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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 Extension for Non-ASCII Mail Message Headers
(Page 2 of 2)
MIME Encoded-Word Syntax
In the MIME non-ASCII header technique,
the value of a regular header is replaced by a MIME encoded-word
that has the following syntax:
The strings =? and ?=
are used to bracket the non-ASCII header, which flags it as a
MIME encoded header to the recipient's e-mail client. The other elements,
separated by ?, indicate how the non-ASCII text is encoded:
- <charset>: The character set used,
such as iso-8859-1.
- <encoding>: Two different encoding
types are defined, each represented by a single letter for brevity:
- <encoded-text>: The non-ASCII text
that has been encoded as ASCII using the encoding type indicated.
As you can see, this method is analogous
to how a non-ASCII message body or body part would be encoded, but the
information about the encoding has been condensed so everything can
fit in a single header line. The <charset> parameter
is somewhat analogous to the Content-Type header for a message
body, but since headers can only contain text, it specifies what kind
of text it is. The <encoding> parameter is clearly
equivalent to the Content-Transfer-Encoding header.
Example Non-ASCII MIME Header
Here's an example of a non-ASCII
header, using the GB2312 character set (for Chinese characters) and
I hope that doesn't say anything
inappropriate; I took it from a piece of spam e-mail I received once!
Key Concept: In addition to its many functions for encoding a variety of data in e-mail message bodies, MIME provides a feature that allows non-ASCII information to be placed into e-mail headers. This is done by encoding the data using either quoted-printable or base64 encoding, and then using a special format for the header value that specifies its encoding and character set. This technique is especially useful for e-mail sent in languages that cannot be represented easily in standard ASCII, such as many Asian languages.
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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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