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Network Working Group K. Moore Internet Draft University of Tennessee 22 March 1993 MIME (Multipurpose Internet Mail Extensions) Part Two: Message Header Extensions for Non-ASCII Text Status of this Memo This document is an Internet Draft. Internet Drafts are working documents of the Internet Engineering Task Force (IETF), its Areas, and its Working Groups. Note that other groups may also distribute working documents as Internet Drafts. Internet Drafts are valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. (The file 1id-abstracts.txt, available via anonymous ftp from nic.ddn.mil, describes the current status of each Internet Draft.) It is not appropriate to use an Internet Draft as reference material or to cite one other than as a "work in progress". This document is a revision of RFC 1342. If approved by the IETF message format extensions working group, it will be submitted to the IESG as a candidate for Draft Standard status. Distribution of this memo is unlimited. Please send comments to ietf-822@dimacs.rutgers.edu. Abstract This memo describes an extension to the message format defined in RFC 1341++ [1], to allow the representation of character sets other than ASCII in RFC 822 message headers. The extensions described were designed to be highly compatible with existing Internet mail handling software, and to be easily implemented in mail readers that support RFC 1341++. Introduction RFC 1341++ describes a mechanism for denoting textual body parts which are coded in various character sets, as well as methods for encoding such body parts as sequences of printable ASCII characters. This memo describes similar techniques to allow the encoding of non- ASCII text in various portions of a RFC 822 [2] message header, in a manner which is unlikely to confuse existing message handling software. Like the encoding techniques described in RFC 1341++, the techniques outlined here were designed to allow the use of non-ASCII characters in message headers in a way which is unlikely to be disturbed by the quirks of existing Internet mail handling programs. In particular, some mail relaying programs are known to (a) delete some message K. Moore [Page 1] Internet Draft Expires 22 September 1993 22 March 1993 header fields while retaining others, (b) rearrange the order of addresses in To or Cc fields, (c) rearrange the (vertical) order of header fields, and/or (d) "wrap" message headers at different places than those in the original message. In addition, some mail reading programs are known to have difficulty correctly parsing message headers which, while legal according to RFC 822, make use of backslash-quoting to "hide" special characters such as "<", ",", or ":", or which exploit other infrequently-used features of that specification. While it is unfortunate that these programs do not correctly interpret RFC 822 headers, to "break" these programs would cause severe operational problems for the Internet mail system. The extensions described in this memo therefore do not rely on little- used features of RFC 822. Instead, certain sequences of "ordinary" printable ASCII characters (which are assumed to be unlikely to otherwise appear in message headers) are reserved for use as encoded data. The characters used in these encodings are restricted to those which do not have special meanings in the context in which the encoded text appears. Notes This memo relies heavily on notation and terms defined RFC 822 and RFC 1341++. In particular, the syntax for the EBNF used in this memo is defined in RFC 822, as well as many of the terms used in the grammar for the header extensions defined here. Successful implementation of this protocol extension requires careful attention to the details of both RFC 822 and RFC 1341++. When the term "ASCII" appears in this memo, it refers to the "7-Bit American Standard Code for Information Interchange", ANSI X3.4-1986. The MIME charset name for this character set is "US-ASCII". When not specifically referring to the MIME charset name, this document uses the term "ASCII", both for brevity and for consistency with RFC 822. However, implementors are warned that the character set name must be spelled "US-ASCII" in MIME message and body part headers. Encodings An "encoded-word" is a sequence of printable ASCII characters that begins with "=?", ends with "?=", and has two "?"s in between. It specifies a character set and an encoding method, and also includes the original text encoded as ASCII characters, according to the rules for that encoding method. A mail composer that implements this specification will provide a means of inputting non-ASCII text in header fields, but will translate these fields (or appropriate portions of these fields) into K. Moore [Page 2] Internet Draft Expires 22 September 1993 22 March 1993 encoded- words before inserting them into the message header. A mail reader that implements this specification will recognize encoded-words when they appear in certain portions of the message header. Instead of displaying the encoded-word "as is", it will reverse the encoding and display the original text in the designated character set. An "encoded-word" is more precisely defined by the following ABNF grammar, using the notation of RFC 822: encoded-word = "=" "?" charset "?" encoding "?" encoded-text "?" "=" charset = token ; legal charsets defined by RFC 1341++ encoding = token ; Either "B" or "Q" token = 1*<Any CHAR except SPACE, CTLs, and tspecials> tspecials = "(" / ")" / "<" / ">" / "@" / "," / ";" / ":" / "\" / <"> / "/" / "[" / "]" / "?" / "." / "=" encoded-text = 1*<Any printable ASCII character other than "?" or SPACE> ; (but see "Use of encoded-words in message ; headers", below) Both "encoding" and "charset" names are case-independent. Thus "ISO-8859-1" is equivalent to "iso-8859-1", and the "Q" encoding may be spelled either "Q" or "q". An encoded-word may not be more than 75 characters long, including charset, encoding, encoded-text, and delimiters. If it is desirable to encode more text than will fit in an encoded-word of 75 characters, multiple encoded-words (separated by SPACE or newline) may be used. While there is no limit to the length of a multiple- line header field, each line of a header field that contains one or more encoded-words is limited to 76 characters. NOTE: These restrictions are included not only to ease interoperability through internetwork mail gateways, but also to impose a limit on the amount of lookahead a header parser must employ (while looking for a final ?= delimiter) before it can decide whether a token is an encoded-word or something else. Initially, the legal values for "encoding" are "Q" and "B". These encodings are described below. The "Q" encoding is recommended for use with Latin character sets, and the "B" encoding for all others. Nevertheless, a mail reader which claims to recognize encoded-words MUST be able to accept either encoding for any character set which it supports. Only a subset of the printable ASCII characters may be used in K. Moore [Page 3] Internet Draft Expires 22 September 1993 22 March 1993 encoded-text. The SPACE character is not allowed, so that the beginning and end of an encoded-word are obvious. The "?" character is used within an encoded-word to separate the various portions of the encoded-word from one another, and thus cannot appear in the encoded-text portion. Other characters are also illegal in certain contexts. For example, an encoded-word in a "phrase" preceeding an address in a From header field may not contain any of the "specials" defined in RFC 822. Finally, certain other characters are disallowed in some contexts, to ensure reliability for messages that pass through internetwork mail gateways. The "B" encoding automatically meets these requirements. The "Q" encoding allows a wide range of printable characters to be used in non-critical locations in the message header (e.g., Subject), with fewer characters available for use in other locations. The "B" encoding The "B" encoding is identical to the "BASE64" encoding defined by RFC 1341++. The "Q" encoding The "Q" encoding is similar to the "Quoted-Printable" content- transfer-encoding defined in RFC 1341++. It is designed to allow text containing mostly ASCII characters to be decipherable on an ASCII terminal without decoding. 1. Any 8-bit value may be represented by a "=" followed by two hexadecimal digits. For example, if the character set in use were ISO-8859-1, the "=" character would thus be encoded as "=3D", and a SPACE by "=20". 2. The 8-bit hexadecimal value 20 (e.g., ISO-8859-1 SPACE) may be represented as "_" (underscore, ASCII 95.). (This character may not pass through some internetwork mail gateways, but its use will greatly enhance readability of "Q" encoded data with mail readers that do not support this encoding.) Note that the "_" always represents hexadecimal 20, even if the SPACE character occupies a different code position in the character set in use. 3. 8-bit values which correspond to printable ASCII characters other than "=", "?", "_" (underscore), and SPACE may be represented as those characters. (But see "Use of encoded-words in message headers", below). K. Moore [Page 4] Internet Draft Expires 22 September 1993 22 March 1993 Character sets In an encoded-word, the character set associated with the unencoded text is specified by a charset. A charset can be any of the character set names allowed in an RFC 1341++ "charset" parameter of a "text/plain" body part, or any character set name registered with IANA for use with the MIME text/plain content-type. (See section 7.1.1 of RFC 1341++ for a list of charsets defined in that document). Some character sets use code-switching techniques to switch between "ASCII mode" and other modes. Display of each encoded-word using such a character set implicitly begins in ASCII mode. If the encoded-text in an encoded-word contains control codes to switch out of ASCII mode, it must also contain additional control codes such that ASCII mode is again selected at the end of the encoded-word. (This rule applies separately to each encoded-word, including adjacent encoded-words within a single header field.) When there is a possibility of using more than one character set to represent the text in an encoded-word, and in the absence of private agreements between sender and recipients of a message, it is recommended that members of the ISO-8859-* series be used in preference to other character sets. Among the various ISO-8859-* character sets, the lowest-numbered set which contains all of the required characters should be used. Use of encoded-words in message headers A sequence of one or more encoded-words is used to represent non- ASCII textual data within a header field. An encoded-word must be separated from an adjacent encoded-word, "word", "text", "ctext", or "special" by a linear white-space character or a newline. When displaying a particular header field that contains multiple encoded-words, any linear-white-space that separates a pair of adjacent encoded-words is ignored. (This is to allow the use of multiple encoded-words to represent long strings of unencoded text, without having to separate encoded-words where spaces occur in the unencoded text.) Each encoded-word must represent an integral number of characters; a character may not be split across adjacent encoded-words. An encoded-word may appear in a message header or body part header according to the following rules: - An encoded-word may replace a "text" token (as defined by RFC 822) in: (1) a Subject or Comments header field, (2) any extension message header field, (3) any user-defined message header field, or (4) any RFC 1341++ body part header field (such as Content-Description) for which the field body contains only "text"s. K. Moore [Page 5] Internet Draft Expires 22 September 1993 22 March 1993 - An encoded-word may appear within a comment delimited by "(" and ")", i.e., wherever a "ctext" is allowed. More precisely, the RFC 822 ABNF definition for "comment" is amended as follows: comment = "(" *(ctext / quoted-pair / comment / encoded-word) ")" A "Q"-encoded encoded-word which appears in a comment MUST NOT contain the characters "(", ")" or "\". - As a replacement for a "word" entity within a "phrase", for example, one that precedes an address in a From, To, or Cc header. The ABNF definition for phrase from RFC 822 thus becomes: phrase = 1*(encoded-word / word) In this case the set of characters that may be used in a "Q"-encoded encoded-word is restricted to: <upper and lower case ASCII letters, decimal digits, "!", "*", "+", "-", "/", "=", and "_" (underscore, ASCII 95.)>. These are the ONLY locations where an encoded-word may appear. In particular, an encoded-word MUST NOT appear in any portion of an "address". In addition, an encoded-word MUST NOT be used in a Received header field. Whenever such words appear in a header being displayed, an enlightened mail reader will decode the text and render it appropriately. Only textual data (printable and white space characters) should be encoded using this scheme. However, since these encoding schemes allow the encoding of arbitrary 8-bit values, mail readers that implement this decoding should also ensure that display of the decoded data on the recipient's terminal will not cause unwanted side-effects. Use of these methods to encode non-textual data (e.g., pictures or sounds) is not defined by this memo. Use of encoded-words to represent strings of purely ASCII characters is allowed, but discouraged. Recognition of encoded-words in message headers. An encoded-word may be distinguished from an ordinary "word", "text", or "ctext", as follows: An encoded-word begins with "=?", ends with "?=", contains exactly four "?" characters including the delimiters, and is followed by a SPACE or newline. If the "word", "text", or "ctext" does not meet the above tests, it should be displayed as it appears in the message header. If the mail reader does not support the character set used, it may either display the encoded-word as ordinary text (i.e., as it appears in K. Moore [Page 6] Internet Draft Expires 22 September 1993 22 March 1993 the header), or it may substitute an appropriate message indicating that the decoded text could not be displayed. Conformance A mail composing program claiming compliance with this specification MUST ensure that any string of printable ASCII characters in a "text" or "ctext" entity within a header, or any "atom" within a "phrase", that begins with "=?" and ends with "?=" be a valid encoded-word. A mail reading program claiming compliance with this specification must be able to distinguish encoded-words from "text", "ctext", or "word"s anytime they appear in appropriate places in message headers. In addition, the program must be able to display the unencoded text if the character set is "US-ASCII". For the ISO-8859-* character sets, the mail reading program must at least be able to display the characters which are also in the ASCII set. Examples From: =?US-ASCII?Q?Keith_Moore?= <moore@cs.utk.edu> To: =?ISO-8859-1?Q?Keld_J=F8rn_Simonsen?= <keld@dkuug.dk> CC: =?ISO-8859-1?Q?Andr=E9_?= Pirard <PIRARD@vm1.ulg.ac.be> Subject: =?ISO-8859-1?B?SWYgeW91IGNhbiByZWFkIHRoaXMgeW8=?= =?ISO-8859-2?B?dSB1bmRlcnN0YW5kIHRoZSBleGFtcGxlLg==?= From: =?ISO-8859-1?Q?Olle_J=E4rnefors?= <ojarnef@admin.kth.se> To: ietf-822@dimacs.rutgers.edu, ojarnef@admin.kth.se Subject: Time for ISO 10646? To: Dave Crocker <dcrocker@mordor.stanford.edu> Cc: ietf-822@dimacs.rutgers.edu, paf@comsol.se From: =?ISO-8859-1?Q?Patrik_F=E4ltstr=F6m?= <paf@nada.kth.se> Subject: Re: RFC-HDR care and feeding From: Nathaniel Borenstein <nsb@thumper.bellcore.com> (=?iso-8859-8?b?7eXs+SDv4SDp7Oj08A==?=) To: Greg Vaudreuil <gvaudre@NRI.Reston.VA.US>, Ned Freed <ned@innosoft.com>, Keith Moore <moore@cs.utk.edu> Subject: Test of new header generator MIME-Version: 1.0 Content-type: text/plain; charset=ISO-8859-1 References [1] Borenstein N., and N. Freed, "MIME (Multipurpose Internet Mail Extensions) Part One: Mechanisms for Specifying and Describing the K. Moore [Page 7] Internet Draft Expires 22 September 1993 22 March 1993 Format of Internet Message Bodies", Internet-Draft RFC 1341++, Bellcore, Innosoft, March 1993. [2] Crocker, D., "Standard for the Format of ARPA Internet Text Messages", RFC 822, UDEL, August 1982. Security Considerations Security issues are not discussed in this memo. Author's Address Keith Moore University of Tennessee 107 Ayres Hall Knoxville TN 37996-1301 EMail: moore@cs.utk.edu K. Moore [Page 8] Internet Draft Expires 22 September 1993 22 March 1993 Appendix - changes from RFC 1342 1. Title changed to say "MIME Part 2". 2. Character sets allowed to include IANA-registered charsets in addition to those defined in RFC 1341++. (X-* charsets are still excluded.) 3. RFC 1342 said, in effect, "don't display a space or newline following an encoded-word". This memo says, "don't display any linear-white-space between adjacent encoded-words." 4. Each encoded-word must now contain an integral number of characters. 5. Added language about charsets that use code-switching techniques. 6. "Compliance" paragraph changed -- =?something?= is now only required to be a valid encoded-word if the =?something?= is either contained in a "text", or is an "atom" within a "phrase". 7. Clarified the 76 character per line limit (it's per line, not per header field). 8. Define "charset" and "encoding" to be case-independent. 9. Added note to explicitly refer the reader to RFC 822 and RFC 1341++. 10. Added note re: usage of "ASCII" vs. "US-ASCII". K. Moore [Page 9]