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rfc:rfc7377

Internet Engineering Task Force (IETF) B. Leiba Request for Comments: 7377 Huawei Technologies Obsoletes: 6237 A. Melnikov Updates: 4466 Isode Limited Category: Standards Track October 2014 ISSN: 2070-1721

                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 delays caused by many round trips and
 not requiring disruption of the currently selected mailbox.  This
 extension also uses MAILBOX, UIDVALIDITY, and TAG fields in ESEARCH
 responses, allowing a client to pipeline the searches if it chooses.
 This document updates RFC 4466 and obsoletes RFC 6237.

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/rfc7377.

Leiba & Melnikov Standards Track [Page 1] RFC 7377 IMAP4 Multimailbox SEARCH Extension October 2014

Copyright Notice

 Copyright (c) 2014 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
 2.3.  Strictness in Search Matches  . . . . . . . . . . . . . . .   7
 2.4.  Server-Side Limitations on Search Volume  . . . . . . . . .   7
 3.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
 4.  Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . .   8
 5.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
 6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
 7.  Changes since RFC 6237  . . . . . . . . . . . . . . . . . . .  10
 8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
 8.1.  Normative References  . . . . . . . . . . . . . . . . . . .  10
 8.2.  Informative References  . . . . . . . . . . . . . . . . . .  11
 Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . .  11
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

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.

Leiba & Melnikov Standards Track [Page 2] RFC 7377 IMAP4 Multimailbox SEARCH Extension October 2014

 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.
 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].
 This extension was previously published in an Experimental RFC.
 There is now implementation experience, giving confidence in the
 protocol, so this document puts the extension on the Standards Track,
 with some minor updates that were informed by the implementation
 experience.  A brief summary of changes is in Section 7.

1.1. Conventions Used in This Document

 In examples, "C:" indicates lines sent by a client that is connected
 to a server, and "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 [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

Leiba & Melnikov Standards Track [Page 3] RFC 7377 IMAP4 Multimailbox SEARCH Extension October 2014

 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.
 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, 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.

Leiba & Melnikov Standards Track [Page 4] RFC 7377 IMAP4 Multimailbox SEARCH Extension October 2014

 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.
 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

Leiba & Melnikov Standards Track [Page 5] RFC 7377 IMAP4 Multimailbox SEARCH Extension October 2014

     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 by
 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.
 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.
 The client SHOULD NOT provide source options that resolve to
 including the same mailbox more than once.  A server can, of course,
 remove the duplicates before processing, but the server MAY return
 "BAD" to an ESEARCH command with duplicate source mailboxes.
 If the server supports the SEARCHRES [RFC5182] extension, then the
 "SAVE" result option is valid only if "selected" is specified or
 defaulted to 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.

Leiba & Melnikov Standards Track [Page 6] RFC 7377 IMAP4 Multimailbox SEARCH Extension October 2014

 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.

2.3. Strictness in Search Matches

 The base IMAP SEARCH command (Section 6.4.4. of [RFC3501]) requires
 strict substring matching in text searches.  Many servers, however,
 use search engines that match strings in different ways, for example,
 matching "swim" to both "swam" and "swum" or only doing full word
 matching (where "swim" will not match "swimming").  This is covered
 by the "Fuzzy Search" extension to IMAP [RFC6203], and that extension
 is compatible with this one and can be combined with it.
 Whether or not Fuzzy Search is implemented or used, this extension
 explicitly allows flexible searching with respect to TEXT and BODY
 searches.  Servers MAY use fuzzy text matching in multimailbox
 searches.

2.4. Server-Side Limitations on Search Volume

 To avoid having a search use more than a reasonable share of server
 resources, servers MAY apply limits that go beyond loop protection,
 such as limits on the number of mailboxes that may be searched at
 once and/or limits on the number or total size of messages searched.
 A server can apply those limits up front, responding with "NO
 [LIMIT]" if a limit is exceeded (see [RFC5530] for information about
 response codes).  Alternatively, a server can process the search and
 terminate it when a limit is exceeded, responding with "OK [LIMIT]"
 and returning partial results.  Note that searches that return
 partial results can cause complexity for client implementations and
 confusion to users.

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).

Leiba & Melnikov Standards Track [Page 7] RFC 7377 IMAP4 Multimailbox SEARCH Extension October 2014

 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
 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.

Leiba & Melnikov Standards Track [Page 8] RFC 7377 IMAP4 Multimailbox SEARCH Extension October 2014

 scope-option-value =  tagged-ext-val
         ; No options defined here.  Syntax for future extensions.
 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 (see also Section 2.4).
 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 exist 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.

Leiba & Melnikov Standards Track [Page 9] RFC 7377 IMAP4 Multimailbox SEARCH Extension October 2014

 If "subtree" is specified, the server MUST defend against loops in
 the hierarchy (see the paragraph about this in Section 2.2).

6. IANA Considerations

 The "Internet Message Access Protocol (IMAP) Capabilities Registry"
 is currently located at
 <http://www.iana.org/assignments/imap-capabilities>.
 IANA has changed the reference for the IMAP capability "MULTISEARCH"
 to point to this document.

7. Changes since RFC 6237

 o  Change to Standards Track.
 o  Added paragraph about duplicate mailboxes.
 o  Added Section 2.3 about Fuzzy Search.

8. References

8.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997,
            <http://www.rfc-editor.org/info/rfc2119>.
 [RFC2978]  Freed, N. and J. Postel, "IANA Charset Registration
            Procedures", BCP 19, RFC 2978, October 2000,
            <http://www.rfc-editor.org/info/rfc2978>.
 [RFC3501]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
            4rev1", RFC 3501, March 2003,
            <http://www.rfc-editor.org/info/rfc3501>.
 [RFC4314]  Melnikov, A., "IMAP4 Access Control List (ACL) Extension",
            RFC 4314, December 2005,
            <http://www.rfc-editor.org/info/rfc4314>.
 [RFC4466]  Melnikov, A. and C. Daboo, "Collected Extensions to IMAP4
            ABNF", RFC 4466, April 2006,
            <http://www.rfc-editor.org/info/rfc4466>.
 [RFC4731]  Melnikov, A. and D. Cridland, "IMAP4 Extension to SEARCH
            Command for Controlling What Kind of Information Is
            Returned", RFC 4731, November 2006,
            <http://www.rfc-editor.org/info/rfc4731>.

Leiba & Melnikov Standards Track [Page 10] RFC 7377 IMAP4 Multimailbox SEARCH Extension October 2014

 [RFC5182]  Melnikov, A., "IMAP Extension for Referencing the Last
            SEARCH Result", RFC 5182, March 2008,
            <http://www.rfc-editor.org/info/rfc5182>.
 [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
            Specifications: ABNF", STD 68, RFC 5234, January 2008,
            <http://www.rfc-editor.org/info/rfc5234>.
 [RFC5267]  Cridland, D. and C. King, "Contexts for IMAP4", RFC 5267,
            July 2008, <http://www.rfc-editor.org/info/rfc5267>.
 [RFC5465]  Gulbrandsen, A., King, C., and A. Melnikov, "The IMAP
            NOTIFY Extension", RFC 5465, February 2009,
            <http://www.rfc-editor.org/info/rfc5465>.
 [RFC5530]  Gulbrandsen, A., "IMAP Response Codes", RFC 5530, May
            2009, <http://www.rfc-editor.org/info/rfc5530>.

8.2. Informative References

 [RFC6203]  Sirainen, T., "IMAP4 Extension for Fuzzy Search", RFC
            6203, March 2011,
            <http://www.rfc-editor.org/info/rfc6203>.

Acknowledgments

 The authors gratefully acknowledge feedback provided by Timo
 Sirainen, Peter Coates, Arnt Gulbrandsen, and Chris Newman.

Authors' Addresses

 Barry Leiba
 Huawei Technologies
 Phone: +1 646 827 0648
 EMail: barryleiba@computer.org
 URI:   http://internetmessagingtechnology.org/
 Alexey Melnikov
 Isode Limited
 14 Castle Mews
 Hampton, Middlesex  TW12 2NP
 United Kingdom
 EMail: Alexey.Melnikov@isode.com
 URI:   http://www.melnikov.ca/

Leiba & Melnikov Standards Track [Page 11]

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