Network Working GroupJ. Reschke
Intended status: Standards TrackAugust 15, 2008
Expires: February 16, 2009

Applicability of RFC 2231 Encoding to Hypertext Transfer Protocol (HTTP) Headers


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By default, message header parameters in Hypertext Transfer Protocol (HTTP) messages can not carry characters outside the ISO-8859-1 character set. RFC 2231 defines an escaping mechanism for use in Multipurpose Internet Mail Extensions (MIME) headers. This document specifies a profile of that encoding suitable for use in HTTP.

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

There are multiple HTTP headers that already use RFC 2231 encoding in practice (Content-Disposition) or might use it in the future (Link). The purpose of this document is to provide a single place where the generic aspects of RFC 2231 encoding in HTTP headers are defined.

Distribution of this document is unlimited. Although this is not a work item of the HTTPbis Working Group, comments should be sent to the Hypertext Transfer Protocol (HTTP) mailing list at, which may be joined by sending a message with subject "subscribe" to

Discussions of the HTTPbis Working Group are archived at <>.

XML versions, latest edits and the issues list for this document are available from <>.

1. Introduction

By default, message header parameters in HTTP ([RFC2616]) messages can not carry characters outside the ISO-8859-1 character set ([ISO-8859-1]). RFC 2231 ([RFC2231]) defines an escaping mechanism for use in MIME headers. This document specifies a profile of that encoding for use in HTTP.

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 augmented BNF notation defined in Section 2.1 of [RFC2616], including its rules for linear whitespace (LWS). [LWS: This needs to be checked.]

Non-ASCII characters used in prose for examples are encoded using the format "Backslash-U with Delimiters", defined in Section 5.1 of [RFC5137].

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).

3. A Profile of RFC 2231 for Use in HTTP

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

In short:

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 ([RFC2616], Section 19.4.7).

Thus in HTTP, senders MUST NOT use parameter continuations, and therefore recipients do not need to support them.

3.2. 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 mandatory-to-implement character encoding, 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 leaving out the character encoding information. The profile defined by this specification does not allow that.

The syntax for parameters is defined in Section 3.6 of [RFC2616]:

  parameter     = attribute "=" 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>

This specification extends the grammar to:

  parameter     = reg-parameter | ext-parameter
  reg-parameter = attribute "=" value
  ext-parameter = attribute "*=" ext-value

  ext-value     = charset  "'" [ language ] "'" value-chars

  charset       = "UTF-8" | "ISO-8859-1" | ext-charset
                ; NOTE: case-insensitive

  ext-charset   = token ; see IANA charset registry
                ; (<>)
  language      = <Language-Tag, defined in [RFC4646], Section 2.1>
  value-chars   = *( pct-encoded | attr-char )

  pct-encoded   = "%" HEXDIG HEXDIG

  attr-char     = ALPHA | DIGIT
                | "-" | "." | "_" | "~" | ":"
                | "!" | "$" | "&" | "+" 
  ALPHA         = %x41-5A | %x61-7A
                ; A-Z | a-z
  DIGIT         = %x30-39
                ; any US-ASCII digit "0".."9"
  HEXDIG        = DIGIT | "A" | "B" | "C" | "D" | "E" | "F"
                ; NOTE: case-insensitive

3.2.1. 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 using ISO-8859-1 into the single octet A3, then percent-encoded. Also note that the space character was encoded as %20, as attr-char does not contain it.

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 using UTF-8 into the octet sequence C2 A3, 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 lower-case and upper-case character, so recipients must understand both, and that the language information is optional, while the character set 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. Although the HTTP/1.1 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 <> for details).

Thus, the RFC 2231 profile defined by this specification does not include this feature.

4. Guidelines for Usage in HTTP Header Definitions

Specifications of HTTP headers that use the extensions defined in Section 3.2 should clearly state that. The best way to achieve this is to normatively reference this specification, and to include the ext-parameter production into the ABNF for that header.

4.1. When to Use the Extension

Section 4.2 of [RFC2277] requires that protocol elements containing text can carry language information. Thus, the ext-parameter production should always be used when the parameter value is of textual nature.

Furthermore, the extension should 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 header that would be restricted to a subset of the Unicode character set).

4.2. Error Handling

Header specifications that include parameters should also specify whether same-named parameters can occur multiple times. If repetitions are not allowed (and this is believed to be the common case), the specification should state whether regular or the extended syntax takes precedence. In the latter case, this could be used by senders to use both formats without breaking recipients that do not understand the syntax.


  foo: bar; title="EURO exchange rates";

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

5. Security Considerations

This document does not discuss security issues and is not believed to raise any security issues not already endemic in HTTP.

6. IANA Considerations

There are no IANA Considerations related to this specification.

7. Acknowledgements

Thanks to Frank Ellermann for help figuring out BNF details.

8. References

8.1. Normative References

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.
Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels”, BCP 14, RFC 2119, March 1997.
Freed, N. and K. Moore, “MIME Parameter Value and Encoded Word Extensions: Character Sets, Languages, and Continuations”, RFC 2231, November 1997.
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.
Yergeau, F., “UTF-8, a transformation format of ISO 10646”, RFC 3629, STD 63, November 2003.
Phillips, A. and M. Davis, “Tags for Identifying Languages”, BCP 47, RFC 4646, September 2006.

8.2. Informative References

Moore, K., “MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text”, RFC 2047, November 1996.
Alvestrand, H., “IETF Policy on Character Sets and Languages”, BCP 18, RFC 2277, January 1998.
Klensin, J., “ASCII Escaping of Unicode Characters”, BCP 137, RFC 5137, February 2008.
American National Standards Institute, “Coded Character Set -- 7-bit American Standard Code for Information Interchange”, ANSI X3.4, 1986.

Author's Address

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

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