HTTP J. Reschke
Internet-Draft greenbytes
Obsoletes: 5987 (if approved) July 8, 2016
Intended status: Standards Track
Expires: January 9, 2017

Indicating Character Encoding and Language for HTTP Header Field Parameters
draft-ietf-httpbis-rfc5987bis-02

Abstract

By default, header field values in Hypertext Transfer Protocol (HTTP) messages cannot directly carry characters outside the US-ASCII coded character set. RFC 2231 defines an encoding mechanism for use in Multipurpose Internet Mail Extensions (MIME) headers. This document specifies an encoding suitable for use in HTTP header fields that is compatible with a profile of the encoding defined in RFC 2231.

Editorial Note (To be removed by RFC Editor before publication)

Discussion of this draft takes place on the HTTPBIS working group mailing list (ietf-http-wg@w3.org), which is archived at <https://lists.w3.org/Archives/Public/ietf-http-wg/>.

Working Group information can be found at <http://httpwg.github.io/>; source code and issues list for this draft can be found at <https://github.com/httpwg/http-extensions>.

The changes in this draft are summarized in Appendix C.

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 January 9, 2017.

Copyright Notice

Copyright (c) 2016 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

By default, header field values in HTTP messages ([RFC7230]) cannot directly carry characters outside the US-ASCII coded character set ([RFC0020]). RFC 2231 ([RFC2231]) defines an encoding mechanism for use in MIME headers. This document specifies an encoding suitable for use in HTTP header fields that is compatible with a profile of the encoding defined in RFC 2231.

This document obsoletes [RFC5987] and moves it to "historic" status; the changes are summarized in Appendix A.

2. Notational Conventions

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].

This specification uses the ABNF (Augmented Backus-Naur Form) notation defined in [RFC5234]. The following core rules are included by reference, as defined in [RFC5234]: ALPHA (letters), DIGIT (decimal 0-9), HEXDIG (hexadecimal 0-9/A-F/a-f), and LWSP (linear whitespace).

This specification uses terminology defined in [RFC6365], namely: "

Note that this differs from RFC 2231, which uses the term "character set" for "character encoding scheme".

3. Comparison to RFC 2231 and Definition of the Encoding

RFC 2231 defines several extensions to MIME. The sections below discuss if and how they apply to HTTP header fields.

In short:

3.1. Parameter Continuations

[RFC2231] defines a mechanism that deals with the length limitations that apply to MIME headers. These limitations do not apply to HTTP ([RFC7231]).

Thus, parameter continuations are not part of the encoding defined by this specification.

3.2. Parameter Value Character Encoding and Language Information

[RFC2231] specifies how to embed language information into parameter values, and also how to encode non-ASCII characters, dealing with restrictions both in MIME and HTTP header field parameters.

However, RFC 2231 does not specify a mandatory-to-implement character encoding, making it hard for senders to decide which encoding to use. Thus, recipients implementing this specification [RFC3629].

Furthermore, RFC 2231 allows the character encoding information to be left out. The encoding defined by this specification does not allow that.

3.2.1. Definition

The presence of extended parameter values usually is indicated by a parameter name ending in an asterisk character. Note however that this is just a convention, and that it needs to be explicitly specified in the definition of the header field using this extension (see Section 4).

The ABNF for extended parameter values is specified below:

  ext-value     = charset  "'" [ language ] "'" value-chars
                ; like RFC 2231's <extended-initial-value>
                ; (see 

The value part of an extended parameter (ext-value) is a token that consists of three parts:

Note that both character encoding names and language tags are restricted to the US-ASCII coded character set, and are matched case-insensitively (see [RFC2978] and [RFC5646]).

Inside the value part, characters not contained in attr-char are encoded into an octet sequence using the specified character encoding. That octet sequence is then percent-encoded as specified in [RFC3986].

Producers [RFC3629]) character encoding. Extension character encodings (mime-charset) are reserved for future use.

3.2.2. Historical Notes

The RFC 7230 token production ([RFC7230]) differs from the production used in RFC 2231 (imported from [RFC2045]) in that curly braces ("{" and "}") are excluded. Thus, these two characters are excluded from the attr-char production as well.

The <mime-charset> ABNF defined here differs from the one in [RFC2978] in that it does not allow the single quote character (see also RFC Errata ID 1912 [Err1912]). In practice, no character encoding names using that character have been registered at the time of this writing.

For backwards compatibility with RFC 2231, the encoding defined by this specification deviates from common parameter syntax in that the quoted-string notation is not allowed. Implementations using generic parser components might not be able to detect the use of quoted-string notation and thus might accept that format, although invalid, as well.

[RFC5987] did require support for ISO-8859-1 ([ISO-8859-1]), too; for compatibility with legacy code, recipients are encouraged to support this encoding as well.

3.2.3. Examples

Non-extended notation, using "token":

  foo: bar; title=Economy

Non-extended notation, using "quoted-string":

  foo: bar; title="US-$ rates"

Extended notation, using the Unicode character U+00A3 ("£", POUND SIGN):

  foo: bar; title*=utf-8'en'

Note: the Unicode pound sign character U+00A3 was encoded into the octet sequence C2 A3 using the UTF-8 character encoding, then percent-encoded. Also, note that the space character was encoded as %20, as it is not contained in attr-char.

Extended notation, using the Unicode characters U+00A3 ("£", POUND SIGN) and U+20AC ("€", EURO SIGN):

  foo: bar; title*=UTF-8''

Note: the Unicode pound sign character U+00A3 was encoded into the octet sequence C2 A3 using the UTF-8 character encoding, then percent-encoded. Likewise, the Unicode euro sign character U+20AC was encoded into the octet sequence E2 82 AC, then percent-encoded. Also note that HEXDIG allows both lowercase and uppercase characters, so recipients must understand both, and that the language information is optional, while the character encoding is not.

3.3. Language Specification in Encoded Words

[RFC2231] extends the encoding defined in [RFC2047] to also support language specification in encoded words. RFC 2616, the now-obsolete HTTP/1.1 specification, did refer to RFC 2047 ([RFC2616]). However, it wasn't clear to which header field it applied. Consequently, the current revision of the HTTP/1.1 specification has deprecated use of the encoding forms defined in RFC 2047 (see [RFC7230]).

Thus, this specification does not include this feature.

4. Guidelines for Usage in HTTP Header Field Definitions

Specifications of HTTP header fields that use the extensions defined in Section 3.2 ought to clearly state that. A simple way to achieve this is to normatively reference this specification, and to include the

For instance:

  foo         = token ";" LWSP title-param
  title-param = "title" LWSP "=" LWSP value
              / "title*" LWSP "=" LWSP ext-value
  ext-value   = <see draft-ietf-httpbis-rfc5987bis, 

[pub]Upon publication as RFC, the string "draft-ietf-httpbis-rfc5987bis" needs to be replaced with the RFC name, and this comment needs to be removed.

4.1. When to Use the Extension

[RFC2277] requires that protocol elements containing human-readable text are able to carry language information. Thus, the

Furthermore, the extension ought to also be used whenever the parameter value needs to carry characters not present in the US-ASCII ([RFC0020]) coded character set (note that it would be unacceptable to define a new parameter that would be restricted to a subset of the Unicode character set).

4.2. Error Handling

Header field specifications need to define whether multiple instances of parameters with identical parmname components are allowed, and how they should be processed. This specification suggests that a parameter using the extended syntax takes precedence. This would allow producers to use both formats without breaking recipients that do not understand the extended syntax yet.

Example:

  foo: bar; title="EURO exchange rates";
            title*=utf-8''

In this case, the sender provides an ASCII version of the title for legacy recipients, but also includes an internationalized version for recipients understanding this specification -- the latter obviously ought to prefer the new syntax over the old one.

5. Security Considerations

The format described in this document makes it possible to transport non-ASCII characters, and thus enables character "spoofing" scenarios, in which a displayed value appears to be something other than it is.

Furthermore, there are known attack scenarios relating to decoding UTF-8.

See [RFC3629] for more information on both topics.

In addition, the extension specified in this document makes it possible to transport multiple language variants for a single parameter, and such use might allow spoofing attacks, where different language versions of the same parameter are not equivalent. Whether this attack is useful as an attack depends on the parameter specified.

6. IANA Considerations

There are no IANA Considerations related to this specification.

7. References

7.1. Normative References

[RFC0020] Cerf, V., "ASCII format for network interchange", STD 80, RFC 20, October 1969.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2978] Freed, N. and J. Postel, "IANA Charset Registration Procedures", BCP 19, RFC 2978, October 2000.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, November 2003.
[RFC3986] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, January 2005.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5646] Phillips, A. and M. Davis, "Tags for Identifying Languages", BCP 47, RFC 5646, September 2009.
[RFC7230] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing", RFC 7230, June 2014.
[RFC7231] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content", RFC 7231, June 2014.

7.2. Informative References

[Err1912] RFC Errata, "Errata ID 1912, RFC 2978", October 2009.
[ISO-8859-1] International Organization for Standardization, "Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1", ISO/IEC 8859-1:1998, 1998.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, November 1996.
[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text", RFC 2047, November 1996.
[RFC2231] Freed, N. and K. Moore, "MIME Parameter Value and Encoded Word Extensions: Character Sets, Languages, and Continuations", RFC 2231, November 1997.
[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and Languages", BCP 18, RFC 2277, January 1998.
[RFC2388] Masinter, L., "Returning Values from Forms: multipart/form-data", RFC 2388, August 1998.
[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.
[RFC5987] Reschke, J., "Character Set and Language Encoding for Hypertext Transfer Protocol (HTTP) Header Field Parameters", RFC 5987, August 2010.
[RFC5988] Nottingham, M., "Web Linking", RFC 5988, October 2010.
[RFC6266] Reschke, J., "Use of the Content-Disposition Header Field in the Hypertext Transfer Protocol (HTTP)", RFC 6266, June 2011.
[RFC6365] Hoffman, P. and J. Klensin, "Terminology Used in Internationalization in the IETF", BCP 166, RFC 6365, September 2011.

Appendix A. Changes from RFC 5987

This section summarizes the changes compared to [RFC5987]:

Appendix B. Implementation Report

The encoding defined in this document currently is used for two different HTTP header fields:

As the encoding is a profile/clarification of the one defined in [RFC2231] in 1997, many user agents already supported it for use in "Content-Disposition" when [RFC5987] got published.

Since the publication of [RFC5987], three more popular desktop user agents have added support for this encoding; see <http://purl.org/NET/http/content-disposition-tests#encoding-2231-char> for details. At this time, the current versions of all major desktop user agents support it.

Note that the implementation in Internet Explorer 9 does not support the ISO-8859-1 character encoding; this document revision acknowledges that UTF-8 is sufficient for expressing all code points, and removes the requirement to support ISO-8859-1.

The "Link" header field, on the other hand, was only recently specified in [RFC5988]. At the time of this writing, no shipping User Agent except Firefox supported the "title*" parameter (starting with release 15).

Appendix C. Change Log (to be removed by RFC Editor before publication)

C.1. Since RFC5987

Only editorial changes for the purpose of starting the revision process (obs5987).

C.2. Since draft-reschke-rfc5987bis-00

Resolved issues "iso-8859-1" and "title" (title simplified). Added and resolved issue "historic5987".

C.3. Since draft-reschke-rfc5987bis-01

Added issues "httpbis", "parmsyntax", "terminology" and "valuesyntax". Closed issue "impls".

C.4. Since draft-reschke-rfc5987bis-02

Resolved issue "terminology".

C.5. Since draft-reschke-rfc5987bis-03

In Section 3.2, pull historical notes into a separate subsection. Resolved issues "valuesyntax" and "parmsyntax".

C.6. Since draft-reschke-rfc5987bis-04

Update status of Firefox support in HTTP Link Header field.

C.7. Since draft-reschke-rfc5987bis-05

Update status of Firefox support in HTTP Link Header field.

C.8. Since draft-reschke-rfc5987bis-06

Update status with respect to Safari 6.

Started work on update with respect to RFC 723x.

C.9. Since draft-ietf-httpbis-rfc5987bis-00

Editorial changes; introducing non-ASCII characters into author's address, acknowledgements, and examples.

C.10. Since draft-ietf-httpbis-rfc5987bis-01

Removed mention of RFC 2616 from Abstract and Introduction.

Reference RFC 20 for US-ASCII.

Do not attempt to define a generic parameter ABNF; just concentrate on the parameter value syntax.

Appendix D. Acknowledgements

Thanks to Martin Dürst and Frank Ellermann for help figuring out ABNF details, to Graham Klyne and Alexey Melnikov for general review, to Chris Newman for pointing out an RFC 2231 incompatibility, and to Benjamin Carlyle, Roar Lauritzsen, Eric Lawrence, and James Manger for implementer's feedback.

Author's Address

Julian F. Reschke greenbytes GmbH Hafenweg 16 Münster, NW 48155 Germany EMail: julian.reschke@greenbytes.de URI: http://greenbytes.de/tech/webdav/