source: draft-ietf-httpbis-content-disp/00/draft-ietf-httpbis-content-disp-00.xml @ 1338

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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-00" 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="September" year="2010" day="3"/>
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://www3.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.04"/>.
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.
110</t>
111
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</section>
140
141<section title="Disposition Type" anchor="disposition.type">
142<t>
143  If the disposition type matches "attachment" (case-insensitively), this indicates that the user agent should not display the response,
144  but directly enter a "save as..." dialog.
145</t>
146<t>
147  On the other hand, if it matches "inline" (case-insensitively), this implies default processing.
148</t>
149<t>
150  Other disposition types SHOULD be handled the same way as "attachment"
151  (see also <xref target="RFC2183"/>, Section 2.8).
152</t>
153</section>
154
155<section title="Disposition Parameter: 'Filename'" anchor="disposition.parameter.filename">
156<t>
157  The parameters "filename" and "filename*", to be matched case-insensitively,
158  provide information on how to construct a filename for storing the message
159  payload.
160</t>
161<t>
162  Depending on the disposition type, this information might be used right away
163  (in the "save as..." interaction caused for the "attachment" disposition type),
164  or later on (for instance, when the user decides to save the contents of the
165  current page being displayed).
166</t>
167<t>
168  "filename" and "filename*" behave the same, except that "filename*" uses
169  the encoding defined in <xref target="RFC5987"/>, allowing the use
170  of characters not present in the ISO-8859-1 character set (<xref target="ISO-8859-1"/>). When both "filename" and "filename*"
171  are present, a recipient SHOULD pick "filename*" and ignore "filename" -
172  this will make it possible to send the same header value to clients
173  that do not support "filename*".
174</t>
175
176<t>
177  It is essential that user agents treat the specified filename as advisory
178  only, thus be very careful in extracting the desired information.
179  In particular:
180  <list style="symbols">
181    <t>
182      When the value contains path separator characters, all but the last
183      segment SHOULD be ignored. This prevents unintentional overwriting
184      of well-known file system location (such as "/etc/passwd").
185    </t>
186    <t>
187      Many platforms do not use Internet Media Types (<xref target="RFC2046"/>)
188      to hold type information in the file system, but rely on filename
189      extensions instead. Trusting the server-provided file extension could
190      introduce a privilege escalation when later on the file is opened locally
191      (consider ".exe"). Thus, recipients need to ensure that a file extension
192      is used that is safe, optimally matching the media type of the received
193      payload.
194    </t>
195    <t>
196      Other aspects recipients need to be aware of are names that have a
197      special meaning in the filesystem or in shell commands, such as "." and "..",
198      "~", "|", and also device names.
199    </t>
200  </list>
201</t>
202</section>
203
204<section title="Disposition Parameter: Extensions" anchor="disposition.parameter.extensionsS">
205<t>
206  To enable future extensions, unknown parameters SHOULD be ignored (see also <xref target="RFC2183"/>, Section 2.8).
207</t>
208</section>
209
210<section title="Extensibility" anchor="extensibility">
211<t>
212  Note that Section 9 of <xref target="RFC2183"/> defines IANA registries both
213  for disposition types and disposition parameters. This registry is
214  shared by different protocols using Content-Disposition, such as MIME and HTTP.
215  Therefore, not all registered values may make sense in the context of HTTP.
216</t>
217</section>
218
219</section> 
220
221<section title="Examples">
222
223<figure>
224<preamble>
225Direct UA to show "save as" dialog, with a filename of "foo.html": 
226</preamble>
227<artwork type="example"><![CDATA[
228Content-Disposition: Attachment; filename=foo.html
229]]></artwork></figure>
230<figure>
231<preamble>
232Direct UA to behave as if the Content-Disposition header field wasn't present,
233but to remember the filename "foo.html" for a subsequent save operation:
234</preamble>
235<artwork type="example"><![CDATA[
236Content-Disposition: INLINE; FILENAME= "foo.html"
237]]></artwork></figure>
238<figure>
239<preamble>
240Direct UA to show "save as" dialog, with a filename of "an example":
241</preamble>
242<artwork type="example"><![CDATA[
243Content-Disposition: Attachment; Filename*=UTF-8'en'an%20example
244]]></artwork>
245<postamble>Note that this example uses the extended encoding defined in
246<xref target="RFC5987"/> to specify that the natural language of the filename
247is English, and also to encode the space character which is not allowed in the
248token production.
249</postamble>
250</figure>
251<figure>
252<preamble>
253Direct UA to show "save as" dialog, with a filename containing the Unicode character  U+20AC (EURO SIGN):
254</preamble>
255<artwork type="example"><![CDATA[
256Content-Disposition: attachment; filename*= UTF-8''%e2%82%ac%20rates
257]]></artwork>
258<postamble>
259  Here, the encoding defined in <xref target="RFC5987"/> is also used to encode the
260  non-ISO-8859-1 character.
261</postamble>
262</figure>
263
264
265<figure>
266<preamble>
267Same as above, but adding the "filename" parameter for compatibility with
268user agents not implementing RFC 5987:
269</preamble>
270<artwork type="example"><![CDATA[
271Content-Disposition: attachment; filename="EURO rates";
272                                 filename*=utf-8''%e2%82%ac%20rates
273]]></artwork>
274<postamble>
275  Note: as of August 2010, many user agents unfortunately did not properly handle
276  unexpected parameters, and some that implement RFC 5987 did not pick
277  the extended parameter when both were present.
278</postamble>
279</figure>
280
281</section>
282
283<section title="Internationalization Considerations" anchor="i18n">
284<t>
285  The "filename*" parameter (<xref target="disposition.parameter.filename"/>),
286  using the encoding defined in <xref target="RFC5987"/>, allows the
287  server to transmit characters outside the ISO-8859-1 character set,
288  and also to optionally specify the language in use.
289</t>
290<t>
291  Future parameters might also require internationalization, in which case
292  the same encoding can be used.
293</t>
294</section>
295
296<section title="Security Considerations" anchor="security.considerations">
297<t>
298  Using server-supplied information for constructing local filenames introduces
299  many risks. These are summarized in <xref target="disposition.parameter.filename"/>.
300</t>
301<t>
302  Furthermore, implementers also ought to be aware of the Security
303  Considerations applying to HTTP (see Section 15 of <xref target="RFC2616"/>), and also the parameter encoding defined in <xref target="RFC5987"/>
304  (see Appendix ).
305</t>
306</section> 
307
308<section title="IANA Considerations" anchor="iana.considerations">
309
310<section title="Registry for Disposition Values and Parameter" anchor="registry">
311<t>
312  This specification does not introduce any changes to the registration
313  procedures for disposition values and parameters that are defined in
314  Section 9 of <xref target="RFC2183"/>.
315</t>
316
317</section>
318
319<section title="Header Field Registration" anchor="header.field.registration"> 
320<t>
321  This document updates the definition of the Content-Disposition HTTP header field
322  in the permanent HTTP header field registry (see <xref target="RFC3864"/>).
323</t>
324<t>
325<list style="hanging">
326  <t hangText="Header field name:">Content-Disposition</t>
327  <t hangText="Applicable protocol:">http</t>
328  <t hangText="Status:">standard</t>
329  <t hangText="Author/Change controller:">IETF</t>
330  <t hangText="Specification document:">this specification (<xref target="header.field.definition"/>)</t>
331</list>
332</t>
333</section>
334
335</section> 
336
337<section title="Acknowledgements">
338<t>
339  Thanks to Rolf Eike Beer, Alfred Hoenes, and Roar Lauritzsen for
340  their valuable feedback.
341</t>
342</section> 
343
344  </middle>
345  <back>
346 
347<references title="Normative References">
348 
349  <reference anchor="RFC2119">
350    <front>
351      <title abbrev="RFC Key Words">Key words for use in RFCs to Indicate Requirement Levels</title>
352      <author initials="S." surname="Bradner" fullname="Scott Bradner">
353        <organization>Harvard University</organization>
354        <address><email>sob@harvard.edu</email></address>
355      </author>
356      <date month="March" year="1997"/>
357      <area>General</area>
358      <keyword>keyword</keyword>
359    </front>
360    <seriesInfo name="BCP" value="14"/>
361    <seriesInfo name="RFC" value="2119"/>
362  </reference>
363
364  <reference anchor="RFC2616">
365    <front>
366      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
367      <author initials="R." surname="Fielding" fullname="R. Fielding">
368        <organization>University of California, Irvine</organization>
369        <address><email>fielding@ics.uci.edu</email></address>
370      </author>
371      <author initials="J." surname="Gettys" fullname="J. Gettys">
372        <organization>W3C</organization>
373        <address><email>jg@w3.org</email></address>
374      </author>
375      <author initials="J." surname="Mogul" fullname="J. Mogul">
376        <organization>Compaq Computer Corporation</organization>
377        <address><email>mogul@wrl.dec.com</email></address>
378      </author>
379      <author initials="H." surname="Frystyk" fullname="H. Frystyk">
380        <organization>MIT Laboratory for Computer Science</organization>
381        <address><email>frystyk@w3.org</email></address>
382      </author>
383      <author initials="L." surname="Masinter" fullname="L. Masinter">
384        <organization>Xerox Corporation</organization>
385        <address><email>masinter@parc.xerox.com</email></address>
386      </author>
387      <author initials="P." surname="Leach" fullname="P. Leach">
388        <organization>Microsoft Corporation</organization>
389        <address><email>paulle@microsoft.com</email></address>
390      </author>
391      <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
392        <organization>W3C</organization>
393        <address><email>timbl@w3.org</email></address>
394      </author>
395      <date month="June" year="1999"/>
396    </front>
397    <seriesInfo name="RFC" value="2616"/>
398  </reference>
399
400  <reference anchor="RFC5987">
401        <front>
402      <title abbrev="RFC2231 Encoding in HTTP">Applicability of RFC 2231
403      Encoding to Hypertext Transfer Protocol (HTTP) Headers</title>
404      <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke">
405        <organization abbrev="greenbytes">greenbytes GmbH</organization>
406        <address>
407          <postal>
408            <street>Hafenweg 16</street>
409            <city>Muenster</city><region>NW</region><code>48155</code>
410            <country>Germany</country>
411          </postal>
412          <email>julian.reschke@greenbytes.de</email>   
413          <uri>http://greenbytes.de/tech/webdav/</uri> 
414        </address>
415      </author>
416      <date month="August" year="2010"/>
417    </front>
418    <seriesInfo name="RFC" value="5987"/>
419  </reference>
420
421
422  <reference anchor="ISO-8859-1">
423    <front>
424      <title>Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1</title>
425      <author>
426        <organization>International Organization for Standardization</organization>
427      </author>
428      <date year="1998"/>
429    </front>
430    <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
431  </reference>
432
433</references>
434 
435<references title="Informative References">
436
437  <reference anchor="RFC2046">
438    <front>
439      <title abbrev="Media Types">Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types</title>
440      <author initials="N." surname="Freed" fullname="Ned Freed">
441        <organization>Innosoft International, Inc.</organization>
442        <address><email>ned@innosoft.com</email></address>
443      </author>
444      <author initials="N." surname="Borenstein" fullname="Nathaniel S. Borenstein">
445        <organization>First Virtual Holdings</organization>
446        <address><email>nsb@nsb.fv.com</email></address>
447      </author>
448      <date month="November" year="1996"/>
449    </front>
450    <seriesInfo name="RFC" value="2046"/>
451  </reference>
452
453  <reference anchor="RFC2047">
454    <front>
455      <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
456      <author initials="K." surname="Moore" fullname="Keith Moore">
457        <organization>University of Tennessee</organization>
458        <address><email>moore@cs.utk.edu</email></address>
459      </author>
460      <date month="November" year="1996"/>
461    </front>
462    <seriesInfo name="RFC" value="2047"/>
463  </reference>
464
465  <reference anchor="RFC2183">
466    <front>
467      <title abbrev="Content-Disposition">Communicating Presentation Information in Internet Messages: The Content-Disposition Header Field</title>
468      <author initials="R." surname="Troost" fullname="Rens Troost">
469        <organization>New Century Systems</organization>
470        <address><email>rens@century.com</email></address>
471      </author>
472      <author initials="S." surname="Dorner" fullname="Steve Dorner">
473        <organization>QUALCOMM Incorporated</organization>
474        <address><email>sdorner@qualcomm.com</email></address>
475      </author>
476      <author initials="K." surname="Moore" fullname="Keith Moore">
477        <organization>Department of Computer Science</organization>
478        <address><email>moore@cs.utk.edu</email></address>
479      </author>
480      <date year="1997" month="August"/>
481    </front>
482    <seriesInfo name="RFC" value="2183"/>
483  </reference>
484
485  <reference anchor="RFC2231">
486    <front>
487      <title abbrev="MIME Value and Encoded Word Extensions">MIME Parameter Value and Encoded Word Extensions: Character Sets, Languages, and Continuations</title>
488      <author initials="N." surname="Freed" fullname="Ned Freed">
489        <organization abbrev="Innosoft">Innosoft International, Inc.</organization>
490        <address><email>ned.freed@innosoft.com</email></address>
491      </author>
492      <author initials="K." surname="Moore" fullname="Keith Moore">
493        <organization>University of Tennessee</organization>
494        <address><email>moore@cs.utk.edu</email></address>
495      </author>
496      <date year="1997" month="November"/>
497    </front>
498    <seriesInfo name="RFC" value="2231"/>
499  </reference>
500
501  <reference anchor="RFC3629">
502    <front>
503      <title>UTF-8, a transformation format of ISO 10646</title>
504      <author initials="F." surname="Yergeau" fullname="F. Yergeau">
505        <organization>Alis Technologies</organization>
506        <address><email>fyergeau@alis.com</email></address>
507      </author>
508      <date month="November" year="2003"/>
509    </front>
510    <seriesInfo name="RFC" value="3629"/>
511    <seriesInfo name="STD" value="63"/>
512  </reference>
513
514  <reference anchor="RFC3864">
515    <front>
516      <title>Registration Procedures for Message Header Fields</title>
517      <author initials="G." surname="Klyne" fullname="G. Klyne">
518        <organization>Nine by Nine</organization>
519        <address><email>GK-IETF@ninebynine.org</email></address>
520      </author>
521      <author initials="M." surname="Nottingham" fullname="M. Nottingham">
522        <organization>BEA Systems</organization>
523        <address><email>mnot@pobox.com</email></address>
524      </author>
525      <author initials="J." surname="Mogul" fullname="J. Mogul">
526        <organization>HP Labs</organization>
527        <address><email>JeffMogul@acm.org</email></address>
528      </author>
529      <date year="2004" month="September"/>
530    </front>
531    <seriesInfo name="BCP" value="90"/>
532    <seriesInfo name="RFC" value="3864"/>
533  </reference>
534
535  <reference anchor="RFC3986">
536   <front>
537    <title abbrev="URI Generic Syntax">Uniform Resource Identifier (URI): Generic Syntax</title>
538    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
539      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
540      <address>
541         <email>timbl@w3.org</email>
542         <uri>http://www.w3.org/People/Berners-Lee/</uri>
543      </address>
544    </author>
545    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
546      <organization abbrev="Day Software">Day Software</organization>
547      <address>
548        <email>fielding@gbiv.com</email>
549        <uri>http://roy.gbiv.com/</uri>
550      </address>
551    </author>
552    <author initials="L." surname="Masinter" fullname="Larry Masinter">
553      <organization abbrev="Adobe Systems">Adobe Systems Incorporated</organization>
554      <address>
555        <email>LMM@acm.org</email>
556        <uri>http://larry.masinter.net/</uri>
557      </address>
558    </author>
559    <date month="January" year="2005"/>
560   </front>
561   <seriesInfo name="RFC" value="3986"/>
562   <seriesInfo name="STD" value="66"/>
563  </reference>
564
565</references>
566
567<section title="Changes from the RFC 2616 Definition" anchor="changes.from.rfc2616">
568<t>
569  Compared to Section 19.5.1 of <xref target="RFC2616"/>, the following
570  normative changes reflecting actual implementations have been made:
571<list style="symbols">
572  <t>
573    According to RFC 2616, the disposition type "attachment" only applies to
574    content of type "application/octet-stream". This restriction has been
575    removed, because user agents in practice do not check the content type, and
576    it also discourages properly declaring the media type.
577  </t>
578
579  <t>
580    RFC 2616 only allows "quoted-string" for the filename parameter. This
581    would be an exceptional parameter syntax, and also doesn't reflect actual
582    use.
583  </t>
584
585  <t>
586    The definition for the disposition type "inline" (<xref target="RFC2183"/>, Section 2.1)
587    has been re-added with a suggestion for its processing.
588  </t>
589  <t>
590    This specification requires support for the extended parameter encoding
591    defined in <xref target="RFC5987"/>.
592  </t>
593</list>
594</t>
595</section>
596
597<section title="Differences compared to RFC 2183" anchor="diffs.compared.to.rfc2183">
598<t>
599  Section 2 of <xref target="RFC2183"/> defines several additional
600  disposition parameters: "creation-date", "modification-date",
601  "quoted-date-time", and "size". These do not appear to be implemented by
602  any user agent, thus have been omitted from this specification.
603</t>
604</section>
605
606<section title="Alternative Approaches to Internationalization" anchor="alternatives">
607<t>
608  By default, HTTP header field parameters cannot carry characters outside
609  the ISO-8859-1 (<xref target="ISO-8859-1"/>) character encoding (see
610  <xref target="RFC2616"/>, Section 2.2). For the "filename"
611  parameter, this of course is an unacceptable restriction.
612</t>
613<t>
614  Unfortunately, user agent implementers have not managed to come up with
615  an interoperable approach, although the IETF Standards Track specifies
616  exactly one solution (<xref target="RFC2231"/>, clarified and profiled for
617  HTTP in <xref target="RFC5987"/>).
618</t>
619<t>
620  For completeness, the sections below describe the various approaches that
621  have been tried, and explains how they are inferior to the RFC 5987
622  encoding used in this specification.
623</t>
624
625<section title="RFC 2047 Encoding" anchor="alternatives.rfc2047">
626<t>
627  RFC 2047 defines an encoding mechanism for
628  header fields, but this encoding is not supposed to be used for
629  header field parameters - see Section 5 of <xref target="RFC2047"/>: 
630</t>
631<t><list>
632  <t>
633    An 'encoded-word' MUST NOT appear within a 'quoted-string'.
634  </t>
635  <t>
636    ...
637  </t>
638  <t>
639    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'.
640  </t>
641</list></t>
642<t>
643  In practice, some user agents implement the encoding, some do not
644  (exposing the encoded string to the user), and some get confused by it.
645</t>
646</section>
647
648<section title="Percent Encoding" anchor="alternatives.percent">
649<t>
650  Some user agents accept percent encoded (<xref target="RFC3986"/>, Section 2.1)
651  sequences of characters encoded using the UTF-8 (<xref target="RFC3629"/>) character encoding.
652</t>
653<t>
654  In practice, this is hard to use because those user agents
655  that do not support it will display the escaped character sequence to the user.
656</t>
657<t>
658  Furthermore, the first user agent to implement this did choose the encoding
659  based on local settings; thus making it very hard to use in multi-lingual
660  environments.
661</t>
662</section>
663
664<section title="Encoding Sniffing" anchor="alternatives.sniff">
665<t>
666  Some user agents inspect the value (which defaults to ISO-8859-1) and
667  switch to UTF-8 when it seems to be more likely to be the correct
668  interpretation.
669</t>
670<t>
671  As with the approaches above, this is not interoperable and furthermore
672  risks misinterpreting the actual value.
673</t>
674</section>
675
676<section title="Implementations" anchor="alternatives.implementations">
677
678<t>
679  Unfortunately, as of August 2010, neither the encoding defined in RFCs 2231
680  and 5789, nor any of the alternate approaches discussed above was
681  implemented interoperably. Thus, this specification recommends the approach
682  defined in RFC 5987, which at least has the advantage of actually being
683  specified properly.
684</t>
685<t>
686  The table below shows the implementation support for the various approaches:
687  <cref anchor="impls">Discuss: should we mention the implementation status
688  of actual UAs in a RFC? Up to the IESG to decide...</cref> 
689</t>
690
691
692<texttable align="left">
693  <ttcol>User Agent</ttcol>
694  <ttcol>RFC 2231/5987</ttcol>
695  <ttcol>RFC 2047</ttcol>
696  <ttcol>Percent Encoding</ttcol>
697  <ttcol>Encoding Sniffing</ttcol>
698 
699  <c>Chrome</c>
700  <c>no</c>
701  <c>yes</c>
702  <c>yes</c>
703  <c>yes</c>
704
705  <c>Firefox</c>
706  <c>yes (*)</c>
707  <c>yes</c>
708  <c>no</c>
709  <c>yes</c>
710
711  <c>Internet Explorer</c>
712  <c>no</c>
713  <c>no</c>
714  <c>yes</c>
715  <c>no</c>
716
717  <c>Konqueror</c>
718  <c>yes</c>
719  <c>no</c>
720  <c>no</c>
721  <c>no</c>
722
723  <c>Opera</c>
724  <c>yes (*)</c>
725  <c>no</c>
726  <c>no</c>
727  <c>no</c>
728
729  <c>Safari</c>
730  <c>no</c>
731  <c>no</c>
732  <c>no</c>
733  <c>yes</c>
734 
735  <postamble>
736  (*) Does
737  not implement the fallback behavior to "filename" described in
738  <xref target="disposition.parameter.filename"/>.
739 
740  </postamble>
741
742</texttable>
743
744</section>
745
746</section>
747
748
749<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
750<section title="Since draft-reschke-rfc2183-in-http-00">
751<t> 
752  Adjust terminology ("header" -&gt; "header field").
753  Update rfc2231-in-http reference.
754</t>
755</section>
756
757
758<section title="Since draft-reschke-rfc2183-in-http-01">
759<t> 
760  Update rfc2231-in-http reference. Actually define the "filename"
761  parameter. Add internationalization considerations.
762  Add examples using the RFC 5987 encoding.
763  Add overview over other approaches, plus a table reporting
764  implementation status.
765  Add and resolve issue "nodep2183".
766  Add issues "asciivsiso",
767  "deplboth", "quoted", and "registry".
768</t>
769</section>
770
771<section title="Since draft-reschke-rfc2183-in-http-02">
772<t>
773  Add and close issue "docfallback".
774  Close issues "asciivsiso", "deplboth", "quoted", and
775  "registry".
776</t>
777</section>
778
779<section title="Since draft-reschke-rfc2183-in-http-03">
780<t>
781  Updated to be a Working Draft of the IETF HTTPbis Working Group.
782</t>
783</section>
784</section>
785
786
787  </back>
788
789</rfc>
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