Application of RFC 2231
Encoding to Hypertext Transfer Protocol (HTTP) Headersgreenbytes GmbHHafenweg 16MuensterNW48155Germanyjulian.reschke@greenbytes.dehttp://greenbytes.de/tech/webdav/
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.
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 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
.
XML versions, latest edits and the issues list for this document
are available from .
A collection of test cases is available at .
By default, message header parameters in HTTP () messages
can not carry characters outside the ISO-8859-1 character set (). RFC 2231
() defines an escaping mechanism for use in MIME headers.
This document specifies a profile of that encoding for use in HTTP.
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 .
This specification uses the ABNF (Augmented Backus-Naur Form) notation defined in
. The following core rules are included by
reference, as defined in , Appendix B.1:
ALPHA (letters), DIGIT (decimal 0-9), HEXDIG (hexadecimal 0-9/A-F/a-f) and
LWSP (linear white space).
Non-ASCII characters used in prose for examples are encoded using
the format "Backslash-U with Delimiters", defined in
Section 5.1 of .
Note that this specification uses the term "character set" for consistency
with other IETF specifications such as RFC 2277 (see , Section 3). A more accurate term would be "character
encoding" (a mapping of code points to octet sequences).
RFC 2231 defines several extensions to MIME. The sections below discuss
if and how they apply to HTTP.
In short:
Parameter Continuations aren't needed (),Character Set and Language Information are useful, therefore a simple subset
is specified (), andLanguage Specifications in Encoded Words aren't needed ().
Section 3 of defines a mechanism that
deals with the length limitations that apply to MIME headers. These
limitations do not apply to HTTP (, Section 19.4.7).
Thus in HTTP, senders MUST NOT use parameter continuations, and
therefore recipients do not need to support them.
Section 4 of 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 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" and "UTF-8"
.
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
(with RFC 2616 implied LWS translated to RFC 5234 LWSP):
This specification extends the grammar to:
Thus, a parameter is either regular parameter (reg-parameter), as previously
defined in Section 3.6 of , 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 a character sequence representing the
actual value (value-chars), separated by single quote
characters.
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 Section 2.1 of .
Producers MUST NOT use character sets other than "UTF-8" ()
or ISO-8859-1 ().
Extension character sets (ext-charset) are reserved for future use.
Section 5 of extends the encoding
defined in to also support language specification
in encoded words.
Although the HTTP/1.1 specification does refer to RFC 2047
(, 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.
Specifications of HTTP headers that use the extensions defined
in should 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.
Note to RFC Editor: in the figure above, please replace "xxxx" by the
RFC number assigned to this specification.
Section 4.2 of requires that protocol
elements containing text can carry language information. Thus, the ext-value
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 ()
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).
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.
Does not work as expected, see <http://greenbytes.de/tech/tc2231/#attfnboth>
and <http://greenbytes.de/tech/tc2231/#attfnboth2>.
This document does not discuss security issues and is not believed to raise
any security issues not already endemic in HTTP.
There are no IANA Considerations related to this specification.
Thanks to Frank Ellermann for help figuring out ABNF details.
Key words for use in RFCs to Indicate Requirement LevelsHarvard Universitysob@harvard.edu
General
keywordHypertext Transfer Protocol -- HTTP/1.1University of California, Irvinefielding@ics.uci.eduW3Cjg@w3.orgCompaq Computer Corporationmogul@wrl.dec.comMIT Laboratory for Computer Sciencefrystyk@w3.orgXerox Corporationmasinter@parc.xerox.comMicrosoft Corporationpaulle@microsoft.comW3Ctimbl@w3.orgUTF-8, a transformation format of ISO 10646Alis Technologiesfyergeau@alis.comTags for Identifying LanguagesYahoo! Inc.addison@inter-locale.comGooglemark.davis@macchiato.comAugmented BNF for Syntax Specifications: ABNFBrandenburg InternetWorking+1.408.246.8253dcrocker@bbiw.netTHUS plc.paul.overell@thus.netInformation technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1International Organization for StandardizationMIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII TextUniversity of Tennesseemoore@cs.utk.eduMIME Parameter Value and Encoded Word Extensions: Character Sets, Languages, and ContinuationsInnosoft International, Inc.ned.freed@innosoft.comUniversity of Tennesseemoore@cs.utk.eduIETF Policy on Character Sets and LanguagesUNINETTHarald.T.Alvestrand@uninett.noUniform Resource Identifier (URI): Generic SyntaxWorld Wide Web Consortiumtimbl@w3.orghttp://www.w3.org/People/Berners-Lee/Day Softwarefielding@gbiv.comhttp://roy.gbiv.com/Adobe Systems IncorporatedLMM@acm.orghttp://larry.masinter.net/ASCII Escaping of Unicode Charactersjohn-ietf@jck.comCoded Character Set -- 7-bit American Standard Code for Information InterchangeAmerican National Standards Institute
Use RFC5234-style ABNF, closer to the one used in RFC 2231.
Make RFC 2231 dependency informative, so this specification can evolve
independantly.
Explain the ABNF in prose.