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. 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. (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 …