RFC855

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.