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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 Content-Type Header and Discrete Media: Types, Subtypes and Parameters
(Page 2 of 5)
Discrete Media Types and Subtypes
As I mentioned in the
preceding topic, MIME supports two basic
structures: simple or complex. A simple message carries only one media
type, such as a piece of text, a picture or an executable file. These
are called discrete media types in MIME. A complex message carries
a composite media type, which may incorporate multiple body parts.
Each body part in turn carries data corresponding to one of the discrete
media types. The top-level media type indicates whether the whole message
carries a discrete media type or a composite type; I will describe the
discrete media types here; the composite types are discussed in the
The RFC 2046 standard (part two of
of five standards that describes MIME)
defines five discrete top-level media types: text, image,
audio, video and application; they each represent
one of the major classes of data commonly transmitted over TCP/IP. Each
of these has one or more subtypes, and some also have parameters that
are used to provide more information about them.
The creators of MIME recognized that
the standard could not describe every media type, and that new ones
would be created in the future. RFC 2048 (part four of the five-standard
MIME set) describes the process by which new media types, subtypes and
parameters can be described and registered with the Internet
Assigned Numbers Authority (IANA).
Thus far, only one new top-level
media type has been created; this is the model top-level type,
defined for CAD modeling files and similar uses, as described in RFC
2077. However, many dozens of new subtypes have been created
over the years, some specified in RFCs and others just registered directly
with IANA. This includes many vendor-specific subtypes,
which are usually identified by either the prefix x- or
vnd. in the subtype name.
Key Concept: The MIME Content-Type header specifies what sort of data is encoded in a MIME message. The header indicates the general form of the messages content through a top-level media type, and the more specific nature of the data through the specification of a subtype. It may also contain optional parameters that provide still more information about the content.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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