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|| 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 Content-Transfer-Encoding Header and Encoding Methods
(Page 1 of 3)
One of the main reasons why MIME
was created was the significant
restrictions that the RFC 822 standard
places on how data in e-mail messages must be formatted. To follow the
rules, messages must be encoded in US ASCII, a 7-bit data representation.
This means that even though each byte can theoretically have any of
256 values, in ASCII only 128 values are valid. Furthermore, lines can
be no longer than 1000 characters including the carriage return and
line feed (CRLF) characters at the end, and those two characters
cannot appear elsewhere.
For some types of data, such as text
files, this is no big deal, but for others it is a serious problem.
This is especially the case with binary data. If you look at the data
in a video clip or MP3 file or executable program, it will appear to
be random gibberish. In fact, such data is not random but
is represented using specific rules, but the data is expressed in raw
binary form, where any 8-bit byte can contain any value from 0 to 255,
which is why it looks like junk to humans. More importantly,
this means that this data does not follow the rules for RFC 822 files
and cannot be sent directly in this form.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
© Copyright 2001-2005 Charles M. Kozierok. All Rights Reserved.
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