Network Working Group J. Reschke
Internet-Draft greenbytes
Intended status: Standards Track August 15, 2008
Expires: February 16, 2009
Applicability of RFC 2231 Encoding to
Hypertext Transfer Protocol (HTTP) Headers
draft-reschke-rfc2231-in-http-00
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on February 16, 2009.
Abstract
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
Reschke Expires February 16, 2009 [Page 1]
Internet-Draft RFC2231 Encoding in HTTP August 2008
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
ietf-http-wg@w3.org [1], which may be joined by sending a message
with subject "subscribe" to ietf-http-wg-request@w3.org [2].
Discussions of the HTTPbis Working Group are archived at
.
XML versions, latest edits and the issues list for this document are
available from
.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 3
3. A Profile of RFC 2231 for Use in HTTP . . . . . . . . . . . . 3
3.1. Parameter Continuations . . . . . . . . . . . . . . . . . 4
3.2. Parameter Value Character Set and Language Information . . 4
3.2.1. Examples . . . . . . . . . . . . . . . . . . . . . . . 5
3.3. Language specification in Encoded Words . . . . . . . . . 6
4. Guidelines for Usage in HTTP Header Definitions . . . . . . . 6
4.1. When to Use the Extension . . . . . . . . . . . . . . . . 6
4.2. Error Handling . . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . . . 7
8.2. Informative References . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8
Intellectual Property and Copyright Statements . . . . . . . . . . 10
Reschke Expires February 16, 2009 [Page 2]
Internet-Draft RFC2231 Encoding in HTTP August 2008
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:
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).
Reschke Expires February 16, 2009 [Page 3]
Internet-Draft RFC2231 Encoding in HTTP August 2008
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 =
token =
This specification extends the grammar to:
Reschke Expires February 16, 2009 [Page 4]
Internet-Draft RFC2231 Encoding in HTTP August 2008
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 =
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.
Reschke Expires February 16, 2009 [Page 5]
Internet-Draft RFC2231 Encoding in HTTP August 2008
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).
Reschke Expires February 16, 2009 [Page 6]
Internet-Draft RFC2231 Encoding in HTTP August 2008
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.
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 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
[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
Reschke Expires February 16, 2009 [Page 7]
Internet-Draft RFC2231 Encoding in HTTP August 2008
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2231] Freed, N. and K. Moore, "MIME Parameter Value and Encoded
Word Extensions:
Character Sets, Languages, and Continuations", RFC 2231,
November 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.
[RFC4646] Phillips, A. and M. Davis, "Tags for Identifying
Languages", BCP 47, RFC 4646, September 2006.
8.2. Informative References
[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
Part Three: Message Header Extensions for Non-ASCII Text",
RFC 2047, November 1996.
[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
Languages", BCP 18, RFC 2277, January 1998.
[RFC5137] Klensin, J., "ASCII Escaping of Unicode Characters",
BCP 137, RFC 5137, February 2008.
[USASCII] American National Standards Institute, "Coded Character
Set -- 7-bit American Standard Code for Information
Interchange", ANSI X3.4, 1986.
URIs
[1]
[2]
Reschke Expires February 16, 2009 [Page 8]
Internet-Draft RFC2231 Encoding in HTTP August 2008
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 Expires February 16, 2009 [Page 9]
Internet-Draft RFC2231 Encoding in HTTP August 2008
Full Copyright Statement
Copyright (C) The IETF Trust (2008).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Reschke Expires February 16, 2009 [Page 10]