MIME Parameter Value and Encoded Word Extensions: Character Sets, Languages, and Continuations
RFC 2231
| Document | Type |
RFC - Proposed Standard
(November 1997; Errata)
Obsoletes RFC 2184
Was draft-freed-pvcsc-new (individual)
|
|
|---|---|---|---|
| Last updated | 2013-03-02 | ||
| Stream | Legacy | ||
| Formats | plain text pdf html bibtex | ||
| Stream | Legacy state | (None) | |
| Consensus Boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
| IESG | IESG state | RFC 2231 (Proposed Standard) | |
| Telechat date | |||
| Responsible AD | (None) | ||
| Send notices to | (None) | ||
Network Working Group N. Freed
Request for Comments: 2231 Innosoft
Updates: 2045, 2047, 2183 K. Moore
Obsoletes: 2184 University of Tennessee
Category: Standards Track November 1997
MIME Parameter Value and Encoded Word Extensions:
Character Sets, Languages, and Continuations
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1997). All Rights Reserved.
1. Abstract
This memo defines extensions to the RFC 2045 media type and RFC 2183
disposition parameter value mechanisms to provide
(1) a means to specify parameter values in character sets
other than US-ASCII,
(2) to specify the language to be used should the value be
displayed, and
(3) a continuation mechanism for long parameter values to
avoid problems with header line wrapping.
This memo also defines an extension to the encoded words defined in
RFC 2047 to allow the specification of the language to be used for
display as well as the character set.
2. Introduction
The Multipurpose Internet Mail Extensions, or MIME [RFC-2045, RFC-
2046, RFC-2047, RFC-2048, RFC-2049], define a message format that
allows for:
Freed & Moore Standards Track [Page 1]
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 [Page 2]
RFC 2231 MIME Value and Encoded Word Extensions November 1997
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.
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
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