RFC 2184 (rfc2184) - Page 2 of 9
MIME Parameter Value and Encoded Word Extensions: Character Sets, Languages, and Continuations
Alternative Format: Original Text Document
RFC 2184 MIME Parameter Value and Encoded Word Extensions August 1997
(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 aloud. The language the text is written in is
needed for this to be done correctly. Some parameter values
may need to be displayed, hence there is a need to allow for
the inclusion of language information.
The last problem on this list is also an issue for the encoded words
defined by RFC 2047, as encoded words are intended primarily for
display purposes.
Freed & Moore Standards Track