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Unformatted text preview: P: Simple Mail Transfer Protocol bit set.
3. base64, which we showed in Figure 28.6.
4. 8bit containing lines of characters, some of which are non-ASCII and have their
eighth bit set.
5. binary encoding, which is 8-bit data that need not contain lines.
Content-Type Subtype Description
Text with simple formatting, such as bold, italic,
underline, and so on.
A clarification, simplification, and refinement of
richtext. text plain
enriched multipart Multiple body parts to be processed sequentially.
Multiple body parts that can be processed in parallel.
An electronic mail digest.
Multiple body parts are present, all with identical
semantic content. message rfc822
externalbody octetapplication stream
postscript Content is another RFC 822 mail message.
Content is a fragment of a mail message.
Content is a pointer to the actual message.
Arbitrary binary data.
A PostScript program. image jpeg
gif ISO 10918 format.
CompuServe's Graphic Interchange Format. audio basic Encoded using 8-bit ISDN //-law format. video mpeg ISO 11172 format. Figure 28.7 MIME content types and subtypes.
Only the first three of these are valid for an RFC 821 MTA, since these three generate a
body containing only NVT ASCII characters. Using extended SMTP with 8BITMIME
support allows 8bit encoding to be used.
Although the content type and encoding are independent, RFC 1521 recommends
quoted-printable for text with non-ASCII data, and base64 for image,
audio, video, and octet-stream application data. This allows maximum
interoperability with RFC 821 conformant MTAs. Also, the multipart and message
content types must be encoded as 7bit. file:///D|/Documents%20and%20Settings/bigini/Doc...omenet2run/tcpip/tcp-ip-illustrated/smtp_sim.htm (20 of 23) [12/09/2001 14.47.52] Chapter 28. SMTP: Simple Mail Transfer Protocol As an example of a multipart content type. Figure 28.8 shows a mail message from
the RFC distribution list. The subtype is mixed, meaning each of the parts should be
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