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.









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RFC 2047               Message Header Extensions           November 1996


  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.



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RFC 2047               Message Header Extensions           November 1996


  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.






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  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.





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  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.



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  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.



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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'.






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  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'.









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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.




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  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



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  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?= <[email protected]>
  To: =?ISO-8859-1?Q?Keld_J=F8rn_Simonsen?= <[email protected]>
  CC: =?ISO-8859-1?Q?Andr=E9?= Pirard <[email protected]>
  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?= <[email protected]>
  To: [email protected], [email protected]
  Subject: Time for ISO 10646?

  To: Dave Crocker <[email protected]>
  Cc: [email protected], [email protected]
  From: =?ISO-8859-1?Q?Patrik_F=E4ltstr=F6m?= <[email protected]>
  Subject: Re: RFC-HDR care and feeding





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  From: Nathaniel Borenstein <[email protected]>
        (=?iso-8859-8?b?7eXs+SDv4SDp7Oj08A==?=)
  To: Greg Vaudreuil <[email protected]>, Ned Freed
     <[email protected]>, Keith Moore <[email protected]>
  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.



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RFC 2047               Message Header Extensions           November 1996


  (=?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.



















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RFC 2047               Message Header Extensions           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: [email protected]






























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RFC 2047               Message Header Extensions           November 1996


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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