Unformatted text preview: ons.
MIME does not require any of the extensions that we've described previously in this
section (extended SMTP or non-ASCII headers). MIME just adds some new headers (in
accordance with RFC 822) that tell the recipient the structure of the body The body can
still be transmitted using NVT ASCII, regardless of the mail contents. While some of the
extensions we've just described might be nice to have along with MIME-the extended
SMTP SIZE command, since MIME messages can become large, and non-ASCII headersthese extensions are not required by MIME. All that's required to exchange MIME
messages with another party is for both ends to have a user agent that understands MIME.
No changes are required in any of the MTAs. MIME defines the five new header fields:
As an example, the following two header lines can appear in an Internet mail message:
Content-Type: TEXT/PLAIN; charset=US-ASCII
The current MIME version is 1.0 and the content type is plain ASCII text, the default for
Internet mail. The word PLAIN is considered a subtype of the content type (TEXT), and
the string charset=US-ASCII is a parameter.
Text is just one of MIME'S seven defined content types. Figure 28.7 summarizes the 16
different content types and subtypes defined in RFC 1521. Numerous parameters can be
specified for certain content types and subtypes.
The content type and the transfer encoding used for the body are independent. The former
is specified by the Content-Type header field, and the latter by the ContentTransfer-Encoding header field. There are five different encoding formats defined
in RFC 1521.
1. 7bit, which is NVT ASCII, the default.
2. quoted-printable, which we saw an example of earlier with non-ASCII
headers. It is useful when only a small fraction of the characters have their eighth file:///D|/Documents%20and%20Settings/bigini/Doc...omenet2run/tcpip/tcp-ip-illustrated/smtp_sim.htm (19 of 23) [12/09/2001 14.47.52] Chapter 28. SMT...
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.
- Spring '12