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By default, message header field 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.
There are multiple HTTP header fields 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 header fields is 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 ietf-http-wg@w3.org, which may be joined by sending a message with subject "subscribe" to ietf-http-wg-request@w3.org.
Discussions of the HTTPbis Working Group are archived at http://lists.w3.org/Archives/Public/ietf-http-wg/.
XML versions, latest edits and the issues list for this document are available from http://greenbytes.de/tech/webdav/#draft-reschke-rfc2231-in-http. A collection of test cases is available at http://greenbytes.de/tech/tc2231/.
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Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved.
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1.
Introduction
2.
Notational Conventions
3.
A Profile of RFC 2231 for Use in HTTP
3.1.
Parameter Continuations
3.2.
Parameter Value Character Set and Language Information
3.2.1.
Examples
3.3.
Language specification in Encoded Words
4.
Guidelines for Usage in HTTP Header Field Definitions
4.1.
When to Use the Extension
4.2.
Error Handling
4.3.
Using Multiple Instances for Internationalization
5.
Security Considerations
6.
IANA Considerations
7.
Acknowledgements
8.
References
8.1.
Normative References
8.2.
Informative References
Appendix A.
Document History and Future Plans (to be removed by RFC Editor before publication)
Appendix B.
Change Log (to be removed by RFC Editor before publication)
B.1.
Since draft-reschke-rfc2231-in-http-00
B.2.
Since draft-reschke-rfc2231-in-http-01
B.3.
Since draft-reschke-rfc2231-in-http-02
B.4.
Since draft-reschke-rfc2231-in-http-03
B.5.
Since draft-reschke-rfc2231-in-http-04
B.6.
Since draft-reschke-rfc2231-in-http-05
B.7.
Since draft-reschke-rfc2231-in-http-06
§
Author's Address
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By default, message header field parameters in HTTP ([RFC2616] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.)) messages can not carry characters outside the ISO-8859-1 character set ([ISO‑8859‑1] (International Organization for Standardization, “Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1,” 1998.)). RFC 2231 ([RFC2231] (Freed, N. and K. Moore, “MIME Parameter Value and Encoded Word Extensions: Character Sets, Languages, and Continuations,” November 1997.)) defines an escaping mechanism for use in MIME headers. This document specifies a profile of that encoding for use in HTTP.
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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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).
This specification uses the ABNF (Augmented Backus-Naur Form) notation defined in [RFC5234] (Crocker, D., Ed. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” January 2008.). The following core rules are included by reference, as defined in [RFC5234] (Crocker, D., Ed. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” January 2008.): ALPHA (letters), DIGIT (decimal 0-9), HEXDIG (hexadecimal 0-9/A-F/a-f) and LWSP (linear white space).
Note that this specification uses the term "character set" for consistency with other IETF specifications such as RFC 2277 (see [RFC2277] (Alvestrand, H., “IETF Policy on Character Sets and Languages,” January 1998.)). A more accurate term would be "character encoding" (a mapping of code points to octet sequences).
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RFC 2231 defines several extensions to MIME. The sections below discuss if and how they apply to HTTP.
In short:
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[RFC2231] (Freed, N. and K. Moore, “MIME Parameter Value and Encoded Word Extensions: Character Sets, Languages, and Continuations,” November 1997.) defines a mechanism that deals with the length limitations that apply to MIME headers. These limitations do not apply to HTTP ([RFC2616] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.)).
Thus in HTTP, senders use parameter continuations, and therefore recipients do not need to support them.
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[RFC2231] (Freed, N. and K. Moore, “MIME Parameter Value and Encoded Word Extensions: Character Sets, Languages, and Continuations,” November 1997.) 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 support the character sets "ISO-8859-1" [ISO‑8859‑1] (International Organization for Standardization, “Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1,” 1998.) and "UTF-8" [RFC3629] (Yergeau, F., “UTF-8, a transformation format of ISO 10646,” November 2003.).
Furthermore, RFC 2231 allows leaving out the character set information. The profile defined by this specification does not allow that.
The syntax for parameters is defined in [RFC2616] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.) (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 attribute = token
value = token / quoted-string
quoted-string = <quoted-string, defined in attribute = token
value = token / quoted-string
quoted-string = <quoted-string, defined in This specification extends the grammar to:
parameter = reg-parameter / ext-parameter reg-parameter = attribute LWSP "=" LWSP value ext-parameter = attribute "*" LWSP "=" LWSP ext-value ext-value = charset "'" [ language ] "'" value-chars ; extended-initial-value, ; defined in
parameter = reg-parameter / ext-parameter reg-parameter = attribute LWSP "=" LWSP value ext-parameter = attribute "*" LWSP "=" LWSP ext-value ext-value = charset "'" [ language ] "'" value-chars ; extended-initial-value, ; defined in
parameter = reg-parameter / ext-parameter reg-parameter = attribute LWSP "=" LWSP value ext-parameter = attribute "*" LWSP "=" LWSP ext-value ext-value = charset "'" [ language ] "'" value-chars ; extended-initial-value, ; defined in
parameter = reg-parameter / ext-parameter reg-parameter = attribute LWSP "=" LWSP value ext-parameter = attribute "*" LWSP "=" LWSP ext-value ext-value = charset "'" [ language ] "'" value-chars ; extended-initial-value, ; defined in
parameter = reg-parameter / ext-parameter reg-parameter = attribute LWSP "=" LWSP value ext-parameter = attribute "*" LWSP "=" LWSP ext-value ext-value = charset "'" [ language ] "'" value-chars ; extended-initial-value, ; defined in
Thus, a parameter is either regular parameter (reg-parameter), as previously defined in [RFC2616] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.), 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 character set name (charset), the 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] (Freed, N. and J. Postel, “IANA Charset Registration Procedures,” October 2000.) and [RFC5646] (Phillips, A., Ed. and M. Davis, Ed., “Tags for Identifying Languages,” September 2009.)).
Inside the value part, characters not contained in attr-char are encoded into an octet sequence using the specified character set. That octet sequence then is percent-encoded as specified in [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.).
Producers use character sets other than "UTF-8" ([RFC3629] (Yergeau, F., “UTF-8, a transformation format of ISO 10646,” November 2003.)) or "ISO-8859-1" ([ISO‑8859‑1] (International Organization for Standardization, “Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1,” 1998.)). Extension character sets (ext-charset) are reserved for future use.
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:
the <mime-charset> ABNF defined here differs from the one in [RFC2978] (Freed, N. and J. Postel, “IANA Charset Registration Procedures,” October 2000.) in that it does not allow the single quote character (see also RFC Editor Errata ID 1912). In practice, no character set names using that character have been registered at the time of this writing.
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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'
foo: bar; title*=iso-8859-1'en'
: 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 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''
foo: bar; title*=UTF-8''
foo: bar; title*=UTF-8''
: 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.
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[RFC2231] (Freed, N. and K. Moore, “MIME Parameter Value and Encoded Word Extensions: Character Sets, Languages, and Continuations,” November 1997.) extends the encoding defined in [RFC2047] (Moore, K., “MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text,” November 1996.) to also support language specification in encoded words. Although the HTTP/1.1 specification does refer to RFC 2047 ([RFC2616] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.)), 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, the RFC 2231 profile defined by this specification does not include this feature.
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Specifications of HTTP header fields that use the extensions defined in Section 3.2 (Parameter Value Character Set and Language Information) should clearly state that. A simple way to achieve this is to normatively reference this specification, and to include the 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 RFCxxxx,
foo-header = "foo" LWSP ":" LWSP token ";" LWSP title-param
title-param = "title" LWSP "=" LWSP value
/ "title*" LWSP "=" LWSP ext-value
ext-value = <see RFCxxxx,[rfcno] (Note to RFC Editor: in the figure above, please replace "xxxx" by the RFC number assigned to this specification.)
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[RFC2277] (Alvestrand, H., “IETF Policy on Character Sets and Languages,” January 1998.) requires that protocol elements containing text can carry language information. Thus, the 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] (American National Standards Institute, “Coded Character Set -- 7-bit American Standard Code for Information Interchange,” 1986.)) 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).
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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 producers to use both formats without breaking recipients that do not understand the syntax.
Example:
foo: bar; title="EURO exchange rates";
title*=utf-8''
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 should prefer the new syntax over the old one.
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.
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It is expected that in many cases, internationalization of parameters in response headers is implemented using server driven content negotiation ([RFC2616] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.)) using the Accept-Language header ([RFC2616] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.)). However, the format described in this specification also allows to use multiple instances providing multiple languages in a single header. Specifications that want to take advantage of this should clearly specify the expected processing by the recipient.
Example:
foo: bar; title*=utf-8'
foo: bar; title*=utf-8'
foo: bar; title*=utf-8'
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This document does not discuss security issues and is not believed to raise any security issues not already endemic in HTTP.
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There are no IANA Considerations related to this specification.
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Thanks to Martin Duerst and Frank Ellermann for help figuring out ABNF details, and to Roar Lauritzsen for implementer's feedback.
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| [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. |
| [RFC3629] | Yergeau, F., “UTF-8, a transformation format of ISO 10646,” RFC 3629, STD 63, November 2003. |
| [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. |
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| [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. |
| [RFC2978] | Freed, N. and J. Postel, “IANA Charset Registration Procedures,” BCP 19, RFC 2978, October 2000. |
| [RFC3986] | Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” RFC 3986, STD 66, January 2005. |
| [USASCII] | American National Standards Institute, “Coded Character Set -- 7-bit American Standard Code for Information Interchange,” ANSI X3.4, 1986. |
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Problems with the internationalization of the HTTP Content-Disposition header field have been known for many years (see test cases at http://greenbytes.de/tech/tc2231/).
During IETF 72 (http://tools.ietf.org/wg/httpbis/minutes?item=minutes72.html), the HTTPbis Working Group shortly discussed how to deal with the underspecification of (1) Content-Disposition, and its (2) internationalization aspects. Back then, there was rough consensus in the room to move the definition into a separate draft.
This specification addresses problem (2), by defining a simple subset of the encoding format defined in RFC 2231. A separate specification, draft-reschke-rfc2183-in-http, is planned to address problem (1). Note that this approach was chosen because Content-Disposition is just an example for an HTTP header field using this kind of encoding. Another example is the currently proposed Link header field (draft-nottingham-http-link-header).
This document is planned to be published on the IETF Standards Track, so that other standards-track level documents can depend on it, such as the new specification of Content-Disposition, or potentially future revisions of the HTTP Link Header specification.
Also note that this document specifies a proper subset of the extensions defined in RFC 2231, but does not normatively refer to it. Thus, RFC 2231 can be revised separately, should the email community decide to.
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Use RFC5234-style ABNF, closer to the one used in RFC 2231.
Make RFC 2231 dependency informative, so this specification can evolve independently.
Explain the ABNF in prose.
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Remove unneeded RFC5137 notation (code point vs character).
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And and resolve issues "charset", "repeats" and "rfc4646".
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And and resolve issue "charsetmatch".
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Add and resolve issues "badseq" and "tokenquotcharset".
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Say "header field" instead of "header" in the context of HTTP.
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Add an appendix discussing document history and future plans, to be removed before publication.
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| Julian F. Reschke | |
| greenbytes GmbH | |
| Hafenweg 16 | |
| Muenster, NW 48155 | |
| Germany | |
| Email: | julian.reschke@greenbytes.de |
| URI: | http://greenbytes.de/tech/webdav/ |