source: draft-ietf-httpbis-content-disp/05/draft-ietf-httpbis-content-disp-05.txt @ 2044

HTTPbis Working Group                                         J. Reschke
Internet-Draft                                                greenbytes
Updates: 2616 (if approved)                            February 17, 2011
Intended status: Standards Track
Expires: August 21, 2011


           Use of the Content-Disposition Header Field in the
                   Hypertext Transfer Protocol (HTTP)
                   draft-ietf-httpbis-content-disp-05

Abstract

   HTTP/1.1 defines the Content-Disposition response header field, but
   points out that it is not part of the HTTP/1.1 Standard.  This
   specification takes over the definition and registration of Content-
   Disposition, as used in HTTP, and clarifies internationalization
   aspects.

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

   This specification is expected to replace the definition of Content-
   Disposition in the HTTP/1.1 specification, as currently revised by
   the IETF HTTPbis working group.  See also
   <http://trac.tools.ietf.org/wg/httpbis/trac/ticket/123>.

   Discussion of this draft should take place on the HTTPBIS working
   group mailing list (ietf-http-wg@w3.org).  The current issues list is
   at <http://trac.tools.ietf.org/wg/httpbis/trac/
   query?component=content-disp> and related documents (including fancy
   diffs) can be found at <http://tools.ietf.org/wg/httpbis/>.

   The changes in this draft are summarized in Appendix D.9.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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."



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   This Internet-Draft will expire on August 21, 2011.

Copyright Notice

   Copyright (c) 2011 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
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   publication of this document.  Please review these documents
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.



































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Notational Conventions . . . . . . . . . . . . . . . . . . . .  4
   3.  Conformance and Error Handling . . . . . . . . . . . . . . . .  4
   4.  Header Field Definition  . . . . . . . . . . . . . . . . . . .  5
     4.1.  Grammar  . . . . . . . . . . . . . . . . . . . . . . . . .  5
     4.2.  Disposition Type . . . . . . . . . . . . . . . . . . . . .  6
     4.3.  Disposition Parameter: 'Filename'  . . . . . . . . . . . .  6
     4.4.  Disposition Parameter: Extensions  . . . . . . . . . . . .  7
     4.5.  Extensibility  . . . . . . . . . . . . . . . . . . . . . .  7
   5.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   6.  Internationalization Considerations  . . . . . . . . . . . . .  8
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
     8.1.  Registry for Disposition Values and Parameter  . . . . . .  9
     8.2.  Header Field Registration  . . . . . . . . . . . . . . . .  9
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  9
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     10.1. Normative References . . . . . . . . . . . . . . . . . . .  9
     10.2. Informative References . . . . . . . . . . . . . . . . . . 10
   Appendix A.  Changes from the RFC 2616 Definition  . . . . . . . . 10
   Appendix B.  Differences compared to RFC 2183  . . . . . . . . . . 11
   Appendix C.  Alternative Approaches to Internationalization  . . . 11
     C.1.  RFC 2047 Encoding  . . . . . . . . . . . . . . . . . . . . 11
     C.2.  Percent Encoding . . . . . . . . . . . . . . . . . . . . . 12
     C.3.  Encoding Sniffing  . . . . . . . . . . . . . . . . . . . . 12
     C.4.  Implementations (to be removed by RFC Editor before
           publication) . . . . . . . . . . . . . . . . . . . . . . . 12
   Appendix D.  Change Log (to be removed by RFC Editor before
                publication)  . . . . . . . . . . . . . . . . . . . . 13
     D.1.  Since draft-reschke-rfc2183-in-http-00 . . . . . . . . . . 13
     D.2.  Since draft-reschke-rfc2183-in-http-01 . . . . . . . . . . 13
     D.3.  Since draft-reschke-rfc2183-in-http-02 . . . . . . . . . . 13
     D.4.  Since draft-reschke-rfc2183-in-http-03 . . . . . . . . . . 13
     D.5.  Since draft-ietf-httpbis-content-disp-00 . . . . . . . . . 13
     D.6.  Since draft-ietf-httpbis-content-disp-01 . . . . . . . . . 13
     D.7.  Since draft-ietf-httpbis-content-disp-02 . . . . . . . . . 13
     D.8.  Since draft-ietf-httpbis-content-disp-03 . . . . . . . . . 14
     D.9.  Since draft-ietf-httpbis-content-disp-04 . . . . . . . . . 14
   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15










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1.  Introduction

   HTTP/1.1 defines the Content-Disposition response header field in
   Section 19.5.1 of [RFC2616], but points out that it is not part of
   the HTTP/1.1 Standard (Section 15.5):

      Content-Disposition is not part of the HTTP standard, but since it
      is widely implemented, we are documenting its use and risks for
      implementers.

   This specification takes over the definition and registration of
   Content-Disposition, as used in HTTP.  Based on interoperability
   testing with existing User Agents, it fully defines a profile of the
   features defined in the Multipurpose Internet Mail Extensions (MIME)
   variant ([RFC2183]) of the header field, and also clarifies
   internationalization aspects.

      Note: this document does not apply to Content-Disposition header
      fields appearing in 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 augmented BNF notation defined in Section
   2.1 of [RFC2616], including its rules for implied linear whitespace
   (LWS).

3.  Conformance and Error Handling

   This specification defines conformance criteria for both senders
   (usually, HTTP origin servers) and recipients (usually, HTTP user
   agents) of the Content-Location header field.  An implementation is
   considered conformant if it complies with all of the requirements
   associated with its role.

   This specification also defines certain forms of the header field-
   value to be invalid, using both ABNF and prose requirements, but it
   does not define special handling of these invalid field-values.

   Sending implementations MUST NOT generate Content-Location header
   fields that are invalid.

   Consuming implementations MAY take steps to recover a usable field-
   value from an invalid header field, but SHOULD NOT reject the message



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   outright, unless this is explicitly desirable behaviour (e.g., the
   implementation is a validator).  As such, the default handling of
   invalid fields is to ignore them.

4.  Header Field Definition

   The Content-Disposition response header field is used to convey
   additional information about how to process the response payload, and
   also can be used to attach additional metadata, such as the filename
   to use when saving the response payload locally.

4.1.  Grammar

     content-disposition = "Content-Disposition" ":"
                            disposition-type *( ";" disposition-parm )

     disposition-type    = "inline" | "attachment" | disp-ext-type
                         ; case-insensitive
     disp-ext-type       = token

     disposition-parm    = filename-parm | disp-ext-parm

     filename-parm       = "filename" "=" value
                         | "filename*" "=" ext-value

     disp-ext-parm       = token "=" value
                         | ext-token "=" ext-value
     ext-token           = <the characters in token, followed by "*">

   Defined in [RFC2616]:

     token         = <token, defined in [RFC2616], Section 2.2>
     quoted-string = <quoted-string, defined in [RFC2616], Section 2.2>
     value         = <value, defined in [RFC2616], Section 3.6>
                   ; token | quoted-string


   Defined in [RFC5987]:

     ext-value   = <ext-value, defined in [RFC5987], Section 3.2>

   Header field values with multiple instances of the same parameter
   name are invalid.

   Note that due to the rules for implied linear whitespace (Section 2.1
   of [RFC2616]), OPTIONAL whitespace can appear between words (token or
   quoted-string) and separator characters.




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   Furthermore note that the format used for ext-value allows specifying
   a natural language; this is of limited use for filenames and is
   likely to be ignored by recipients.

4.2.  Disposition Type

   If the disposition type matches "attachment" (case-insensitively),
   this indicates that the user agent should prompt the user to save the
   response locally, rather than process it normally (as per its media
   type).

   On the other hand, if it matches "inline" (case-insensitively), this
   implies default processing.

   Unknown or unhandled disposition types SHOULD be handled by
   recipients the same way as "attachment" (see also [RFC2183], Section
   2.8).

4.3.  Disposition Parameter: 'Filename'

   The parameters "filename" and "filename*", to be matched case-
   insensitively, provide information on how to construct a filename for
   storing the message payload.

   Depending on the disposition type, this information might be used
   right away (in the "save as..." interaction caused for the
   "attachment" disposition type), or later on (for instance, when the
   user decides to save the contents of the current page being
   displayed).

   The parameters "filename" and "filename*" differ only in that
   "filename*" uses the encoding defined in [RFC5987], allowing the use
   of characters not present in the ISO-8859-1 character set
   ([ISO-8859-1]).

   Many user agent implementations predating this specification do not
   understand the "filename*" parameter.  Therefore, when both
   "filename" and "filename*" are present in a single header field
   value, recipients SHOULD pick "filename*" and ignore "filename".
   This way, senders can avoid special-casing specific user agents by
   sending both the more expressive "filename*" parameter, and the
   "filename" parameter as fallback for legacy recipients (see Section 5
   for an example).

   It is essential that user agents treat the specified filename as
   advisory only, thus be very careful in extracting the desired
   information.  In particular:




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   o  When the value contains path separator characters ("\" or "/"),
      recipients SHOULD ignore all but the last path segment.  This
      prevents unintentional overwriting of well-known file system
      locations (such as "/etc/passwd").

   o  Many platforms do not use Internet Media Types ([RFC2046]) to hold
      type information in the file system, but rely on filename
      extensions instead.  Trusting the server-provided file extension
      could introduce a privilege escalation when the saved file is
      later opened (consider ".exe").  Thus, recipients need to ensure
      that a file extension is used that is safe, optimally matching the
      media type of the received payload.

   o  Recipients are advised to strip or replace character sequences
      that are known to cause confusion both in user interfaces and in
      filenames, such as control characters and leading and trailing
      whitespace.

   o  Other aspects recipients need to be aware of are names that have a
      special meaning in the file system or in shell commands, such as
      "." and "..", "~", "|", and also device names.

      Note: Many user agents do not properly handle escape characters
      when using the quoted-string form.  Furthermore, some user agents
      erroneously try to perform unescaping of "percent" escapes (see
      Appendix C.2), and thus might misinterpret filenames containing
      the percent character followed by two hex digits.

4.4.  Disposition Parameter: Extensions

   To enable future extensions, recipients SHOULD ignore unrecognized
   parameters (see also [RFC2183], Section 2.8).

4.5.  Extensibility

   Note that Section 9 of [RFC2183] defines IANA registries both for
   disposition types and disposition parameters.  This registry is
   shared by different protocols using Content-Disposition, such as MIME
   and HTTP.  Therefore, not all registered values may make sense in the
   context of HTTP.

5.  Examples

   Direct UA to show "save as" dialog, with a filename of
   "example.html":

   Content-Disposition: Attachment; filename=example.html




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   Direct UA to behave as if the Content-Disposition header field wasn't
   present, but to remember the filename "an example.html" for a
   subsequent save operation:

     Content-Disposition: INLINE; FILENAME= "an example.html"

   Note: this uses the quoted-string form so that the space character
   can be included.

   Direct UA to show "save as" dialog, with a filename containing the
   Unicode character U+20AC (EURO SIGN):

     Content-Disposition: attachment;
                          filename*= UTF-8''%e2%82%ac%20rates

   Here, the encoding defined in [RFC5987] is also used to encode the
   non-ISO-8859-1 character.

   Same as above, but adding the "filename" parameter for compatibility
   with user agents not implementing RFC 5987:

     Content-Disposition: attachment;
                          filename="EURO rates";
                          filename*=utf-8''%e2%82%ac%20rates

   Note: as of February 2011, those user agents that do not support the
   RFC 5987 encoding ignore "filename*" when it occurs after "filename".
   Unfortunately, some user agents that do support RFC 5987 do pick the
   "filename" rather than the "filename*" parameter when it occurs
   first; it is expected that this situation is going to improve soon.

6.  Internationalization Considerations

   The "filename*" parameter (Section 4.3), using the encoding defined
   in [RFC5987], allows the server to transmit characters outside the
   ISO-8859-1 character set, and also to optionally specify the language
   in use.

   Future parameters might also require internationalization, in which
   case the same encoding can be used.

7.  Security Considerations

   Using server-supplied information for constructing local filenames
   introduces many risks.  These are summarized in Section 4.3.

   Furthermore, implementers also ought to be aware of the Security
   Considerations applying to HTTP (see Section 15 of [RFC2616]), and



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   also the parameter encoding defined in [RFC5987] (see Section 5).

8.  IANA Considerations

8.1.  Registry for Disposition Values and Parameter

   This specification does not introduce any changes to the registration
   procedures for disposition values and parameters that are defined in
   Section 9 of [RFC2183].

8.2.  Header Field Registration

   This document updates the definition of the Content-Disposition HTTP
   header field in the permanent HTTP header field registry (see
   [RFC3864]).

   Header field name:  Content-Disposition

   Applicable protocol:  http

   Status:  standard

   Author/Change controller:  IETF

   Specification document:  this specification (Section 4)

9.  Acknowledgements

   Thanks to Adam Barth, Rolf Eike Beer, Bjoern Hoehrmann, Alfred
   Hoenes, Roar Lauritzsen, Henrik Nordstrom, and Mark Nottingham for
   their valuable feedback.

10.  References

10.1.  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.




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   [RFC5987]     Reschke, J., "Character Set and Language Encoding for
                 Hypertext Transfer Protocol (HTTP) Header Field
                 Parameters", RFC 5987, August 2010.

10.2.  Informative References

   [RFC2046]     Freed, N. and N. Borenstein, "Multipurpose Internet
                 Mail Extensions (MIME) Part Two: Media Types",
                 RFC 2046, November 1996.

   [RFC2047]     Moore, K., "MIME (Multipurpose Internet Mail
                 Extensions) Part Three: Message Header Extensions for
                 Non-ASCII Text", RFC 2047, November 1996.

   [RFC2183]     Troost, R., Dorner, S., and K. Moore, "Communicating
                 Presentation Information in Internet Messages: The
                 Content-Disposition Header Field", RFC 2183,
                 August 1997.

   [RFC2231]     Freed, N. and K. Moore, "MIME Parameter Value and
                 Encoded Word Extensions: Character Sets, Languages, and
                 Continuations", RFC 2231, November 1997.

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

   [RFC3864]     Klyne, G., Nottingham, M., and J. Mogul, "Registration
                 Procedures for Message Header Fields", BCP 90,
                 RFC 3864, September 2004.

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

Appendix A.  Changes from the RFC 2616 Definition

   Compared to Section 19.5.1 of [RFC2616], the following normative
   changes reflecting actual implementations have been made:

   o  According to RFC 2616, the disposition type "attachment" only
      applies to content of type "application/octet-stream".  This
      restriction has been removed, because user agents in practice do
      not check the content type, and it also discourages properly
      declaring the media type.

   o  RFC 2616 only allows "quoted-string" for the filename parameter.
      This would be an exceptional parameter syntax, and also doesn't
      reflect actual use.



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   o  The definition for the disposition type "inline" ([RFC2183],
      Section 2.1) has been re-added with a suggestion for its
      processing.

   o  This specification requires support for the extended parameter
      encoding defined in [RFC5987].

Appendix B.  Differences compared to RFC 2183

   Section 2 of [RFC2183] defines several additional disposition
   parameters: "creation-date", "modification-date", "quoted-date-time",
   and "size".  The majority of user agents does not implement these,
   thus they have been omitted from this specification.

Appendix C.  Alternative Approaches to Internationalization

   By default, HTTP header field parameters cannot carry characters
   outside the ISO-8859-1 ([ISO-8859-1]) character encoding (see
   [RFC2616], Section 2.2).  For the "filename" parameter, this of
   course is an unacceptable restriction.

   Unfortunately, user agent implementers have not managed to come up
   with an interoperable approach, although the IETF Standards Track
   specifies exactly one solution ([RFC2231], clarified and profiled for
   HTTP in [RFC5987]).

   For completeness, the sections below describe the various approaches
   that have been tried, and explains how they are inferior to the RFC
   5987 encoding used in this specification.

C.1.  RFC 2047 Encoding

   RFC 2047 defines an encoding mechanism for header fields, but this
   encoding is not supposed to be used for header field parameters - see
   Section 5 of [RFC2047]:

      An 'encoded-word' MUST NOT appear within a 'quoted-string'.

      ...

      An 'encoded-word' MUST NOT be used in parameter of a MIME Content-
      Type or Content-Disposition field, or in any structured field body
      except within a 'comment' or 'phrase'.

   In practice, some user agents implement the encoding, some do not
   (exposing the encoded string to the user), and some get confused by
   it.




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C.2.  Percent Encoding

   Some user agents accept percent encoded ([RFC3986], Section 2.1)
   sequences of characters.  The character encoding being used for
   decoding depends on various factors, including the encoding of the
   referring page, the user agent's locale, its configuration, and also
   the actual value of the parameter.

   In practice, this is hard to use because those user agents that do
   not support it will display the escaped character sequence to the
   user.  For those user agents that do implement this it is difficult
   to predict what character encoding they actually expect.

C.3.  Encoding Sniffing

   Some user agents inspect the value (which defaults to ISO-8859-1 for
   the quoted-string form) and switch to UTF-8 when it seems to be more
   likely to be the correct interpretation.

   As with the approaches above, this is not interoperable and
   furthermore risks misinterpreting the actual value.

C.4.  Implementations (to be removed by RFC Editor before publication)

   Unfortunately, as of February 2011, neither the encoding defined in
   RFCs 2231 and 5987, nor any of the alternate approaches discussed
   above was implemented interoperably.  Thus, this specification
   recommends the approach defined in RFC 5987, which at least has the
   advantage of actually being specified properly.

   The table below shows the implementation support for the various
   approaches:

   +---------------+------------+--------+--------------+--------------+
   | User Agent    | RFC        | RFC    | Percent      | Encoding     |
   |               | 2231/5987  | 2047   | Encoding     | Sniffing     |
   +---------------+------------+--------+--------------+--------------+
   | Chrome        | yes        | yes    | yes          | yes          |
   | Firefox       | yes (*)    | yes    | no           | yes          |
   | Internet      | yes (**)   | no     | yes          | no           |
   | Explorer      |            |        |              |              |
   | Konqueror     | yes        | no     | no           | no           |
   | Opera         | yes        | no     | no           | no           |
   | Safari        | no         | no     | no           | yes          |
   +---------------+------------+--------+--------------+--------------+

   (*) Does not implement the fallback behavior to "filename" described
   in Section 4.3.



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   (**) Starting with IE9RC, but only implements UTF-8.

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

   Note: the issues names in the change log entries for
   draft-reschke-rfc2183-in-http refer to <http://greenbytes.de/tech/
   webdav/draft-reschke-rfc2183-in-http-issues.html>.

D.1.  Since draft-reschke-rfc2183-in-http-00

   Adjust terminology ("header" -> "header field").  Update rfc2231-in-
   http reference.

D.2.  Since draft-reschke-rfc2183-in-http-01

   Update rfc2231-in-http reference.  Actually define the "filename"
   parameter.  Add internationalization considerations.  Add examples
   using the RFC 5987 encoding.  Add overview over other approaches,
   plus a table reporting implementation status.  Add and resolve issue
   "nodep2183".  Add issues "asciivsiso", "deplboth", "quoted", and
   "registry".

D.3.  Since draft-reschke-rfc2183-in-http-02

   Add and close issue "docfallback".  Close issues "asciivsiso",
   "deplboth", "quoted", and "registry".

D.4.  Since draft-reschke-rfc2183-in-http-03

   Updated to be a Working Draft of the IETF HTTPbis Working Group.

D.5.  Since draft-ietf-httpbis-content-disp-00

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/242>: "handling of
      unknown disposition types"

   Slightly updated the notes about the proposed fallback behavior.

D.6.  Since draft-ietf-httpbis-content-disp-01

   Various editorial improvements.

D.7.  Since draft-ietf-httpbis-content-disp-02

   Closed issues:




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   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/244>: "state that
      repeating parameters are invalid"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/245>: "warn about
      %xx in filenames being misinterpreted"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/246>: "mention
      control chars when talking about postprecessing the filename
      parameter"

   Update Appendix C.4; Opera 10.63 RC implements the recommended
   fallback behavior.

D.8.  Since draft-ietf-httpbis-content-disp-03

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/252>:
      "'modification-date' *is* implemented in Konq 4.5"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/253>: "clarify what
      LWS means for the Content-Disp grammar"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/258>: "Avoid passive
      voice in message requirements"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/263>: "text about
      historical percent-decoding unclear"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/264>: "add
      explanation of language tagging"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/265>: "Clarify that
      C-D spec does not apply to multipart upload"

D.9.  Since draft-ietf-httpbis-content-disp-04

   Updated implementation information (Chrome 9 implements RFC 5987, IE
   9 RC implements it for UTF-8 only).

   Clarify who requirements are on, add a section discussing conformance
   and handling of invalid field values in general.

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/272>: "Path
      Separator Characters"




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Internet-Draft         Content-Disposition in HTTP         February 2011


Index

   C
      Content-Disposition header  5

   H
      Headers
         Content-Disposition  5

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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