RFC 2184 (rfc2184) - Page 3 of 9

MIME Parameter Value and Encoded Word Extensions: Character Sets, Languages, and Continuations

RFC 2184    MIME Parameter Value and Encoded Word Extensions August 1997


   This document defines extensions that address all of these
   limitations. All of these extensions are implemented in a fashion
   that is completely compatible at a syntactic level with existing MIME
   implementations. In addition, the extensions are designed to have as
   little impact as possible on existing uses of MIME.

   IMPORTANT NOTE: These mechanisms end up being somewhat gibbous when
   they actually are used. As such, use of these mechanisms should not
   be used lightly; they should be reserved for situations where a real
   need for them exists.

2.1.  Requirements notation

   This document occasionally uses terms that appear in capital letters.
   When the terms "MUST", "SHOULD", "MUST NOT", "SHOULD NOT", and "MAY"
   appear capitalized, they are being used to indicate particular
   requirements of this specification. A discussion of the meanings of
   these terms appears in [RFC-2119].


3.  Parameter Value Continuations

   Long MIME media type or disposition parameter values do not interact
   well with header line wrapping conventions.  In particular, proper
   header line wrapping depends on there being places where linear
   whitespace (LWSP) is allowed, which may or may not be present in a
   parameter value, and even if present may not be recognizable as such
   since specific knowledge of parameter value syntax may not be
   available to the agent doing the line wrapping. The result is that
   long parameter values may end up getting truncated or otherwise
   damaged by incorrect line wrapping implementations.

   A mechanism is therefore needed to break up parameter values into
   smaller units that are amenable to line wrapping. Any such mechanism
   MUST be compatible with existing MIME processors. This means that

    (1)   the mechanism MUST NOT change the syntax of MIME media
          type and disposition lines, and

    (2)   the mechanism MUST NOT depend on parameter ordering
          since MIME states that parameters are not order sensitive.
          Note that while MIME does prohibit modification of MIME
          headers during transport, it is still possible that parameters
          will be reordered when user agent level processing is done.







Freed & Moore               Standards Track