[Docs] [txt|pdf|xml|html] [Tracker] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 RFC 5987

Network Working Group                                         J. Reschke
Internet-Draft                                                greenbytes
Intended status: Standards Track                       December 27, 2009
Expires: June 30, 2010


                  Application of RFC 2231 Encoding to
            Hypertext Transfer Protocol (HTTP) Header Fields
                    draft-reschke-rfc2231-in-http-07

Abstract

   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.

Editorial Note (To be removed by RFC Editor before publication)

   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 [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
   <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/>.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering



Reschke                   Expires June 30, 2010                 [Page 1]

Internet-Draft          RFC2231 Encoding in HTTP           December 2009


   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 June 30, 2010.

Copyright Notice

   Copyright (c) 2009 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 BSD License.




















Reschke                   Expires June 30, 2010                 [Page 2]

Internet-Draft          RFC2231 Encoding in HTTP           December 2009


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Notational Conventions . . . . . . . . . . . . . . . . . . . .  4
   3.  A Profile of RFC 2231 for Use in HTTP  . . . . . . . . . . . .  4
     3.1.  Parameter Continuations  . . . . . . . . . . . . . . . . .  4
     3.2.  Parameter Value Character Set and Language Information . .  5
       3.2.1.  Examples . . . . . . . . . . . . . . . . . . . . . . .  7
     3.3.  Language specification in Encoded Words  . . . . . . . . .  8
   4.  Guidelines for Usage in HTTP Header Field Definitions  . . . .  8
     4.1.  When to Use the Extension  . . . . . . . . . . . . . . . .  8
     4.2.  Error Handling . . . . . . . . . . . . . . . . . . . . . .  9
     4.3.  Using Multiple Instances for Internationalization  . . . .  9
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 10
   Appendix A.  Document History and Future Plans (to be removed
                by RFC Editor before publication) . . . . . . . . . . 11
   Appendix B.  Change Log (to be removed by RFC Editor before
                publication)  . . . . . . . . . . . . . . . . . . . . 12
     B.1.  Since draft-reschke-rfc2231-in-http-00 . . . . . . . . . . 12
     B.2.  Since draft-reschke-rfc2231-in-http-01 . . . . . . . . . . 12
     B.3.  Since draft-reschke-rfc2231-in-http-02 . . . . . . . . . . 12
     B.4.  Since draft-reschke-rfc2231-in-http-03 . . . . . . . . . . 12
     B.5.  Since draft-reschke-rfc2231-in-http-04 . . . . . . . . . . 12
     B.6.  Since draft-reschke-rfc2231-in-http-05 . . . . . . . . . . 12
     B.7.  Since draft-reschke-rfc2231-in-http-06 . . . . . . . . . . 12
   Appendix C.  Open issues (to be removed by RFC Editor prior to
                publication)  . . . . . . . . . . . . . . . . . . . . 13
     C.1.  edit . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13

















Reschke                   Expires June 30, 2010                 [Page 3]

Internet-Draft          RFC2231 Encoding in HTTP           December 2009


1.  Introduction

   By default, message header field 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 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 white space).

   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).

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).




Reschke                   Expires June 30, 2010                 [Page 4]

Internet-Draft          RFC2231 Encoding in HTTP           December 2009


   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 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 leaving out the character set
   information.  The profile defined by this specification does not
   allow that.

   The syntax for parameters is defined in Section 3.6 of [RFC2616]
   (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 [RFC2616], Section 2.2>
     token         = <token, defined in [RFC2616], Section 2.2>

   This specification extends the grammar to:





















Reschke                   Expires June 30, 2010                 [Page 5]

Internet-Draft          RFC2231 Encoding in HTTP           December 2009


     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 [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

     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
                   / "-" / "." / "_" / "~" / ":"
                   / "!" / "$" / "&" / "+"

   Thus, a parameter is either 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



Reschke                   Expires June 30, 2010                 [Page 6]

Internet-Draft          RFC2231 Encoding in HTTP           December 2009


   encoded into an octet sequence using the specified character set.
   That octet sequence then is percent-encoded as specified in Section
   2.1 of [RFC3986].

   Producers MUST NOT use character sets other than "UTF-8" ([RFC3629])
   or "ISO-8859-1" ([ISO-8859-1]).  Extension character sets (ext-
   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 <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 Editor Errata ID 1912 [3]).  In
      practice, no character set names using that character have been
      registered at the time of this writing.

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 it is not contained
   in attr-char.





Reschke                   Expires June 30, 2010                 [Page 7]

Internet-Draft          RFC2231 Encoding in HTTP           December 2009


   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 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, the RFC 2231 profile defined by this specification does not
   include this feature.


4.  Guidelines for Usage in HTTP Header Field Definitions

   Specifications of HTTP header fields that use the extensions defined
   in Section 3.2 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 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, Section 3.2>

   [[rfcno: Note to RFC Editor: in the figure above, please replace
   "xxxx" by the RFC number assigned to this specification.]]

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-value production
   should always be used when the parameter value is of textual nature.



Reschke                   Expires June 30, 2010                 [Page 8]

Internet-Draft          RFC2231 Encoding in HTTP           December 2009


   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 parameter that would be restricted to a subset of the
   Unicode character set).

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
   producers 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 recipients, but also includes an internationalized version for
   recipients understanding this specification -- the latter obviously
   should 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.

4.3.  Using Multiple Instances for Internationalization

   It is expected that in many cases, internationalization of parameters
   in response headers is implemented using server driven content
   negotiation ([RFC2616], Section 12.1) using the Accept-Language
   header ([RFC2616], Section 14.4).  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'en'Document%20Title;
               title*=utf-8'de'Titel%20des%20Dokuments






Reschke                   Expires June 30, 2010                 [Page 9]

Internet-Draft          RFC2231 Encoding in HTTP           December 2009


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 Martin Duerst and Frank Ellermann for help figuring out
   ABNF details, and to Roar Lauritzsen for implementer's feedback.


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
              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.

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.



Reschke                   Expires June 30, 2010                [Page 10]

Internet-Draft          RFC2231 Encoding in HTTP           December 2009


   [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.

URIs

   [1]  <mailto:ietf-http-wg@w3.org>

   [2]  <mailto:ietf-http-wg-request@w3.org?subject=subscribe>

   [3]  <http://www.rfc-editor.org/errata_search.php?eid=1912>


Appendix A.  Document History and Future Plans (to be removed by RFC
             Editor before publication)

   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



Reschke                   Expires June 30, 2010                [Page 11]

Internet-Draft          RFC2231 Encoding in HTTP           December 2009


   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.


Appendix B.  Change Log (to be removed by RFC Editor before publication)

B.1.  Since draft-reschke-rfc2231-in-http-00

   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.

B.2.  Since draft-reschke-rfc2231-in-http-01

   Remove unneeded RFC5137 notation (code point vs character).

B.3.  Since draft-reschke-rfc2231-in-http-02

   And and resolve issues "charset", "repeats" and "rfc4646".

B.4.  Since draft-reschke-rfc2231-in-http-03

   And and resolve issue "charsetmatch".

B.5.  Since draft-reschke-rfc2231-in-http-04

   Add and resolve issues "badseq" and "tokenquotcharset".

B.6.  Since draft-reschke-rfc2231-in-http-05

   Say "header field" instead of "header" in the context of HTTP.

B.7.  Since draft-reschke-rfc2231-in-http-06

   Add an appendix discussing document history and future plans, to be
   removed before publication.



Reschke                   Expires June 30, 2010                [Page 12]

Internet-Draft          RFC2231 Encoding in HTTP           December 2009


Appendix C.  Open issues (to be removed by RFC Editor prior to
             publication)

C.1.  edit

   Type: edit

   julian.reschke@greenbytes.de (2009-04-17): Umbrella issue for
   editorial fixes/enhancements.


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 June 30, 2010                [Page 13]


Html markup produced by rfcmarkup 1.108, available from http://tools.ietf.org/tools/rfcmarkup/