RFC 2231 (rfc2231) - Page 2 of 10
MIME Parameter Value and Encoded Word Extensions: Character Sets, Languages, and Continuations
Alternative Format: Original Text Document
RFC 2231 MIME Value and Encoded Word Extensions November 1997
(1) textual message bodies in character sets other than
US-ASCII,
(2) non-textual message bodies,
(3) multi-part message bodies, and
(4) textual header information in character sets other than
US-ASCII.
MIME is now widely deployed and is used by a variety of Internet
protocols, including, of course, Internet email. However, MIME's
success has resulted in the need for additional mechanisms that were
not provided in the original protocol specification.
In particular, existing MIME mechanisms provide for named media type
(content-type field) parameters as well as named disposition
(content-disposition field). A MIME media type may specify any
number of parameters associated with all of its subtypes, and any
specific subtype may specify additional parameters for its own use. A
MIME disposition value may specify any number of associated
parameters, the most important of which is probably the attachment
disposition's filename parameter.
These parameter names and values end up appearing in the content-type
and content-disposition header fields in Internet email. This
inherently imposes three crucial limitations:
(1) Lines in Internet email header fields are folded
according to RFC 822 folding rules. This makes long
parameter values problematic.
(2) MIME headers, like the RFC 822 headers they often
appear in, are limited to 7bit US-ASCII, and the
encoded-word mechanisms of RFC 2047 are not available
to parameter values. This makes it impossible to have
parameter values in character sets other than US-ASCII
without specifying some sort of private per-parameter
encoding.
(3) It has recently become clear that character set
information is not sufficient to properly display some
sorts of information -- language information is also
needed [RFC-2130]. For example, support for
handicapped users may require reading text string
Freed & Moore Standards Track