Applications Area Working Group A. Melnikov Internet-Draft Isode Limited Updates: 2046 (if approved) J. Reschke Intended status: Standards Track greenbytes Expires: August 4, 2012 February 1, 2012 Update to MIME regarding Charset Parameter Handling in Textual Media Types draft-ietf-appsawg-mime-default-charset-00 Abstract 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. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. 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Melnikov & Reschke Expires August 4, 2012 [Page 1] Internet-Draft MIME Charset Default Update February 2012 Table of Contents 1. Introduction and Overview . . . . . . . . . . . . . . . 3 2. Conventions Used in This Document . . . . . . . . . . . 3 3. New rules for default charset parameter values for text/* media types . . . . . . . . . . . . . . . . . . 3 4. Default charset parameter value for text/plain media type . . . . . . . . . . . . . . . . . . . . . . 4 5. Security Considerations . . . . . . . . . . . . . . . . 4 6. IANA Considerations . . . . . . . . . . . . . . . . . . 4 7. References . . . . . . . . . . . . . . . . . . . . . . 4 7.1. Normative References . . . . . . . . . . . . . . . . . 4 7.2. Informative References . . . . . . . . . . . . . . . . 5 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . . 5 Authors' Addresses . . . . . . . . . . . . . . . . . . 5 Melnikov & Reschke Expires August 4, 2012 [Page 2] Internet-Draft MIME Charset Default Update February 2012 1. Introduction and Overview [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. [[anchor2: 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. 2. Conventions Used in This Document 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]. 3. New rules for default charset parameter values for text/* media types Section 4.1.2 of [RFC2046] says: The default character set, which must be assumed in the absence of a charset parameter, is US-ASCII. As explained in the Introduction section this rule is considered to be outdated, so this document replaces it with the following set of rules: 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. Melnikov & Reschke Expires August 4, 2012 [Page 3] Internet-Draft MIME Charset Default Update February 2012 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. 4. Default charset parameter value for text/plain media type The default charset parameter value for text/plain is unchanged from [RFC2046] and remains as "US-ASCII". 5. Security Considerations TBD. Guessing of default charset is a security problem. Conflicting information in-band vs out-of-band is also a security problem. 6. IANA Considerations This document asks IANA to update the "text" subregistry of the Media Types registry to additionally point to this document. 7. References 7.1. Normative References Melnikov & Reschke Expires August 4, 2012 [Page 4] Internet-Draft MIME Charset Default Update February 2012 [RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types", RFC 2046, November 1996. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, November 2003. 7.2. Informative References [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. [RFC2854] Connolly, D. and L. Masinter, "The 'text/html' Media Type", RFC 2854, June 2000. [RFC3023] Murata, M., St. Laurent, S., and D. Kohn, "XML Media Types", RFC 3023, January 2001. Appendix A. Acknowledgements 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. Authors' Addresses Alexey Melnikov Isode Limited 5 Castle Business Village 36 Station Road Hampton, Middlesex TW12 2BX UK Email: Alexey.Melnikov@isode.com Julian F. Reschke greenbytes GmbH Hafenweg 16 Muenster, NW 48155 Germany Melnikov & Reschke Expires August 4, 2012 [Page 5] Internet-Draft MIME Charset Default Update February 2012 Email: julian.reschke@greenbytes.de URI: http://greenbytes.de/tech/webdav/ Melnikov & Reschke Expires August 4, 2012 [Page 6]