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RFC 2047 - MIME (Multipurpose Internet Mail Extensions) Part Thr


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Network Working Group                                           K. Moore
Request for Comments: 2047                       University of Tennessee
Obsoletes: 1521, 1522, 1590                                November 1996
Category: Standards Track

        MIME (Multipurpose Internet Mail Extensions) Part Three:
              Message Header Extensions for Non-ASCII Text

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   STD 11, RFC 822, defines a message representation protocol specifying
   considerable detail about US-ASCII message headers, and leaves the
   message content, or message body, as flat US-ASCII text.  This set of
   documents, collectively called the Multipurpose Internet Mail
   Extensions, or MIME, redefines the format of messages to allow for

   (1) textual message bodies in character sets other than US-ASCII,

   (2) an extensible set of different formats for non-textual message
       bodies,

   (3) multi-part message bodies, and

   (4) textual header information in character sets other than US-ASCII.

   These documents are based on earlier work documented in RFC 934, STD
   11, and RFC 1049, but extends and revises them.  Because RFC 822 said
   so little about message bodies, these documents are largely
   orthogonal to (rather than a revision of) RFC 822.

   This particular document is the third document in the series.  It
   describes extensions to RFC 822 to allow non-US-ASCII text data in
   Internet mail header fields.

   Other documents in this series include:

   + RFC 2045, which specifies the various headers used to describe
     the structure of MIME messages.

   + RFC 2046, which defines the general structure of the MIME media
     typing system and defines an initial set of media types,

   + RFC 2048, which specifies various IANA registration procedures
     for MIME-related facilities, and

   + RFC 2049, which describes MIME conformance criteria and
     provides some illustrative examples of MIME message formats,
     acknowledgements, and the bibliography.

   These documents are revisions of RFCs 1521, 1522, and 1590, which
   themselves were revisions of RFCs 1341 and 1342.  An appendix in RFC
   2049 describes differences and changes from previous versions.

1. Introduction

   RFC 2045 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 US-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 2045, 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
   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
   (known as "encoded-words") are reserved for use as encoded data.  The
   syntax of encoded-words is such that they are unlikely to
   "accidentally" appear as normal text in message headers.
   Furthermore, the characters used in encoded-words are restricted to
   those which do not have special meanings in the context in which the
   encoded-word appears.

   Generally, 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 graphic 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
   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.

NOTES

   This memo relies heavily on notation and terms defined RFC 822 and
   RFC 2045.  In particular, the syntax for the ABNF used in this memo
   is defined in RFC 822, as well as many of the terminal or nonterminal
   symbols from RFC 822 are used in the grammar for the header
   extensions defined here.  Among the symbols defined in RFC 822 and
   referenced in this memo are: 'addr-spec', 'atom', 'CHAR', 'comment',
   'CTLs', 'ctext', 'linear-white-space', 'phrase', 'quoted-pair'.
   'quoted-string', 'SPACE', and 'word'.  Successful implementation of
   this protocol extension requires careful attention to the RFC 822
   definitions of these terms.

   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.

   This memo specifies a protocol for the representation of non-ASCII
   text in message headers.  It specifically DOES NOT define any
   translation between "8-bit headers" and pure ASCII headers, nor is
   any such translation assumed to be possible.

2. Syntax of encoded-words

   An 'encoded-word' is defined by the following ABNF grammar.  The
   notation of RFC 822 is used, with the exception that white space
   characters MUST NOT appear between components of an 'encoded-word'.

   encoded-word = "=?" charset "?" encoding "?" encoded-text "?="

   charset = token    ; see section 3

   encoding = token   ; see section 4

   token = 1*<Any CHAR except SPACE, CTLs, and especials>

   especials = "(" / ")" / "<" / ">" / "@" / "," / ";" / ":" / "
               <"> / "/" / "[" / "]" / "?" / "." / "="

   encoded-text = 1*<Any printable ASCII character other than "?"
                     or SPACE>
                  ; (but see "Use of encoded-words in message
                  ; headers", section 5)

   Both 'encoding' and 'charset' names are case-independent.  Thus the
   charset name "ISO-8859-1" is equivalent to "iso-8859-1", and the
   encoding named "Q" 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-word's (separated by CRLF SPACE) 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-word's is limited to 76 characters.

   The length restrictions are included both to ease interoperability
   through internetwork mail gateways, and 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.

   IMPORTANT: 'encoded-word's are designed to be recognized as 'atom's
   by an RFC 822 parser.  As a consequence, unencoded white space
   characters (such as SPACE and HTAB) are FORBIDDEN within an
   'encoded-word'.  For example, the character sequence

      =?iso-8859-1?q?this is some text?=

   would be parsed as four 'atom's, rather than as a single 'atom' (by
   an RFC 822 parser) or 'encoded-word' (by a parser which understands
   'encoded-words').  The correct way to encode the string "this is some
   text" is to encode the SPACE characters as well, e.g.

      =?iso-8859-1?q?this=20is=20some=20text?=

   The characters which may appear in 'encoded-text' are further
   restricted by the rules in section 5.

3. Character sets

   The 'charset' portion of an 'encoded-word' specifies the character
   set associated with the unencoded text.  A 'charset' can be any of
   the character set names allowed in an MIME "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.

   Some character sets use code-switching techniques to switch between
   "ASCII mode" and other modes.  If unencoded text in an 'encoded-word'
   contains a sequence which causes the charset interpreter to switch
   out of ASCII mode, it MUST 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-word's 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.

4. Encodings

   Initially, the legal values for "encoding" are "Q" and "B".  These
   encodings are described below.  The "Q" encoding is recommended for
   use when most of the characters to be encoded are in the ASCII
   character set; otherwise, the "B" encoding should be used.
   Nevertheless, a mail reader which claims to recognize 'encoded-word's
   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
   'encoded-text'.  Space and tab characters are 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' preceding 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.

4.1. The "B" encoding

   The "B" encoding is identical to the "BASE64" encoding defined by RFC
   2045.

4.2. The "Q" encoding

   The "Q" encoding is similar to the "Quoted-Printable" content-
   transfer-encoding defined in RFC 2045.  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".  (Upper case should be used for
       hexadecimal digits "A" through "F".)

   (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 "=", "?", and "_" (underscore), MAY be represented as those
       characters.  (But see section 5 for restrictions.)  In
       particular, SPACE and TAB MUST NOT be represented as themselves
       within encoded words.

5. Use of encoded-words in message headers

   An 'encoded-word' may appear in a message header or body part header
   according to the following rules:

(1) An 'encoded-word' may replace a 'text' token (as defined by RFC 822)
    in any Subject or Comments header field, any extension message
    header field, or any MIME body part field for which the field body
    is defined as '*text'.  An 'encoded-word' may also appear in any
    user-defined ("X-") message or body part header field.

    Ordinary ASCII text and 'encoded-word's may appear together in the
    same header field.  However, an 'encoded-word' that appears in a
    header field defined as '*text' MUST be separated from any adjacent
    'encoded-word' or 'text' by 'linear-white-space'.

(2) 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 "
    'encoded-word' that appears in a 'comment' MUST be separated from
    any adjacent 'encoded-word' or 'ctext' by 'linear-white-space'.

    It is important to note that 'comment's are only recognized inside
    "structured" field bodies.  In fields whose bodies are defined as
    '*text', "(" and ")" are treated as ordinary characters rather than
    comment delimiters, and rule (1) of this section applies.  (See RFC
    822, sections 3.1.2 and 3.1.3)

(3) 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.)>.  An 'encoded-word' that appears within a
    'phrase' MUST be separated from any adjacent 'word', 'text' or
    'special' by 'linear-white-space'.

   These are the ONLY locations where an 'encoded-word' may appear.  In
   particular:

   + An 'encoded-word' MUST NOT appear in any portion of an 'addr-spec'.

   + An 'encoded-word' MUST NOT appear within a 'quoted-string'.

   + An 'encoded-word' MUST NOT be used in a Received header field.

   + An 'encoded-word' MUST NOT be used in parameter of a MIME
     Content-Type or Content-Disposition field, or in any structured
     field body except within a 'comment' or 'phrase'.

   The 'encoded-text' in an 'encoded-word' must be self-contained;
   'encoded-text' MUST NOT be continued from one 'encoded-word' to
   another.  This implies that the 'encoded-text' portion of a "B"
   'encoded-word' will be a multiple of 4 characters long; for a "Q"
   'encoded-word', any "=" character that appears in the 'encoded-text'
   portion will be followed by two hexadecimal characters.

   Each 'encoded-word' MUST encode an integral number of octets.  The
   'encoded-text' in each 'encoded-word' must be well-formed according
   to the encoding specified; the 'encoded-text' may not be continued in
   the next 'encoded-word'.  (For example, "=?charset?Q?=?=
   =?charset?Q?AB?=" would be illegal, because the two hex digits "AB"
   must follow the "=" in the same 'encoded-word'.)

   Each 'encoded-word' MUST represent an integral number of characters.
   A multi-octet character may not be split across adjacent 'encoded-
   word's.

   Only printable and white space character data should be encoded using
   this scheme.  However, since these encoding schemes allow the
   encoding of arbitrary octet 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-word's to
   represent strings of purely ASCII characters is allowed, but
   discouraged.  In rare cases it may be necessary to encode ordinary
   text that looks like an 'encoded-word'.

6. Support of 'encoded-word's by mail readers

6.1. Recognition of 'encoded-word's in message headers

   A mail reader must parse the message and body part headers according
   to the rules in RFC 822 to correctly recognize 'encoded-word's.

   'encoded-word's are to be recognized as follows:

   (1) Any message or body part header field defined as '*text', or any
       user-defined header field, should be parsed as follows: Beginning
       at the start of the field-body and immediately following each
       occurrence of 'linear-white-space', each sequence of up to 75
       printable characters (not containing any 'linear-white-space')
       should be examined to see if it is an 'encoded-word' according to
       the syntax rules in section 2.  Any other sequence of printable
       characters should be treated as ordinary ASCII text.

   (2) Any header field not defined as '*text' should be parsed
       according to the syntax rules for that header field.  However,
       any 'word' that appears within a 'phrase' should be treated as an
       'encoded-word' if it meets the syntax rules in section 2.
       Otherwise it should be treated as an ordinary 'word'.

   (3) Within a 'comment', any sequence of up to 75 printable characters
       (not containing 'linear-white-space'), that meets the syntax
       rules in section 2, should be treated as an 'encoded-word'.
       Otherwise it should be treated as normal comment text.

   (4) A MIME-Version header field is NOT required to be present for
       'encoded-word's to be interpreted according to this
       specification.  One reason for this is that the mail reader is
       not expected to parse the entire message header before displaying
       lines that may contain 'encoded-word's.

6.2. Display of 'encoded-word's

   Any 'encoded-word's so recognized are decoded, and if possible, the
   resulting unencoded text is displayed in the original character set.

   NOTE: Decoding and display of encoded-words occurs *after* a
   structured field body is parsed into tokens.  It is therefore
   possible to hide 'special' characters in encoded-words which, when
   displayed, will be indistinguishable from 'special' characters in the
   surrounding text.  For this and other reasons, it is NOT generally
   possible to translate a message header containing 'encoded-word's to
   an unencoded form which can be parsed by an RFC 822 mail reader.

   When displaying a particular header field that contains multiple
   'encoded-word's, any 'linear-white-space' that separates a pair of
   adjacent 'encoded-word's is ignored.  (This is to allow the use of
   multiple 'encoded-word's to represent long strings of unencoded text,
   without having to separate 'encoded-word's where spaces occur in the
   unencoded text.)

   In the event other encodings are defined in the future, and the mail
   reader does not support the encoding used, it may either (a) display
   the 'encoded-word' as ordinary text, or (b) substitute an appropriate
   message indicating that the text could not be decoded.

   If the mail reader does not support the character set used, it may
   (a) display the 'encoded-word' as ordinary text (i.e., as it appears
   in the header), (b) make a "best effort" to display using such
   characters as are available, or (c) substitute an appropriate message
   indicating that the decoded text could not be displayed.

   If the character set being used employs code-switching techniques,
   display of the encoded text implicitly begins in "ASCII mode".  In
   addition, the mail reader must ensure that the output device is once
   again in "ASCII mode" after the 'encoded-word' is displayed.

6.3. Mail reader handling of incorrectly formed 'encoded-word's

   It is possible that an 'encoded-word' that is legal according to the
   syntax defined in section 2, is incorrectly formed according to the
   rules for the encoding being used.   For example:

   (1) An 'encoded-word' which contains characters which are not legal
       for a particular encoding (for example, a "-" in the "B"
       encoding, or a SPACE or HTAB in either the "B" or "Q" encoding),
       is incorrectly formed.

   (2) Any 'encoded-word' which encodes a non-integral number of
       characters or octets is incorrectly formed.

   A mail reader need not attempt to display the text associated with an
   'encoded-word' that is incorrectly formed.  However, a mail reader
   MUST NOT prevent the display or handling of a message because an
   'encoded-word' is incorrectly formed.

7. Conformance

   A mail composing program claiming compliance with this specification
   MUST ensure that any string of non-white-space printable ASCII
   characters within a '*text' or '*ctext' that begins with "=?" and
   ends with "?=" be a valid 'encoded-word'.  ("begins" means: at the

   start of the field-body, immediately following 'linear-white-space',
   or immediately following a "(" for an 'encoded-word' within '*ctext';
   "ends" means: at the end of the field-body, immediately preceding
   'linear-white-space', or immediately preceding a ")" for an
   'encoded-word' within '*ctext'.)  In addition, any 'word' within a
   'phrase' that begins with "=?" and ends with "?=" must be a valid
   'encoded-word'.

   A mail reading program claiming compliance with this specification
   must be able to distinguish 'encoded-word's from 'text', 'ctext', or
   'word's, according to the rules in section 6, anytime they appear in
   appropriate places in message headers.  It must support both the "B"
   and "Q" encodings for any character set which it supports.  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.

8. Examples

   The following are examples of message headers containing 'encoded-
   word's:

   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==?=

      Note: In the first 'encoded-word' of the Subject field above, the
      last "=" at the end of the 'encoded-text' is necessary because each
      'encoded-word' must be self-contained (the "=" character completes a
      group of 4 base64 characters representing 2 octets).  An additional
      octet could have been encoded in the first 'encoded-word' (so that
      the encoded-word would contain an exact multiple of 3 encoded
      octets), except that the second 'encoded-word' uses a different
      'charset' than the first one.

   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

   The following examples illustrate how text containing 'encoded-word's
   which appear in a structured field body.  The rules are slightly
   different for fields defined as '*text' because "(" and ")" are not
   recognized as 'comment' delimiters.  [Section 5, paragraph (1)].

   In each of the following examples, if the same sequence were to occur
   in a '*text' field, the "displayed as" form would NOT be treated as
   encoded words, but be identical to the "encoded form".  This is
   because each of the encoded-words in the following examples is
   adjacent to a "(" or ")" character.

   encoded form                                displayed as
   ---------------------------------------------------------------------
   (=?ISO-8859-1?Q?a?=)                        (a)

   (=?ISO-8859-1?Q?a?= b)                      (a b)

           Within a 'comment', white space MUST appear between an
           'encoded-word' and surrounding text.  [Section 5,
           paragraph (2)].  However, white space is not needed between
           the initial "(" that begins the 'comment', and the
           'encoded-word'.

   (=?ISO-8859-1?Q?a?= =?ISO-8859-1?Q?b?=)     (ab)

           White space between adjacent 'encoded-word's is not
           displayed.

   (=?ISO-8859-1?Q?a?=  =?ISO-8859-1?Q?b?=)    (ab)

        Even multiple SPACEs between 'encoded-word's are ignored
        for the purpose of display.

   (=?ISO-8859-1?Q?a?=                         (ab)
       =?ISO-8859-1?Q?b?=)

           Any amount of linear-space-white between 'encoded-word's,
           even if it includes a CRLF followed by one or more SPACEs,
           is ignored for the purposes of display.

   (=?ISO-8859-1?Q?a_b?=)                      (a b)

           In order to cause a SPACE to be displayed within a portion
           of encoded text, the SPACE MUST be encoded as part of the
           'encoded-word'.

   (=?ISO-8859-1?Q?a?= =?ISO-8859-2?Q?_b?=)    (a b)

           In order to cause a SPACE to be displayed between two strings
           of encoded text, the SPACE MAY be encoded as part of one of
           the 'encoded-word's.

9. References

   [RFC 822] Crocker, D., "Standard for the Format of ARPA Internet Text
       Messages", STD 11, RFC 822, UDEL, August 1982.

   [RFC 2049] Borenstein, N., and N. Freed, "Multipurpose Internet Mail
       Extensions (MIME) Part Five: Conformance Criteria and Examples",
       RFC 2049, November 1996.

   [RFC 2045] Borenstein, N., and N. Freed, "Multipurpose Internet Mail
       Extensions (MIME) Part One: Format of Internet Message Bodies",
       RFC 2045, November 1996.

   [RFC 2046] Borenstein N., and N. Freed, "Multipurpose Internet Mail
       Extensions (MIME) Part Two: Media Types", RFC 2046,
       November 1996.

   [RFC 2048] Freed, N., Klensin, J., and J. Postel, "Multipurpose
       Internet Mail Extensions (MIME) Part Four: Registration
       Procedures", RFC 2048, November 1996.

10. Security Considerations

   Security issues are not discussed in this memo.

11. Acknowledgements

   The author wishes to thank Nathaniel Borenstein, Issac Chan, Lutz
   Donnerhacke, Paul Eggert, Ned Freed, Andreas M. Kirchwitz, Olle
   Jarnefors, Mike Rosin, Yutaka Sato, Bart Schaefer, and Kazuhiko
   Yamamoto, for their helpful advice, insightful comments, and
   illuminating questions in response to earlier versions of this
   specification.

12. Author's Address

   Keith Moore
   University of Tennessee
   107 Ayres Hall
   Knoxville TN 37996-1301

   EMail: moore@cs.utk.edu

Appendix - changes since RFC 1522 (in no particular order)

   + explicitly state that the MIME-Version is not requried to use
     'encoded-word's.

   + add explicit note that SPACEs and TABs are not allowed within
     'encoded-word's, explaining that an 'encoded-word' must look like an
     'atom' to an RFC822 parser.values, to be precise).

   + add examples from Olle Jarnefors (thanks!) which illustrate how
     encoded-words with adjacent linear-white-space are displayed.

   + explicitly list terms defined in RFC822 and referenced in this memo

   + fix transcription typos that caused one or two lines and a couple of
     characters to disappear in the resulting text, due to nroff quirks.

   + clarify that encoded-words are allowed in '*text' fields in both
     RFC822 headers and MIME body part headers, but NOT as parameter
     values.

   + clarify the requirement to switch back to ASCII within the encoded
     portion of an 'encoded-word', for any charset that uses code switching
     sequences.

   + add a note about 'encoded-word's being delimited by "(" and ")"
     within a comment, but not in a *text (how bizarre!).

   + fix the Andre Pirard example to get rid of the trailing "_" after
     the =E9.  (no longer needed post-1342).

   + clarification: an 'encoded-word' may appear immediately following
     the initial "(" or immediately before the final ")" that delimits a
     comment, not just adjacent to "(" and ")" *within* *ctext.

   + add a note to explain that a "B" 'encoded-word' will always have a
     multiple of 4 characters in the 'encoded-text' portion.

   + add note about the "=" in the examples

   + note that processing of 'encoded-word's occurs *after* parsing, and
     some of the implications thereof.

   + explicitly state that you can't expect to translate between
     1522 and either vanilla 822 or so-called "8-bit headers".

   + explicitly state that 'encoded-word's are not valid within a
     'quoted-string'.

 

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