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Network Working Group                                           K. MooreRequest for Comments: 2047                       University of TennesseeObsoletes:1521,1522,1590                                November 1996Category: Standards TrackMIME (Multipurpose Internet Mail Extensions) Part Three:Message Header Extensions for Non-ASCII TextStatus 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 inRFC 934, STD   11, andRFC 1049, but extends and revises them.  BecauseRFC 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 toRFC 822 to allow non-US-ASCII text data in   Internet mail header fields.Moore                       Standards Track                     [Page 1]

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 inRFC2049 describes differences and changes from previous versions.1. IntroductionRFC 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 aRFC 822 [2] message header, in a manner   which is unlikely to confuse existing message handling software.   Like the encoding techniques described inRFC 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 toRFC 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   interpretRFC 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 ofRFC 822.Moore                       Standards Track                     [Page 2]

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 definedRFC 822 andRFC 2045.  In particular, the syntax for the ABNF used in this memo   is defined inRFC 822, as well as many of the terminal or nonterminal   symbols fromRFC 822 are used in the grammar for the header   extensions defined here.  Among the symbols defined inRFC 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 theRFC 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 withRFC 822.   However, implementors are warned that the character set name must be   spelled "US-ASCII" in MIME message and body part headers.Moore                       Standards Track                     [Page 3]

RFC 2047               Message Header Extensions           November 1996   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 ofRFC 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    ; seesection 3   encoding = token   ; seesection 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.Moore                       Standards Track                     [Page 4]

RFC 2047               Message Header Extensions           November 1996   IMPORTANT: 'encoded-word's are designed to be recognized as 'atom's   by anRFC 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   anRFC 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 insection 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.Moore                       Standards Track                     [Page 5]

RFC 2047               Message Header Extensions           November 1996   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 inRFC 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 byRFC2045.4.2. The "Q" encoding   The "Q" encoding is similar to the "Quoted-Printable" content-   transfer-encoding defined inRFC 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 seesection 5 for restrictions.)  In       particular, SPACE and TAB MUST NOT be represented as themselves       within encoded words.Moore                       Standards Track                     [Page 6]

RFC 2047               Message Header Extensions           November 19965. 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 byRFC 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, theRFC822 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.  (SeeRFC822, sections3.1.2 and3.1.3)(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' fromRFC 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'.Moore                       Standards Track                     [Page 7]

RFC 2047               Message Header Extensions           November 1996   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'.Moore                       Standards Track                     [Page 8]

RFC 2047               Message Header Extensions           November 19966. Support of 'encoded-word's by mail readers6.1. Recognition of 'encoded-word's in message headers   A mail reader must parse the message and body part headers according   to the rules inRFC 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 insection 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 insection 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 insection 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 anRFC 822 mail reader.Moore                       Standards Track                     [Page 9]

RFC 2047               Message Header Extensions           November 1996   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 insection 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 theMoore                       Standards Track                    [Page 10]

RFC 2047               Message Header Extensions           November 1996   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 insection 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 feedingMoore                       Standards Track                    [Page 11]

RFC 2047               Message Header Extensions           November 1996   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.Moore                       Standards Track                    [Page 12]

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.Moore                       Standards Track                    [Page 13]

RFC 2047               Message Header Extensions           November 199610. 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.eduMoore                       Standards Track                    [Page 14]

RFC 2047               Message Header Extensions           November 1996Appendix - changes sinceRFC 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 anRFC822 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 inRFC822 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 bothRFC822 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'.Moore                       Standards Track                    [Page 15]