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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 Content-Transfer-Encoding Header and Encoding Methods
(Page 2 of 3)
MIME Encoding Methods
To send non-ASCII data in MIME, it
is necessary that it be encoded. The Content-Transfer-Encoding
header is used to specify how a MIME message or body part has been encoded,
so that it can be decoded by its recipient. The following types of encoding
- 7bit: This indicates that the message
is already in ASCII form compatible with RFC 822. It is the default
and is what is assumed if no Content-Transfer-Encoding header
- 8bit / binary: These synonymous values
mean the message has been encoded directly in 8-bit binary form. Yes,
I did just say that this would violate the rules of RFC 822. These options
appear to have been included to support future mechanisms for transporting
binary data directly. RFC 1652 describes an SMTP extension that discusses
this in part: SMTP Service Extension for 8bit-MIMEtransport
(sic, there is no space between MIME and transport).
However, the standard is clear that this still does not allow the transfer
of raw binary data using SMTP and RFC 822.
- quoted-printable: This is a special encoding
that is used when most of the data is ASCII text, but when it contains
certain violations of the rules of RFC 822. These illegal sections are
converted using special encoding rules so the data as a whole is consistent
with RFC 822.
- base64: An encoding used to allow arbitrary
binary data to be represented in ASCII form. The data is then sent as
ASCII and decoded back into binary form by the recipient.
The quoted-printable and base64
encodings are the most interesting ones, because they are what allow
non-RFC-822 data to be sent using RFC 822.
Key Concept: MIME supports four encoding methods: 7bit, 8bit (binary), quoted-printable and base64. 7bit encoding is standard ASCII and is used for text; quoted-printable encoding is for output that is mostly text but has some special characters that must be encoded; base64 is used for arbitrary binary files. (The 8bit encoding method is defined in MIME but not used for RFC 822 messages.)
This encoding method is used when
the message is mostly ASCII; only the problem bytes are
encoded. The result is that RFC 822 compatibility is achieved while
maintaining most of the data as regular text so it can still be easily
understood by a human.
An example of where this would be
letters with tildes or accents, such as those used in French or Spanish.
Another would be a text message formed using an editor that inserts
carriage return characters in the middle of a line. Most of the message
is still text. The quoted-printable encoding can be used here,
with the carriage return characters represented as =0D (the
hexadecimal value of the character prepended by an equal sign). RFC
2046 contains more details on how this is done.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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