MIME
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Multipurpose Internet Mail Extensions (MIME) is an Internet Standard for the format of e-mail. Virtually all Internet e-mail is transmitted via SMTP in MIME format. Internet e-mail is so closely associated with the SMTP and MIME standards that it is sometimes called SMTP/MIME e-mail.
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Introduction
The basic Internet e-mail transmission protocol, SMTP, supports only 7-bit ASCII characters (see also 8BITMIME). This effectively limits Internet e-mail to messages which, when transmitted, include only the characters used for the English language. MIME defines mechanisms for sending other kinds of information in e-mail, including text in languages other than English using character encodings other than ASCII as well as 8-bit binary content such as files containing images, sounds, movies, and computer programs. MIME is also a fundamental component of communication protocols such as HTTP, which requires that data be transmitted in the context of e-mail-like messages, even though the data may not actually be e-mail.
Mapping messages into and out of MIME format is typically done automatically by an email client or by proprietary mail servers when sending or receiving Internet (SMTP/MIME) e-mail.
The basic format of Internet e-mail is defined in RFC 2822 (http://www.faqs.org/rfcs/rfc2822.html), which is an updated version of RFC 822. These standards specify the familiar formats for text e-mail headers and body and rules pertaining to commonly used header fields such as "To:", "Subject:", "From:", and "Date:". MIME defines a collection of e-mail headers for specifying additional attributes of a message including content type, and defines a set of transfer encodings which can be used to represent 8-bit binary data using characters from the 7-bit ASCII character set. MIME also specifies rules for encoding non-ASCII characters in e-mail message headers, such as "Subject:", allowing these header fields to contain non-English characters.
MIME is extensible. Its definition includes a method to register new content types and other MIME attribute values.
One of the explicit goals of the MIME definition was to not require changes to pre-existing e-mail servers or clients. This goal is achieved by allowing all MIME message attributes to be optional, with default values making a non-MIME message likely to be interpreted correctly by a MIME-capable client. In addition, a simple MIME text message is likely to be interpreted correctly by a non-MIME client although it has e-mail headers the non-MIME client won't know how to interpret.
MIME headers
MIME-Version
The presence of this header indicates the message is MIME-formatted. The value is typically "1.0" so this header appears as
MIME-Version: 1.0
Content-type
This header indicates the type and subtype of the message content, for example
Content-type: text/plain
The combination of type and subtype is generally called a MIME type. A large number of file formats have registered MIME types. Any text type has an additional charset parameter that can be included to indicate the character encoding. A very large number of character encodings have registered MIME charset names.
Although originally defined for MIME e-mail, the content-type header and MIME type registry is reused in other Internet protocols such as HTTP.
Through the use of the multipart type, MIME allows messages to have parts arranged in a tree structure where the leaf nodes are any non-multipart content type and the non-leaf nodes are any of a variety of multipart types. This mechanism supports:
- simple text messages using text/plain (the default value for "Content-type:")
- text plus attachments (multipart/mixed with a text/plain part and other non-text parts). A MIME message including an attached file generally indicates the file's original name with the "Content-disposition:" header, so the type of file is indicated both by the MIME content-type and the (usually OS-specific) filename extension.
- reply with original attached (multipart/mixed with a text/plain part and the original message as a message/rfc822 part)
- alternative content, such as a message sent in both plain text and another format such as HTML (multipart/alternative with the same content in text/plain and text/html forms)
- many other message constructs
Content-Transfer-Encoding
MIME (RFC 2045) defines a set of methods for representing binary data in ASCII text format. The content-transfer-encoding: MIME header indicates the method that has been used. Typically encountered values for this header include:
- 7bit - up to 998 octets per line of the code range [1..127]\{CR, NL}. This is the default value.
- 8bit - up to 998 octets per line of the code range [1..255]\{CR, NL}. This can be used only if both the sending and receiving mail transfer agents support the 8BITMIME transport SMTP extension.
- binary - any sequence of octets
- quoted-printable - used for text data consisting primarily of US-ASCII characters
- base64 - used for arbitrary binary data
Encoded-Word
Through the use of MIME (RFC 2047) encoded-words non-ASCII text can be included in e-mail header fields, such as "Subject:". The character set for the content of these fields defaults to ASCII, so any non-ASCII text must be encoded. An encoded-word is a string of ASCII characters indicating both the original character set and the content-transfer-encoding used to map the original characters into ASCII characters.
Multipart Example
A MIME multipart message contains a boundary in the "Content-type:" header; this boundary, which must not occur in any of the parts, is placed between the parts, and at the beginning and end of the body of the message, as follows:
Content-type: multipart/mixed; boundary="frontier" MIME-version: 1.0 --frontier Content-type: text/plain This is the body of the message. --frontier Content-type: application/octet-stream Content-transfer-encoding: base64 gajwO4+n2Fy4FV3V7zD9awd7uG8/TITP/vIocxXnnf/5mjgQjcipBUL1b3uyLwAVtBLOP4nV LdIAhSzlZnyLAF8na0n7g6OSeej7aqIl3NIXCfxDsPsY6NQjSvV77j4hWEjlF/aglS6ghfju FgRr+OX8QZMI1OmR4rUJUS7xgoknalqj3HJvaOpeb3CFlNI9VGZYz6H6zuQBOWZzNB8glwpC --frontier--
See also
- DIME — Microsoft proposed a technology called DIME which would have streamlined MIME for use in web services. A specification for the standard was submitted to the Internet Engineering Task Force (IETF) but, according to the IETF web site, the standard has been withdrawn and never made RFC status.
References
- RFC 2045 - Full text of RFC 2045 (http://www.ietf.org/rfc/rfc2045.txt) Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies. N. Freed, N. Borenstein. November 1996. (Format: TXT=72932 bytes) (Obsoletes RFC 1521, RFC 1522, RFC 1590) (Updated by RFC 2184, RFC 2231) (Status: DRAFT STANDARD)
- RFC 2046 - Full text of RFC 2046 (http://www.ietf.org/rfc/rfc2046.txt) Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types. N. Freed, N. Borenstein. November 1996. (Format: TXT=105854 bytes) (Obsoletes RFC 1521, RFC 1522, RFC 1590) (Updated by RFC 2646) (Status: DRAFT STANDARD)
- RFC 2047 - Full text of RFC 2047 (http://www.ietf.org/rfc/rfc2047.txt) MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text. K. Moore. November 1996. (Format: TXT=33262 bytes) (Obsoletes RFC 1521, RFC 1522, RFC 1590) (Updated by RFC 2184, RFC 2231) (Status: DRAFT STANDARD)
- RFC 2048 - Full text of RFC 2048 (http://www.ietf.org/rfc/rfc2048.txt) Multipurpose Internet Mail Extensions (MIME) Part Four: Registration Procedures. N. Freed, J. Klensin, Jon Postel. November 1996. (Format: TXT=45033 bytes) (Obsoletes RFC 1521, RFC 1522, RFC 1590) (Updated by RFC 3023) (Also BCP0013) (Status: BEST CURRENT PRACTICE)
- RFC 2049 - Full text of RFC 2049 (http://www.ietf.org/rfc/rfc2049.txt) Multipurpose Internet Mail Extensions (MIME) Part Five: Conformance Criteria and Examples. N. Freed, N. Borenstein. November 1996. (Format: TXT=51207 bytes) (Obsoletes RFC 1521, RFC 1522, RFC 1590) (Status: DRAFT STANDARD)
External links
- Blog tracking Internet mail related drafts and standards (http://db.org/)
- List of IANA registered MIME Media Types (http://www.iana.org/assignments/media-types/)
- List of Character Sets (http://www.iana.org/assignments/character-sets)
- http://wiki.debian.net/index.cgi?MIMEca:MIME
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