rfc2152.txt

来自「RFC 的详细文档！」· 文本 代码 · 共 844 行 · 第 1/2 页

Network Working Group                                       D. Goldsmith
Request for Comments: 2152                          Apple Computer, Inc.
Obsoletes: RFC 1642                                             M. Davis
Category: Informational                                   Taligent, Inc.
                                                                May 1997


                                 UTF-7

              A Mail-Safe Transformation Format of Unicode

Status of this Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.

Abstract

   The Unicode Standard, version 2.0, and ISO/IEC 10646-1:1993(E) (as
   amended) jointly define a character set (hereafter referred to as
   Unicode) which encompasses most of the world's writing systems.
   However, Internet mail (STD 11, RFC 822) currently supports only 7-
   bit US ASCII as a character set. MIME (RFC 2045 through 2049) extends
   Internet mail to support different media types and character sets,
   and thus could support Unicode in mail messages. MIME neither defines
   Unicode as a permitted character set nor specifies how it would be
   encoded, although it does provide for the registration of additional
   character sets over time.

   This document describes a transformation format of Unicode that
   contains only 7-bit ASCII octets and is intended to be readable by
   humans in the limiting case that the document consists of characters
   from the US-ASCII repertoire. It also specifies how this
   transformation format is used in the context of MIME and RFC 1641,
   "Using Unicode with MIME".

Motivation

   Although other transformation formats of Unicode exist and could
   conceivably be used in this context (most notably UTF-8, also known
   as UTF-2 or UTF-FSS), they suffer the disadvantage that they use
   octets in the range decimal 128 through 255 to encode Unicode
   characters outside the US-ASCII range. Thus, in the context of mail,
   those octets must themselves be encoded. This requires putting text
   through two successive encoding processes, and leads to a significant
   expansion of characters outside the US-ASCII range, putting non-
   English speakers at a disadvantage. For example, using UTF-8 together



Goldsmith & Davis            Informational                      [Page 1]

RFC 2152                         UTF-7                          May 1997


   with the Quoted-Printable content transfer encoding of MIME
   represents US-ASCII characters in one octet, but other characters may
   require up to nine octets.

Overview

   UTF-7 encodes Unicode characters as US-ASCII octets, together with
   shift sequences to encode characters outside that range. For this
   purpose, one of the characters in the US-ASCII repertoire is reserved
   for use as a shift character.

   Many mail gateways and systems cannot handle the entire US-ASCII
   character set (those based on EBCDIC, for example), and so UTF-7
   contains provisions for encoding characters within US-ASCII in a way
   that all mail systems can accomodate.

   UTF-7 should normally be used only in the context of 7 bit
   transports, such as mail. In other contexts, straight Unicode or
   UTF-8 is preferred.

   See RFC 1641, "Using Unicode with MIME" for the overall specification
   on usage of Unicode transformation formats with MIME.

Definitions

   First, the definition of Unicode:

      The 16 bit character set Unicode is defined by "The Unicode
      Standard, Version 2.0". This character set is identical with the
      character repertoire and coding of the international standard
      ISO/IEC 10646-1:1993(E); Coded Representation Form=UCS-2;
      Subset=300; Implementation Level=3, including the first 7
      amendments to 10646 plus editorial corrections.

      Note. Unicode 2.0 further specifies the use and interaction of
      these character codes beyond the ISO standard. However, any valid
      10646 sequence is a valid Unicode sequence, and vice versa;
      Unicode supplies interpretations of sequences on which the ISO
      standard is silent as to interpretation.

   Next, some handy definitions of US-ASCII character subsets:

      Set D (directly encoded characters) consists of the following
      characters (derived from RFC 1521, Appendix B, which no longer
      appears in RFC 2045): the upper and lower case letters A through Z
      and a through z, the 10 digits 0-9, and the following nine special
      characters (note that "+" and "=" are omitted):




Goldsmith & Davis            Informational                      [Page 2]

RFC 2152                         UTF-7                          May 1997


               Character   ASCII & Unicode Value (decimal)
                  '           39
                  (           40
                  )           41
                  ,           44
                  -           45
                  .           46
                  /           47
                  :           58
                  ?           63

      Set O (optional direct characters) consists of the following
      characters (note that "\" and "~" are omitted):

               Character   ASCII & Unicode Value (decimal)
                  !           33
                  "           34
                  #           35
                  $           36
                  %           37
                  &           38
                  *           42
                  ;           59
                  <           60
                  =           61
                  >           62
                  @           64
                  [           91
                  ]           93
                  ^           94
                  _           95
                  '           96
                  {           123
                  |           124
                  }           125

   Rationale. The characters "\" and "~" are omitted because they are
   often redefined in variants of ASCII.

   Set B (Modified Base 64) is the set of characters in the Base64
   alphabet defined in RFC 2045, excluding the pad character "="
   (decimal value 61).









Goldsmith & Davis            Informational                      [Page 3]

RFC 2152                         UTF-7                          May 1997


   Rationale. The pad character = is excluded because UTF-7 is designed
   for use within header fields as set forth in RFC 2047. Since the only
   readable encoding in RFC 2047 is "Q" (based on RFC 2045's Quoted-
   Printable), the "=" character is not available for use (without a lot
   of escape sequences). This was very unfortunate but unavoidable. The
   "=" character could otherwise have been used as the UTF-7 escape
   character as well (rather than using "+").

   Note that all characters in US-ASCII have the same value in Unicode
   when zero-extended to 16 bits.

UTF-7 Definition

   A UTF-7 stream represents 16-bit Unicode characters using 7-bit US-
   ASCII octets as follows:

      Rule 1: (direct encoding) Unicode characters in set D above may be
      encoded directly as their ASCII equivalents. Unicode characters in
      Set O may optionally be encoded directly as their ASCII
      equivalents, bearing in mind that many of these characters are
      illegal in header fields, or may not pass correctly through some
      mail gateways.

      Rule 2: (Unicode shifted encoding) Any Unicode character sequence
      may be encoded using a sequence of characters in set B, when
      preceded by the shift character "+" (US-ASCII character value
      decimal 43). The "+" signals that subsequent octets are to be
      interpreted as elements of the Modified Base64 alphabet until a
      character not in that alphabet is encountered. Such characters
      include control characters such as carriage returns and line
      feeds; thus, a Unicode shifted sequence always terminates at the
      of a line. As a special case, if the sequence terminates with the
      character "-" (US-ASCII decimal 45) then that character is
      absorbed; other terminating characters are not absorbed and are
      processed normally.

      Note that if the first character after the shifted sequence is "-"
      then an extra "-" must be present to terminate the shifted
      sequence so that the actual "-" is not itself absorbed.

      Rationale. A terminating character is necessary for cases where
      the next character after the Modified Base64 sequence is part of
      character set B or is itself the terminating character. It can
      also enhance readability by delimiting encoded sequences.







Goldsmith & Davis            Informational                      [Page 4]

RFC 2152                         UTF-7                          May 1997


      Also as a special case, the sequence "+-" may be used to encode
      the character "+". A "+" character followed immediately by any
      character other than members of set B or "-" is an ill-formed
      sequence.

      Unicode is encoded using Modified Base64 by first converting
      Unicode 16-bit quantities to an octet stream (with the most
      significant octet first). Surrogate pairs (UTF-16) are converted
      by treating each half of the pair as a separate 16 bit quantity
      (i.e., no special treatment). Text with an odd number of octets is
      ill-formed. ISO 10646 characters outside the range addressable via
      surrogate pairs cannot be encoded.

      Rationale. ISO/IEC 10646-1:1993(E) specifies that when characters
      the UCS-2 form are serialized as octets, that the most significant
      octet appear first.  This is also in keeping with common network
      practice of choosing a canonical format for transmission.

      Rationale. The policy for code point allocation within ISO 10646
      and Unicode is that the repertoires be kept synchronized. No code
      points will be allocated in ISO 10646 outside the range
      addressable by surrogate pairs.

      Next, the octet stream is encoded by applying the Base64 content
      transfer encoding algorithm as defined in RFC 2045, modified to
      omit the "=" pad character. Instead, when encoding, zero bits are
      added to pad to a Base64 character boundary. When decoding, any
      bits at the end of the Modified Base64 sequence that do not
      constitute a complete 16-bit Unicode character are discarded. If
      such discarded bits are non-zero the sequence is ill-formed.

      Rationale. The pad character "=" is not used when encoding
      Modified Base64 because of the conflict with its use as an escape
      character for the Q content transfer encoding in RFC 2047 header
      fields, as mentioned above.

      Rule 3: The space (decimal 32), tab (decimal 9), carriage return
      (decimal 13), and line feed (decimal 10) characters may be
      directly represented by their ASCII equivalents. However, note
      that MIME content transfer encodings have rules concerning the use
      of such characters. Usage that does not conform to the
      restrictions of RFC 822, for example, would have to be encoded
      using MIME content transfer encodings other than 7bit or 8bit,
      such as quoted-printable, binary, or base64.

   Given this set of rules, Unicode characters which may be encoded via
   rules 1 or 3 take one octet per character, and other Unicode
   characters are encoded on average with 2 2/3 octets per character



Goldsmith & Davis            Informational                      [Page 5]

RFC 2152                         UTF-7                          May 1997


   plus one octet to switch into Modified Base64 and an optional octet
   to switch out.

      Example. The Unicode sequence "A<NOT IDENTICAL TO><ALPHA>."
      (hexadecimal 0041,2262,0391,002E) may be encoded as follows:

            A+ImIDkQ.

      Example. The Unicode sequence "Hi Mom -<WHITE SMILING FACE>-!"
      (hexadecimal 0048, 0069, 0020, 004D, 006F, 006D, 0020, 002D, 263A,
       002D, 0021) may be encoded as follows:

            Hi Mom -+Jjo--!

      Example. The Unicode sequence representing the Han characters for
      the Japanese word "nihongo" (hexadecimal 65E5,672C,8A9E) may be
      encoded as follows:

            +ZeVnLIqe-

Use of Character Set UTF-7 Within MIME

   Character set UTF-7 is safe for mail transmission and therefore may
   be used with any content transfer encoding in MIME (except where line
   length and line break restrictions are violated). Specifically, the 7
   bit encoding for bodies and the Q encoding for headers are both
   acceptable. The MIME character set tag is UTF-7. This signifies any
   version of Unicode equal to or greater than 2.0.

      Example. Here is a text portion of a MIME message containing the
      Unicode sequence "Hi Mom <WHITE SMILING FACE>!" (hexadecimal 0048,
      0069, 0020, 004D, 006F, 006D, 0020, 263A, 0021).

      Content-Type: text/plain; charset=UTF-7

      Hi Mom +Jjo-!

      Example. Here is a text portion of a MIME message containing the
      Unicode sequence representing the Han characters for the Japanese
      word "nihongo" (hexadecimal 65E5,672C,8A9E).

      Content-Type: text/plain; charset=UTF-7

      +ZeVnLIqe-

      Example. Here is a text portion of a MIME message containing the
      Unicode sequence "A<NOT IDENTICAL TO><ALPHA>." (hexadecimal
      0041,2262,0391,002E).



Goldsmith & Davis            Informational                      [Page 6]

RFC 2152                         UTF-7                          May 1997


      Content-Type: text/plain; charset=utf-7

      A+ImIDkQ.

      Example. Here is a text portion of a MIME message containing the
      Unicode sequence "Item 3 is <POUND SIGN>1."  (hexadecimal 0049,
      0074, 0065, 006D, 0020, 0033, 0020, 0069, 0073, 0020, 00A3, 0031,
      002E).

      Content-Type: text/plain; charset=UTF-7

      Item 3 is +AKM-1.

   Note that to achieve the best interoperability with systems that may
   not support Unicode or MIME, when preparing text for mail
   transmission line breaks should follow Internet conventions. This
   means that lines should be short and terminated with the proper SMTP
   CRLF sequence. Unicode LINE SEPARATOR (hexadecimal 2028) and
   PARAGRAPH SEPARATOR (hexadecimal 2029) should be converted to SMTP
   line breaks. Ideally, this would be handled transparently by a
   Unicode-aware user agent.

   This preparation is not absolutely necessary, since UTF-7 and the
   appropriate MIME content transfer encoding can handle text that does
   not follow Internet conventions, but readability by systems without
   Unicode or MIME will be impaired. See RFC 2045 for a discussion of
   mail interoperability issues.

   Lines should never be broken in the middle of a UTF-7 shifted
   sequence, since such sequences may not cross line breaks. Therefore,
   UTF-7 encoding should take place after line breaking. If a line
   containing a shifted sequence is too long after encoding, a MIME
   content transfer encoding such as Quoted Printable can be used to
   encode the text. Another possibility is to perform line breaking and
   UTF-7 encoding at the same time, so that lines containing shifted
   sequences already conform to length restrictions.

Discussion

   In this section we will motivate the introduction of UTF-7 as opposed
   to the alternative of using the existing transformation formats of
   Unicode (e.g., UTF-8) with MIME's content transfer encodings. Before
   discussing this, it will be useful to list some assumptions about
   character frequency within typical natural language text strings that
   we use to estimate typical storage requirements:

   1. Most Western European languages use roughly 7/8 of their letters
      from US-ASCII and 1/8 from Latin 1 (ISO-8859-1).



Goldsmith & Davis            Informational                      [Page 7]

RFC 2152                         UTF-7                          May 1997


   2. Most non-Roman alphabet-based languages (e.g., Greek) use about
      1/6 of their letters from ASCII (since white space is in the 7-bit
      area) and the rest from their alphabets.

   3. East Asian ideographic-based languages (including Japanese) use
      essentially all of their characters from the Han or CJK syllabary
      area.

   4. Non-directly encoded punctuation characters do not occur
      frequently enough to affect the results.

   Notice that current 8 bit standards, such as ISO-8859-x, require use
   of a content transfer encoding. For comparison with the subsequent
   discussion, the costs break down as follows (note that many of these
   figures are approximate since they depend on the exact composition of
   the text):

   8859-x in Base64

      Text type          Average octets/character
      All                      1.33

   8859-x in Quoted Printable