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Keywords: IMAP
Internet Engineering Task Force (IETF) A. Gulbrandsen
Request for Comments: 6851
Category: Standards Track N. Freed, Ed.
ISSN: 2070-1721 Oracle
January 2013
Internet Message Access Protocol (IMAP) - MOVE Extension
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
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6851.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Gulbrandsen & Freed Standards Track [Page 1]
RFC 6851 IMAP - MOVE Extension January 2013
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. Additionally,
servers with certain types of back-end message stores might have
efficient ways of moving messages, which don't involve the actual
copying of data. Such efficiencies are often not available to the
COPY/STORE/EXPUNGE process.
The MOVE extension is present in any IMAP implementation that 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.
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RFC 6851 IMAP - MOVE Extension January 2013
3. MOVE and UID MOVE
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.
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RFC 6851 IMAP - MOVE Extension January 2013
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
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 as 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 foo
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 or UID
MOVE.
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
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RFC 6851 IMAP - MOVE Extension January 2013
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.
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, Access Control List (ACL)
The ACL rights [RFC4314] required for MOVE and 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] SHOULD send COPYUID in response
to a UID MOVE command. For additional information see Section 3 of
[RFC4315].
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
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RFC 6851 IMAP - MOVE Extension January 2013
updated per-mailbox modification sequence using the HIGHESTMODSEQ
response code (defined in [RFC4551]) in the tagged or untagged OK
response.
When one or more messages are moved to a target mailbox, if the
server is capable of storing modification sequences for the mailbox,
the server MUST generate and assign new modification sequence numbers
to the moved messages that are higher than the highest modification
sequence of the messages originally in the mailbox.
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 that could
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RFC 6851 IMAP - MOVE Extension January 2013
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.
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 has added MOVE to the "IMAP 4 Capabilities" registry,
<http://www.iana.org/assignments/imap4-capabilities>.
8. Acknowledgments
This document is dedicated to the memory of Mark Crispin, the
inventor of the IMAP protocol, author of the IMAP protocol
specification [RFC3501], and contributor to many other email
specifications in the IETF.
An extension like this has been proposed many times, by many people.
This document is based on several of those proposals, 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.
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RFC 6851 IMAP - MOVE Extension January 2013
[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.
[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.
Authors' Addresses
Arnt Gulbrandsen
Schweppermannstr. 8
D-81671 Muenchen
Germany
Fax: +49 89 4502 9758
EMail: arnt@gulbrandsen.priv.no
Ned Freed (editor)
Oracle
800 Royal Oaks
Monrovia, CA 91016-6347
USA
EMail: ned+ietf@mrochek.com
Gulbrandsen & Freed Standards Track [Page 8]