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rfc:rfc4731

Network Working Group A. Melnikov Request for Comments: 4731 Isode Ltd Category: Standards Track D. Cridland

                                                 Inventure Systems Ltd
                                                         November 2006
         IMAP4 Extension to SEARCH Command for Controlling
                What Kind of Information Is Returned

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The IETF Trust (2006).

Abstract

 This document extends IMAP (RFC 3501) SEARCH and UID SEARCH commands
 with several result options, which can control what kind of
 information is returned. The following result options are defined:
 minimal value, maximal value, all found messages, and number of found
 messages.

Table of Contents

 1. Introduction ....................................................2
 2. Conventions Used in This Document ...............................2
 3. IMAP Protocol Changes ...........................................2
    3.1. New SEARCH/UID SEARCH Result Options .......................2
    3.2. Interaction with CONDSTORE extension .......................4
 4. Formal Syntax ...................................................5
 5. Security Considerations .........................................6
 6. IANA Considerations .............................................6
 7. Normative References ............................................6
 8. Acknowledgments .................................................6

Melnikov & Cridland Standards Track [Page 1] RFC 4731 IMAP4 Extension to SEARCH November 2006

1. Introduction

 [IMAPABNF] extended SEARCH and UID SEARCH commands with result
 specifiers (also known as result options), which can control what
 kind of information is returned.
 A server advertising the ESEARCH capability supports the following
 result options:  minimal value, maximal value, all found messages,
 and number of found messages.  These result options allow clients to
 get SEARCH results in more convenient forms, while also saving
 bandwidth required to transport the results, for example, by finding
 the first unseen message or returning the number of unseen or deleted
 messages.  Also, when a single MIN or a single MAX result option is
 specified, servers can optimize execution of SEARCHes.

2. Conventions Used in This Document

 In examples, "C:" and "S:" indicate lines sent by the client and
 server, respectively.
 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 [KEYWORDS].

3. IMAP Protocol Changes

3.1. New SEARCH/UID SEARCH Result Options

 The SEARCH/UID SEARCH commands are extended to allow for the
 following result options:
    MIN
       Return the lowest message number/UID that satisfies the SEARCH
       criteria.
       If the SEARCH results in no matches, the server MUST NOT
       include the MIN result option in the ESEARCH response; however,
       it still MUST send the ESEARCH response.
    MAX
       Return the highest message number/UID that satisfies the SEARCH
       criteria.
       If the SEARCH results in no matches, the server MUST NOT
       include the MAX result option in the ESEARCH response; however,
       it still MUST send the ESEARCH response.

Melnikov & Cridland Standards Track [Page 2] RFC 4731 IMAP4 Extension to SEARCH November 2006

    ALL
       Return all message numbers/UIDs that satisfy the SEARCH
       criteria.  Unlike regular (unextended) SEARCH, the messages are
       always returned using the sequence-set syntax.  A sequence-set
       representation may be more compact and can be used as is in a
       subsequent command that accepts sequence-set.  Note, the client
       MUST NOT assume that messages/UIDs will be listed in any
       particular order.
       If the SEARCH results in no matches, the server MUST NOT
       include the ALL result option in the ESEARCH response; however,
       it still MUST send the ESEARCH response.
    COUNT
       Return number of the messages that satisfy the SEARCH criteria.
       This result option MUST always be included in the ESEARCH
       response.
 If one or more result options described above are specified, the
 extended SEARCH command MUST return a single ESEARCH response
 [IMAPABNF], instead of the SEARCH response.
 An extended UID SEARCH command MUST cause an ESEARCH response with
 the UID indicator present.
 Note that future extensions to this document can allow servers to
 return multiple ESEARCH responses for a single extended SEARCH
 command.  These extensions will have to describe how results from
 multiple ESEARCH responses are to be amalgamated.
 If the list of result options is empty, that requests the server to
 return an ESEARCH response instead of the SEARCH response.  This is
 equivalent to "(ALL)".
    Example:    C: A282 SEARCH RETURN (MIN COUNT) FLAGGED
                   SINCE 1-Feb-1994 NOT FROM "Smith"
                S: * ESEARCH (TAG "A282") MIN 2 COUNT 3
                S: A282 OK SEARCH completed
    Example:    C: A283 SEARCH RETURN () FLAGGED
                   SINCE 1-Feb-1994 NOT FROM "Smith"
                S: * ESEARCH (TAG "A283") ALL 2,10:11
                S: A283 OK SEARCH completed
 The following example demonstrates finding the first unseen message
 as returned in the UNSEEN response code on a successful SELECT
 command:

Melnikov & Cridland Standards Track [Page 3] RFC 4731 IMAP4 Extension to SEARCH November 2006

    Example:    C: A284 SEARCH RETURN (MIN) UNSEEN
                S: * ESEARCH (TAG "A284") MIN 4
                S: A284 OK SEARCH completed
 The following example demonstrates that if the ESEARCH UID indicator
 is present, all data in the ESEARCH response is referring to UIDs;
 for example, the MIN result specifier will be followed by a UID.
    Example:    C: A285 UID SEARCH RETURN (MIN MAX) 1:5000
                S: * ESEARCH (TAG "A285") UID MIN 7 MAX 3800
                S: A285 OK SEARCH completed
 The following example demonstrates returning the number of deleted
 messages:
    Example:    C: A286 SEARCH RETURN (COUNT) DELETED
                S: * ESEARCH (TAG "A286") COUNT 15
                S: A286 OK SEARCH completed

3.2. Interaction with CONDSTORE extension

 When the server supports both the ESEARCH and the CONDSTORE
 [CONDSTORE] extension, and the client requests one or more result
 option described in section 3.1 together with the MODSEQ search
 criterion in the same SEARCH/UID SEARCH command, then the server MUST
 return the ESEARCH response containing the MODSEQ result option
 (described in the following paragraph) instead of the extended SEARCH
 response described in section 3.5 of [CONDSTORE].
 If the SEARCH/UID SEARCH command contained a single MIN or MAX result
 option, the MODSEQ result option contains the mod-sequence for the
 found message.  If the SEARCH/UID SEARCH command contained both MIN
 and MAX result options and no ALL/COUNT option, the MODSEQ result
 option contains the highest mod-sequence for the two returned
 messages.  Otherwise the MODSEQ result option contains the highest
 mod-sequence for all messages being returned.
 Example: The following example demonstrates how Example 15 from
 [CONDSTORE] would look in the presence of one or more result option:
       C: a1 SEARCH RETURN (MIN) MODSEQ "/flags/\\draft"
           all 620162338
       S: * ESEARCH (TAG "a1") MIN 2 MODSEQ 917162488
       S: a1 OK Search complete
       C: a2 SEARCH RETURN (MAX) MODSEQ "/flags/\\draft"
           all 620162338
       S: * ESEARCH (TAG "a2") MAX 23 MODSEQ 907162321

Melnikov & Cridland Standards Track [Page 4] RFC 4731 IMAP4 Extension to SEARCH November 2006

       S: a2 OK Search complete
       C: a3 SEARCH RETURN (MIN MAX) MODSEQ "/flags/\\draft"
           all 620162338
       S: * ESEARCH (TAG "a3") MIN 2 MAX 23 MODSEQ 917162488
       S: a3 OK Search complete
       C: a4 SEARCH RETURN (MIN COUNT) MODSEQ "/flags/\\draft"
           all 620162338
       S: * ESEARCH (TAG "a4") MIN 2 COUNT 10 MODSEQ 917162500
       S: a4 OK Search complete

4. Formal Syntax

 The following syntax specification uses the Augmented Backus-Naur
 Form (ABNF) notation as specified in [ABNF].
 Non-terminals referenced but not defined below are as defined by
 [IMAP4], [CONDSTORE], or [IMAPABNF].
 Except as noted otherwise, all alphabetic characters are case-
 insensitive.  The use of upper or lowercase characters to define
 token strings is for editorial clarity only.  Implementations MUST
 accept these strings in a case-insensitive fashion.
   capability         =/ "ESEARCH"
   search-return-data = "MIN" SP nz-number /
                        "MAX" SP nz-number /
                        "ALL" SP sequence-set /
                        "COUNT" SP number
                        ;; conforms to the generic
                        ;; search-return-data syntax defined
                        ;; in [IMAPABNF]
   search-return-opt  = "MIN" / "MAX" / "ALL" / "COUNT"
                        ;; conforms to generic search-return-opt
                        ;; syntax defined in [IMAPABNF]
   When the CONDSTORE [CONDSTORE] IMAP extension is also supported,
   the ABNF is updated as follows:
   search-return-data =/ "MODSEQ" SP mod-sequence-value
                        ;; mod-sequence-value is defined
                        ;; in [CONDSTORE]

Melnikov & Cridland Standards Track [Page 5] RFC 4731 IMAP4 Extension to SEARCH November 2006

5. Security Considerations

 In the general case, the IMAP SEARCH/UID SEARCH commands can be CPU
 and/or IO intensive, and are seen by some as a potential attack point
 for denial of service attacks, so some sites/implementations even
 disable them entirely.  This is quite unfortunate, as SEARCH command
 is one of the best examples demonstrating IMAP advantage over POP3.
 The ALL and COUNT return options don't change how SEARCH is working
 internally; they only change how information about found messages is
 returned.  MIN and MAX SEARCH result options described in this
 document can lighten the load on IMAP servers that choose to optimize
 SEARCHes containing only one or both of them.
 It is believed that this extension doesn't raise any additional
 security concerns not already discussed in [IMAP4].

6. IANA Considerations

 IMAP4 capabilities are registered by publishing a standards track RFC
 or an IESG-approved experimental RFC.  The registry is currently
 located at <http://www.iana.org/assignments/imap4-capabilities>.
 This document defines the ESEARCH IMAP capability, which IANA added
 to the registry.

7. Normative References

 [KEYWORDS]  Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.
 [IMAP4]     Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
             4rev1", RFC 3501, March 2003.
 [ABNF]      Crocker, D. (Ed.) and P. Overell , "Augmented BNF for
             Syntax Specifications: ABNF", RFC 4234, October 2005.
 [IMAPABNF]  Melnikov, A. and C. Daboo, "Collected Extensions to IMAP4
             ABNF", RFC 4466, April 2006..
 [CONDSTORE] Melnikov, A. and S. Hole, "IMAP Extension for Conditional
             STORE", RFC 4551, June 2006.

8. Acknowledgments

 Thanks to Michael Wener, Arnt Gulbrandsen, Cyrus Daboo, Mark Crispin,
 and Pete Maclean for comments and corrections.

Melnikov & Cridland Standards Track [Page 6] RFC 4731 IMAP4 Extension to SEARCH November 2006

Authors' Addresses

 Alexey Melnikov
 Isode Limited
 5 Castle Business Village
 36 Station Road
 Hampton, Middlesex, TW12 2BX
 UK
 EMail: Alexey.Melnikov@isode.com
 Dave A. Cridland
 Inventure Systems Limited
 EMail: dave.cridland@inventuresystems.co.uk
 URL: http://invsys.co.uk/dave/

Melnikov & Cridland Standards Track [Page 7] RFC 4731 IMAP4 Extension to SEARCH November 2006

Full Copyright Statement

 Copyright (C) The IETF Trust (2006).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
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Melnikov & Cridland Standards Track [Page 8]

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