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9 TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)
9 TCP/IP Key Applications and Application Protocols
9 TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)
9 TCP/IP Electronic Mail System: Concepts and Protocols (RFC 822, MIME, SMTP, POP3, IMAP)
9 TCP/IP Electronic Mail Message Formats and Message Processing: RFC 822 and MIME
9 TCP/IP Enhanced Electronic Mail Message Format: Multipurpose Internet Mail Extensions (MIME)
MIME Composite Media Types: Multipart and Encapsulated Message Structures
(Page 2 of 6)
MIME Multipart Message Type
The multipart media type is
the more common of the two, and for good reason: it is an incredibly
powerful mechanism. It allows one message to contain many different
kinds of information that can be used in different ways. Each piece
of data is encoded separately as a MIME body part and the parts
are combined into a single e-mail message. How these parts are used
depends on the semantics of the message, which is indicated by the MIME
subtype. RFC 2046 describes several of these, and a few new ones have
also been defined by the IANA registration scheme described in the previous
MIME Multipart Message Subtypes
The following are the most common
multipart media subtypes, and how they are used. The first four are
defined in RFC 2046, and the others as indicated:
- multipart/mixed: Indicates that
the body parts are not really related, but have just been bundled for
transport in a single message for convenience. For example, this might
be used by someone to send an office memo along with a vacation snapshot
just for fun. This subtype is also used sometimes when the parts are
related but the relationship is communicated to the recipient in some
other way (such as a description in a distinct body part).
- multipart/alternative: Specifies
that the body parts are alternative representations of the same information.
The recipient decodes the parts and chooses the one that is best suited
to his or her needs. A common use of this is in sending HTML-encoded
e-mail. Some e-mail clients can't display HTML, so it is courteous to
send a multipart/alternative message containing the message in
both HTML and plain text forms.
The alternatives should be placed in the message in increasing order
of preference, meaning that the preferred format goes last. In the case
of a document that includes plain text and rich text alternativessuch
as the example above with plain text and HTML versions of a documentthe
plainest format should go first and the fanciest last.
- multipart/parallel: Tells the recipient
that the body parts should all be displayed at the same time (in
parallel). One example of how this might be used would be if someone
sent an audio clip along with explanatory text to be displayed alongside
it as it played.
- multipart/digest: This is used
to allow a message to carry a digest, such as a collection of other
- multipart/related: Indicates specifically
that the body parts are related to each other. Special parameters are
used to provide more information on how they are to be interpreted.
This subtype was defined in RFC 2387.
- multipart/encrypted: Used for encrypted
data. The first body part contains information on how the data is to
be decrypted, and the second contains the data itself. This subtype
was defined in RFC 1847.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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