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+Internet Engineering Task Force (IETF) B. Leiba
+Request for Comments: 6237 Huawei Technologies
+Updates: 4466 A. Melnikov
+Category: Experimental Isode Limited
+ISSN: 2070-1721 May 2011
+
+
+ IMAP4 Multimailbox SEARCH Extension
+
+Abstract
+
+ The IMAP4 specification allows the searching of only the selected
+ mailbox. A user often wants to search multiple mailboxes, and a
+ client that wishes to support this must issue a series of SELECT and
+ SEARCH commands, waiting for each to complete before moving on to the
+ next. This extension allows a client to search multiple mailboxes
+ with one command, limiting the round trips and waiting for various
+ searches to complete, and not requiring disruption of the currently
+ selected mailbox. This extension also uses MAILBOX and TAG fields in
+ ESEARCH responses, allowing a client to pipeline the searches if it
+ chooses. This document updates RFC 4466.
+
+Status of This Memo
+
+ This document is not an Internet Standards Track specification; it is
+ published for examination, experimental implementation, and
+ evaluation.
+
+ This document defines an Experimental Protocol for the Internet
+ community. 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). Not
+ all documents approved by the IESG are a candidate for any level of
+ Internet Standard; see 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/rfc6237.
+
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+Leiba & Melnikov Experimental [Page 1]
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+RFC 6237 IMAP4 Multimailbox SEARCH Extension May 2011
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+Copyright Notice
+
+ Copyright (c) 2011 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.
+
+Table of Contents
+
+ 1. Introduction ....................................................2
+ 1.1. Conventions Used in This Document ..........................3
+ 2. New ESEARCH Command .............................................3
+ 2.1. The ESEARCH Response .......................................4
+ 2.2. Source Options: Specifying Mailboxes to Search .............5
+ 3. Examples ........................................................6
+ 4. Formal Syntax ...................................................7
+ 5. Security Considerations .........................................8
+ 6. IANA Considerations .............................................9
+ 7. Acknowledgements ................................................9
+ 8. Normative References ............................................9
+
+1. Introduction
+
+ The IMAP4 specification allows the searching of only the selected
+ mailbox. A user often wants to search multiple mailboxes, and a
+ client that wishes to support this must issue a series of SELECT and
+ SEARCH commands, waiting for each to complete before moving on to the
+ next. The commands can't be pipelined, because the server might run
+ them in parallel, and the untagged SEARCH responses could not then be
+ distinguished from each other.
+
+ This extension allows a client to search multiple mailboxes with one
+ command, and includes MAILBOX and TAG fields in the ESEARCH response,
+ yielding the following advantages:
+
+ o A single command limits the number of round trips needed to search
+ a set of mailboxes.
+
+ o A single command eliminates the need to wait for one search to
+ complete before starting the next.
+
+
+
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+RFC 6237 IMAP4 Multimailbox SEARCH Extension May 2011
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+ o A single command allows the server to optimize the search, if it
+ can.
+
+ o A command that is not dependent upon the selected mailbox
+ eliminates the need to disrupt the selection state or to open
+ another IMAP connection.
+
+ o The MAILBOX, UIDVALIDITY, and TAG fields in the responses allow a
+ client to distinguish which responses go with which search (and
+ which mailbox). A client can safely pipeline these search
+ commands without danger of confusion. The addition of the MAILBOX
+ and UIDVALIDITY fields updates the search-correlator item defined
+ in [RFC4466].
+
+1.1. Conventions Used in This Document
+
+ In examples, "C:" indicates lines sent by a client that is connected
+ to a server. "S:" indicates lines sent by the server to the client.
+
+ 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 RFC 2119 [RFC2119].
+
+2. New ESEARCH Command
+
+ Arguments: OPTIONAL source options
+ OPTIONAL result options
+ OPTIONAL charset specification (see [RFC2978])
+ searching criteria (one or more)
+
+ Responses: REQUIRED untagged response: ESEARCH
+
+ Result: OK -- search completed
+ NO -- error: cannot search that charset or criteria
+ BAD -- command unknown or arguments invalid
+
+ This section defines a new ESEARCH command, which works similarly to
+ the UID SEARCH command described in Section 2.6.1 of [RFC4466]
+ (initially described in Section 6.4.4 of [RFC3501] and extended by
+ [RFC4731]).
+
+ The ESEARCH command further extends searching by allowing for
+ optional source and result options. This document does not define
+ any new result options (see Section 3.1 of [RFC4731]). A server that
+ supports this extension includes "MULTISEARCH" in its IMAP capability
+ string.
+
+
+
+
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+ Because there has been confusion about this, it is worth pointing out
+ that with ESEARCH, as with *any* SEARCH or UID SEARCH command, it
+ MUST NOT be considered an error if the search terms include a range
+ of message numbers that extends (or, in fact, starts) beyond the end
+ of the mailbox. For example, a client might want to establish a
+ rolling window through the search results this way:
+
+ C: tag1 UID ESEARCH FROM "frobozz" 1:100
+
+ ...followed later by this:
+
+ C: tag1 UID ESEARCH FROM "frobozz" 101:200
+
+ ...and so on. This tells the server to match only the first hundred
+ messages in the mailbox the first time, the second hundred the second
+ time, etc. In fact, it might likely allow the server to optimize the
+ search significantly. In the above example, whether the mailbox
+ contains 50 or 150 or 250 messages, neither of the search commands
+ shown will result in an error. It is up to the client to know when
+ to stop moving its search window.
+
+2.1. The ESEARCH Response
+
+ In response to an ESEARCH command, the server MUST return ESEARCH
+ responses [RFC4731] (that is, not SEARCH responses). Because message
+ numbers are not useful for mailboxes that are not selected, the
+ responses MUST contain information about UIDs, not message numbers.
+ This is true even if the source options specify that only the
+ selected mailbox be searched.
+
+ Presence of a source option in the absence of a result option implies
+ the "ALL" result option (see Section 3.1 of [RFC4731]). Note that
+ this is not the same as the result from the SEARCH command described
+ in the IMAP base protocol [RFC3501].
+
+ Source options describe which mailboxes must be searched for
+ messages. An ESEARCH command with source options does not affect
+ which mailbox, if any, is currently selected, regardless of which
+ mailboxes are searched.
+
+ For each mailbox satisfying the source options, a single ESEARCH
+ response MUST be returned if any messages in that mailbox match the
+ search criteria. An ESEARCH response MUST NOT be returned for
+ mailboxes that contain no matching messages. This is true even when
+ result options such as MIN, MAX, and COUNT are specified (see
+ Section 3.1 of [RFC4731]), and the values returned (lowest UID
+ matched, highest UID matched, and number of messages matched,
+ respectively) apply to the mailbox reported in that ESEARCH response.
+
+
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+ Note that it is possible for an ESEARCH command to return *no*
+ untagged responses (no ESEARCH responses at all), in the case that
+ there are no matches to the search in any of the mailboxes that
+ satisfy the source options. Clients can detect this situation by
+ finding the tagged OK response without having received any matching
+ untagged ESEARCH responses.
+
+ Each ESEARCH response MUST contain the MAILBOX, TAG, and UIDVALIDITY
+ correlators. Correlators allow clients to issue several ESEARCH
+ commands at once (pipelined). If the SEARCHRES [RFC5182] extension
+ is used in an ESEARCH command, that ESEARCH command MUST be executed
+ by the server after all previous SEARCH/ESEARCH commands have
+ completed and before any subsequent SEARCH/ESEARCH commands are
+ executed. The server MAY perform consecutive ESEARCH commands in
+ parallel as long as none of them use the SEARCHRES extension.
+
+2.2. Source Options: Specifying Mailboxes to Search
+
+ The source options, if present, MUST contain a mailbox specifier as
+ defined in the IMAP NOTIFY extension [RFC5465], Section 6 (using the
+ "filter-mailboxes" ABNF item), with the following differences:
+
+ 1. The "selected-delayed" specifier is not valid here.
+
+ 2. A "subtree-one" specifier is added. The "subtree" specifier
+ results in a search of the specified mailbox and all selectable
+ mailboxes that are subordinate to it, through an indefinitely
+ deep hierarchy. The "subtree-one" specifier results in a search
+ of the specified mailbox and all selectable child mailboxes, one
+ hierarchy level down.
+
+ If "subtree" is specified, the server MUST defend against loops in
+ the hierarchy (for example, those caused by recursive file-system
+ links within the message store). The server SHOULD do this by
+ keeping track of the mailboxes that have been searched, and
+ terminating the hierarchy traversal when a repeat is found. If it
+ cannot do that, it MAY do it by limiting the hierarchy depth.
+
+ If the source options are not present, the value "selected" is
+ assumed -- that is, only the currently selected mailbox is searched.
+
+ The "personal" source option is a particularly convenient way to
+ search all of the current user's mailboxes. Note that there is no
+ way to use wildcard characters to search all mailboxes; the
+ "mailboxes" source option does not do wildcard expansion.
+
+
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+ If the source options include (or default to) "selected", the IMAP
+ session MUST be in "selected" state. If the source options specify
+ other mailboxes and NOT "selected", then the IMAP session MUST be in
+ either "selected" or "authenticated" state. If the session is not in
+ a correct state, the ESEARCH command MUST return a "BAD" result.
+
+ If the server supports the SEARCHRES [RFC5182] extension, then the
+ "SAVE" result option is valid *only* if "selected" is specified or
+ defaulted as the sole mailbox to be searched. If any source option
+ other than "selected" is specified, the ESEARCH command MUST return a
+ "BAD" result.
+
+ If the server supports the CONTEXT=SEARCH and/or CONTEXT=SORT
+ extension [RFC5267], then the following additional rules apply:
+
+ o The CONTEXT return option (Section 4.2 of [RFC5267]) can be used
+ with an ESEARCH command.
+
+ o If the UPDATE return option is used (Section 4.3 of [RFC5267]), it
+ MUST apply ONLY to the currently selected mailbox. If UPDATE is
+ used and there is no mailbox currently selected, the ESEARCH
+ command MUST return a "BAD" result.
+
+ o The PARTIAL search return option (Section 4.4 of [RFC5267]) can be
+ used and applies to each mailbox searched by the ESEARCH command.
+
+ If the server supports the Access Control List (ACL) [RFC4314]
+ extension, then the logged-in user is required to have the "r" right
+ for each mailbox she wants to search. In addition, any mailboxes
+ that are not explicitly named (accessed through "personal" or
+ "subtree", for example) are required to have the "l" right.
+ Mailboxes matching the source options for which the logged-in user
+ lacks sufficient rights MUST be ignored by the ESEARCH command
+ processing. In particular, ESEARCH responses MUST NOT be returned
+ for those mailboxes.
+
+3. Examples
+
+ In the following example, note that two ESEARCH commands are
+ pipelined, and that the server is running them in parallel,
+ interleaving a response to the second search amid the responses to
+ the first (watch the tags).
+
+ C: tag1 ESEARCH IN (mailboxes "folder1" subtree "folder2") unseen
+ C: tag2 ESEARCH IN (mailboxes "folder1" subtree-one "folder2")
+ subject "chad"
+ S: * ESEARCH (TAG "tag1" MAILBOX "folder1" UIDVALIDITY 1) UID ALL
+ 4001,4003,4005,4007,4009
+
+
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+ S: * ESEARCH (TAG "tag2" MAILBOX "folder1" UIDVALIDITY 1) UID ALL
+ 3001:3004,3788
+ S: * ESEARCH (TAG "tag1" MAILBOX "folder2/banana" UIDVALIDITY 503)
+ UID ALL 3002,4004
+ S: * ESEARCH (TAG "tag1" MAILBOX "folder2/peach" UIDVALIDITY 3) UID
+ ALL 921691
+ S: tag1 OK done
+ S: * ESEARCH (TAG "tag2" MAILBOX "folder2/salmon" UIDVALIDITY
+ 1111111) UID ALL 50003,50006,50009,50012
+ S: tag2 OK done
+
+4. Formal Syntax
+
+ The following syntax specification uses the Augmented Backus-Naur
+ Form (ABNF) as described in [RFC5234]. Terms not defined here are
+ taken from [RFC3501], [RFC5465], or [RFC4466].
+
+ command-auth =/ esearch
+ ; Update definition from IMAP base [RFC3501].
+ ; Add new "esearch" command.
+
+ command-select =/ esearch
+ ; Update definition from IMAP base [RFC3501].
+ ; Add new "esearch" command.
+
+ filter-mailboxes-other =/ ("subtree-one" SP one-or-more-mailbox)
+ ; Update definition from IMAP Notify [RFC5465].
+ ; Add new "subtree-one" selector.
+
+ filter-mailboxes-selected = "selected"
+ ; Update definition from IMAP Notify [RFC5465].
+ ; We forbid the use of "selected-delayed".
+
+ one-correlator = ("TAG" SP tag-string) / ("MAILBOX" SP astring) /
+ ("UIDVALIDITY" SP nz-number)
+ ; Each correlator MUST appear exactly once.
+
+ scope-option = scope-option-name [SP scope-option-value]
+ ; No options defined here. Syntax for future extensions.
+
+ scope-option-name = tagged-ext-label
+ ; No options defined here. Syntax for future extensions.
+
+ scope-option-value = tagged-ext-val
+ ; No options defined here. Syntax for future extensions.
+
+
+
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+ scope-options = scope-option *(SP scope-option)
+ ; A given option may only appear once.
+ ; No options defined here. Syntax for future extensions.
+
+ esearch = "ESEARCH" [SP esearch-source-opts]
+ [SP search-return-opts] SP search-program
+
+ search-correlator = SP "(" one-correlator *(SP one-correlator) ")"
+ ; Updates definition in IMAP4 ABNF [RFC4466].
+
+ esearch-source-opts = "IN" SP "(" source-mbox [SP
+ "(" scope-options ")"] ")"
+
+ source-mbox = filter-mailboxes *(SP filter-mailboxes)
+ ; "filter-mailboxes" is defined in IMAP Notify [RFC5465].
+ ; See updated definition of filter-mailboxes-other, above.
+ ; See updated definition of filter-mailboxes-selected, above.
+
+5. Security Considerations
+
+ This new IMAP ESEARCH command allows a single command to search many
+ mailboxes at once. On the one hand, a client could do that by
+ sending many IMAP SEARCH commands. On the other hand, this makes it
+ easier for a client to overwork a server, by sending a single command
+ that results in an expensive search of tens of thousands of
+ mailboxes. Server implementations need to be aware of that, and
+ provide mechanisms that prevent a client from adversely affecting
+ other users. Limitations on the number of mailboxes that may be
+ searched in one command, and/or on the server resources that will be
+ devoted to responding to a single client, are reasonable limitations
+ for an implementation to impose.
+
+ Implementations MUST, of course, apply access controls appropriately,
+ limiting a user's access to ESEARCH in the same way its access is
+ limited for any other IMAP commands. This extension has no data-
+ access risks beyond what may be there in the unextended IMAP
+ implementation.
+
+ Mailboxes matching the source options for which the logged-in user
+ lacks sufficient rights MUST be ignored by the ESEARCH command
+ processing (see the paragraph about this in Section 2.2). In
+ particular, any attempt to distinguish insufficient access from
+ non-existent mailboxes may expose information about the mailbox
+ hierarchy that isn't otherwise available to the client.
+
+ If "subtree" is specified, the server MUST defend against loops in
+ the hierarchy (see the paragraph about this in Section 2.2).
+
+
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+6. IANA Considerations
+
+ IMAP4 capabilities are registered by publishing a Standards Track or
+ IESG-approved Experimental RFC. The "IMAP 4 Capabilities" registry
+ is currently located here:
+
+ http://www.iana.org/
+
+ This document defines the IMAP capability "MULTISEARCH", and IANA has
+ added it to the registry.
+
+7. Acknowledgements
+
+ The authors gratefully acknowledge feedback provided by Timo
+ Sirainen, Peter Coates, and Arnt Gulbrandsen.
+
+8. Normative References
+
+ [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
+ Requirement Levels", BCP 14, RFC 2119, March 1997.
+
+ [RFC2978] Freed, N. and J. Postel, "IANA Charset Registration
+ Procedures", BCP 19, RFC 2978, October 2000.
+
+ [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.
+
+ [RFC4466] Melnikov, A. and C. Daboo, "Collected Extensions to IMAP4
+ ABNF", RFC 4466, April 2006.
+
+ [RFC4731] Melnikov, A. and D. Cridland, "IMAP4 Extension to SEARCH
+ Command for Controlling What Kind of Information Is
+ Returned", RFC 4731, November 2006.
+
+ [RFC5182] Melnikov, A., "IMAP Extension for Referencing the Last
+ SEARCH Result", RFC 5182, March 2008.
+
+ [RFC5234] Crocker, D., Ed., and P. Overell, "Augmented BNF for
+ Syntax Specifications: ABNF", STD 68, RFC 5234,
+ January 2008.
+
+ [RFC5267] Cridland, D. and C. King, "Contexts for IMAP4", RFC 5267,
+ July 2008.
+
+
+
+
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+Leiba & Melnikov Experimental [Page 9]
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+ [RFC5465] Gulbrandsen, A., King, C., and A. Melnikov, "The IMAP
+ NOTIFY Extension", RFC 5465, February 2009.
+
+Authors' Addresses
+
+ Barry Leiba
+ Huawei Technologies
+
+ Phone: +1 646 827 0648
+ EMail: barryleiba@computer.org
+ URI: http://internetmessagingtechnology.org/
+
+
+ Alexey Melnikov
+ Isode Limited
+ 5 Castle Business Village
+ 36 Station Road
+ Hampton, Middlesex TW12 2BX
+ UK
+
+ EMail: Alexey.Melnikov@isode.com
+ URI: http://www.melnikov.ca/
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