RFC5987

Internet Engineering Task Force (IETF) J. Reschke Request for Comments: 5987 greenbytes Category: Standards Track August 2010 ISSN: 2070-1721

            Character Set and Language Encoding for
   Hypertext Transfer Protocol (HTTP) Header Field Parameters

Abstract

By default, message header field parameters in Hypertext Transfer Protocol (HTTP) messages cannot carry characters outside the ISO- 8859-1 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.

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 5741.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc5987.

Copyright Notice

Copyright (c) 2010 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.

Introduction

By default, message header field parameters in HTTP (RFC2616) messages cannot carry characters outside the ISO-8859-1 character set ([ISO-8859-1]). 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.

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

  Note: this encoding does not apply to message payloads transmitted
  over HTTP, such as when using the media type "multipart/form-data"
  (RFC2388).

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, Appendix B.1: ALPHA (letters), DIGIT (decimal 0-9), HEXDIG (hexadecimal 0-9/A-F/a-f), and LWSP (linear whitespace).

Note that this specification uses the term "character set" for consistency with other IETF specifications such as RFC 2277 (see RFC2277, Section 3). A more accurate term would be "character encoding" (a mapping of code points to octet sequences).

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:

o Parameter Continuations aren't needed (Section 3.1),

o Character Set and Language Information are useful, therefore a

  simple subset is specified (Section 3.2), and

o Language Specifications in Encoded Words aren't needed

  (Section 3.3).

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 (RFC2616, Section 19.4.7).

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

Parameter Value Character Set 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 parameters.

However, RFC 2231 does not specify a mandatory-to-implement character set, making it hard for senders to decide which character set to use. Thus, recipients implementing this specification MUST support the character sets "ISO-8859-1" [ISO-8859-1] and "UTF-8" RFC3629.

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

Definition

The syntax for parameters is defined in Section 3.6 of RFC2616 (with RFC 2616 implied LWS translated to RFC 5234 LWSP):

 parameter     = attribute LWSP "=" LWSP value

 attribute     = token
 value         = token / quoted-string

 quoted-string = <quoted-string, defined in RFC2616, Section 2.2>
 token         = <token, defined in RFC2616, Section 2.2>

In order to include character set and language information, this specification modifies the RFC 2616 grammar to be:

 parameter     = reg-parameter / ext-parameter

 reg-parameter = parmname LWSP "=" LWSP value

 ext-parameter = parmname "*" LWSP "=" LWSP ext-value

 parmname      = 1*attr-char

 ext-value     = charset  "'" [ language ] "'" value-chars
               ; like RFC 2231's <extended-initial-value>
               ; (see RFC2231, Section 7)

 charset       = "UTF-8" / "ISO-8859-1" / 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, defined in 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 ( "*" / "'" / "%" )

Thus, a parameter is either a regular parameter (reg-parameter), as previously defined in Section 3.6 of RFC2616, or an extended parameter (ext-parameter).

Extended parameters are those where the left-hand side of the assignment ends with an asterisk character.

The value part of an extended parameter (ext-value) is a token that consists of three parts: the REQUIRED character set name (charset), the OPTIONAL language information (language), and a character sequence representing the actual value (value-chars), separated by single quote characters. Note that both character set names and language tags are restricted to the US-ASCII character set, and are matched case-insensitively (see RFC2978, Section 2.3 and RFC5646, Section 2.1.1).

Inside the value part, characters not contained in attr-char are encoded into an octet sequence using the specified character set. That octet sequence is then percent-encoded as specified in Section 2.1 of RFC3986.

Producers MUST use either the "UTF-8" (RFC3629) or the "ISO-8859-1" ([ISO-8859-1]) character set. Extension character sets (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,

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

  Note: the RFC 2616 token production (RFC2616, Section 2.2)
  differs from the production used in RFC 2231 (imported from
  Section 5.1 of RFC2045) in that curly braces ("{" and "}") are
  excluded.  Thus, these two characters are excluded from the attr-
  char production as well.

  Note: 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 set names using that character have been
  registered at the time of this writing.

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*=iso-8859-1'en'%A3%20rates

Note: the Unicode pound sign character U+00A3 was encoded into the single octet A3 using the ISO-8859-1 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%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, 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 set is not.

Language Specification in Encoded Words

Section 5 of RFC2231 extends the encoding defined in RFC2047 to also support language specification in encoded words. Although the HTTP/1.1 specification does refer to RFC 2047 (RFC2616, Section 2.2), it's not clear to which header field exactly it applies, and whether it is implemented in practice (see <http://tools.ietf.org/wg/httpbis/trac/ticket/111> for details).

Thus, this specification does not include this feature.

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 that header field.

For instance:

 foo-header  = "foo" LWSP ":" LWSP token ";" LWSP title-param
 title-param = "title" LWSP "=" LWSP value
             / "title*" LWSP "=" LWSP ext-value
 ext-value   = <see RFC 5987, 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 parmname components, as the
  processing of continuations would become ambiguous.  Thus,
  specifications using this extension are advised to disallow this
  case for compatibility with RFC 2231.

When to Use the Extension

Section 4.2 of RFC2277 requires that protocol elements containing human-readable text are able to carry language information. Thus, the ext-value production ought to be always used when the parameter value is of 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 ([USASCII]) 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).

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%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.

  Note: at the time of this writing, many implementations failed to
  ignore the form they do not understand, or prioritize the ASCII
  form although the extended syntax was present.

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 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 useful as an attack depends on the parameter specified.

Acknowledgements

Thanks to Martin Duerst 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 and Roar Lauritzsen for implementer's feedback.

References

Normative References

[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.

RFC2119 Bradner, S., "Key words for use in RFCs to Indicate

             Requirement Levels", BCP 14, RFC 2119, March 1997.

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.

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", RFC 3629, STD 63, November 2003.

RFC3986 Berners-Lee, T., Fielding, R., and L. Masinter,

             "Uniform Resource Identifier (URI): Generic Syntax",
             RFC 3986, STD 66, January 2005.

RFC5234 Crocker, D., Ed. and P. Overell, "Augmented BNF for

             Syntax Specifications: ABNF", STD 68, RFC 5234,
             January 2008.

RFC5646 Phillips, A., Ed. and M. Davis, Ed., "Tags for

             Identifying Languages", BCP 47, RFC 5646,
             September 2009.

[USASCII] American National Standards Institute, "Coded Character

             Set -- 7-bit American Standard Code for Information
             Interchange", ANSI X3.4, 1986.

Informative References

[Err1912] RFC Errata, Errata ID 1912, RFC 2978,

             <http://www.rfc-editor.org>.

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.

Author's Address

Julian F. Reschke greenbytes GmbH Hafenweg 16 Muenster, NW 48155 Germany

EMail: [email protected] URI: http://greenbytes.de/tech/webdav/