One document matched: draft-reschke-rfc2183-in-http-03.txt
Differences from draft-reschke-rfc2183-in-http-02.txt
Network Working Group J. Reschke
Internet-Draft greenbytes
Updates: 2616 (if approved) August 30, 2010
Intended status: Standards Track
Expires: March 3, 2011
Use of the Content-Disposition Header Field in the
Hypertext Transfer Protocol (HTTP)
draft-reschke-rfc2183-in-http-03
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>.
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-rfc2183-in-http>. A
collection of test cases is available at
<http://greenbytes.de/tech/tc2231/>.
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
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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 3, 2011.
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
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described in the Simplified BSD License.
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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 . . . . . . . . 10
Appendix B. Differences compared to RFC 2183 . . . . . . . . . . 10
Appendix C. Alternative Approaches to Internationalization . . . 10
C.1. RFC 2047 Encoding . . . . . . . . . . . . . . . . . . . . 11
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
Appendix E. Resolved issues (to be removed by RFC Editor
before publication) . . . . . . . . . . . . . . . . . 12
E.1. quoted . . . . . . . . . . . . . . . . . . . . . . . . . . 13
E.2. asciivsiso . . . . . . . . . . . . . . . . . . . . . . . . 13
E.3. deplboth . . . . . . . . . . . . . . . . . . . . . . . . . 13
E.4. registry . . . . . . . . . . . . . . . . . . . . . . . . . 13
E.5. docfallback . . . . . . . . . . . . . . . . . . . . . . . 14
Appendix F. Open issues (to be removed by RFC Editor prior to
publication) . . . . . . . . . . . . . . . . . . . . 14
F.1. edit . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
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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
implementors.
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 Section 5).
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.
URIs
[1] <mailto:ietf-http-wg@w3.org>
[2] <mailto:ietf-http-wg-request@w3.org?subject=subscribe>
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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.
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.
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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.
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.
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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...]]
+---------------+------------+--------+--------------+--------------+
| 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".
Appendix E. Resolved issues (to be removed by RFC Editor before
publication)
Issues that were either rejected or resolved in this version of this
document.
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E.1. quoted
In Section 3.1:
Type: change
julian.reschke@greenbytes.de (2010-08-23): Can value be quoted-pair
as well? It is "value" only in RFC 2183, but "quoted-string" only in
2616. UAs seem to handle quoted-strings, although some have trouble
unescaping backslashes.
julian.reschke@greenbytes.de (2010-08-24): Actually, "value" is
"token" or "quoted-string", both in RFC 2616 and RFC 2183 (by
reference to RFC 2045). The only problem is that RFC 2616 uses
quoted-string instead of value in the definition for the filename
parameter. This is a bug in 2616.
Resolution (2010-08-24): Note the change in "Changes from the RFC
2616 Definition".
E.2. asciivsiso
In Section 3.3:
Type: change
julian.reschke@greenbytes.de (2010-08-24): We should be consistent
about what RFC 2616 defaults to (ASCII vs ISO-8859-1).
Resolution (2010-08-30): Say "ISO-8859-1", and also make the ISO-
8859-1 ref normative.
E.3. deplboth
In Section 4:
Type: change
julian.reschke@greenbytes.de (2010-08-24): Add an example that uses
both "filename" and "filename*" and mention current UA behavior.
Resolution (2010-08-27): Add the example, and mention the issues with
it.
E.4. registry
In Section 7.1:
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Type: change
julian.reschke@greenbytes.de (2010-08-23): The registry technically
is for the MIME header, but has been used for C-D in other protocols
already. What's missing are instructions that new registrations
should state which protocol they're for. Do we want to attempt to
modify the registry?
Resolution (2010-08-30): Add a section about extensibility explaining
the existing registries.
E.5. docfallback
In Section C.4:
Type: edit
julian.reschke@greenbytes.de (2010-08-30): Describe the
implementation quality of the fallback behavior.
Resolution (2010-08-30): Done.
Appendix F. Open issues (to be removed by RFC Editor prior to
publication)
F.1. edit
Type: edit
julian.reschke@greenbytes.de (2009-10-16): Umbrella issue for
editorial fixes/enhancements.
Index
C
Content-Disposition header 4
H
Headers
Content-Disposition 4
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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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