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Internet Engineering Task Force (IETF)                        J. Reschke
Request for Comments: 8187                                    greenbytes
Obsoletes: 5987                                           September 2017
Category: Standards Track
ISSN: 2070-1721

Indicating Character Encoding and Language for HTTP Header Field

Parameters

Abstract

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

This document obsoletes RFC 5987.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8187.

Copyright Notice

Copyright © 2017 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 (https://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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Notational Conventions  . . . . . . . . . . . . . . . . . . .   3
   3.  Comparison to RFC 2231 and Definition of the Encoding . . . .   3
     3.1.  Parameter Continuations . . . . . . . . . . . . . . . . .   4
     3.2.  Parameter Value Character Encoding and Language
           Information . . . . . . . . . . . . . . . . . . . . . . .   4
       3.2.1.  Definition  . . . . . . . . . . . . . . . . . . . . .   4
       3.2.2.  Historical Notes  . . . . . . . . . . . . . . . . . .   6
       3.2.3.  Examples  . . . . . . . . . . . . . . . . . . . . . .   6
     3.3.  Language Specification in Encoded Words . . . . . . . . .   7
   4.  Guidelines for Usage in HTTP Header Field Definitions . . . .   7
     4.1.  When to Use the Extension . . . . . . . . . . . . . . . .   8
     4.2.  Error Handling  . . . . . . . . . . . . . . . . . . . . .   8
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Appendix A.  Changes from RFC 5987  . . . . . . . . . . . . . . .  12
   Appendix B.  Implementation Report  . . . . . . . . . . . . . . .  12
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  13
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  13

1. Introduction

Use of characters outside the US-ASCII coded character set ([RFC0020]) in HTTP header fields ([RFC7230]) is non-trivial:

  • The HTTP specification discourages use of non-US-ASCII characters in field values, placing them into the "obs-text" Augmented Backus-Naur Form (ABNF) production ([RFC7230], Section 3.2).
  • Furthermore, it stays silent about default character encoding schemes for field values, so any use of non-US-ASCII characters would need to be specific to the field definition or would require some other kind of out-of-band information.
  • Finally, some APIs assume a default character encoding scheme in order to map from the octet sequences (obtained from the HTTP message) to character sequences: for instance, the XMLHttpRequest API ([XMLHttpRequest]) uses the Interface Definition Language type "ByteString", effectively resulting in the ISO-8859-1 character encoding scheme ([ISO-8859-1]) being used.

On the other hand, RFC 2231 defines an encoding mechanism for parameters inside MIME header fields ([RFC2231]), which, as opposed to HTTP messages, do need to be sent over non-binary transports. This document specifies an encoding suitable for use in HTTP header fields that is compatible with a simplified profile of the encoding defined in RFC 2231. It can be applied to any HTTP header field that uses the common "parameter" ("name=value") syntax.

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

Note: In the remainder of this document, RFC 2231 is only referenced for the purpose of explaining the choice of features that were adopted; therefore, they are purely informative.

Note: This encoding does not apply to message payloads transmitted over HTTP, such as when using the media type "multipart/form-data" ([RFC7578]).

2. Notational Conventions

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

   This specification uses the ABNF notation defined in [RFC5234].  The
   following core rules are included by reference, as defined in
   [RFC5234], Appendix B.1: 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: "character encoding scheme" (abbreviated to "character encoding" below), "charset", and "coded character set".

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:

  • Parameter Continuations aren't needed (Section 3.1),
  • Character Encoding and Language Information are useful, therefore a simple subset is specified (Section 3.2), and
  • Language Specifications in Encoded Words aren't needed (Section 3.3).

3.1. Parameter Continuations

Section 3 of [RFC2231] defines a mechanism that deals with the length limitations that apply to MIME headers. These limitations do not apply to HTTP ([RFC7231], Appendix A.6).

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

3.2. Parameter Value Character Encoding and Language Information

Section 4 of [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 MUST support the "UTF-8" character encoding [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 is usually indicated by a parameter name ending in an asterisk character. However, note that this is just a convention, and that the extended parameter values need 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 [RFC2231], Section 7)
     
     charset       = "UTF-8" / mime-charset
     
     mime-charset  = 1*mime-charsetc
     mime-charsetc = ALPHA / DIGIT
                   / "!" / "#" / "$" / "%" / "&"
                   / "+" / "-" / "^" / "_" / "`"
                   / "{" / "}" / "~"
                   ; as <mime-charset> in Section 2.3 of [RFC2978]
                   ; except that the single quote is not included
                   ; SHOULD be registered in the IANA charset registry
     
     language      = <Language-Tag, see [RFC5646], Section 2.1>
     
     value-chars   = *( pct-encoded / attr-char )
     
     pct-encoded   = "%" HEXDIG HEXDIG
                   ; see [RFC3986], Section 2.1
     
     attr-char     = ALPHA / DIGIT
                   / "!" / "#" / "$" / "&" / "+" / "-" / "."
                   / "^" / "_" / "`" / "|" / "~"
                   ; token except ( "*" / "'" / "%" )

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

  1. the REQUIRED character encoding name (charset),
   2.  the OPTIONAL language information (language), and
  1. a character sequence representing the actual value (value-chars), separated by single quote characters.

Note that both character encoding names and language tags are restricted to the US-ASCII coded character set and are matched case- insensitively (see Section 2.3 of [RFC2978] and Section 2.1.1 of [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 Section 2.1 of [RFC3986].

Producers MUST use the "UTF-8" ([RFC3629]) character encoding. Extension character encodings (mime-charset) are reserved for future use.

Note: Recipients should be prepared to handle encoding errors, such as malformed or incomplete percent escape sequences, or non-decodable octet sequences, in a robust manner. This specification does not mandate any specific behavior; for instance, the following strategies are all acceptable:

  • ignoring the parameter,
      *  stripping a non-decodable octet sequence, and
  • substituting a non-decodable octet sequence by a replacement character, such as the Unicode character U+FFFD (Replacement Character).

3.2.2. Historical Notes

The RFC 7230 token production ([RFC7230], Section 3.2.6) differs from the production used in RFC 2231 (imported from Section 5.1 of [RFC2045]) in that curly braces (i.e., "{" 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 Section 2.3 of [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'%C2%A3%20rates

Note: The Unicode pound sign character U+00A3 was encoded into the octet sequence C2 A3 using the UTF-8 character encoding, and 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''%c2%a3%20and%20%e2%82%ac%20rates

Note: The Unicode pound sign character U+00A3 was encoded into the octet sequence C2 A3 using the UTF-8 character encoding, and then percent-encoded. Likewise, the Unicode euro sign character U+20AC was encoded into the octet sequence E2 82 AC, and 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

Section 5 of [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], Section 2.2). 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 Section 3.2.4 of [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 ext-value production into the ABNF for specific header field parameters.

For instance: 

     foo         = token ";" LWSP title-param
     title-param = "title" LWSP "=" LWSP value
                 / "title*" LWSP "=" LWSP ext-value
     ext-value   = <see RFC 8187, Section 3.2>

Note: The Parameter Value Continuation feature defined in Section 3 of [RFC2231] makes it impossible to have multiple instances of extended parameters with identical names, as the processing of continuations would become ambiguous. Thus, specifications using this extension are advised to disallow this case for compatibility with RFC 2231.

Note: This specification does not automatically assign a new interpretation to parameter names ending in an asterisk. As pointed out above, it's up to the specification for the non-extended parameter to "opt in" to the syntax defined here. That being said, some existing implementations are known to automatically switch to using this notation when a parameter name ends with an asterisk; thus, using parameter names ending in an asterisk for something else is likely to cause interoperability problems.

4.1. When to Use the Extension

Section 4.2 of [RFC2277] requires that protocol elements containing human-readable text be able to carry language information. Thus, the ext-value production ought to always be used when the parameter value is of a textual nature and its language is known.

Furthermore, the extension ought to also be used whenever the parameter value needs to carry characters not present in the US-ASCII coded character set ([RFC0020]); 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 names 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''%e2%82%ac%20exchange%20rates

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 related to decoding UTF-8.

See Section 10 of [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 effective as an attack depends on the parameter specified.

6. IANA Considerations

This document does not require any IANA actions.

7. References

7.1. Normative References

   [RFC0020]  Cerf, V., "ASCII format for network interchange", STD 80,
   
              RFC 20, DOI 10.17487/RFC0020, October 1969,
              <https://www.rfc-editor.org/info/rfc20>.
   
   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.
   
   [RFC2978]  Freed, N. and J. Postel, "IANA Charset Registration
              Procedures", BCP 19, RFC 2978, DOI 10.17487/RFC2978,
              October 2000, <https://www.rfc-editor.org/info/rfc2978>.
   
   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <https://www.rfc-editor.org/info/rfc3629>.
   
   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.
   
   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.
   
   [RFC5646]  Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
              Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646,
              September 2009, <https://www.rfc-editor.org/info/rfc5646>.
   
   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <https://www.rfc-editor.org/info/rfc7230>.
   
   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <https://www.rfc-editor.org/info/rfc7231>.

7.2. Informative References

   [Err1912]  RFC Errata, "Erratum ID 1912, RFC 2978",
              <https://www.rfc-editor.org/errata/eid1912>.

[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, DOI 10.17487/RFC2045, November 1996,
              <https://www.rfc-editor.org/info/rfc2045>.
   
   [RFC2047]  Moore, K., "MIME (Multipurpose Internet Mail Extensions)
              Part Three: Message Header Extensions for Non-ASCII Text",
              RFC 2047, DOI 10.17487/RFC2047, November 1996,
              <https://www.rfc-editor.org/info/rfc2047>.
   
   [RFC2231]  Freed, N. and K. Moore, "MIME Parameter Value and Encoded
              Word Extensions: Character Sets, Languages, and
              Continuations", RFC 2231, DOI 10.17487/RFC2231, November
              1997, <https://www.rfc-editor.org/info/rfc2231>.
   
   [RFC2277]  Alvestrand, H., "IETF Policy on Character Sets and
              Languages", BCP 18, RFC 2277, DOI 10.17487/RFC2277,
              January 1998, <https://www.rfc-editor.org/info/rfc2277>.
   
   [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,
              DOI 10.17487/RFC2616, June 1999,
              <https://www.rfc-editor.org/info/rfc2616>.
   
   [RFC5987]  Reschke, J., "Character Set and Language Encoding for
              Hypertext Transfer Protocol (HTTP) Header Field
              Parameters", RFC 5987, DOI 10.17487/RFC5987, August 2010,
              <https://www.rfc-editor.org/info/rfc5987>.
   
   [RFC5988]  Nottingham, M., "Web Linking", RFC 5988,
              DOI 10.17487/RFC5988, October 2010,
              <https://www.rfc-editor.org/info/rfc5988>.
   
   [RFC6266]  Reschke, J., "Use of the Content-Disposition Header Field
              in the Hypertext Transfer Protocol (HTTP)", RFC 6266,
              DOI 10.17487/RFC6266, June 2011,
              <https://www.rfc-editor.org/info/rfc6266>.
   
   [RFC6365]  Hoffman, P. and J. Klensin, "Terminology Used in
              Internationalization in the IETF", BCP 166, RFC 6365,
              DOI 10.17487/RFC6365, September 2011,
              <https://www.rfc-editor.org/info/rfc6365>.
   
   [RFC7578]  Masinter, L., "Returning Values from Forms: multipart/
              form-data", RFC 7578, DOI 10.17487/RFC7578, July 2015,
              <https://www.rfc-editor.org/info/rfc7578>.
   
   [RFC7616]  Shekh-Yusef, R., Ed., Ahrens, D., and S. Bremer, "HTTP
              Digest Access Authentication", RFC 7616,
              DOI 10.17487/RFC7616, September 2015,
              <https://www.rfc-editor.org/info/rfc7616>.
   
   [RFC8053]  Oiwa, Y., Watanabe, H., Takagi, H., Maeda, K., Hayashi,
              T., and Y. Ioku, "HTTP Authentication Extensions for
              Interactive Clients", RFC 8053, DOI 10.17487/RFC8053,
              January 2017, <https://www.rfc-editor.org/info/rfc8053>.

[XMLHttpRequest]

WhatWG, "XMLHttpRequest", <https://xhr.spec.whatwg.org/>.

Appendix A. Changes from RFC 5987

This section summarizes the changes compared to [RFC5987]:

  • The document title was changed to "Indicating Character Encoding and Language for HTTP Header Field Parameters".
  • The introduction was rewritten to better explain the issues around non-ASCII characters in field values.
  • The requirement to support the "ISO-8859-1" encoding was removed.
  • This document no longer attempts to redefine a generic "parameter" ABNF (it turned out that there really isn't a generic definition of parameters in HTTP; for instance, there are subtle differences with respect to whitespace handling).
  • A note about defects in error handling in current implementations was removed, as it was no longer accurate.

Appendix B. Implementation Report

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

   o  "Authentication-Control", defined in [RFC8053],
  • "Authorization" (as used in HTTP Digest Authentication, defined in [RFC7616]),
   o  "Content-Disposition", defined in [RFC6266], and

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] was 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 more recently specified in [RFC5988]. At the time of this writing, no user agent except Firefox supported the "title*" parameter (starting with release 15).

Section 3.4 of [RFC7616] defines the "username*" parameter for use in HTTP Digest Authentication. At the time of writing, no user agent implemented this extension.

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 implementers feedback.

Furthermore, thanks to the members of the IETF HTTP Working Group for the feedback specific to this update of RFC 5987.

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/

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