rfc2183.txt - rohrpost - A commandline mail client to change the world as we see it.
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rfc2183.txt (23158B)
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7 Network Working Group R. Troost
8 Request for Comments: 2183 New Century Systems
9 Updates: 1806 S. Dorner
10 Category: Standards Track QUALCOMM Incorporated
11 K. Moore, Editor
12 University of Tennessee
13 August 1997
14
15
16 Communicating Presentation Information in
17 Internet Messages:
18 The Content-Disposition Header Field
19
20 Status of this Memo
21
22 This document specifies an Internet standards track protocol for the
23 Internet community, and requests discussion and suggestions for
24 improvements. Please refer to the current edition of the "Internet
25 Official Protocol Standards" (STD 1) for the standardization state
26 and status of this protocol. Distribution of this memo is unlimited.
27
28 Abstract
29
30 This memo provides a mechanism whereby messages conforming to the
31 MIME specifications [RFC 2045, RFC 2046, RFC 2047, RFC 2048, RFC
32 2049] can convey presentational information. It specifies the
33 "Content-Disposition" header field, which is optional and valid for
34 any MIME entity ("message" or "body part"). Two values for this
35 header field are described in this memo; one for the ordinary linear
36 presentation of the body part, and another to facilitate the use of
37 mail to transfer files. It is expected that more values will be
38 defined in the future, and procedures are defined for extending this
39 set of values.
40
41 This document is intended as an extension to MIME. As such, the
42 reader is assumed to be familiar with the MIME specifications, and
43 [RFC 822]. The information presented herein supplements but does not
44 replace that found in those documents.
45
46 This document is a revision to the Experimental protocol defined in
47 RFC 1806. As compared to RFC 1806, this document contains minor
48 editorial updates, adds new parameters needed to support the File
49 Transfer Body Part, and references a separate specification for the
50 handling of non-ASCII and/or very long parameter values.
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58 Troost, et. al. Standards Track [Page 1]
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60 RFC 2183 Content-Disposition August 1997
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63 1. Introduction
64
65 MIME specifies a standard format for encapsulating multiple pieces of
66 data into a single Internet message. That document does not address
67 the issue of presentation styles; it provides a framework for the
68 interchange of message content, but leaves presentation issues solely
69 in the hands of mail user agent (MUA) implementors.
70
71 Two common ways of presenting multipart electronic messages are as a
72 main document with a list of separate attachments, and as a single
73 document with the various parts expanded (displayed) inline. The
74 display of an attachment is generally construed to require positive
75 action on the part of the recipient, while inline message components
76 are displayed automatically when the message is viewed. A mechanism
77 is needed to allow the sender to transmit this sort of presentational
78 information to the recipient; the Content-Disposition header provides
79 this mechanism, allowing each component of a message to be tagged
80 with an indication of its desired presentation semantics.
81
82 Tagging messages in this manner will often be sufficient for basic
83 message formatting. However, in many cases a more powerful and
84 flexible approach will be necessary. The definition of such
85 approaches is beyond the scope of this memo; however, such approaches
86 can benefit from additional Content-Disposition values and
87 parameters, to be defined at a later date.
88
89 In addition to allowing the sender to specify the presentational
90 disposition of a message component, it is desirable to allow her to
91 indicate a default archival disposition; a filename. The optional
92 "filename" parameter provides for this. Further, the creation-date,
93 modification-date, and read-date parameters allow preservation of
94 those file attributes when the file is transmitted over MIME email.
95
96 NB: The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
97 SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
98 document, are to be interpreted as described in [RFC 2119].
99
100 2. The Content-Disposition Header Field
101
102 Content-Disposition is an optional header field. In its absence, the
103 MUA may use whatever presentation method it deems suitable.
104
105 It is desirable to keep the set of possible disposition types small
106 and well defined, to avoid needless complexity. Even so, evolving
107 usage will likely require the definition of additional disposition
108 types or parameters, so the set of disposition values is extensible;
109 see below.
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114 Troost, et. al. Standards Track [Page 2]
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116 RFC 2183 Content-Disposition August 1997
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119 In the extended BNF notation of [RFC 822], the Content-Disposition
120 header field is defined as follows:
121
122 disposition := "Content-Disposition" ":"
123 disposition-type
124 *(";" disposition-parm)
125
126 disposition-type := "inline"
127 / "attachment"
128 / extension-token
129 ; values are not case-sensitive
130
131 disposition-parm := filename-parm
132 / creation-date-parm
133 / modification-date-parm
134 / read-date-parm
135 / size-parm
136 / parameter
137
138 filename-parm := "filename" "=" value
139
140 creation-date-parm := "creation-date" "=" quoted-date-time
141
142 modification-date-parm := "modification-date" "=" quoted-date-time
143
144 read-date-parm := "read-date" "=" quoted-date-time
145
146 size-parm := "size" "=" 1*DIGIT
147
148 quoted-date-time := quoted-string
149 ; contents MUST be an RFC 822 `date-time'
150 ; numeric timezones (+HHMM or -HHMM) MUST be used
151
152
153
154 NOTE ON PARAMETER VALUE LENGHTS: A short (length <= 78 characters)
155 parameter value containing only non-`tspecials' characters SHOULD be
156 represented as a single `token'. A short parameter value containing
157 only ASCII characters, but including `tspecials' characters, SHOULD
158 be represented as `quoted-string'. Parameter values longer than 78
159 characters, or which contain non-ASCII characters, MUST be encoded as
160 specified in [RFC 2184].
161
162 `Extension-token', `parameter', `tspecials' and `value' are defined
163 according to [RFC 2045] (which references [RFC 822] in the definition
164 of some of these tokens). `quoted-string' and `DIGIT' are defined in
165 [RFC 822].
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170 Troost, et. al. Standards Track [Page 3]
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172 RFC 2183 Content-Disposition August 1997
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174
175 2.1 The Inline Disposition Type
176
177 A bodypart should be marked `inline' if it is intended to be
178 displayed automatically upon display of the message. Inline
179 bodyparts should be presented in the order in which they occur,
180 subject to the normal semantics of multipart messages.
181
182 2.2 The Attachment Disposition Type
183
184 Bodyparts can be designated `attachment' to indicate that they are
185 separate from the main body of the mail message, and that their
186 display should not be automatic, but contingent upon some further
187 action of the user. The MUA might instead present the user of a
188 bitmap terminal with an iconic representation of the attachments, or,
189 on character terminals, with a list of attachments from which the
190 user could select for viewing or storage.
191
192 2.3 The Filename Parameter
193
194 The sender may want to suggest a filename to be used if the entity is
195 detached and stored in a separate file. If the receiving MUA writes
196 the entity to a file, the suggested filename should be used as a
197 basis for the actual filename, where possible.
198
199 It is important that the receiving MUA not blindly use the suggested
200 filename. The suggested filename SHOULD be checked (and possibly
201 changed) to see that it conforms to local filesystem conventions,
202 does not overwrite an existing file, and does not present a security
203 problem (see Security Considerations below).
204
205 The receiving MUA SHOULD NOT respect any directory path information
206 that may seem to be present in the filename parameter. The filename
207 should be treated as a terminal component only. Portable
208 specification of directory paths might possibly be done in the future
209 via a separate Content-Disposition parameter, but no provision is
210 made for it in this draft.
211
212 Current [RFC 2045] grammar restricts parameter values (and hence
213 Content-Disposition filenames) to US-ASCII. We recognize the great
214 desirability of allowing arbitrary character sets in filenames, but
215 it is beyond the scope of this document to define the necessary
216 mechanisms. We expect that the basic [RFC 1521] `value'
217 specification will someday be amended to allow use of non-US-ASCII
218 characters, at which time the same mechanism should be used in the
219 Content-Disposition filename parameter.
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226 Troost, et. al. Standards Track [Page 4]
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228 RFC 2183 Content-Disposition August 1997
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231 Beyond the limitation to US-ASCII, the sending MUA may wish to bear
232 in mind the limitations of common filesystems. Many have severe
233 length and character set restrictions. Short alphanumeric filenames
234 are least likely to require modification by the receiving system.
235
236 The presence of the filename parameter does not force an
237 implementation to write the entity to a separate file. It is
238 perfectly acceptable for implementations to leave the entity as part
239 of the normal mail stream unless the user requests otherwise. As a
240 consequence, the parameter may be used on any MIME entity, even
241 `inline' ones. These will not normally be written to files, but the
242 parameter could be used to provide a filename if the receiving user
243 should choose to write the part to a file.
244
245 2.4 The Creation-Date parameter
246
247 The creation-date parameter MAY be used to indicate the date at which
248 the file was created. If this parameter is included, the paramter
249 value MUST be a quoted-string which contains a representation of the
250 creation date of the file in [RFC 822] `date-time' format.
251
252 UNIX and POSIX implementors are cautioned that the `st_ctime' file
253 attribute of the `stat' structure is not the creation time of the
254 file; it is thus not appropriate as a source for the creation-date
255 parameter value.
256
257 2.5 The Modification-Date parameter
258
259 The modification-date parameter MAY be used to indicate the date at
260 which the file was last modified. If the modification-date parameter
261 is included, the paramter value MUST be a quoted-string which
262 contains a representation of the last modification date of the file
263 in [RFC 822] `date-time' format.
264
265 2.6 The Read-Date parameter
266
267 The read-date parameter MAY be used to indicate the date at which the
268 file was last read. If the read-date parameter is included, the
269 parameter value MUST be a quoted-string which contains a
270 representation of the last-read date of the file in [RFC 822] `date-
271 time' format.
272
273 2.7 The Size parameter
274
275 The size parameter indicates an approximate size of the file in
276 octets. It can be used, for example, to pre-allocate space before
277 attempting to store the file, or to determine whether enough space
278 exists.
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282 Troost, et. al. Standards Track [Page 5]
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284 RFC 2183 Content-Disposition August 1997
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287 2.8 Future Extensions and Unrecognized Disposition Types
288
289 In the likely event that new parameters or disposition types are
290 needed, they should be registered with the Internet Assigned Numbers
291 Authority (IANA), in the manner specified in Section 9 of this memo.
292
293 Once new disposition types and parameters are defined, there is of
294 course the likelihood that implementations will see disposition types
295 and parameters they do not understand. Furthermore, since x-tokens
296 are allowed, implementations may also see entirely unregistered
297 disposition types and parameters.
298
299 Unrecognized parameters should be ignored. Unrecognized disposition
300 types should be treated as `attachment'. The choice of `attachment'
301 for unrecognized types is made because a sender who goes to the
302 trouble of producing a Content-Disposition header with a new
303 disposition type is more likely aiming for something more elaborate
304 than inline presentation.
305
306 Unless noted otherwise in the definition of a parameter, Content-
307 Disposition parameters are valid for all dispositions. (In contrast
308 to MIME content-type parameters, which are defined on a per-content-
309 type basis.) Thus, for example, the `filename' parameter still means
310 the name of the file to which the part should be written, even if the
311 disposition itself is unrecognized.
312
313 2.9 Content-Disposition and Multipart
314
315 If a Content-Disposition header is used on a multipart body part, it
316 applies to the multipart as a whole, not the individual subparts.
317 The disposition types of the subparts do not need to be consulted
318 until the multipart itself is presented. When the multipart is
319 displayed, then the dispositions of the subparts should be respected.
320
321 If the `inline' disposition is used, the multipart should be
322 displayed as normal; however, an `attachment' subpart should require
323 action from the user to display.
324
325 If the `attachment' disposition is used, presentation of the
326 multipart should not proceed without explicit user action. Once the
327 user has chosen to display the multipart, the individual subpart
328 dispositions should be consulted to determine how to present the
329 subparts.
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338 Troost, et. al. Standards Track [Page 6]
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340 RFC 2183 Content-Disposition August 1997
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343 2.10 Content-Disposition and the Main Message
344
345 It is permissible to use Content-Disposition on the main body of an
346 [RFC 822] message.
347
348 3. Examples
349
350 Here is a an example of a body part containing a JPEG image that is
351 intended to be viewed by the user immediately:
352
353 Content-Type: image/jpeg
354 Content-Disposition: inline
355 Content-Description: just a small picture of me
356
357 <jpeg data>
358
359 The following body part contains a JPEG image that should be
360 displayed to the user only if the user requests it. If the JPEG is
361 written to a file, the file should be named "genome.jpg". The
362 recipient's user might also choose to set the last-modified date of
363 the stored file to date in the modification-date parameter:
364
365 Content-Type: image/jpeg
366 Content-Disposition: attachment; filename=genome.jpeg;
367 modification-date="Wed, 12 Feb 1997 16:29:51 -0500";
368 Content-Description: a complete map of the human genome
369
370 <jpeg data>
371
372 The following is an example of the use of the `attachment'
373 disposition with a multipart body part. The user should see text-
374 part-1 immediately, then take some action to view multipart-2. After
375 taking action to view multipart-2, the user will see text-part-2
376 right away, and be required to take action to view jpeg-1. Subparts
377 are indented for clarity; they would not be so indented in a real
378 message.
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394 Troost, et. al. Standards Track [Page 7]
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396 RFC 2183 Content-Disposition August 1997
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399 Content-Type: multipart/mixed; boundary=outer
400 Content-Description: multipart-1
401
402 --outer
403 Content-Type: text/plain
404 Content-Disposition: inline
405 Content-Description: text-part-1
406
407 Some text goes here
408
409 --outer
410 Content-Type: multipart/mixed; boundary=inner
411 Content-Disposition: attachment
412 Content-Description: multipart-2
413
414 --inner
415 Content-Type: text/plain
416 Content-Disposition: inline
417 Content-Description: text-part-2
418
419 Some more text here.
420
421 --inner
422 Content-Type: image/jpeg
423 Content-Disposition: attachment
424 Content-Description: jpeg-1
425
426 <jpeg data>
427 --inner--
428 --outer--
429
430 4. Summary
431
432 Content-Disposition takes one of two values, `inline' and
433 `attachment'. `Inline' indicates that the entity should be
434 immediately displayed to the user, whereas `attachment' means that
435 the user should take additional action to view the entity.
436
437 The `filename' parameter can be used to suggest a filename for
438 storing the bodypart, if the user wishes to store it in an external
439 file.
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450 Troost, et. al. Standards Track [Page 8]
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452 RFC 2183 Content-Disposition August 1997
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455 5. Security Considerations
456
457 There are security issues involved any time users exchange data.
458 While these are not to be minimized, neither does this memo change
459 the status quo in that regard, except in one instance.
460
461 Since this memo provides a way for the sender to suggest a filename,
462 a receiving MUA must take care that the sender's suggested filename
463 does not represent a hazard. Using UNIX as an example, some hazards
464 would be:
465
466 + Creating startup files (e.g., ".login").
467
468 + Creating or overwriting system files (e.g., "/etc/passwd").
469
470 + Overwriting any existing file.
471
472 + Placing executable files into any command search path
473 (e.g., "~/bin/more").
474
475 + Sending the file to a pipe (e.g., "| sh").
476
477 In general, the receiving MUA should not name or place the file such
478 that it will get interpreted or executed without the user explicitly
479 initiating the action.
480
481 It is very important to note that this is not an exhaustive list; it
482 is intended as a small set of examples only. Implementors must be
483 alert to the potential hazards on their target systems.
484
485 6. References
486
487 [RFC 2119]
488 Bradner, S., "Key words for use in RFCs to Indicate Requirement
489 Levels", RFC 2119, March 1997.
490
491 [RFC 2184]
492 Freed, N. and K. Moore, "MIME Parameter value and Encoded Words:
493 Character Sets, Lanaguage, and Continuations", RFC 2184, August
494 1997.
495
496 [RFC 2045]
497 Freed, N. and N. Borenstein, "MIME (Multipurpose Internet Mail
498 Extensions) Part One: Format of Internet Message Bodies", RFC
499 2045, December 1996.
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506 Troost, et. al. Standards Track [Page 9]
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508 RFC 2183 Content-Disposition August 1997
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510
511 [RFC 2046]
512 Freed, N. and N. Borenstein, "MIME (Multipurpose Internet Mail
513 Extensions) Part Two: Media Types", RFC 2046, December 1996.
514
515 [RFC 2047]
516 Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part
517 Three: Message Header Extensions for non-ASCII Text", RFC 2047,
518 December 1996.
519
520 [RFC 2048]
521 Freed, N., Klensin, J. and J. Postel, "MIME (Multipurpose
522 Internet Mail Extensions) Part Four: Registration Procedures",
523 RFC 2048, December 1996.
524
525 [RFC 2049]
526 Freed, N. and N. Borenstein, "MIME (Multipurpose Internet Mail
527 Extensions) Part Five: Conformance Criteria and Examples", RFC
528 2049, December 1996.
529
530 [RFC 822]
531 Crocker, D., "Standard for the Format of ARPA Internet Text
532 Messages", STD 11, RFC 822, UDEL, August 1982.
533
534 7. Acknowledgements
535
536 We gratefully acknowledge the help these people provided during the
537 preparation of this draft:
538
539 Nathaniel Borenstein
540 Ned Freed
541 Keith Moore
542 Dave Crocker
543 Dan Pritchett
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564 RFC 2183 Content-Disposition August 1997
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567 8. Authors' Addresses
568
569 You should blame the editor of this version of the document for any
570 changes since RFC 1806:
571
572 Keith Moore
573 Department of Computer Science
574 University of Tennessee, Knoxville
575 107 Ayres Hall
576 Knoxville TN 37996-1301
577 USA
578
579 Phone: +1 (423) 974-5067
580 Fax: +1 (423) 974-8296
581 Email: moore@cs.utk.edu
582
583
584 The authors of RFC 1806 are:
585
586 Rens Troost
587 New Century Systems
588 324 East 41st Street #804
589 New York, NY, 10017 USA
590
591 Phone: +1 (212) 557-2050
592 Fax: +1 (212) 557-2049
593 EMail: rens@century.com
594
595
596 Steve Dorner
597 QUALCOMM Incorporated
598 6455 Lusk Boulevard
599 San Diego, CA 92121
600 USA
601
602 EMail: sdorner@qualcomm.com
603
604
605 9. Registration of New Content-Disposition Values and Parameters
606
607 New Content-Disposition values (besides "inline" and "attachment")
608 may be defined only by Internet standards-track documents, or in
609 Experimental documents approved by the Internet Engineering Steering
610 Group.
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618 Troost, et. al. Standards Track [Page 11]
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620 RFC 2183 Content-Disposition August 1997
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623 New content-disposition parameters may be registered by supplying the
624 information in the following template and sending it via electronic
625 mail to IANA@IANA.ORG:
626
627 To: IANA@IANA.ORG
628 Subject: Registration of new Content-Disposition parameter
629
630 Content-Disposition parameter name:
631
632 Allowable values for this parameter:
633 (If the parameter can only assume a small number of values,
634 list each of those values. Otherwise, describe the values
635 that the parameter can assume.)
636 Description:
637 (What is the purpose of this parameter and how is it used?)
638
639 10. Changes since RFC 1806
640
641 The following changes have been made since the earlier version of
642 this document, published in RFC 1806 as an Experimental protocol:
643
644 + Updated references to MIME documents. In some cases this
645 involved substituting a reference to one of the current MIME
646 RFCs for a reference to RFC 1521; in other cases, a reference to
647 RFC 1521 was simply replaced with the word "MIME".
648
649 + Added a section on registration procedures, since none of the
650 procedures in RFC 2048 seemed to be appropriate.
651
652 + Added new parameter types: creation-date, modification-date,
653 read-date, and size.
654
655
656 + Incorporated a reference to draft-freed-pvcsc-* for encoding
657 long or non-ASCII parameter values.
658
659 + Added reference to RFC 2119 to define MUST, SHOULD, etc.
660 keywords.
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