Internet DRAFT - draft-ietf-imapmove-command
draft-ietf-imapmove-command
Network Working Group A. Gulbrandsen
Internet-Draft
Intended status: Standards Track N. Freed
Expires: June 2, 2013 Oracle
November 29, 2012
Internet Message Access Protocol (IMAP) - MOVE Extension
draft-ietf-imapmove-command-05
Abstract
This document defines an IMAP extension consisting of two new
commands, MOVE and UID MOVE, that are used to move messages from one
mailbox to another.
Status of this Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on June 2, 2013.
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1. Introduction
This document defines an IMAP [RFC3501] extension to facilitate
moving messages from one mailbox to another. This is accomplished by
defining a new MOVE command and extending the UID command to allow
UID MOVE.
A move function is not provided in the base IMAP specification, so
clients have instead had to use a combination of the COPY, STORE, and
EXPUNGE commands to perform this very common operation.
Implementors have long pointed out some shortcomings with this
approach. Because the moving of a message is not an atomic process,
interruptions can leave messages in intermediate states. Because
multiple clients can be accessing the mailboxes at the same time,
clients can see messages in intermediate states even without
interruptions. If the source mailbox contains other messages that
are flagged for deletion, the third step can have the side effect of
expunging more than just the set of moved messages. And servers with
certain types of back-end message stores might have efficient ways of
moving messages, which don't involve actual copying of data. Such
efficiencies are often not available to the COPY/STORE/EXPUNGE
process.
The MOVE extension is present in any IMAP4 implementation which
returns "MOVE" as one of the supported capabilities to the CAPABILITY
command.
2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Formal syntax is specified using ABNF [RFC5234].
Example lines prefaced by "C:" are sent by the client and ones
prefaced by "S:" by the server.
3. MOVE and UID MOVE
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3.1. MOVE Command
Arguments: sequence set
mailbox name
Responses: no specific responses for this command
Result: OK - move completed
NO - move error: can't move those messages or to that name
BAD - command unknown or arguments invalid
3.2. UID MOVE Command
This extends the first form of the UID command (see [RFC3501],
Section 6.4.8) to add the MOVE command, defined above, as a valid
argument.
3.3. Semantics of MOVE and UID MOVE
The MOVE command takes two arguments: a message set (sequence numbers
for MOVE, UIDs for UID MOVE) and a named mailbox. Each message
included in the set is moved, rather than copied, from the selected
(source) mailbox to the named (target) mailbox.
This means that a new message is created in the target mailbox, with
a new UID, the original message is removed from the source mailbox,
and it appears to the client as a single action. This has the same
effect for each message as this sequence:
1. [UID] COPY
2. [UID] STORE +FLAGS.SILENT \DELETED
3. UID EXPUNGE
Although the effect of the MOVE is the same as the preceding steps,
the semantics are not identical: The intermediate states produced by
those steps do not occur, and the response codes are different. In
particular, though the COPY and EXPUNGE response codes will be
returned, response codes for a STORE MUST NOT be generated and the
\DELETED flag MUST NOT be set for any message.
Because a MOVE applies to a set of messages, it might fail partway
through the set. Regardless of whether the command is successful in
moving the entire set, each individual message SHOULD either be moved
or unaffected. The server MUST leave each message in a state where
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it is in at least one of the source or target mailboxes (no message
can be lost or orphaned). The server SHOULD NOT leave any message in
both mailboxes (it would be bad for a partial failure to result in a
bunch of duplicate messages). This is true even if the server
returns a tagged NO response to the command.
Because of the similarity of MOVE to COPY, extensions that affect
COPY affect MOVE in the same way. Response codes such TRYCREATE (see
[RFC3501] Section 6.4.7), as well as those defined by extensions, are
sent as appropriate. See Section 4 for more information about how
MOVE interacts with other IMAP extensions.
An example:
C: a UID MOVE 42:69 forble
S: * OK [COPYUID 432432 42:69 1202:1229]
S: * 22 EXPUNGE
S: (more expunges)
S: a OK Done
Note that the server may send unrelated EXPUNGE responses as well, if
any happen to have been expunged at the same time; this is normal
IMAP operation.
Implementers will need to read [RFC4315] to understand what UID
EXPUNGE does, though full implementation of [RFC4315] is not
necessary.
Note that moving a message to the currently selected mailbox (that
is, where the source and target mailboxes are the same) is allowed
when copying the message to the currently selected mailbox is
allowed.
The server may send EXPUNGE (or VANISHED) responses before the tagged
response, so the client cannot safely send more commands with message
sequence number arguments while the server is processing MOVE. The
UID MOVE command does not have this limitation.
Both MOVE and UID MOVE can be pipelined with other commands, but care
has to be taken. Both commands modify sequence numbers and also
allow unrelated EXPUNGE responses. The renumbering of other messages
in the source mailbox following any EXPUNGE response can be
surprising, and makes it unsafe to pipeline any command that relies
on message sequence numbers after a MOVE or UID MOVE. Similarly,
MOVE cannot be pipelined with a command that might cause message
renumbering. See [RFC3501], Section 5.5, for more information about
ambiguities as well as handling requirements for both clients and
servers.
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4. Interaction with other extensions
This section describes how MOVE interacts with some other IMAP
extensions.
4.1. RFC 2087, QUOTA
The QUOTA extension (defined by [RFC2087]) may interact with MOVE, on
some servers, in the sense that a MOVE command may succeed where COPY
would cause a quota overrun.
4.2. RFC 4314, ACL
The ACL rights [RFC4314] required for UID MOVE are the union of the
ACL rights required for UID STORE, UID COPY and UID EXPUNGE.
4.3. RFC 4315, UIDPLUS
Servers supporting UIDPLUS [RFC4315] MUST send COPYUID in response to
a UID MOVE command.
Servers implementing UIDPLUS are also advised to send the COPYUID
response code in an untagged OK before sending EXPUNGE or moved
responses. (Sending COPYUID in the tagged OK, as described in the
UIDPLUS specification, means that clients first receive an EXPUNGE
for a message and afterwards COPYUID for the same message. It can be
unnecessarily difficult to process that sequence usefully.)
4.4. RFC 5162, QRESYNC
The QRESYNC extension [RFC5162] states that the server SHOULD send
VANISHED rather than EXPUNGE in response to the UID EXPUNGE command.
The same requirement applies to MOVE, and a QRESYNC-enabled client
needs to handle both VANISHED and EXPUNGE responses to a UID MOVE
command.
If the server is capable of storing modification sequences for the
selected mailbox, it MUST increment the per-mailbox mod-sequence if
at least one message was permanently moved due to the execution of
the MOVE/UID MOVE command. For each permanently removed message, the
server MUST remember the incremented mod-sequence and corresponding
UID. If at least one message was moved, the server MUST send the
updated per-mailbox modification sequence using the HIGHESTMODSEQ
response code (defined in [RFC4551]) in the tagged or untagged OK
response.
When a message is moved to a target mailbox, if the server is capable
of storing modification sequences for the mailbox, the server MUST
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generate a new modification sequence that is higher than the highest
modification sequence of all messages in the mailbox and assign it to
the moved message.
4.5. IMAP Events in Sieve
MOVE applies to IMAP events in Sieve [RFC6785] in the same way as
COPY does. Therefore, MOVE can cause a Sieve script to be invoked
with the imap.cause set to "COPY". Because MOVE does not cause flags
to be changed, a MOVE command will not result in a script invocation
with the imap.cause set to "FLAG".
5. Formal Syntax
The following syntax specification uses the Augmented Backus-Naur
Form (ABNF) notation as specified in [RFC5234]. [RFC3501] defines
the non-terminals "capability", "command-select", "sequence-set" and
"mailbox".
Except as noted otherwise, all alphabetic characters are case-
insensitive. The use of upper or lower case characters to define
token strings is for editorial clarity only. Implementations MUST
accept these strings in a case-insensitive fashion.
capability =/ "MOVE"
command-select =/ move
move = "MOVE" SP sequence-set SP mailbox
uid = "UID" SP (copy / fetch / search / store / move)
6. Security Considerations
MOVE does not introduce any new capabilities to IMAP, and this limits
the security impact. However, the transactional semantics of MOVE
may interact with specific implementations in ways that could have
unexpected consequences. For example, moving messages between
mailboxes under the quota root may require temporary suspension of
quota checking.
An additional area of concern is interaction with antispam,
antivirus, and other security scanning and auditing mechanisms.
Different mailboxes may have different security policies which could
interact with MOVE in complex ways. Scanning with updated rules may
also be required when messages are moved even when the underlying
policy has not changed.
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MOVE does relieve a problem with the base specification, since client
authors currently have to devise and implement complicated algorithms
to handle partial failures of the STORE/COPY/EXPUNGE trio.
Incomplete or improper implementation of these algorithms can lead to
mail loss.
7. IANA Considerations
The IANA is requested to add MOVE to the "IMAP 4 Capabilities"
registry, http://www.iana.org/assignments/imap4-capabilities.
8. Acknowledgments
An extension like this has been proposed many times, by many people.
This document is based on several of those, most recently that by
Witold Krecicki. Witold, Benoit Claise, Adrien W. de Croy, Stephen
Farrell, Bron Gondwana, Dan Karp, Christian Ketterer, Murray
Kucherawy, Jan Kundrat, Barry Leiba, Alexey Melnikov, Kathleen
Moriarty, Zoltan Ordogh, Pete Resnick, Timo Sirainen, Michael
Slusarz, and others provided valuable comments.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, March 2003.
[RFC4314] Melnikov, A., "IMAP4 Access Control List (ACL) Extension",
RFC 4314, December 2005.
[RFC4315] Crispin, M., "Internet Message Access Protocol (IMAP) -
UIDPLUS extension", RFC 4315, December 2005.
[RFC4551] Melnikov, A. and S. Hole, "IMAP Extension for Conditional
STORE Operation or Quick Flag Changes Resynchronization",
RFC 4551, June 2006.
[RFC5162] Melnikov, A., Cridland, D., and C. Wilson, "IMAP4
Extensions for Quick Mailbox Resynchronization", RFC 5162,
March 2008.
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[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
9.2. Informative References
[RFC2087] Myers, J., "IMAP4 QUOTA extension", RFC 2087,
January 1997.
[RFC6785] Leiba, B., "Support for Internet Message Access Protocol
(IMAP) Events in Sieve", RFC 6785, November 2012.
Appendix A. Change History
RFC Editor: Please delete this section from the final RFC.
A.1. Changes since -00
1. Fixed two bad nouns. Mailboxes aren't messages.
2. Adrien's server can easily do UID MOVE but not so easily MSN-
based moves.
A.2. Changes since -01
1. Changed to Informative, on Barry's suggestion. Or did I ask him?
Whatever.
2. Removed the 'reasons to avoid', it was doubleplusungood.
A.3. Changes since draft-gulbrandsen-imap-move-02
1. Various wording changes from Barry's review.
2. Open issue: Delete the \deleted rule?
3. Back to PS, informative didn't fly in the IESG
4. Turned into a WG document in order to get write access to the
IMAP4 capabilities registry
5. Mention VANISHED in 5162
6. Added bad boilerplate to please idnits. This document contains
no code.
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A.4. Changes since -00
1. Added MSN-based move. The consensus seems mildly in favor. I
think. We'll see once this is posted.
2. Advise sending COPYUID earlier, to help clients. Requiring out
of order processing is unnecessarily nasty.
3. Note that moving to the source inbox has to work. I think it
does have to work, but this is a draft, it says so on every page.
A.5. Changes since -01
1. (Issue tracker #1) Changed command-select ABNF to conform with
the conventions used in RFC 3501.
2. (Issue tracker #2) Banned overlapped pipelined MOVE and UID
MOVE.
3. (Issue tracker #3) Added section about interaction with IMAP
Sieve.
4. (Issue tracker #4) Revised security considerations.
5. (Issue tracker #5) Removed text that characterized MOVE as the
same as COPY/STORE/EXPUNGE.
6. (Issue tracker #6) RFC 4314 is now a normative reference.
7. (Issue tracker #7) Major rewrite of the command description text
as a result of AD review.
8. (Issue tracker #8) Revised abstract.
9. (Issue tracker #9) Added text saying partial failures are
allowed.
10. (Issue tracker #10) Some additional tweaks to the security
considerations section were made.
11. The abstract and introduction were out of whack as a result of
other changes, so some revisions were made to bring them back
into sync.
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A.6. Changes since -02
1. Corrected various typos, clarified several paragraphs discussing
MOVE semantics.
2. Added the usual text about the extension only being available
when it is reported by the CAPABILITIES command.
3. Revised the text about QRESYNC to make clear what the
requirements are.
4. Removed a suggestion about MUA/user behavior from the discussion
of the QUOTA extension.
5. Updated the main and running title to conform to other IMAP RFCs.
A.7. Changes since -03
1. Corrected another bad noun. Responses aren't messages.
2. More minor editorial changes.
3. The text on command pipelining has been completely rewritten.
4. Added text stating that the effects of setting the \Deleted flag
aren't visible.
A.8. Changes since -04
1. Added some additional requirements text for QRESYNC.
2. Took some text from the charter and added it to the introduction
in order to clarify why this extension is useful.
Authors' Addresses
Arnt Gulbrandsen
Schweppermannstr. 8
D-81671 Muenchen
Germany
Fax: +49 89 4502 9758
Email: arnt@gulbrandsen.priv.no
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Ned Freed (editor)
Oracle
800 Royal Oaks
Monrovia, CA 91016-6347
USA
Email: ned+ietf@mrochek.com
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