Network Working Group
Request for Comments: 855
Obsoletes: NIC 18640
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:
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.
The default assumptions for hosts which do not implement, or use, the option must be described.
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.
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.
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.