source: draft-ietf-httpbis-content-disp/03/draft-ietf-httpbis-content-disp-03.xml @ 1338

Last change on this file since 1338 was 1056, checked in by julian.reschke@…, 10 years ago

Prepare publication of draft-ietf-httpbis-content-disp-03 on 2010-10-25.

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1<?xml version="1.0" encoding="UTF-8"?>
2<!--
3    This XML document is the output of clean-for-DTD.xslt; a tool that strips
4    extensions to RFC2629(bis) from documents for processing with xml2rfc.
5-->
6<?xml-stylesheet type='text/xsl' href='../../draft-ietf-httpbis/myxml2rfc.xslt' ?>
7<?rfc toc="yes"?>
8<?rfc symrefs="yes"?>
9<?rfc sortrefs="yes"?>
10<?rfc compact="yes"?>
11<?rfc comments="yes"?>
12<?rfc inline="yes"?>
13<?rfc subcompact="no"?>
14<?rfc rfcedstyle="yes"?>
15<!DOCTYPE rfc
16  PUBLIC "" "rfc2629.dtd">
17<rfc ipr="trust200902" docName="draft-ietf-httpbis-content-disp-03" category="std" updates="2616">
18        <front>
19  <title abbrev="Content-Disposition in HTTP">Use of the Content-Disposition Header Field
20  in the Hypertext Transfer Protocol (HTTP)</title>
21  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke">
22    <organization abbrev="greenbytes">greenbytes GmbH</organization>
23    <address>
24      <postal>
25        <street>Hafenweg 16</street>
26        <city>Muenster</city><region>NW</region><code>48155</code>
27        <country>Germany</country>
28      </postal>
29      <email>julian.reschke@greenbytes.de</email>       
30      <uri>http://greenbytes.de/tech/webdav/</uri>     
31    </address>
32  </author>
33
34  <date month="October" year="2010" day="25"/>
35  <workgroup>HTTPbis Working Group</workgroup>
36 
37  <abstract>
38    <t>
39      HTTP/1.1 defines the Content-Disposition response header field,
40      but points out that it is not part of the HTTP/1.1 Standard.
41      This specification takes over the definition and registration of
42      Content-Disposition, as used in HTTP, and clarifies internationalization
43      aspects.
44    </t>
45  </abstract>
46 
47  <note title="Editorial Note (To be removed by RFC Editor before publication)">
48    <t>
49      This specification is expected to replace the definition of Content-Disposition
50      in the HTTP/1.1 specification, as currently revised by the IETF HTTPbis
51      working group. See also <eref target="http://trac.tools.ietf.org/wg/httpbis/trac/ticket/123"/>.
52    </t>
53    <t>
54      Discussion of this draft should take place on the HTTPBIS working group
55      mailing list (ietf-http-wg@w3.org). The current issues list is
56      at <eref target="http://trac.tools.ietf.org/wg/httpbis/trac/query?component=content-disp"/>
57      and related documents (including fancy diffs) can be found at
58      <eref target="http://tools.ietf.org/wg/httpbis/"/>.
59    </t>
60    <t>
61      The changes in this draft are summarized in <xref target="changes.since.02"/>.
62    </t>
63  </note>
64  </front>
65
66  <middle>
67
68<section title="Introduction" anchor="introduction">
69<t>
70  HTTP/1.1 defines the Content-Disposition response header field in Section 19.5.1 of <xref target="RFC2616"/>,
71  but points out that it is not part of the HTTP/1.1 Standard (Section 15.5):
72</t>
73<t><list>
74  <t>
75    Content-Disposition is not part of the HTTP standard, but since it is
76    widely implemented, we are documenting its use and risks for implementers.
77  </t>
78</list></t>
79<t>
80  This specification takes over the definition and registration of
81  Content-Disposition, as used in HTTP.
82  Based on interoperability testing with existing User Agents,
83  it fully defines a profile of the
84  features defined in the Multipurpose Internet Mail Extensions (MIME) variant (<xref target="RFC2183"/>) of the
85  header field, and also clarifies internationalization
86  aspects.
87</t>
88</section> 
89
90<section title="Notational Conventions">
91<t>
92  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
93  "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document
94  are to be interpreted as described in <xref target="RFC2119"/>.
95</t>
96<t>
97  This specification uses the augmented BNF notation defined in
98  Section 2.1 of <xref target="RFC2616"/>, including its rules for
99  linear whitespace (LWS).
100</t>
101</section> 
102
103<section title="Header Field Definition" anchor="header.field.definition">
104  <iref item="Headers" subitem="Content-Disposition" primary="true"/>
105  <iref item="Content-Disposition header" primary="true"/>
106<t>
107  The Content-Disposition response header field is used to convey additional
108  information about how to process the response payload, and also can be used
109  to attach additional metadata, such as the filename to use when saving the
110  response payload locally.
111</t>
112
113<section title="Grammar">
114<figure><artwork type="abnf2616"><![CDATA[
115  content-disposition = "Content-Disposition" ":"
116                         disposition-type *( ";" disposition-parm )
117
118  disposition-type    = "inline" | "attachment" | disp-ext-type
119                      ; case-insensitive
120  disp-ext-type       = token
121
122  disposition-parm    = filename-parm | disp-ext-parm
123
124  filename-parm       = "filename" "=" value
125                      | "filename*" "=" ext-value
126 
127  disp-ext-parm       = token "=" value
128                      | ext-token "=" ext-value
129  ext-token           = <the characters in token, followed by "*">
130]]></artwork></figure>
131
132<figure><preamble>Defined in <xref target="RFC2616"/>:</preamble><artwork type="abnf2616"><![CDATA[
133  token       = <token, defined in [RFC2616], Section 2.2>
134  value       = <value, defined in [RFC2616], Section 3.6>
135]]></artwork></figure>
136<figure><preamble>Defined in <xref target="RFC5987"/>:</preamble><artwork type="abnf2616"><![CDATA[
137  ext-value   = <ext-value, defined in [RFC5987], Section 3.2>
138]]></artwork></figure>
139
140<t>
141  Parameter names MUST NOT be repeated; a header field value with
142  multiple instances of the same parameter SHOULD be treated as invalid.
143</t>
144
145</section>
146
147<section title="Disposition Type" anchor="disposition.type">
148<t>
149  If the disposition type matches "attachment" (case-insensitively), this
150  indicates that the user agent should prompt the user to save the response
151  locally, rather than process it normally (as per its media type).
152</t>
153<t>
154  On the other hand, if it matches "inline" (case-insensitively), this implies
155  default processing.
156</t>
157<t>
158  Unknown or unhandled disposition types SHOULD be handled the same way as
159  "attachment" (see also <xref target="RFC2183"/>, Section 2.8).
160</t>
161</section>
162
163<section title="Disposition Parameter: 'Filename'" anchor="disposition.parameter.filename">
164<t>
165  The parameters "filename" and "filename*", to be matched case-insensitively,
166  provide information on how to construct a filename for storing the message
167  payload.
168</t>
169<t>
170  Depending on the disposition type, this information might be used right away
171  (in the "save as..." interaction caused for the "attachment" disposition type),
172  or later on (for instance, when the user decides to save the contents of the
173  current page being displayed).
174</t>
175<t>
176  The parameters "filename" and "filename*" differ only in that "filename*" uses
177  the encoding defined in <xref target="RFC5987"/>, allowing the use
178  of characters not present in the ISO-8859-1 character set (<xref target="ISO-8859-1"/>).
179</t>
180<t>
181  Many user agent implementations predating this specification
182  do not understand the "filename*" parameter. Therefore, when both "filename"
183  and "filename*" are present in a single header field value, recipients
184  SHOULD pick "filename*" and ignore "filename". This way, senders
185  can avoid special-casing specific user agents by sending both the
186  more expressive "filename*" parameter, and the "filename" parameter
187  as fallback for legacy recipients (see <xref target="examples"/> for
188  an example).
189</t>
190<t>
191  It is essential that user agents treat the specified filename as advisory
192  only, thus be very careful in extracting the desired information.
193  In particular:
194  <list style="symbols">
195    <t>
196      When the value contains path separator characters, all but the last
197      segment SHOULD be ignored. This prevents unintentional overwriting
198      of well-known file system location (such as "/etc/passwd").
199    </t>
200    <t>
201      Many platforms do not use Internet Media Types (<xref target="RFC2046"/>)
202      to hold type information in the file system, but rely on filename
203      extensions instead. Trusting the server-provided file extension could
204      introduce a privilege escalation when the saved file is later opened
205      (consider ".exe"). Thus, recipients need to ensure that a file extension
206      is used that is safe, optimally matching the media type of the received
207      payload.
208    </t>
209    <t>
210      Recipients are advised to strip or replace character sequences that are
211      known to cause confusion both in user interfaces and in filenames, such as
212      control characters and leading and trailing whitespace.
213    </t>
214    <t>
215      Other aspects recipients need to be aware of are names that have a
216      special meaning in the file system or in shell commands, such as "." and "..",
217      "~", "|", and also device names.
218    </t>
219  </list>
220</t>
221<t><list>
222  <t>
223    Note: Many user agents do not properly handle escape characters
224    when using the quoted-string form. Furthermore, some user agents
225    erroneously try to perform unescaping of "percent" escapes (see
226    <xref target="alternatives.percent"/>), and thus might misinterpret
227    filenames containing the percent character followed by two hex digits.
228  </t>
229</list></t>
230</section>
231
232<section title="Disposition Parameter: Extensions" anchor="disposition.parameter.extensions">
233<t>
234  To enable future extensions, unknown parameters SHOULD be ignored (see also <xref target="RFC2183"/>, Section 2.8).
235</t>
236</section>
237
238<section title="Extensibility" anchor="extensibility">
239<t>
240  Note that Section 9 of <xref target="RFC2183"/> defines IANA registries both
241  for disposition types and disposition parameters. This registry is
242  shared by different protocols using Content-Disposition, such as MIME and HTTP.
243  Therefore, not all registered values may make sense in the context of HTTP.
244</t>
245</section>
246
247</section> 
248
249<section title="Examples" anchor="examples">
250
251<figure>
252<preamble>
253Direct UA to show "save as" dialog, with a filename of "example.html": 
254</preamble>
255<artwork type="example"><![CDATA[
256Content-Disposition: Attachment; filename=example.html
257]]></artwork></figure>
258<figure>
259<preamble>
260Direct UA to behave as if the Content-Disposition header field wasn't present,
261but to remember the filename "example.html" for a subsequent save operation:
262</preamble>
263<artwork type="example"><![CDATA[
264  Content-Disposition: INLINE; FILENAME= "example.html"
265  ]]></artwork></figure>
266<figure>
267<preamble>
268Direct UA to show "save as" dialog, with a filename of "an example":
269</preamble>
270<artwork type="example"><![CDATA[
271Content-Disposition: Attachment; Filename*=UTF-8'en'an%20example
272]]></artwork>
273<postamble>Note that this example uses the extended encoding defined in
274<xref target="RFC5987"/> to specify that the natural language of the filename
275is English, and also to encode the space character which is not allowed in the
276token production.
277</postamble>
278</figure>
279<figure>
280<preamble>
281Direct UA to show "save as" dialog, with a filename containing the Unicode character  U+20AC (EURO SIGN):
282</preamble>
283<artwork type="example"><![CDATA[
284  Content-Disposition: attachment;
285                       filename*= UTF-8''%e2%82%ac%20rates
286  ]]></artwork>
287<postamble>
288  Here, the encoding defined in <xref target="RFC5987"/> is also used to encode the
289  non-ISO-8859-1 character.
290</postamble>
291</figure>
292<figure>
293<preamble>
294Same as above, but adding the "filename" parameter for compatibility with
295user agents not implementing RFC 5987:
296</preamble>
297<artwork type="example"><![CDATA[
298  Content-Disposition: attachment;
299                       filename="EURO rates";
300                       filename*=utf-8''%e2%82%ac%20rates
301  ]]></artwork>
302<postamble>
303  Note: as of October 2010, those user agents that do not support the RFC 5987
304  encoding ignore "filename*" when it occurs after "filename". Unfortunately,
305  some user agents that do support RFC 5987 do pick the "filename" rather
306  than the "filename*" parameter when it occurs first; it is expected that
307  this situation is going to improve soon.
308</postamble>
309</figure>
310
311</section>
312
313<section title="Internationalization Considerations" anchor="i18n">
314<t>
315  The "filename*" parameter (<xref target="disposition.parameter.filename"/>),
316  using the encoding defined in <xref target="RFC5987"/>, allows the
317  server to transmit characters outside the ISO-8859-1 character set,
318  and also to optionally specify the language in use.
319</t>
320<t>
321  Future parameters might also require internationalization, in which case
322  the same encoding can be used.
323</t>
324</section>
325
326<section title="Security Considerations" anchor="security.considerations">
327<t>
328  Using server-supplied information for constructing local filenames introduces
329  many risks. These are summarized in <xref target="disposition.parameter.filename"/>.
330</t>
331<t>
332  Furthermore, implementers also ought to be aware of the Security
333  Considerations applying to HTTP (see Section 15 of <xref target="RFC2616"/>), and also the parameter encoding defined in <xref target="RFC5987"/>
334  (see Section 5).
335</t>
336</section> 
337
338<section title="IANA Considerations" anchor="iana.considerations">
339
340<section title="Registry for Disposition Values and Parameter" anchor="registry">
341<t>
342  This specification does not introduce any changes to the registration
343  procedures for disposition values and parameters that are defined in
344  Section 9 of <xref target="RFC2183"/>.
345</t>
346</section>
347
348<section title="Header Field Registration" anchor="header.field.registration"> 
349<t>
350  This document updates the definition of the Content-Disposition HTTP header field
351  in the permanent HTTP header field registry (see <xref target="RFC3864"/>).
352</t>
353<t>
354<list style="hanging">
355  <t hangText="Header field name:">Content-Disposition</t>
356  <t hangText="Applicable protocol:">http</t>
357  <t hangText="Status:">standard</t>
358  <t hangText="Author/Change controller:">IETF</t>
359  <t hangText="Specification document:">this specification (<xref target="header.field.definition"/>)</t>
360</list>
361</t>
362</section>
363
364</section> 
365
366<section title="Acknowledgements">
367<t>
368  Thanks to Adam Barth, Rolf Eike Beer, Bjoern Hoehrmann, Alfred Hoenes, Roar Lauritzsen,
369  Henrik Nordstrom, and Mark Nottingham for their valuable feedback.
370</t>
371</section> 
372
373  </middle>
374  <back>
375 
376<references title="Normative References">
377 
378  <reference anchor="RFC2119">
379    <front>
380      <title abbrev="RFC Key Words">Key words for use in RFCs to Indicate Requirement Levels</title>
381      <author initials="S." surname="Bradner" fullname="Scott Bradner">
382        <organization>Harvard University</organization>
383        <address><email>sob@harvard.edu</email></address>
384      </author>
385      <date month="March" year="1997"/>
386      <area>General</area>
387      <keyword>keyword</keyword>
388    </front>
389    <seriesInfo name="BCP" value="14"/>
390    <seriesInfo name="RFC" value="2119"/>
391  </reference>
392
393  <reference anchor="RFC2616">
394    <front>
395      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
396      <author initials="R." surname="Fielding" fullname="R. Fielding">
397        <organization>University of California, Irvine</organization>
398        <address><email>fielding@ics.uci.edu</email></address>
399      </author>
400      <author initials="J." surname="Gettys" fullname="J. Gettys">
401        <organization>W3C</organization>
402        <address><email>jg@w3.org</email></address>
403      </author>
404      <author initials="J." surname="Mogul" fullname="J. Mogul">
405        <organization>Compaq Computer Corporation</organization>
406        <address><email>mogul@wrl.dec.com</email></address>
407      </author>
408      <author initials="H." surname="Frystyk" fullname="H. Frystyk">
409        <organization>MIT Laboratory for Computer Science</organization>
410        <address><email>frystyk@w3.org</email></address>
411      </author>
412      <author initials="L." surname="Masinter" fullname="L. Masinter">
413        <organization>Xerox Corporation</organization>
414        <address><email>masinter@parc.xerox.com</email></address>
415      </author>
416      <author initials="P." surname="Leach" fullname="P. Leach">
417        <organization>Microsoft Corporation</organization>
418        <address><email>paulle@microsoft.com</email></address>
419      </author>
420      <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
421        <organization>W3C</organization>
422        <address><email>timbl@w3.org</email></address>
423      </author>
424      <date month="June" year="1999"/>
425    </front>
426    <seriesInfo name="RFC" value="2616"/>
427  </reference>
428
429  <reference anchor="RFC5987">
430        <front>
431      <title>Character Set and Language Encoding for Hypertext Transfer Protocol (HTTP) Header Field Parameters</title>
432      <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke">
433        <organization abbrev="greenbytes">greenbytes GmbH</organization>
434        <address>
435          <postal>
436            <street>Hafenweg 16</street>
437            <city>Muenster</city><region>NW</region><code>48155</code>
438            <country>Germany</country>
439          </postal>
440          <email>julian.reschke@greenbytes.de</email>   
441          <uri>http://greenbytes.de/tech/webdav/</uri> 
442        </address>
443      </author>
444      <date month="August" year="2010"/>
445    </front>
446    <seriesInfo name="RFC" value="5987"/>
447  </reference>
448
449  <reference anchor="ISO-8859-1">
450    <front>
451      <title>Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1</title>
452      <author>
453        <organization>International Organization for Standardization</organization>
454      </author>
455      <date year="1998"/>
456    </front>
457    <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
458  </reference>
459
460</references>
461 
462<references title="Informative References">
463
464  <reference anchor="RFC2046">
465    <front>
466      <title abbrev="Media Types">Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types</title>
467      <author initials="N." surname="Freed" fullname="Ned Freed">
468        <organization>Innosoft International, Inc.</organization>
469        <address><email>ned@innosoft.com</email></address>
470      </author>
471      <author initials="N." surname="Borenstein" fullname="Nathaniel S. Borenstein">
472        <organization>First Virtual Holdings</organization>
473        <address><email>nsb@nsb.fv.com</email></address>
474      </author>
475      <date month="November" year="1996"/>
476    </front>
477    <seriesInfo name="RFC" value="2046"/>
478  </reference>
479
480  <reference anchor="RFC2047">
481    <front>
482      <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
483      <author initials="K." surname="Moore" fullname="Keith Moore">
484        <organization>University of Tennessee</organization>
485        <address><email>moore@cs.utk.edu</email></address>
486      </author>
487      <date month="November" year="1996"/>
488    </front>
489    <seriesInfo name="RFC" value="2047"/>
490  </reference>
491
492  <reference anchor="RFC2183">
493    <front>
494      <title abbrev="Content-Disposition">Communicating Presentation Information in Internet Messages: The Content-Disposition Header Field</title>
495      <author initials="R." surname="Troost" fullname="Rens Troost">
496        <organization>New Century Systems</organization>
497        <address><email>rens@century.com</email></address>
498      </author>
499      <author initials="S." surname="Dorner" fullname="Steve Dorner">
500        <organization>QUALCOMM Incorporated</organization>
501        <address><email>sdorner@qualcomm.com</email></address>
502      </author>
503      <author initials="K." surname="Moore" fullname="Keith Moore">
504        <organization>Department of Computer Science</organization>
505        <address><email>moore@cs.utk.edu</email></address>
506      </author>
507      <date year="1997" month="August"/>
508    </front>
509    <seriesInfo name="RFC" value="2183"/>
510  </reference>
511
512  <reference anchor="RFC2231">
513    <front>
514      <title abbrev="MIME Value and Encoded Word Extensions">MIME Parameter Value and Encoded Word Extensions: Character Sets, Languages, and Continuations</title>
515      <author initials="N." surname="Freed" fullname="Ned Freed">
516        <organization abbrev="Innosoft">Innosoft International, Inc.</organization>
517        <address><email>ned.freed@innosoft.com</email></address>
518      </author>
519      <author initials="K." surname="Moore" fullname="Keith Moore">
520        <organization>University of Tennessee</organization>
521        <address><email>moore@cs.utk.edu</email></address>
522      </author>
523      <date year="1997" month="November"/>
524    </front>
525    <seriesInfo name="RFC" value="2231"/>
526  </reference>
527
528  <reference anchor="RFC3629">
529    <front>
530      <title>UTF-8, a transformation format of ISO 10646</title>
531      <author initials="F." surname="Yergeau" fullname="F. Yergeau">
532        <organization>Alis Technologies</organization>
533        <address><email>fyergeau@alis.com</email></address>
534      </author>
535      <date month="November" year="2003"/>
536    </front>
537    <seriesInfo name="STD" value="63"/>
538    <seriesInfo name="RFC" value="3629"/>
539  </reference>
540
541  <reference anchor="RFC3864">
542    <front>
543      <title>Registration Procedures for Message Header Fields</title>
544      <author initials="G." surname="Klyne" fullname="G. Klyne">
545        <organization>Nine by Nine</organization>
546        <address><email>GK-IETF@ninebynine.org</email></address>
547      </author>
548      <author initials="M." surname="Nottingham" fullname="M. Nottingham">
549        <organization>BEA Systems</organization>
550        <address><email>mnot@pobox.com</email></address>
551      </author>
552      <author initials="J." surname="Mogul" fullname="J. Mogul">
553        <organization>HP Labs</organization>
554        <address><email>JeffMogul@acm.org</email></address>
555      </author>
556      <date year="2004" month="September"/>
557    </front>
558    <seriesInfo name="BCP" value="90"/>
559    <seriesInfo name="RFC" value="3864"/>
560  </reference>
561
562  <reference anchor="RFC3986">
563   <front>
564    <title abbrev="URI Generic Syntax">Uniform Resource Identifier (URI): Generic Syntax</title>
565    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
566      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
567      <address>
568         <email>timbl@w3.org</email>
569         <uri>http://www.w3.org/People/Berners-Lee/</uri>
570      </address>
571    </author>
572    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
573      <organization abbrev="Day Software">Day Software</organization>
574      <address>
575        <email>fielding@gbiv.com</email>
576        <uri>http://roy.gbiv.com/</uri>
577      </address>
578    </author>
579    <author initials="L." surname="Masinter" fullname="Larry Masinter">
580      <organization abbrev="Adobe Systems">Adobe Systems Incorporated</organization>
581      <address>
582        <email>LMM@acm.org</email>
583        <uri>http://larry.masinter.net/</uri>
584      </address>
585    </author>
586    <date month="January" year="2005"/>
587   </front>
588   <seriesInfo name="STD" value="66"/>
589   <seriesInfo name="RFC" value="3986"/>
590  </reference>
591
592</references>
593
594<section title="Changes from the RFC 2616 Definition" anchor="changes.from.rfc2616">
595<t>
596  Compared to Section 19.5.1 of <xref target="RFC2616"/>, the following
597  normative changes reflecting actual implementations have been made:
598<list style="symbols">
599  <t>
600    According to RFC 2616, the disposition type "attachment" only applies to
601    content of type "application/octet-stream". This restriction has been
602    removed, because user agents in practice do not check the content type, and
603    it also discourages properly declaring the media type.
604  </t>
605  <t>
606    RFC 2616 only allows "quoted-string" for the filename parameter. This
607    would be an exceptional parameter syntax, and also doesn't reflect actual
608    use.
609  </t>
610  <t>
611    The definition for the disposition type "inline" (<xref target="RFC2183"/>, Section 2.1)
612    has been re-added with a suggestion for its processing.
613  </t>
614  <t>
615    This specification requires support for the extended parameter encoding
616    defined in <xref target="RFC5987"/>.
617  </t>
618</list>
619</t>
620</section>
621
622<section title="Differences compared to RFC 2183" anchor="diffs.compared.to.rfc2183">
623<t>
624  Section 2 of <xref target="RFC2183"/> defines several additional
625  disposition parameters: "creation-date", "modification-date",
626  "quoted-date-time", and "size". These do not appear to be implemented by
627  any user agent, thus have been omitted from this specification.
628</t>
629</section>
630
631<section title="Alternative Approaches to Internationalization" anchor="alternatives">
632<t>
633  By default, HTTP header field parameters cannot carry characters outside
634  the ISO-8859-1 (<xref target="ISO-8859-1"/>) character encoding (see
635  <xref target="RFC2616"/>, Section 2.2). For the "filename"
636  parameter, this of course is an unacceptable restriction.
637</t>
638<t>
639  Unfortunately, user agent implementers have not managed to come up with
640  an interoperable approach, although the IETF Standards Track specifies
641  exactly one solution (<xref target="RFC2231"/>, clarified and profiled for
642  HTTP in <xref target="RFC5987"/>).
643</t>
644<t>
645  For completeness, the sections below describe the various approaches that
646  have been tried, and explains how they are inferior to the RFC 5987
647  encoding used in this specification.
648</t>
649
650<section title="RFC 2047 Encoding" anchor="alternatives.rfc2047">
651<t>
652  RFC 2047 defines an encoding mechanism for
653  header fields, but this encoding is not supposed to be used for
654  header field parameters - see Section 5 of <xref target="RFC2047"/>: 
655</t>
656<t><list>
657  <t>
658    An 'encoded-word' MUST NOT appear within a 'quoted-string'.
659  </t>
660  <t>
661    ...
662  </t>
663  <t>
664    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'.
665  </t>
666</list></t>
667<t>
668  In practice, some user agents implement the encoding, some do not
669  (exposing the encoded string to the user), and some get confused by it.
670</t>
671</section>
672
673<section title="Percent Encoding" anchor="alternatives.percent">
674<t>
675  Some user agents accept percent encoded (<xref target="RFC3986"/>, Section 2.1)
676  sequences of characters encoded using the UTF-8 (<xref target="RFC3629"/>) character encoding.
677</t>
678<t>
679  In practice, this is hard to use because those user agents
680  that do not support it will display the escaped character sequence to the user.
681</t>
682<t>
683  Furthermore, the first user agent to implement this did choose the encoding
684  based on local settings; thus making it very hard to use in multi-lingual
685  environments.
686</t>
687</section>
688
689<section title="Encoding Sniffing" anchor="alternatives.sniff">
690<t>
691  Some user agents inspect the value (which defaults to ISO-8859-1) and
692  switch to UTF-8 when it seems to be more likely to be the correct
693  interpretation.
694</t>
695<t>
696  As with the approaches above, this is not interoperable and furthermore
697  risks misinterpreting the actual value.
698</t>
699</section>
700
701<section title="Implementations (to be removed by RFC Editor before publication)" anchor="alternatives.implementations">
702<t>
703  Unfortunately, as of October 2010, neither the encoding defined in RFCs 2231
704  and 5987, nor any of the alternate approaches discussed above was
705  implemented interoperably. Thus, this specification recommends the approach
706  defined in RFC 5987, which at least has the advantage of actually being
707  specified properly.
708</t>
709<t>
710  The table below shows the implementation support for the various approaches:
711</t>
712<texttable align="left">
713  <ttcol>User Agent</ttcol>
714  <ttcol>RFC 2231/5987</ttcol>
715  <ttcol>RFC 2047</ttcol>
716  <ttcol>Percent Encoding</ttcol>
717  <ttcol>Encoding Sniffing</ttcol>
718 
719  <c>Chrome</c>
720  <c>no</c>
721  <c>yes</c>
722  <c>yes</c>
723  <c>yes</c>
724
725  <c>Firefox</c>
726  <c>yes (*)</c>
727  <c>yes</c>
728  <c>no</c>
729  <c>yes</c>
730
731  <c>Internet Explorer</c>
732  <c>no</c>
733  <c>no</c>
734  <c>yes</c>
735  <c>no</c>
736
737  <c>Konqueror</c>
738  <c>yes</c>
739  <c>no</c>
740  <c>no</c>
741  <c>no</c>
742
743  <c>Opera</c>
744  <c>yes</c>
745  <c>no</c>
746  <c>no</c>
747  <c>no</c>
748
749  <c>Safari</c>
750  <c>no</c>
751  <c>no</c>
752  <c>no</c>
753  <c>yes</c>
754 
755  <postamble>
756  (*) Does not implement the fallback behavior to "filename" described in
757  <xref target="disposition.parameter.filename"/>.
758  </postamble>
759
760</texttable>
761
762</section>
763
764</section>
765
766
767<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
768<t>
769  Note: the issues names in the change log entries for draft-reschke-rfc2183-in-http
770  refer to <eref target="http://greenbytes.de/tech/webdav/draft-reschke-rfc2183-in-http-issues.html"/>.
771</t>
772
773<section title="Since draft-reschke-rfc2183-in-http-00">
774<t> 
775  Adjust terminology ("header" -&gt; "header field").
776  Update rfc2231-in-http reference.
777</t>
778</section>
779
780<section title="Since draft-reschke-rfc2183-in-http-01">
781<t> 
782  Update rfc2231-in-http reference. Actually define the "filename"
783  parameter. Add internationalization considerations.
784  Add examples using the RFC 5987 encoding.
785  Add overview over other approaches, plus a table reporting
786  implementation status.
787  Add and resolve issue "nodep2183".
788  Add issues "asciivsiso",
789  "deplboth", "quoted", and "registry".
790</t>
791</section>
792
793<section title="Since draft-reschke-rfc2183-in-http-02">
794<t>
795  Add and close issue "docfallback".
796  Close issues "asciivsiso", "deplboth", "quoted", and
797  "registry".
798</t>
799</section>
800
801<section title="Since draft-reschke-rfc2183-in-http-03">
802<t>
803  Updated to be a Working Draft of the IETF HTTPbis Working Group.
804</t>
805</section>
806
807<section title="Since draft-ietf-httpbis-content-disp-00" anchor="changes.since.00">
808<t>
809  Closed issues:
810  <list style="symbols">
811    <t>
812      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/242"/>:
813      "handling of unknown disposition types"
814    </t>
815  </list>
816</t>
817<t>
818  Slightly updated the notes about the proposed fallback behavior.
819</t>
820</section>
821
822<section title="Since draft-ietf-httpbis-content-disp-01" anchor="changes.since.01">
823<t>
824  Various editorial improvements.
825</t>
826</section>
827
828<section title="Since draft-ietf-httpbis-content-disp-02" anchor="changes.since.02">
829<t>
830  Closed issues:
831  <list style="symbols">
832    <t>
833      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/244"/>:
834      "state that repeating parameters are invalid"
835    </t>
836    <t>
837      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/245"/>:
838      "warn about %xx in filenames being misinterpreted"
839    </t>
840    <t>
841      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/246"/>:
842      "mention control chars when talking about postprecessing the filename parameter"
843    </t>
844  </list>
845</t>
846<t>
847  Update <xref target="alternatives.implementations"/>; Opera 10.63 RC
848  implements the recommended fallback behavior.
849</t>
850</section>
851</section>
852
853
854  </back>
855
856</rfc>
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