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






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