source: draft-ietf-httpbis-content-disp/00/draft-ietf-httpbis-content-disp.txt @ 2200

HTTPbis Working Group                                         J. Reschke
Internet-Draft                                                greenbytes
Updates: 2616 (if approved)                            September 3, 2010
Intended status: Standards Track
Expires: March 7, 2011


           Use of the Content-Disposition Header Field in the
                   Hypertext Transfer Protocol (HTTP)
                   draft-ietf-httpbis-content-disp-00

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://www3.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/>.

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."

   This Internet-Draft will expire on March 7, 2011.



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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.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Notational Conventions . . . . . . . . . . . . . . . . . . . .  4
   3.  Header Field Definition  . . . . . . . . . . . . . . . . . . .  4
     3.1.  Grammar  . . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.2.  Disposition Type . . . . . . . . . . . . . . . . . . . . .  5
     3.3.  Disposition Parameter: 'Filename'  . . . . . . . . . . . .  5
     3.4.  Disposition Parameter: Extensions  . . . . . . . . . . . .  6
     3.5.  Extensibility  . . . . . . . . . . . . . . . . . . . . . .  6
   4.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   5.  Internationalization Considerations  . . . . . . . . . . . . .  7
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
     7.1.  Registry for Disposition Values and Parameter  . . . . . .  8
     7.2.  Header Field Registration  . . . . . . . . . . . . . . . .  8
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  8
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     9.1.  Normative References . . . . . . . . . . . . . . . . . . .  8
     9.2.  Informative References . . . . . . . . . . . . . . . . . .  9
   Appendix A.  Changes from the RFC 2616 Definition  . . . . . . . .  9
   Appendix B.  Differences compared to RFC 2183  . . . . . . . . . . 10
   Appendix C.  Alternative Approaches to Internationalization  . . . 10
     C.1.  RFC 2047 Encoding  . . . . . . . . . . . . . . . . . . . . 10
     C.2.  Percent Encoding . . . . . . . . . . . . . . . . . . . . . 11
     C.3.  Encoding Sniffing  . . . . . . . . . . . . . . . . . . . . 11
     C.4.  Implementations  . . . . . . . . . . . . . . . . . . . . . 11
   Appendix D.  Change Log (to be removed by RFC Editor before
                publication)  . . . . . . . . . . . . . . . . . . . . 12
     D.1.  Since draft-reschke-rfc2183-in-http-00 . . . . . . . . . . 12
     D.2.  Since draft-reschke-rfc2183-in-http-01 . . . . . . . . . . 12
     D.3.  Since draft-reschke-rfc2183-in-http-02 . . . . . . . . . . 12
     D.4.  Since draft-reschke-rfc2183-in-http-03 . . . . . . . . . . 12



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   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12


















































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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.

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).

3.  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.




















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3.1.  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>
     value       = <value, defined in [RFC2616], Section 3.6>

   Defined in [RFC5987]:

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

3.2.  Disposition Type

   If the disposition type matches "attachment" (case-insensitively),
   this indicates that the user agent should not display the response,
   but directly enter a "save as..." dialog.

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

   Other disposition types SHOULD be handled the same way as
   "attachment" (see also [RFC2183], Section 2.8).

3.3.  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



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   user decides to save the contents of the current page being
   displayed).

   "filename" and "filename*" behave the same, except 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]).  When both "filename" and "filename*" are present, a
   recipient SHOULD pick "filename*" and ignore "filename" - this will
   make it possible to send the same header value to clients that do not
   support "filename*".

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

   o  When the value contains path separator characters, all but the
      last segment SHOULD be ignored.  This prevents unintentional
      overwriting of well-known file system location (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 later on the file is
      opened locally (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  Other aspects recipients need to be aware of are names that have a
      special meaning in the filesystem or in shell commands, such as
      "." and "..", "~", "|", and also device names.

3.4.  Disposition Parameter: Extensions

   To enable future extensions, unknown parameters SHOULD be ignored
   (see also [RFC2183], Section 2.8).

3.5.  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.







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4.  Examples

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

   Content-Disposition: Attachment; filename=foo.html

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

   Content-Disposition: INLINE; FILENAME= "foo.html"

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

   Content-Disposition: Attachment; Filename*=UTF-8'en'an%20example

   Note that this example uses the extended encoding defined in
   [RFC5987] to specify that the natural language of the filename is
   English, and also to encode the space character which is not allowed
   in the token production.

   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 August 2010, many user agents unfortunately did not
   properly handle unexpected parameters, and some that implement RFC
   5987 did not pick the extended parameter when both were present.

5.  Internationalization Considerations

   The "filename*" parameter (Section 3.3), 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.



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6.  Security Considerations

   Using server-supplied information for constructing local filenames
   introduces many risks.  These are summarized in Section 3.3.

   Furthermore, implementers also ought to be aware of the Security
   Considerations applying to HTTP (see Section 15 of [RFC2616]), and
   also the parameter encoding defined in [RFC5987] (see Appendix ).

7.  IANA Considerations

7.1.  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].

7.2.  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 3)

8.  Acknowledgements

   Thanks to Rolf Eike Beer, Alfred Hoenes, and Roar Lauritzsen for
   their valuable feedback.

9.  References

9.1.  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



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                 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.

   [RFC5987]     Reschke, J., "Applicability of RFC 2231 Encoding to
                 Hypertext Transfer Protocol (HTTP) Headers", RFC 5987,
                 August 2010.

9.2.  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.

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

   [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",
                 RFC 3986, STD 66, 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



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      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.

   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".  These do not appear to be implemented by any user agent,
   thus 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'.

      ...





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      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.

C.2.  Percent Encoding

   Some user agents accept percent encoded ([RFC3986], Section 2.1)
   sequences of characters encoded using the UTF-8 ([RFC3629]) character
   encoding.

   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.

   Furthermore, the first user agent to implement this did choose the
   encoding based on local settings; thus making it very hard to use in
   multi-lingual environments.

C.3.  Encoding Sniffing

   Some user agents inspect the value (which defaults to ISO-8859-1) 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

   Unfortunately, as of August 2010, neither the encoding defined in
   RFCs 2231 and 5789, 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: [[impls: Discuss: should we mention the implementation
   status of actual UAs in a RFC?  Up to the IESG to decide...]]









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   +---------------+------------+--------+--------------+--------------+
   | User Agent    | RFC        | RFC    | Percent      | Encoding     |
   |               | 2231/5987  | 2047   | Encoding     | Sniffing     |
   +---------------+------------+--------+--------------+--------------+
   | Chrome        | no         | yes    | yes          | yes          |
   | Firefox       | yes (*)    | yes    | no           | yes          |
   | Internet      | no         | 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 3.3.

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

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.

Index

   C
      Content-Disposition header  4

   H
      Headers



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         Content-Disposition  4

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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