rfc2045.txt

穿越防火墙技术代码

   corruption of the data, it is probably more sensible to use the
   base64 encoding rather than the quoted-printable encoding.

   NOTE: Several kinds of substrings cannot be generated according to
   the encoding rules for the quoted-printable content-transfer-
   encoding, and hence are formally illegal if they appear in the output
   of a quoted-printable encoder. This note enumerates these cases and
   suggests ways to handle such illegal substrings if any are
   encountered in quoted-printable data that is to be decoded.

    (1)   An "=" followed by two hexadecimal digits, one or both
          of which are lowercase letters in "abcdef", is formally
          illegal. A robust implementation might choose to
          recognize them as the corresponding uppercase letters.

    (2)   An "=" followed by a character that is neither a
          hexadecimal digit (including "abcdef") nor the CR
          character of a CRLF pair is illegal.  This case can be
          the result of US-ASCII text having been included in a
          quoted-printable part of a message without itself
          having been subjected to quoted-printable encoding.  A
          reasonable approach by a robust implementation might be
          to include the "=" character and the following
          character in the decoded data without any
          transformation and, if possible, indicate to the user
          that proper decoding was not possible at this point in
          the data.

    (3)   An "=" cannot be the ultimate or penultimate character
          in an encoded object.  This could be handled as in case
          (2) above.

    (4)   Control characters other than TAB, or CR and LF as
          parts of CRLF pairs, must not appear. The same is true
          for octets with decimal values greater than 126.  If
          found in incoming quoted-printable data by a decoder, a
          robust implementation might exclude them from the
          decoded data and warn the user that illegal characters
          were discovered.

    (5)   Encoded lines must not be longer than 76 characters,
          not counting the trailing CRLF. If longer lines are
          found in incoming, encoded data, a robust
          implementation might nevertheless decode the lines, and
          might report the erroneous encoding to the user.






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   WARNING TO IMPLEMENTORS:  If binary data is encoded in quoted-
   printable, care must be taken to encode CR and LF characters as "=0D"
   and "=0A", respectively.  In particular, a CRLF sequence in binary
   data should be encoded as "=0D=0A".  Otherwise, if CRLF were
   represented as a hard line break, it might be incorrectly decoded on
   platforms with different line break conventions.

   For formalists, the syntax of quoted-printable data is described by
   the following grammar:

     quoted-printable := qp-line *(CRLF qp-line)

     qp-line := *(qp-segment transport-padding CRLF)
                qp-part transport-padding

     qp-part := qp-section
                ; Maximum length of 76 characters

     qp-segment := qp-section *(SPACE / TAB) "="
                   ; Maximum length of 76 characters

     qp-section := [*(ptext / SPACE / TAB) ptext]

     ptext := hex-octet / safe-char

     safe-char := <any octet with decimal value of 33 through
                  60 inclusive, and 62 through 126>
                  ; Characters not listed as "mail-safe" in
                  ; RFC 2049 are also not recommended.

     hex-octet := "=" 2(DIGIT / "A" / "B" / "C" / "D" / "E" / "F")
                  ; Octet must be used for characters > 127, =,
                  ; SPACEs or TABs at the ends of lines, and is
                  ; recommended for any character not listed in
                  ; RFC 2049 as "mail-safe".

     transport-padding := *LWSP-char
                          ; Composers MUST NOT generate
                          ; non-zero length transport
                          ; padding, but receivers MUST
                          ; be able to handle padding
                          ; added by message transports.

   IMPORTANT:  The addition of LWSP between the elements shown in this
   BNF is NOT allowed since this BNF does not specify a structured
   header field.





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6.8.  Base64 Content-Transfer-Encoding

   The Base64 Content-Transfer-Encoding is designed to represent
   arbitrary sequences of octets in a form that need not be humanly
   readable.  The encoding and decoding algorithms are simple, but the
   encoded data are consistently only about 33 percent larger than the
   unencoded data.  This encoding is virtually identical to the one used
   in Privacy Enhanced Mail (PEM) applications, as defined in RFC 1421.

   A 65-character subset of US-ASCII is used, enabling 6 bits to be
   represented per printable character. (The extra 65th character, "=",
   is used to signify a special processing function.)

   NOTE:  This subset has the important property that it is represented
   identically in all versions of ISO 646, including US-ASCII, and all
   characters in the subset are also represented identically in all
   versions of EBCDIC. Other popular encodings, such as the encoding
   used by the uuencode utility, Macintosh binhex 4.0 [RFC-1741], and
   the base85 encoding specified as part of Level 2 PostScript, do not
   share these properties, and thus do not fulfill the portability
   requirements a binary transport encoding for mail must meet.

   The encoding process represents 24-bit groups of input bits as output
   strings of 4 encoded characters.  Proceeding from left to right, a
   24-bit input group is formed by concatenating 3 8bit input groups.
   These 24 bits are then treated as 4 concatenated 6-bit groups, each
   of which is translated into a single digit in the base64 alphabet.
   When encoding a bit stream via the base64 encoding, the bit stream
   must be presumed to be ordered with the most-significant-bit first.
   That is, the first bit in the stream will be the high-order bit in
   the first 8bit byte, and the eighth bit will be the low-order bit in
   the first 8bit byte, and so on.

   Each 6-bit group is used as an index into an array of 64 printable
   characters.  The character referenced by the index is placed in the
   output string.  These characters, identified in Table 1, below, are
   selected so as to be universally representable, and the set excludes
   characters with particular significance to SMTP (e.g., ".", CR, LF)
   and to the multipart boundary delimiters defined in RFC 2046 (e.g.,
   "-").











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                    Table 1: The Base64 Alphabet

     Value Encoding  Value Encoding  Value Encoding  Value Encoding
         0 A            17 R            34 i            51 z
         1 B            18 S            35 j            52 0
         2 C            19 T            36 k            53 1
         3 D            20 U            37 l            54 2
         4 E            21 V            38 m            55 3
         5 F            22 W            39 n            56 4
         6 G            23 X            40 o            57 5
         7 H            24 Y            41 p            58 6
         8 I            25 Z            42 q            59 7
         9 J            26 a            43 r            60 8
        10 K            27 b            44 s            61 9
        11 L            28 c            45 t            62 +
        12 M            29 d            46 u            63 /
        13 N            30 e            47 v
        14 O            31 f            48 w         (pad) =
        15 P            32 g            49 x
        16 Q            33 h            50 y

   The encoded output stream must be represented in lines of no more
   than 76 characters each.  All line breaks or other characters not
   found in Table 1 must be ignored by decoding software.  In base64
   data, characters other than those in Table 1, line breaks, and other
   white space probably indicate a transmission error, about which a
   warning message or even a message rejection might be appropriate
   under some circumstances.

   Special processing is performed if fewer than 24 bits are available
   at the end of the data being encoded.  A full encoding quantum is
   always completed at the end of a body.  When fewer than 24 input bits
   are available in an input group, zero bits are added (on the right)
   to form an integral number of 6-bit groups.  Padding at the end of
   the data is performed using the "=" character.  Since all base64
   input is an integral number of octets, only the following cases can
   arise: (1) the final quantum of encoding input is an integral
   multiple of 24 bits; here, the final unit of encoded output will be
   an integral multiple of 4 characters with no "=" padding, (2) the
   final quantum of encoding input is exactly 8 bits; here, the final
   unit of encoded output will be two characters followed by two "="
   padding characters, or (3) the final quantum of encoding input is
   exactly 16 bits; here, the final unit of encoded output will be three
   characters followed by one "=" padding character.

   Because it is used only for padding at the end of the data, the
   occurrence of any "=" characters may be taken as evidence that the
   end of the data has been reached (without truncation in transit).  No



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   such assurance is possible, however, when the number of octets
   transmitted was a multiple of three and no "=" characters are
   present.

   Any characters outside of the base64 alphabet are to be ignored in
   base64-encoded data.

   Care must be taken to use the proper octets for line breaks if base64
   encoding is applied directly to text material that has not been
   converted to canonical form.  In particular, text line breaks must be
   converted into CRLF sequences prior to base64 encoding.  The
   important thing to note is that this may be done directly by the
   encoder rather than in a prior canonicalization step in some
   implementations.

   NOTE: There is no need to worry about quoting potential boundary
   delimiters within base64-encoded bodies within multipart entities
   because no hyphen characters are used in the base64 encoding.

7.  Content-ID Header Field

   In constructing a high-level user agent, it may be desirable to allow
   one body to make reference to another.  Accordingly, bodies may be
   labelled using the "Content-ID" header field, which is syntactically
   identical to the "Message-ID" header field:

     id := "Content-ID" ":" msg-id

   Like the Message-ID values, Content-ID values must be generated to be
   world-unique.

   The Content-ID value may be used for uniquely identifying MIME
   entities in several contexts, particularly for caching data
   referenced by the message/external-body mechanism.  Although the
   Content-ID header is generally optional, its use is MANDATORY in
   implementations which generate data of the optional MIME media type
   "message/external-body".  That is, each message/external-body entity
   must have a Content-ID field to permit caching of such data.

   It is also worth noting that the Content-ID value has special
   semantics in the case of the multipart/alternative media type.  This
   is explained in the section of RFC 2046 dealing with
   multipart/alternative.








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8.  Content-Description Header Field

   The ability to associate some descriptive information with a given
   body is often desirable.  For example, it may be useful to mark an
   "image" body as "a picture of the Space Shuttle Endeavor."  Such text
   may be placed in the Content-Description header field.  This header
   field is always optional.

     description := "Content-Description" ":" *text

   The descript