rfc1521_mime.txt

邮件编码格式说明。

               registered with IANA, as specified in
               appendix E>

     x-token := <The two characters "X-" or "x-" followed, with
                 no intervening white space, by any token>

     subtype := token ; case-insensitive

     parameter := attribute "=" value

     attribute := token   ; case-insensitive

     value := token / quoted-string

     token  :=  1*<any (ASCII) CHAR except SPACE, CTLs,
                   or tspecials>

     tspecials :=  "(" / ")" / "<" / ">" / "@"
                /  "," / ";" / ":" / "\" / <">
                /  "/" / "[" / "]" / "?" / "="
               ; Must be in quoted-string,
               ; to use within parameter values



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   Note that the definition of "tspecials" is the same as the RFC 822
   definition of "specials" with the addition of the three characters
   "/", "?", and "=", and the removal of ".".

   Note also that a subtype specification is MANDATORY.  There are no
   default subtypes.

   The type, subtype, and parameter names are not case sensitive.  For
   example, TEXT, Text, and TeXt are all equivalent.  Parameter values
   are normally case sensitive, but certain parameters are interpreted
   to be case-insensitive, depending on the intended use.  (For example,
   multipart boundaries are case-sensitive, but the "access-type" for
   message/External-body is not case-sensitive.)

   Beyond this syntax, the only constraint on the definition of subtype
   names is the desire that their uses must not conflict.  That is, it
   would be undesirable to have two different communities using
   "Content-Type: application/foobar" to mean two different things.  The
   process of defining new content-subtypes, then, is not intended to be
   a mechanism for imposing restrictions, but simply a mechanism for
   publicizing the usages. There are, therefore, two acceptable
   mechanisms for defining new Content-Type subtypes:

            1.  Private values (starting with "X-") may be
                defined bilaterally between two cooperating
                agents without outside registration or
                standardization.

            2.  New standard values must be documented,
                registered with, and approved by IANA, as
                described in Appendix E.  Where intended for
                public use, the formats they refer to must
                also be defined by a published specification,
                and possibly offered for standardization.

   The seven standard initial predefined Content-Types are detailed in
   the bulk of this document.  They are:

    text -- textual information.  The primary subtype,
         "plain", indicates plain (unformatted) text.  No
         special software is required to get the full
         meaning of the text, aside from support for the
         indicated character set.  Subtypes are to be used
         for enriched text in forms where application
         software may enhance the appearance of the text,
         but such software must not be required in order to
         get the general idea of the content.  Possible
         subtypes thus include any readable word processor



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         format.  A very simple and portable subtype,
         richtext, was defined in RFC 1341, with a future
         revision expected.

    multipart -- data consisting of multiple parts of
         independent data types.  Four initial subtypes
         are defined, including the primary "mixed"
         subtype, "alternative" for representing the same
         data in multiple formats, "parallel" for parts
         intended to be viewed simultaneously, and "digest"
         for multipart entities in which each part is of
         type "message".

    message -- an encapsulated message.  A body of
         Content-Type "message" is itself all or part of a
         fully formatted RFC 822 conformant message which
         may contain its own different Content-Type header
         field.  The primary subtype is "rfc822".  The
         "partial" subtype is defined for partial messages,
         to permit the fragmented transmission of bodies
         that are thought to be too large to be passed
         through mail transport facilities.  Another
         subtype, "External-body", is defined for
         specifying large bodies by reference to an
         external data source.

    image -- image data.  Image requires a display device
         (such as a graphical display, a printer, or a FAX
         machine) to view the information.  Initial
         subtypes are defined for two widely-used image
         formats, jpeg and gif.

    audio -- audio data, with initial subtype "basic".
         Audio requires an audio output device (such as a
         speaker or a telephone) to "display" the contents.

    video -- video data.  Video requires the capability to
         display moving images, typically including
         specialized hardware and software.  The initial
         subtype is "mpeg".

    application -- some other kind of data, typically
         either uninterpreted binary data or information to
         be processed by a mail-based application.  The
         primary subtype, "octet-stream", is to be used in
         the case of uninterpreted binary data, in which
         case the simplest recommended action is to offer
         to write the information into a file for the user.



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         An additional subtype, "PostScript", is defined
         for transporting PostScript documents in bodies.
         Other expected uses for "application" include
         spreadsheets, data for mail-based scheduling
         systems, and languages for "active"
         (computational) email.  (Note that active email
         and other application data may entail several
         security considerations, which are discussed later
         in this memo, particularly in the context of
         application/PostScript.)

   Default RFC 822 messages are typed by this protocol as plain text in
   the US-ASCII character set, which can be explicitly specified as
   "Content-type: text/plain; charset=us-ascii".  If no Content-Type is
   specified, this default is assumed.  In the presence of a MIME-
   Version header field, a receiving User Agent can also assume that
   plain US-ASCII text was the sender's intent.  In the absence of a
   MIME-Version specification, plain US-ASCII text must still be
   assumed, but the sender's intent might have been otherwise.

      RATIONALE: In the absence of any Content-Type header field or
      MIME-Version header field, it is impossible to be certain that a
      message is actually text in the US-ASCII character set, since it
      might well be a message that, using the conventions that predate
      this document, includes text in another character set or non-
      textual data in a manner that cannot be automatically recognized
      (e.g., a uuencoded compressed UNIX tar file).  Although there is
      no fully acceptable alternative to treating such untyped messages
      as "text/plain; charset=us-ascii", implementors should remain
      aware that if a message lacks both the MIME-Version and the
      Content-Type header fields, it may in practice contain almost
      anything.

   It should be noted that the list of Content-Type values given here
   may be augmented in time, via the mechanisms described above, and
   that the set of subtypes is expected to grow substantially.

   When a mail reader encounters mail with an unknown Content-type
   value, it should generally treat it as equivalent to
   "application/octet-stream", as described later in this document.

5.    The Content-Transfer-Encoding Header Field

   Many Content-Types which could usefully be transported via email are
   represented, in their "natural" format, as 8-bit character or binary
   data.  Such data cannot be transmitted over some transport protocols.
   For example, RFC 821 restricts mail messages to 7-bit US-ASCII data
   with lines no longer than 1000 characters.



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   It is necessary, therefore, to define a standard mechanism for re-
   encoding such data into a 7-bit short-line format.  This document
   specifies that such encodings will be indicated by a new "Content-
   Transfer-Encoding" header field.  The Content-Transfer-Encoding field
   is used to indicate the type of transformation that has been used in
   order to represent the body in an acceptable manner for transport.

   Unlike Content-Types, a proliferation of Content-Transfer-Encoding
   values is undesirable and unnecessary.  However, establishing only a
   single Content-Transfer-Encoding mechanism does not seem possible.
   There is a tradeoff between the desire for a compact and efficient
   encoding of largely-binary data and the desire for a readable
   encoding of data that is mostly, but not entirely, 7-bit data.  For
   this reason, at least two encoding mechanisms are necessary: a
   "readable" encoding and a "dense" encoding.

   The Content-Transfer-Encoding field is designed to specify an
   invertible mapping between the "native" representation of a type of
   data and a representation that can be readily exchanged using 7 bit
   mail transport protocols, such as those defined by RFC 821 (SMTP).
   This field has not been defined by any previous standard. The field's
   value is a single token specifying the type of encoding, as
   enumerated below.  Formally:

   encoding := "Content-Transfer-Encoding" ":" mechanism

   mechanism :=     "7bit"  ;  case-insensitive
                  / "quoted-printable"
                  / "base64"
                  / "8bit"
                  / "binary"
                  / x-token

   These values are not case sensitive.  That is, Base64 and BASE64 and
   bAsE64 are all equivalent.  An encoding type of 7BIT requires that
   the body is already in a seven-bit mail-ready representation.  This
   is the default value -- that is, "Content-Transfer-Encoding: 7BIT" is
   assumed if the Content-Transfer-Encoding header field is not present.

   The values "8bit", "7bit", and "binary" all mean that NO encoding has
   been performed. However, they are potentially useful as indications
   of the kind of data contained in the object, and therefore of the
   kind of encoding that might need to be performed for transmission in
   a given transport system.  In particular:

       "7bit" means that the data is all represented as short
            lines of US-ASCII data.




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       "8bit" means that the lines are short, but there may be
            non-ASCII characters (octets with the high-order
            bit set).

       "Binary" means that not only may non-ASCII characters
            be present, but also that the lines are not
            necessarily short enough for SMTP transport.

   The difference between "8bit" (or any other conceivable bit-width
   token) and the "binary" token is that "binary" does not require
   adherence to any limits on line length or to the SMTP CRLF semantics,
   while the bit-width tokens do require such adherence.  If the body
   contains data in any bit-width other than 7-bit, the appropriate
   bit-width Content-Transfer-Encoding token must be used (e.g., "8bit"