Update to MIME regarding Charset Parameter Handling in Textual Media Types

This document changes RFC 2046 rules regarding default charset parameter values for text/* media types to better align with common usage by existing clients and servers.¶

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[RFC2046] specified that the default charset parameter (i.e. the value used when it is not specified) is "US-ASCII". [RFC2616] changed the default for use by HTTP to be "ISO-8859-1". This encoding is not very common for new text/* media types and a special rule in HTTP adds confusion about which specification ([RFC2046] or [RFC2616]) is authoritative in regards to the default charset for text/* media types. [rfc.comment.1: At the time of writing of this document the IETF HTTPBIS WG is working on an update to RFC 2616 which removes the default charset of "ISO-8859-1" for "text/*" media types. It is expected that the set of HTTPBIs documents will reference this document in order to use the updated rules of default charset in "text/*" media types.] ¶

Many complex text subtypes such as text/html [RFC2854] and text/xml [RFC3023] have internal (to their format) means of describing the charset. Many existing User Agents ignore the default of "US-ASCII" rule for at least text/html and text/xml.¶

This document changes RFC 2046 rules regarding default charset parameter values for text/* media types to better align with common usage by existing clients and servers.¶

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].¶

"The default character set, which must be assumed in the absence of a charset parameter, is US-ASCII."¶

Each subtype of the "text" media type which uses the "charset" parameter can define its own default value for the "charset" parameter, including absence of any default.¶

In order to improve interoperability with deployed agents, "text/*" media type definitions SHOULD either a) specify that the "charset" parameter is not used for the defined subtype, because the charset information is transported inside the payload (as in "text/xml") or b) require explicit unconditional inclusion of the "charset" parameter eliminating the need for a default value. In accordance with option (a), above, "text/*" media types that can transport charset information inside the corresponding payloads, specifically including "text/html" and "text/xml", SHOULD NOT specify the use of a "charset" parameter, nor any default value, in order to avoid conflicting interpretations should the charset parameter value and the value specified in the payload disagree.¶

New subtypes of the "text" media type, thus, SHOULD NOT define a default "charset" value. If there is a strong reason to do so despite this advice, they SHOULD use the "UTF-8" [RFC3629] charset as the default.¶

Specifications of how to specify the "charset" parameter, and what default value, if any, is used, are subtype-specific, NOT protocol- specific. Protocols that use MIME, therefore, MUST NOT override default charset values for "text/*" media types to be different for their specific protocol. The protocol definitions MUST leave that to the subtype definitions.¶

The default charset parameter value for text/plain is unchanged from [RFC2046] and remains as "US-ASCII".¶

TBD. Guessing of default charset is a security problem. Conflicting information in-band vs out-of-band is also a security problem.¶

This document asks IANA to update the "text" subregistry of the Media Types registry to additionally point to this document.¶

Many thanks to Ned Freed and John Klensin for comments and ideas that motivated creation of this document, and to Barry Leiba for suggested text.¶