Since its publication in 1982, STD 11, RFC 822 [RFC-822] has defined
the standard format of textual mail messages on the Internet. Its
success has been such that the RFC 822 format has been adopted,
wholly or partially, well beyond the confines of the Internet and the
Internet SMTP transport defined by STD 10, RFC 821 [RFC-821]. As the
format has seen wider use, a number of limitations have proven
increasingly restrictive for the user community.
RFC 822 was intended to specify a format for text messages. As such,
non-text messages, such as multimedia messages that might include
audio or images, are simply not mentioned. Even in the case of text,
however, RFC 822 is inadequate for the needs of mail users whose
languages require the use of character sets richer than US ASCII
[US-ASCII]. Since RFC 822 does not specify mechanisms for mail
containing audio, video, Asian language text, or even text in most
European languages, additional specifications are needed.
One of the notable limitations of RFC 821/822 based mail systems is
the fact that they limit the contents of electronic mail messages to
relatively short lines of seven-bit ASCII. This forces users to
convert any non-textual data that they may wish to send into seven-
bit bytes representable as printable ASCII characters before invoking
a local mail UA (User Agent, a program with which human users send
and receive mail). Examples of such encodings currently used in the
Internet include pure hexadecimal, uuencode, the 3-in-4 base 64
scheme specified in RFC 1421, the Andrew Toolkit Representation
[ATK], and many others.
The limitations of RFC 822 mail become even more apparent as gateways
are designed to allow for the exchange of mail messages between RFC
822 hosts and X.400 hosts. X.400 [X400] specifies mechanisms for the
inclusion of non-textual body parts within electronic mail messages.
The current standards for the mapping of X.400 messages to RFC 822
messages specify either that X.400 non-textual body parts must be
converted to (not encoded in) an ASCII format, or that they must be
discarded, notifying the RFC 822 user that discarding has occurred.
This is clearly undesirable, as information that a user may wish to
receive is lost. Even though a user's UA may not have the capability
of dealing with the non-textual body part, the user might have some
mechanism external to the UA that can extract useful information from
the body part. Moreover, it does not allow for the fact that the
message may eventually be gatewayed back into an X.400 message
handling system (i.e., the X.400 message is "tunneled" through
Internet mail), where the non-textual information would definitely
become useful again.
This document describes several mechanisms that combine to solve most
of these problems without introducing any serious incompatibilities
with the existing world of RFC 822 mail. In particular, it
describes:
- A MIME-Version header field, which uses a version number to
declare a message to be conformant with this specification and
allows mail processing agents to distinguish between such
messages and those generated by older or non-conformant software,
which is presumed to lack such a field.
- A Content-Type header field, generalized from RFC 1049 [RFC-1049],
which can be used to specify the type and subtype of data in the
body of a message and to fully specify the native representation
(encoding) of such data.
- A "text" Content-Type value, which can be used to represent
textual information in a number of character sets and
formatted text description languages in a standardized
manner.
- A "multipart" Content-Type value, which can be used to
combine several body parts, possibly of differing types of
data, into a single message.
- An "application" Content-Type value, which can be used to
transmit application data or binary data, and hence, among
other uses, to implement an electronic mail file transfer
service.
- A "message" Content-Type value, for encapsulating another
mail message.
- An "image" Content-Type value, for transmitting still image
(picture) data.
- An "audio" Content-Type value, for transmitting audio or
voice data.
- A "video" Content-Type value, for transmitting video or
moving image data, possibly with audio as part of the
composite video data format.
- A Content-Transfer-Encoding header field, which can be used to
specify an auxiliary encoding that was applied to the data in
order to allow it to pass through mail transport mechanisms which
may have data or character set limitations.
- Two additional header fields that can be used to further describe
the data in a message body, the Content-ID and Content-
Description header fields.
MIME has been carefully designed as an extensible mechanism, and it
is expected that the set of content-type/subtype pairs and their
associated parameters will grow significantly with time. Several
other MIME fields, notably including character set names, are likely
to have new values defined over time. In order to ensure that the
set of such values is developed in an orderly, well-specified, and
public manner, MIME defines a registration process which uses the
Internet Assigned Numbers Authority (IANA) as a central registry for
such values.
Appendix E
provides details about how IANA registration
is accomplished.
Finally, to specify and promote interoperability,
Appendix A of this
document provides a basic applicability statement for a subset of the
above mechanisms that defines a minimal level of "conformance" with
this document.
- HISTORICAL NOTE:
-
Several of the mechanisms described in this
document may seem somewhat strange or even baroque at first
reading. It is important to note that compatibility with existing
standards AND robustness across existing practice were two of the
highest priorities of the working group that developed this
document. In particular, compatibility was always favored over
elegance.
MIME was first defined and published as RFCs 1341 and 1342 [RFC-1341]
[RFC-1342]. This document is a relatively minor updating of RFC
1341, and is intended to supersede it. The differences between this
document and RFC 1341 are summarized in
Appendix H. Please refer to
the current edition of the "IAB Official Protocol Standards" for the
standardization state and status of this protocol. Several other RFC
documents will be of interest to the MIME implementor, in particular
[RFC 1343], [RFC-1344], and [RFC-1345].