Update to MIME regarding Charset Parameter Handling in Textual Media Types
draft-ietf-appsawg-mime-default-charset-02

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.

Editorial Note (To be removed by RFC Editor)

Discussion of this draft should take place on the Apps Area Working Group mailing list (apps-discuss@ietf.org), which is archived at http://www.ietf.org/mail-archive/web/apps-discuss.

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/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on October 21, 2012.

Copyright Notice

Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

• 1. Introduction and Overview
• 2. Conventions Used in This Document
• 3. New rules for default charset parameter values for text/* media types
• 4. Default charset parameter value for text/plain media type
• 5. Security Considerations
• 6. IANA Considerations
• 7. References
• 7.1. Normative References
• 7.2. Informative References
• Appendix A. Acknowledgements
• Authors' Addresses

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.

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. It does not change the defaults for any currently registered media type.

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 the absence of any default.

In order to improve interoperability with deployed agents, "text/*" media type registrations SHOULD either

1. specify that the "charset" parameter is not used for the defined subtype, because the charset information is transported inside the payload (such as in "text/xml"), or
2. require explicit unconditional inclusion of the "charset" parameter eliminating the need for a default value.

In accordance with option (a), above, registrations for "text/*" media types that can transport charset information inside the corresponding payloads (such as "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 covering 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

Guessing of the charset parameter can lead to security issues such as content buffer overflows, denial of services or bypass of filtering mechanisms. However, this document does not promote guessing, but encourages use of charset information that is specified by the sender.

Conflicting information in-band vs out-of-band can also lead to similar security problems, and this document recommends the use of charset information which is more likely to be correct (for example, in-band over out-of-band).

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

7.2. Informative References

Appendix A. Acknowledgements

Many thanks to Ned Freed and John Klensin for comments and ideas that motivated creation of this document, and to Carsten Bormann, Murray S. Kucherawy, Barry Leiba, and Henri Sivonen for feedback and text suggestions.

Authors' Addresses