rfc2046.txt

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   NOTE:  These "preamble" and "epilogue" areas are generally not used
   because of the lack of proper typing of these parts and the lack of
   clear semantics for handling these areas at gateways, particularly
   X.400 gateways.  However, rather than leaving the preamble area
   blank, many MIME implementations have found this to be a convenient



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RFC 2046                      Media Types                  November 1996


   place to insert an explanatory note for recipients who read the
   message with pre-MIME software, since such notes will be ignored by
   MIME-compliant software.

   NOTE:  Because boundary delimiters must not appear in the body parts
   being encapsulated, a user agent must exercise care to choose a
   unique boundary parameter value.  The boundary parameter value in the
   example above could have been the result of an algorithm designed to
   produce boundary delimiters with a very low probability of already
   existing in the data to be encapsulated without having to prescan the
   data.  Alternate algorithms might result in more "readable" boundary
   delimiters for a recipient with an old user agent, but would require
   more attention to the possibility that the boundary delimiter might
   appear at the beginning of some line in the encapsulated part.  The
   simplest boundary delimiter line possible is something like "---",
   with a closing boundary delimiter line of "-----".

   As a very simple example, the following multipart message has two
   parts, both of them plain text, one of them explicitly typed and one
   of them implicitly typed:

     From: Nathaniel Borenstein <[e-mail]>
     To: Ned Freed <[e-mail]>
     Date: Sun, 21 Mar 1993 23:56:48 -0800 (PST)
     Subject: Sample message
     MIME-Version: 1.0
     Content-type: multipart/mixed; boundary="simple boundary"

     This is the preamble.  It is to be ignored, though it
     is a handy place for composition agents to include an
     explanatory note to non-MIME conformant readers.

     --simple boundary

     This is implicitly typed plain US-ASCII text.
     It does NOT end with a linebreak.
     --simple boundary
     Content-type: text/plain; charset=us-ascii

     This is explicitly typed plain US-ASCII text.
     It DOES end with a linebreak.

     --simple boundary--

     This is the epilogue.  It is also to be ignored.






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RFC 2046                      Media Types                  November 1996


   The use of a media type of "multipart" in a body part within another
   "multipart" entity is explicitly allowed.  In such cases, for obvious
   reasons, care must be taken to ensure that each nested "multipart"
   entity uses a different boundary delimiter.  See RFC 2049 for an
   example of nested "multipart" entities.

   The use of the "multipart" media type with only a single body part
   may be useful in certain contexts, and is explicitly permitted.

   NOTE: Experience has shown that a "multipart" media type with a
   single body part is useful for sending non-text media types.  It has
   the advantage of providing the preamble as a place to include
   decoding instructions.  In addition, a number of SMTP gateways move
   or remove the MIME headers, and a clever MIME decoder can take a good
   guess at multipart boundaries even in the absence of the Content-Type
   header and thereby successfully decode the message.

   The only mandatory global parameter for the "multipart" media type is
   the boundary parameter, which consists of 1 to 70 characters from a
   set of characters known to be very robust through mail gateways, and
   NOT ending with white space. (If a boundary delimiter line appears to
   end with white space, the white space must be presumed to have been
   added by a gateway, and must be deleted.)  It is formally specified
   by the following BNF:

     boundary := 0*69<bchars> bcharsnospace

     bchars := bcharsnospace / " "

     bcharsnospace := DIGIT / ALPHA / "'" / "(" / ")" /
                      "+" / "_" / "," / "-" / "." /
                      "/" / ":" / "=" / "?"

   Overall, the body of a "multipart" entity may be specified as
   follows:

     dash-boundary := "--" boundary
                      ; boundary taken from the value of
                      ; boundary parameter of the
                      ; Content-Type field.

     multipart-body := [preamble CRLF]
                       dash-boundary transport-padding CRLF
                       body-part *encapsulation
                       close-delimiter transport-padding
                       [CRLF epilogue]





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RFC 2046                      Media Types                  November 1996


     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.

     encapsulation := delimiter transport-padding
                      CRLF body-part

     delimiter := CRLF dash-boundary

     close-delimiter := delimiter "--"

     preamble := discard-text

     epilogue := discard-text

     discard-text := *(*text CRLF) *text
                     ; May be ignored or discarded.

     body-part := MIME-part-headers [CRLF *OCTET]
                  ; Lines in a body-part must not start
                  ; with the specified dash-boundary and
                  ; the delimiter must not appear anywhere
                  ; in the body part.  Note that the
                  ; semantics of a body-part differ from
                  ; the semantics of a message, as
                  ; described in the text.

     OCTET := <any 0-255 octet value>

   IMPORTANT:  The free insertion of linear-white-space and RFC 822
   comments between the elements shown in this BNF is NOT allowed since
   this BNF does not specify a structured header field.

   NOTE:  In certain transport enclaves, RFC 822 restrictions such as
   the one that limits bodies to printable US-ASCII characters may not
   be in force. (That is, the transport domains may exist that resemble
   standard Internet mail transport as specified in RFC 821 and assumed
   by RFC 822, but without certain restrictions.) The relaxation of
   these restrictions should be construed as locally extending the
   definition of bodies, for example to include octets outside of the
   US-ASCII range, as long as these extensions are supported by the
   transport and adequately documented in the Content- Transfer-Encoding
   header field.  However, in no event are headers (either message
   headers or body part headers) allowed to contain anything other than
   US-ASCII characters.



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RFC 2046                      Media Types                  November 1996


   NOTE:  Conspicuously missing from the "multipart" type is a notion of
   structured, related body parts. It is recommended that those wishing
   to provide more structured or integrated multipart messaging
   facilities should define subtypes of multipart that are syntactically
   identical but define relationships between the various parts. For
   example, subtypes of multipart could be defined that include a
   distinguished part which in turn is used to specify the relationships
   between the other parts, probably referring to them by their
   Content-ID field.  Old implementations will not recognize the new
   subtype if this approach is used, but will treat it as
   multipart/mixed and will thus be able to show the user the parts that
   are recognized.

5.1.2.  Handling Nested Messages and Multiparts

   The "message/rfc822" subtype defined in a subsequent section of this
   document has no terminating condition other than running out of data.
   Similarly, an improperly truncated "multipart" entity may not have
   any terminating boundary marker, and can turn up operationally due to
   mail system malfunctions.

   It is essential that such entities be handled correctly when they are
   themselves imbedded inside of another "multipart" structure.  MIME
   implementations are therefore required to recognize outer level
   boundary markers at ANY level of inner nesting.  It is not sufficient
   to only check for the next expected marker or other terminating
   condition.

5.1.3.  Mixed Subtype

   The "mixed" subtype of "multipart" is intended for use when the body
   parts are independent and need to be bundled in a particular order.
   Any "multipart" subtypes that an implementation does not recognize
   must be treated as being of subtype "mixed".

5.1.4.  Alternative Subtype

   The "multipart/alternative" type is syntactically identical to
   "multipart/mixed", but the semantics are different.  In particular,
   each of the body parts is an "alternative" version of the same
   information.

   Systems should recognize that the content of the various parts are
   interchangeable.  Systems should choose the "best" type based on the
   local environment and references, in some cases even through user
   interaction.  As with "multipart/mixed", the order of body parts is
   significant.  In this case, the alternatives appear in an order of
   increasing faithfulness to the original content.  In general, the



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RFC 2046                      Media Types                  November 1996


   best choice is the LAST part of a type supported by the recipient
   system's local environment.

   "Multipart/alternative" may be used, for example, to send a message
   in a fancy text format in such a way that it can easily be displayed
   anywhere:

     From: Nathaniel Borenstein <[e-mail]>
     To: Ned Freed <[e-mail]>
     Date: Mon, 22 Mar 1993 09:41:09 -0800 (PST)
     Subject: Formatted text mail
     MIME-Version: 1.0
     Content-Type: multipart/alternative; boundary=boundary42

     --boundary42
     Content-Type: text/plain; charset=us-ascii

       ... plain text version of message goes here ...

     --boundary42
     Content-Type: text/enriched

       ... RFC 1896 text/enriched version of same message
           goes here ...

     --boundary42
     Content-Type: application/x-whatever

       ... fanciest version of same message goes here ...

     --boundary42--

   In this example, users whose mail systems understood the
   "application/x-whatever" format would see only the fancy version,
   while other users would see only the enriched or plain text version,
   depending on