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IETF RFC 5987
Character Set and Language Encoding for Hypertext Transfer Protocol (HTTP) Header Field Parameters
Last modified on Friday, August 13th, 2010
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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/RFC 5987.
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.
Reschke Standards Track PAGE 1 
RFC 5987 Charset/Language Encoding in HTTP August 2010
Table of Contents
1. Introduction ....................................................2
2. Notational Conventions ..........................................2
3. Comparison to RFC 2231 and Definition of the Encoding ...........3
3.1. Parameter Continuations ....................................3
3.2. Parameter Value Character Set and Language Information .....3
3.2.1. Definition ..........................................3
3.2.2. Examples ............................................6
3.3. Language Specification in Encoded Words ....................6
4. Guidelines for Usage in HTTP Header Field Definitions ...........7
4.1. When to Use the Extension ..................................7
4.2. Error Handling .............................................7
5. Security Considerations .........................................8
6. Acknowledgements ................................................8
7. References ......................................................8
7.1. Normative References .......................................8
7.2. Informative References .....................................9
1. Introduction
By default, message header field parameters in HTTP ([RFC 2616])
messages cannot carry characters outside the ISO-8859-1 character set
([ISO-8859-1]). RFC 2231 ([RFC 2231]) 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"
([RFC 2388]).
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 [RFC 2119].
This specification uses the ABNF (Augmented Backus-Naur Form)
notation defined in [RFC 5234]. The following core rules are included
by reference, as defined in [RFC 5234], Appendix B.1: ALPHA (letters),
DIGIT (decimal 0-9), HEXDIG (hexadecimal 0-9/A-F/a-f), and LWSP
(linear whitespace).
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RFC 5987 Charset/Language Encoding in HTTP August 2010
Note that this specification uses the term "character set" for
consistency with other IETF specifications such as RFC 2277 (see
[RFC 2277], Section 3). A more accurate term would be "character
encoding" (a mapping of code points to octet sequences).
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:
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).
3.1. Parameter Continuations
Section 3 of [RFC 2231] defines a mechanism that deals with the length
limitations that apply to MIME headers. These limitations do not
apply to HTTP ([RFC 2616], Section 19.4.7).
Thus, parameter continuations are not part of the encoding defined by
this specification.
3.2. Parameter Value Character Set and Language Information
Section 4 of [RFC 2231] 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" [RFC 3629].
Furthermore, RFC 2231 allows the character set information to be left
out. The encoding defined by this specification does not allow that.
3.2.1. Definition
The syntax for parameters is defined in Section 3.6 of [RFC 2616]
(with RFC 2616 implied LWS translated to RFC 5234 LWSP):
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RFC 5987 Charset/Language Encoding in HTTP August 2010
parameter = attribute LWSP "=" LWSP value
attribute = token
value = token / quoted-string
quoted-string = <quoted-string, defined in [RFC 2616], Section 2.2>
token = <token, defined in [RFC 2616], 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 [RFC 2231], Section 7)
charset = "UTF-8" / "ISO-8859-1" / mime-charset
mime-charset = 1*mime-charsetc
mime-charsetc = ALPHA / DIGIT
/ "!" / "#" / "$" / "%" / "&"
/ "+" / "-" / "^" / "_" / "`"
/ "{" / "}" / "~"
; as <mime-charset> in Section 2.3 of [RFC 2978]
; except that the single quote is not included
; SHOULD be registered in the IANA charset registry
language = <Language-Tag, defined in [RFC 5646], Section 2.1>
value-chars = *( pct-encoded / attr-char )
pct-encoded = "%" HEXDIG HEXDIG
; see [RFC 3986], Section 2.1
attr-char = ALPHA / DIGIT
/ "!" / "#" / "$" / "&" / "+" / "-" / "."
/ "^" / "_" / "`" / "|" / "~"
; token except ( "*" / "'" / "%" )
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RFC 5987 Charset/Language Encoding in HTTP August 2010
Thus, a parameter is either a regular parameter (reg-parameter), as
previously defined in Section 3.6 of [RFC 2616], 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 [RFC 2978], Section 2.3 and [RFC 5646],
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 [RFC 3986].
Producers MUST use either the "UTF-8" ([RFC 3629]) 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 ([RFC 2616], Section 2.2)
differs from the production used in RFC 2231 (imported from
Section 5.1 of [RFC 2045]) in that curly braces ("{" and "}") are
excluded. Thus, these two characters are excluded from the attr-
char production as well.
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RFC 5987 Charset/Language Encoding in HTTP August 2010
Note: the <mime-charset> ABNF defined here differs from the one in
Section 2.3 of [RFC 2978] 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.
3.2.2. 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.
3.3. Language Specification in Encoded Words
Section 5 of [RFC 2231] extends the encoding defined in [RFC 2047] to
also support language specification in encoded words. Although the
HTTP/1.1 specification does refer to RFC 2047 ([RFC 2616], 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.
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RFC 5987 Charset/Language Encoding in HTTP August 2010
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 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 [RFC 2231] 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.
4.1. When to Use the Extension
Section 4.2 of [RFC 2277] 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).
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''%e2%82%ac%20exchange%20rates
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RFC 5987 Charset/Language Encoding in HTTP August 2010
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.
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 Section 10 of [RFC 3629] 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. 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.
7. References
7.1. 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.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Reschke Standards Track PAGE 8
RFC 5987 Charset/Language Encoding in HTTP August 2010
[RFC 2616] 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.
[RFC 2978] Freed, N. and J. Postel, "IANA Charset Registration
Procedures", BCP 19, RFC 2978, October 2000.
[RFC 3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 3629, STD 63, November 2003.
[RFC 3986] Berners-Lee, T., Fielding, R., and L. Masinter,
"Uniform Resource Identifier (URI): Generic Syntax",
RFC 3986, STD 66, January 2005.
[RFC 5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", STD 68, RFC 5234,
January 2008.
[RFC 5646] 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.
7.2. Informative References
[Err1912] RFC Errata, Errata ID 1912, RFC 2978,
<http://www.rfc-editor.org>.
[RFC 2045] Freed, N. and N. Borenstein, "Multipurpose Internet
Mail Extensions (MIME) Part One: Format of Internet
Message Bodies", RFC 2045, November 1996.
[RFC 2047] Moore, K., "MIME (Multipurpose Internet Mail
Extensions) Part Three: Message Header Extensions for
Non-ASCII Text", RFC 2047, November 1996.
[RFC 2231] Freed, N. and K. Moore, "MIME Parameter Value and
Encoded Word Extensions: Character Sets, Languages, and
Continuations", RFC 2231, November 1997.
[RFC 2277] Alvestrand, H., "IETF Policy on Character Sets and
Languages", BCP 18, RFC 2277, January 1998.
[RFC 2388] Masinter, L., "Returning Values from Forms: multipart/
form-data", RFC 2388, August 1998.
Reschke Standards Track PAGE 9
RFC 5987 Charset/Language Encoding in HTTP August 2010
Author's Address
Julian F. Reschke
greenbytes GmbH
Hafenweg 16
Muenster, NW 48155
Germany
EMail: julian.reschke@greenbytes.de
URI: http://greenbytes.de/tech/webdav/
Reschke Standards Track PAGE 10
Character Set and Language Encoding for Hypertext Transfer Protocol (HTTP) Header Field Parameters
RFC TOTAL SIZE: 20108 bytes
PUBLICATION DATE: Friday, August 13th, 2010
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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