    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/rfc5987.

    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.

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    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 ([RFC2616])
       messages cannot carry characters outside the ISO-8859-1 character set
       ([ISO-8859-1]).  RFC 2231 ([RFC2231]) 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"
          ([RFC2388]).

    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 ABNF (Augmented Backus-Naur Form)
       notation defined in [RFC5234].  The following core rules are included
       by reference, as defined in [RFC5234], 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
       [RFC2277], 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 [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, parameter continuations are not part of the encoding defined by
       this specification.

    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 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" [RFC3629].

       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 [RFC2616]
       (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 [RFC2616], Section 2.2>
         token         = <token, defined in [RFC2616], 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 [RFC2231], 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 [RFC2978]
                       ; except that the single quote is not included
                       ; SHOULD be registered in the IANA charset registry

         language      = <Language-Tag, defined in [RFC5646], Section 2.1>

         value-chars   = *( pct-encoded / attr-char )

         pct-encoded   = "%" HEXDIG HEXDIG
                       ; see [RFC3986], 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 [RFC2616], 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 [RFC2978], Section 2.3 and [RFC5646],
       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 [RFC3986].

       Producers MUST use either the "UTF-8" ([RFC3629]) 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 ([RFC2616], Section 2.2)
          differs from the production used in RFC 2231 (imported from
          Section 5.1 of [RFC2045]) in that curly braces ("{" and "}") are
          excluded.  Thus, these two characters are excluded from the attr-
          char production as well.

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          Note: the <mime-charset> ABNF defined here differs from the one in
          Section 2.3 of [RFC2978] 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 [RFC2231] extends the encoding defined in [RFC2047] to
       also support language specification in encoded words.  Although the
       HTTP/1.1 specification 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
       <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 [RFC2231] 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 [RFC2277] 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 [RFC3629] 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.

       [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate
                     Requirement Levels", BCP 14, RFC 2119, March 1997.

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    RFC 5987            Charset/Language Encoding in HTTP        August 2010

       [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.

       [RFC2978]     Freed, N. and J. Postel, "IANA Charset Registration
                     Procedures", BCP 19, RFC 2978, October 2000.

       [RFC3629]     Yergeau, F., "UTF-8, a transformation format of ISO
                     10646", RFC 3629, STD 63, November 2003.

       [RFC3986]     Berners-Lee, T., Fielding, R., and L. Masinter,
                     "Uniform Resource Identifier (URI): Generic Syntax",
                     RFC 3986, STD 66, January 2005.

       [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.

       [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>.

       [RFC2045]     Freed, N. and N. Borenstein, "Multipurpose Internet
                     Mail Extensions (MIME) Part One: Format of Internet
                     Message Bodies", RFC 2045, November 1996.

       [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.

       [RFC2388]     Masinter, L., "Returning Values from Forms: multipart/
                     form-data", RFC 2388, August 1998.

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    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/

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