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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 Message Format Overview, Motivation, History and Standards
(Page 2 of 3)
The idea behind MIME is both clever
and elegantwhich means I like it! RFC 822 restricts e-mail messages
to be ASCII text, but that doesn't mean that we cannot define a more
specific structure for how that ASCII text is created. Instead of just
letting the user type an ASCII text message, we can use ASCII text characters
to actually encode non-text information (commonly called attachments).
Using this technique, MIME allows regular RFC 822 e-mail messages to
carry the following:
- Non-text information, including graphic files,
multimedia clips and all the other non-text data examples I listed earlier;
- Arbitrary binary files, including executable
programs and files stored in proprietary formats (for example, AutoCAD
files, Adobe Acrobat PDF files and so forth);
- Text messages that use character sets other than
ASCII. This even includes the ability to use
non-ASCII characters in the headers of
RFC 822 e-mail messages.
MIME even goes one step beyond this,
by actually defining a structure that allows multiple
files to be encoded into a single e-mail message,
including files of different types. For example, someone working on
a budget analysis could send one e-mail message that includes a text
message, a Powerpoint presentation, and a spreadsheet containing the
budget figures. This capability has greatly expanded e-mails usefulness
All of this is accomplished through
special encoding rules that transform non-ASCII files and information
into an ASCII form. Headers are added to the message to indicate how
the information is encoded. The encoded message can then be sent through
the system like any other message. SMTP and the other protocols that
handle mail pay no attention to the message body, so they don't even
know MIME has been used.
The only changes required to the
e-mail software is adding support for MIME to e-mail client programs:
both the sender and receiver must support MIME to encode and decode
the messages. Support for MIME was not widespread when MIME was first
developed, but the value of the technique is so significant that it
is present in nearly all e-mail client software today. Furthermore,
most clients today can also use the information in MIME headers to not
only decode non-text information but pass it to the appropriate application
for presentation to the user.
Key Concept: The use of the RFC 822 message format ensures that all devices are able to read each others e-mail messages, but it has a critical limitation: it only supports plain, ASCII text. This is insufficient for the needs of modern internetworks, yet reliance on the RFC 822 standard would have made replacing it difficult. Instead, a new standard called the Multipurpose Internet Mail Extensions (MIME) was defined. MIME specifies several methods that allow e-mail messages to contain multimedia content, binary files, and text files using non-ASCII character sets, all while still adhering to the RFC 822 message format. MIME also further expands e-mails flexibility by allowing multiple files or pieces of content to be sent in a single message.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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