The TCP/IP Guide - MIME Message Format Overview, Motivation, History and Standards

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MIME was first described in a set of two standards, RFC 1341 and RFC 1342, published in June 1992. These were updated by RFCs 1521 and 1522 in September 1993. In March 1994, a supplemental standard was published, RFC 1590, which specified the procedure for defining new MIME media types.

Work continued on MIME through the mid-1990s, and in November 1996 the standards were revised again. This time, the documents were completely restructured and published as a set of 5 individual standards. This was done to improve the readability of the information. These standards are shown in Table 241.

**Table 241: Multipurpose Internet Mail Extensions (MIME) Standards**
RFC Number	Name	Description
2045	Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies	Describes the fundamental concepts behind MIME and the structure of MIME messages.
2046	Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types	Explains the concept of MIME media types and subtypes and describes some of the kinds of media whose encoding is defined in the MIME standards.
2047	MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text	Describes how RFC 822 headers can be modified to carry non-ASCII text.
2048	Multipurpose Internet Mail Extensions (MIME) Part Four: Registration Procedures	Discusses how organizations can register additional media types for use with MIME.
2049	Multipurpose Internet Mail Extensions (MIME) Part Five: Conformance Criteria and Examples	Provides additional implementation information and examples of how MIME can be used.

Since the time that these five “primary” MIME standards came out, numerous additional RFCs have been published. These have defined various extensions to MIME itself, including additional MIME header types and new media types. Notable examples are RFCs 2183 and 2557, which define the MIME Content-Disposition and Content-Location headers, respectively. Some other MIME capabilities are actually defined as part of other technologies that use MIME; for example, the first HTTP standard, RFC 1945 defines the Content-Length header. Numerous other RFCs define new media types/subtypes, too many to list here.