imapext-2007
view docs/rfc/rfc2180.txt @ 0:ada5e610ab86
imap-2007e
| author | yuuji@gentei.org |
|---|---|
| date | Mon, 14 Sep 2009 15:17:45 +0900 |
| parents | |
| children |
line source
7 Network Working Group M. Gahrns
8 Request for Comments: 2180 Microsoft
9 Category: Informational July 1997
12 IMAP4 Multi-Accessed Mailbox Practice
14 Status of this Memo
16 This memo provides information for the Internet community. This memo
17 does not specify an Internet standard of any kind. Distribution of
18 this memo is unlimited.
20 1. Abstract
22 IMAP4[RFC-2060] is rich client/server protocol that allows a client
23 to access and manipulate electronic mail messages on a server.
24 Within the protocol framework, it is possible to have differing
25 results for particular client/server interactions. If a protocol does
26 not allow for this, it is often unduly restrictive.
28 For example, when multiple clients are accessing a mailbox and one
29 attempts to delete the mailbox, an IMAP4 server may choose to
30 implement a solution based upon server architectural constraints or
31 individual preference.
33 With this flexibility comes greater client responsibility. It is not
34 sufficient for a client to be written based upon the behavior of a
35 particular IMAP server. Rather the client must be based upon the
36 behavior allowed by the protocol.
38 By documenting common IMAP4 server practice for the case of
39 simultaneous client access to a mailbox, we hope to ensure the widest
40 amount of inter-operation between IMAP4 clients and servers.
42 The behavior described in this document reflects the practice of some
43 existing servers or behavior that the consensus of the IMAP mailing
44 list has deemed to be reasonable. The behavior described within this
45 document is believed to be [RFC-2060] compliant. However, this
46 document is not meant to define IMAP4 compliance, nor is it an
47 exhaustive list of valid IMAP4 behavior. [RFC-2060] must always be
48 consulted to determine IMAP4 compliance, especially for server
49 behavior not described within this document.
58 Gahrns Informational [Page 1]
60 RFC 2180 IMAP4 Multi-Accessed Mailbox Practice July 1997
63 2. Conventions used in this document
65 In examples,"C1:", "C2:" and "C3:" indicate lines sent by 3 different
66 clients (client #1, client #2 and client #3) that are connected to a
67 server. "S1:", "S2:" and "S3:" indicated lines sent by the server to
68 client #1, client #2 and client #3 respectively.
70 A shared mailbox, is a mailbox that can be used by multiple users.
72 A multi-accessed mailbox, is a mailbox that has multiple clients
73 simultaneously accessing it.
75 A client is said to have accessed a mailbox after a successful SELECT
76 or EXAMINE command.
78 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
79 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
80 document are to be interpreted as described in [RFC-2119].
83 3. Deletion/Renaming of a multi-accessed mailbox
85 If an external agent or multiple clients are accessing a mailbox,
86 care must be taken when handling the deletion or renaming of the
87 mailbox. Following are some strategies an IMAP server may choose to
88 use when dealing with this situation.
91 3.1. The server MAY fail the DELETE/RENAME command of a multi-accessed
92 mailbox
94 In some cases, this behavior may not be practical. For example, if a
95 large number of clients are accessing a shared mailbox, the window in
96 which no clients have the mailbox accessed may be small or non-
97 existent, effectively rendering the mailbox undeletable or
98 unrenamable.
100 Example:
102 <Client #1 and Client #2 have mailbox FOO accessed. Client #1 tries
103 to DELETE the mailbox and is refused>
105 C1: A001 DELETE FOO
106 S1: A001 NO Mailbox FOO is in use by another user.
114 Gahrns Informational [Page 2]
116 RFC 2180 IMAP4 Multi-Accessed Mailbox Practice July 1997
119 3.2. The server MAY allow the DELETE command of a multi-accessed
120 mailbox, but keep the information in the mailbox available for
121 those clients that currently have access to the mailbox.
123 When all clients have finished accessing the mailbox, it is
124 permanently removed. For clients that do not already have access to
125 the mailbox, the 'ghosted' mailbox would not be available. For
126 example, it would not be returned to these clients in a subsequent
127 LIST or LSUB command and would not be a valid mailbox argument to any
128 other IMAP command until the reference count of clients accessing the
129 mailbox reached 0.
131 In some cases, this behavior may not be desirable. For example if
132 someone created a mailbox with offensive or sensitive information,
133 one might prefer to have the mailbox deleted and all access to the
134 information contained within removed immediately, rather than
135 continuing to allow access until the client closes the mailbox.
137 Furthermore, this behavior, may prevent 'recycling' of the same
138 mailbox name until all clients have finished accessing the original
139 mailbox.
141 Example:
143 <Client #1 and Client #2 have mailbox FOO selected. Client #1 DELETEs
144 mailbox FOO>
146 C1: A001 DELETE FOO
147 S1: A001 OK Mailbox FOO is deleted.
149 <Client #2 is still able to operate on the deleted mailbox>
151 C2: B001 STORE 1 +FLAGS (\Seen)
152 S2: * 1 FETCH FLAGS (\Seen)
153 S2: B001 OK STORE completed
155 <Client #3 which did not have access to the mailbox prior to the
156 deletion by client #1 does not have access to the mailbox>
158 C3: C001 STATUS FOO (MESSAGES)
159 S3: C001 NO Mailbox does not exist
161 <Nor is client #3 able to create a mailbox with the name FOO, while
162 the reference count is non zero>
164 C3: C002 CREATE FOO
165 S3: C002 NO Mailbox FOO is still in use. Try again later.
170 Gahrns Informational [Page 3]
172 RFC 2180 IMAP4 Multi-Accessed Mailbox Practice July 1997
175 <Client #2 closes its access to the mailbox, no other clients have
176 access to the mailbox FOO and reference count becomes 0>
178 C2: B002 CLOSE
179 S2: B002 OK CLOSE Completed
181 <Now that the reference count on FOO has reached 0, the mailbox name
182 can be recycled>
184 C3: C003 CREATE FOO
185 S3: C003 OK CREATE Completed
188 3.3. The server MAY allow the DELETE/RENAME of a multi-accessed
189 mailbox, but disconnect all other clients who have the mailbox
190 accessed by sending a untagged BYE response.
192 A server may often choose to disconnect clients in the DELETE case,
193 but may choose to implement a "friendlier" method for the RENAME
194 case.
196 Example:
198 <Client #1 and Client #2 have mailbox FOO accessed. Client #1 DELETEs
199 the mailbox FOO>
201 C1: A002 DELETE FOO
202 S1: A002 OK DELETE completed.
204 <Server disconnects all other users of the mailbox>
205 S2: * BYE Mailbox FOO has been deleted.
208 3.4. The server MAY allow the RENAME of a multi-accessed mailbox by
209 simply changing the name attribute on the mailbox.
211 Other clients that have access to the mailbox can continue issuing
212 commands such as FETCH that do not reference the mailbox name.
213 Clients would discover the renaming the next time they referred to
214 the old mailbox name. Some servers MAY choose to include the
215 [NEWNAME] response code in their tagged NO response to a command that
216 contained the old mailbox name, as a hint to the client that the
217 operation can succeed if the command is issued with the new mailbox
218 name.
226 Gahrns Informational [Page 4]
228 RFC 2180 IMAP4 Multi-Accessed Mailbox Practice July 1997
231 Example:
233 <Client #1 and Client #2 have mailbox FOO accessed. Client #1 RENAMEs
234 the mailbox.>
236 C1: A001 RENAME FOO BAR
237 S1: A001 OK RENAME completed.
239 <Client #2 is still able to do operations that do not reference the
240 mailbox name>
242 C2: B001 FETCH 2:4 (FLAGS)
243 S2: * 2 FETCH . . .
244 S2: * 3 FETCH . . .
245 S2: * 4 FETCH . . .
246 S2: B001 OK FETCH completed
248 <Client #2 is not able to do operations that reference the mailbox
249 name>
251 C2: B002 APPEND FOO {300} C2: Date: Mon, 7 Feb 1994
252 21:52:25 0800 (PST) C2: . . . S2: B002 NO [NEWNAME FOO
253 BAR] Mailbox has been renamed
256 4. Expunging of messages on a multi-accessed mailbox
258 If an external agent or multiple clients are accessing a mailbox,
259 care must be taken when handling the EXPUNGE of messages. Other
260 clients accessing the mailbox may be in the midst of issuing a
261 command that depends upon message sequence numbers. Because an
262 EXPUNGE response can not be sent while responding to a FETCH, STORE
263 or SEARCH command, it is not possible to immediately notify the
264 client of the EXPUNGE. This can result in ambiguity if the client
265 issues a FETCH, STORE or SEARCH operation on a message that has been
266 EXPUNGED.
269 4.1. Fetching of expunged messages
271 Following are some strategies an IMAP server may choose to use when
272 dealing with a FETCH command on expunged messages.
282 Gahrns Informational [Page 5]
284 RFC 2180 IMAP4 Multi-Accessed Mailbox Practice July 1997
287 Consider the following scenario:
289 - Client #1 and Client #2 have mailbox FOO selected.
290 - There are 7 messages in the mailbox.
291 - Messages 4:7 are marked for deletion.
292 - Client #1 issues an EXPUNGE, to expunge messages 4:7
295 4.1.1. The server MAY allow the EXPUNGE of a multi-accessed mailbox but
296 keep the messages available to satisfy subsequent FETCH commands
297 until it is able to send an EXPUNGE response to each client.
299 In some cases, the behavior of keeping "ghosted" messages may not be
300 desirable. For example if a message contained offensive or sensitive
301 information, one might prefer to instantaneously remove all access to
302 the information, regardless of whether another client is in the midst
303 of accessing it.
305 Example: (Building upon the scenario outlined in 4.1.)
307 <Client #2 is still able to access the expunged messages because the
308 server has kept a 'ghosted' copy of the messages until it is able to
309 notify client #2 of the EXPUNGE>
311 C2: B001 FETCH 4:7 RFC822
312 S2: * 4 FETCH RFC822 . . . (RFC822 info returned)
313 S2: * 5 FETCH RFC822 . . . (RFC822 info returned)
314 S2: * 6 FETCH RFC822 . . . (RFC822 info returned)
315 S2: * 7 FETCH RFC822 . . . (RFC822 info returned)
316 S2: B001 OK FETCH Completed
318 <Client #2 issues a command where it can get notified of the EXPUNGE>
320 C2: B002 NOOP
321 S2: * 4 EXPUNGE
322 S2: * 4 EXPUNGE
323 S2: * 4 EXPUNGE
324 S2: * 4 EXPUNGE
325 S2: * 3 EXISTS
326 S2: B002 OK NOOP Complete
328 <Client #2 no longer has access to the expunged messages>
330 C2: B003 FETCH 4:7 RFC822
331 S2: B003 NO Messages 4:7 are no longer available.
338 Gahrns Informational [Page 6]
340 RFC 2180 IMAP4 Multi-Accessed Mailbox Practice July 1997
343 4.1.2 The server MAY allow the EXPUNGE of a multi-accessed mailbox,
344 and on subsequent FETCH commands return FETCH responses only for
345 non-expunged messages and a tagged NO.
347 After receiving a tagged NO FETCH response, the client SHOULD issue a
348 NOOP command so that it will be informed of any pending EXPUNGE
349 responses. The client may then either reissue the failed FETCH
350 command, or by examining the EXPUNGE response from the NOOP and the
351 FETCH response from the FETCH, determine that the FETCH failed
352 because of pending expunges.
354 Example: (Building upon the scenario outlined in 4.1.)
356 <Client #2 attempts to FETCH a mix of expunged and non-expunged
357 messages. A FETCH response is returned only for then non-expunged
358 messages along with a tagged NO>
360 C2: B001 FETCH 3:5 ENVELOPE
361 S2: * 3 FETCH ENVELOPE . . . (ENVELOPE info returned)
362 S2: B001 NO Some of the requested messages no longer exist
364 <Upon receiving a tagged NO FETCH response, Client #2 issues a NOOP
365 to be informed of any pending EXPUNGE responses>
367 C2: B002 NOOP
368 S2: * 4 EXPUNGE
369 S2: * 4 EXPUNGE
370 S2: * 4 EXPUNGE
371 S2: * 4 EXPUNGE
372 S2: * 3 EXISTS
373 S2: B002 OK NOOP Completed.
375 <By receiving a FETCH response for message 3, and an EXPUNGE response
376 that indicates messages 4:7 have been expunged, the client does not
377 need to re-issue the FETCH>
394 Gahrns Informational [Page 7]
396 RFC 2180 IMAP4 Multi-Accessed Mailbox Practice July 1997
399 4.1.3 The server MAY allow the EXPUNGE of a multi-accessed mailbox, and
400 on subsequent FETCH commands return the usual FETCH responses for
401 non-expunged messages, "NIL FETCH Responses" for expunged
402 messages, and a tagged OK response.
404 If all of the messages in the subsequent FETCH command have been
405 expunged, the server SHOULD return only a tagged NO. In this case,
406 the client SHOULD issue a NOOP command so that it will be informed of
407 any pending EXPUNGE responses. The client may then either reissue
408 the failed FETCH command, or by examining the EXPUNGE response from
409 the NOOP, determine that the FETCH failed because of pending
410 expunges.
412 "NIL FETCH responses" are a representation of empty data as
413 appropriate for the FETCH argument specified.
415 Example:
417 * 1 FETCH (ENVELOPE (NIL NIL NIL NIL NIL NIL NIL NIL NIL NIL))
418 * 1 FETCH (FLAGS ())
419 * 1 FETCH (INTERNALDATE "00-Jan-0000 00:00:00 +0000")
420 * 1 FETCH (RFC822 "")
421 * 1 FETCH (RFC822.HEADER "")
422 * 1 FETCH (RFC822.TEXT "")
423 * 1 FETCH (RFC822.SIZE 0)
424 * 1 FETCH (BODY ("TEXT" "PLAIN" NIL NIL NIL "7BIT" 0 0)
425 * 1 FETCH (BODYSTRUCTURE ("TEXT" "PLAIN" NIL NIL NIL "7BIT" 0 0)
426 * 1 FETCH (BODY[<section>] "")
427 * 1 FETCH (BODY[<section>]<partial> "")
429 In some cases, a client may not be able to distinguish between "NIL
430 FETCH responses" received because a message was expunged and those
431 received because the data actually was NIL. For example, a * 5
432 FETCH (FLAGS ()) response could be received if no flags were set on
433 message 5, or because message 5 was expunged. In a case of potential
434 ambiguity, the client SHOULD issue a command such as NOOP to force
435 the sending of the EXPUNGE responses to resolve any ambiguity.
437 Example: (Building upon the scenario outlined in 4.1.)
439 <Client #2 attempts to access a mix of expunged and non-expunged
440 messages. Normal data is returned for non-expunged message, "NIL
441 FETCH responses" are returned for expunged messages>
450 Gahrns Informational [Page 8]
452 RFC 2180 IMAP4 Multi-Accessed Mailbox Practice July 1997
455 C2: B002 FETCH 3:5 ENVELOPE
456 S2: * 3 FETCH ENVELOPE . . . (ENVELOPE info returned)
457 S2: * 4 FETCH ENVELOPE (NIL NIL NIL NIL NIL NIL NIL NIL
458 NIL NIL)
459 S2: * 5 FETCH ENVELOPE (NIL NIL NIL NIL NIL NIL NIL NIL
460 NIL NIL)
461 S2: B002 OK FETCH Completed
463 <Client #2 attempts to FETCH only expunged messages and receives a
464 tagged NO response>
466 C2: B002 FETCH 4:7 ENVELOPE
467 S2: B002 NO Messages 4:7 have been expunged.
470 4.1.4 To avoid the situation altogether, the server MAY fail the
471 EXPUNGE of a multi-accessed mailbox
473 In some cases, this behavior may not be practical. For example, if a
474 large number of clients are accessing a shared mailbox, the window in
475 which no clients have the mailbox accessed may be small or non-
476 existent, effectively rendering the message unexpungeable.
479 4.2. Storing of expunged messages
481 Following are some strategies an IMAP server may choose to use when
482 dealing with a STORE command on expunged messages.
485 4.2.1 If the ".SILENT" suffix is used, and the STORE completed
486 successfully for all the non-expunged messages, the server SHOULD
487 return a tagged OK.
489 Example: (Building upon the scenario outlined in 4.1.)
491 <Client #2 tries to silently STORE flags on expunged and non-
492 expunged messages. The server sets the flags on the non-expunged
493 messages and returns OK>
495 C2: B001 STORE 1:7 +FLAGS.SILENT (\SEEN)
496 S2: B001 OK
506 Gahrns Informational [Page 9]
508 RFC 2180 IMAP4 Multi-Accessed Mailbox Practice July 1997
511 4.2.2. If the ".SILENT" suffix is not used, and only expunged messages
512 are referenced, the server SHOULD return only a tagged NO.
514 Example: (Building upon the scenario outlined in 4.1.)
516 <Client #2 tries to STORE flags only on expunged messages>
518 C2: B001 STORE 5:7 +FLAGS (\SEEN)
519 S2: B001 NO Messages have been expunged
522 4.2.3. If the ".SILENT" suffix is not used, and a mixture of expunged
523 and non-expunged messages are referenced, the server MAY set the
524 flags and return a FETCH response for the non-expunged messages
525 along with a tagged NO.
527 After receiving a tagged NO STORE response, the client SHOULD issue a
528 NOOP command so that it will be informed of any pending EXPUNGE
529 responses. The client may then either reissue the failed STORE
530 command, or by examining the EXPUNGE responses from the NOOP and
531 FETCH responses from the STORE, determine that the STORE failed
532 because of pending expunges.
534 Example: (Building upon the scenario outlined in 4.1.)
536 <Client #2 tries to STORE flags on a mixture of expunged and non-
537 expunged messages>
539 C2: B001 STORE 1:7 +FLAGS (\SEEN)
540 S2: * FETCH 1 FLAGS (\SEEN)
541 S2: * FETCH 2 FLAGS (\SEEN)
542 S2: * FETCH 3 FLAGS (\SEEN)
543 S2: B001 NO Some of the messages no longer exist.
545 C2: B002 NOOP
546 S2: * 4 EXPUNGE
547 S2: * 4 EXPUNGE
548 S2: * 4 EXPUNGE
549 S2: * 4 EXPUNGE
550 S2: * 3 EXISTS
551 S2: B002 OK NOOP Completed.
553 <By receiving FETCH responses for messages 1:3, and an EXPUNGE
554 response that indicates messages 4:7 have been expunged, the client
555 does not need to re-issue the STORE>
562 Gahrns Informational [Page 10]
564 RFC 2180 IMAP4 Multi-Accessed Mailbox Practice July 1997
567 4.2.4. If the ".SILENT" suffix is not used, and a mixture of expunged
568 and non-expunged messages are referenced, the server MAY return
569 an untagged NO and not set any flags.
571 After receiving a tagged NO STORE response, the client SHOULD issue a
572 NOOP command so that it will be informed of any pending EXPUNGE
573 responses. The client would then re-issue the STORE command after
574 updating its message list per any EXPUNGE response.
576 If a large number of clients are accessing a shared mailbox, the
577 window in which there are no pending expunges may be small or non-
578 existent, effectively disallowing a client from setting the flags on
579 all messages at once.
581 Example: (Building upon the scenario outlined in 4.1.)
583 <Client #2 tries to STORE flags on a mixture of expunged and non-
584 expunged messages>
586 C2: B001 STORE 1:7 +FLAGS (\SEEN)
587 S2: B001 NO Some of the messages no longer exist.
589 <Client #2 issues a NOOP to be informed of the EXPUNGED messages>
591 C2: B002 NOOP
592 S2: * 4 EXPUNGE
593 S2: * 4 EXPUNGE
594 S2: * 4 EXPUNGE
595 S2: * 4 EXPUNGE
596 S2: * 3 EXISTS
597 S2: B002 OK NOOP Completed.
599 <Client #2 updates its message list and re-issues the STORE on only
600 those messages that have not been expunged>
602 C2: B003 STORE 1:3 +FLAGS (\SEEN) S2: * FETCH 1 FLAGS
603 (\SEEN) S2: * FETCH 2 FLAGS (\SEEN) S2: * FETCH 3 FLAGS
604 (\SEEN) S2: B003 OK STORE Completed
607 4.3. Searching of expunged messages
609 A server MAY simply not return a search response for messages that
610 have been expunged and it has not been able to inform the client
611 about. If a client was expecting a particular message to be returned
612 in a search result, and it was not, the client SHOULD issue a NOOP
613 command to see if the message was expunged by another client.
618 Gahrns Informational [Page 11]
620 RFC 2180 IMAP4 Multi-Accessed Mailbox Practice July 1997
623 4.4 Copying of expunged messages
625 COPY is the only IMAP4 sequence number command that is safe to allow
626 an EXPUNGE response on. This is because a client is not permitted to
627 cascade several COPY commands together. A client is required to wait
628 and confirm that the copy worked before issuing another one.
630 4.4.1 The server MAY disallow the COPY of messages in a multi-access
631 mailbox that contains expunged messages.
633 Pending EXPUNGE response(s) MUST be returned to the COPY command.
635 Example:
637 C: A001 COPY 2,4,6,8 FRED
638 S: * 4 EXPUNGE
639 S: A001 NO COPY rejected, because some of the requested
640 messages were expunged
642 Note: Non of the above messages are copied because if a COPY command
643 is unsuccessful, the server MUST restore the destination mailbox to
644 its state before the COPY attempt.
647 4.4.2 The server MAY allow the COPY of messages in a multi-access
648 mailbox that contains expunged messages.
650 Pending EXPUNGE response(s) MUST be returned to the COPY command.
651 Messages that are copied are messages corresponding to sequence
652 numbers before any EXPUNGE response.
654 Example:
656 C: A001 COPY 2,4,6,8 FRED
657 S: * 3 EXPUNGE
658 S: A001 OK COPY completed
660 In the above example, the messages that are copied to FRED are
661 messages 2,4,6,8 at the start of the COPY command. These are
662 equivalent to messages 2,3,5,7 at the end of the COPY command. The
663 EXPUNGE response can't take place until after the messages from the
664 COPY command are identified (because of the "no expunge while no
665 commands in progress" rule).
674 Gahrns Informational [Page 12]
676 RFC 2180 IMAP4 Multi-Accessed Mailbox Practice July 1997
679 Example:
681 C: A001 COPY 2,4,6,8 FRED
682 S: * 4 EXPUNGE
683 S: A001 OK COPY completed
685 In the above example, message 4 was copied before it was expunged,
686 and MUST appear in the destination mailbox FRED.
689 5. Security Considerations
691 This document describes behavior of servers that use the IMAP4
692 protocol, and as such, has the same security considerations as
693 described in [RFC-2060].
695 In particular, some described server behavior does not allow for the
696 immediate deletion of information when a mailbox is accessed by
697 multiple clients. This may be a consideration when dealing with
698 sensitive information where immediate deletion would be preferred.
701 6. References
703 [RFC-2060], Crispin, M., "Internet Message Access Protocol - Version
704 4rev1", RFC 2060, University of Washington, December 1996.
706 [RFC-2119], Bradner, S., "Key words for use in RFCs to Indicate
707 Requirement Levels", RFC 2119, Harvard University, March 1997.
710 7. Acknowledgments
712 This document is the result of discussions on the IMAP4 mailing list
713 and is meant to reflect consensus of this group. In particular,
714 Raymond Cheng, Mark Crispin, Jim Evans, Erik Forsberg, Steve Hole,
715 Mark Keasling, Barry Leiba, Syd Logan, John Mani, Pat Moran, Larry
716 Osterman, Chris Newman, Bart Schaefer, Vladimir Vulovic, and Jack De
717 Winter were active participants in this discussion or made
718 suggestions to this document.
730 Gahrns Informational [Page 13]
732 RFC 2180 IMAP4 Multi-Accessed Mailbox Practice July 1997
735 8. Author's Address
737 Mike Gahrns
738 Microsoft
739 One Microsoft Way
740 Redmond, WA, 98072
742 Phone: (206) 936-9833
743 EMail: mikega@microsoft.com
786 Gahrns Informational [Page 14]