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|| The TCP/IP Guide|
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 3 of 6)
Multipart Message Encoding
You can see just from these different
subtypes how much flexibility the multipart type provides to
MIME, and there are others. In all cases, the same syntax is used to
encode the constituent body parts into a single message. The basic process
is as follows:
- Each individual piece of data is processed
as if it were to be transmitted as the body of a discrete media type
MIME message. This includes the specification of appropriate headers,
such as Content-Type, Content-ID and Content-Transfer-Encoding,
- A special boundary delimiter
is chosen to separate the body parts. It must be selected so that it
will not appear in any of the body parts; a random string is sometimes
used. It is prepended with two dashes (--) when placed in
the message to reduce the chance of it being mistaken for data.
- The multipart message is assembled.
It consists of a preamble text area, then a boundary line, followed
by the first body part. Each subsequent body part is separated from
the previous one with another boundary line. After the last body part,
another boundary line appears, followed by an epilogue text area.
- The special parameter boundary
is included in the Content-Type header of the message as a whole,
to tell the recipient what pattern separates the body parts.
Key Concept: MIME multipart messages are formed by first processing each individual data component to create a MIME body part. Each can have a distinct encoding method and set of headers, as if it were a separate MIME message. These body parts are then combined into a single multipart message, and separated with a boundary delimiter. The identity of the delimiter is inserted into the boundary parameter of the Content-Type header, so the recipient can easily separate the individual body parts upon receipt of the message.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
© Copyright 2001-2005 Charles M. Kozierok. All Rights Reserved.
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