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 Basic Structures and Headers 1 2 3 4 5 MIME Composite Media Types: Multipart and Encapsulated Message Structures

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 next topic.

The RFC 2046 standard (part two of the set 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.

MIME Basic Structures and Headers 1 2 3 4 5 MIME Composite Media Types: Multipart and Encapsulated Message Structures