documentation for
some cases of experimentation, but in general documentation will be
required prior to code assignment.  Options will be publicized by
publishing their documentation as RFCs; inventors of options may, of
course, publicize them in other ways as well.

   Option codes will be assigned by:

      Jonathan B. Postel
      University of Southern California
      Information Sciences Institute (USC-ISI)
      4676 Admiralty Way
      Marina Del Rey, California 90291
      (213) 822-1511

      Mailbox = POSTEL@USC-ISIF

Documentation of options should contain at least the following sections:

   Section 1 - Command Name and Option Code

   Section 2 - Command Meanings

      The meaning of each possible TELNET command relevant to this
      option should be described.  Note that for complex options, where
      "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.

   Thus, for option "ABC", which requires subnegotiation, the formats of
   the TELNET commands are:

      IAC WILL ABC

         Offer to use option ABC (or favorable acknowledgment of other
         party's request)

      IAC DO ABC

         Request for other party to use option ABC (or favorable
         acknowledgment of other party's offer)

      IAC SB ABC <parameters> IAC SE

         One step of subnegotiation, used by either party.

   Designers of options requiring "subnegotiation" must take great care
   to avoid unending loops in the subnegotiation process.  For example,
   if each party can accept any value of a parameter, and both parties
   suggest parameters with different values, then one is likely to have
   an infinite oscillation of "acknowledgments" (where each receiver
   believes it is only acknowledging the new proposals of the other).
   Finally, if parameters in an option "subnegotiation" include a byte
   with a value of 255, it is necessary to double this byte in
   accordance the general TELNET rules.