IMAP4 Multimailbox SEARCH Extension
RFC 6237

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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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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   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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   [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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