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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]
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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]
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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]
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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]
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- 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]
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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]
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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]
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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]