IMAP4 Multimailbox SEARCH Extension
RFC 7377

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]