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Versions: 00 01 02 03 RFC 6203

Message Organization Working Group                           T. Sirainen
Internet-Draft                                         November 16, 2010
Intended status: Standards Track
Expires: May 20, 2011

                    IMAP4 Extension for Fuzzy Search


   This document describes an IMAP protocol extension enabling server to
   perform searches with inexact matching and assigning relevancy scores
   for matched messages.


   A revised version of this draft document will be submitted to the RFC
   editor as a Proposed Standard for the Internet Community.  Discussion
   and suggestions for improvement are requested, and should be sent to

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on May 20, 2011.

Copyright Notice

   Copyright (c) 2010 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

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   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.

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",
   document are to be interpreted as described in RFC 2119 [Kwds].

2.  Introduction

   When humans perform searches in IMAP clients, they typically want to
   see the most relevant search results first.  IMAP servers are able to
   do this in the most efficient way when they're free to internally
   decide how searches should match messages.  This document describes a
   new SEARCH=FUZZY extension that provides such functionality.

3.  The FUZZY Search Key

   FUZZY search key takes another search key as its argument.  Server is
   allowed to perform all matching in an implementation-defined manner
   for this search key, including ignoring the active comparator as
   defined by [RFC5255].  Typically this would be used to search for
   strings, for example:

   S: * SEARCH 1 5 10
   S: A1 OK Search completed.

   Besides matching messages with subject "IMAP break", the above search
   may also match messages with subjects "broken IMAP", "IMAP is
   broken", or anything else the server decides that might be a good

   This example does a fuzzy SUBJECT search, but a non-fuzzy FROM

   C: A2 SEARCH FUZZY SUBJECT work FROM user@example.com
   S: * SEARCH 1 4

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   S: A2 OK Search completed.

   It is implementation-defined how server handles multiple separate
   FUZZY search keys.

4.  Relevancy Scores for Search Results

   Servers SHOULD assign a search relevancy score for each matched
   message when the FUZZY search key is given.  Relevancy scores are
   given in range 1-100, where 100 is the highest relevancy.  The
   relevancy scores SHOULD use the full 1-100 range, so that clients can
   show them to users in a meaningful way, such as a percentage value.

   As the name already tells, relevancy scores specify how relevant to
   the search the matched message is.  It's not necessarily the same as
   how precisely the message matched.  For example a message whose
   subject matches fuzzily the search string might get a higher
   relevancy score than a message whose body had the exact string in the
   middle of a sentence.  When multiple search keys are matched fuzzily,
   it's server-dependent on how the relevancy score is calculated.

   If server also advertises the ESEARCH capability as defined by
   [ESEARCH], the relevancy scores can be retrieved using the new
   RELEVANCY return option for SEARCH:

   S: * ESEARCH (TAG "B2") ALL 1,5,10 RELEVANCY (4 99 42)
   S: B1 OK Search completed.

   The RELEVANCY return option MUST NOT be used unless FUZZY search key
   is also given.  Note that SEARCH results aren't sorted by relevancy,
   SORT is needed for that.

5.  Fuzzy matching with non-string search keys

   Fuzzy matching is not limited to just string matching.  All search
   keys SHOULD be matched fuzzily, although what exactly that means for
   different search keys is left up to server implementations to decide
   -- including deciding that fuzzy matching is meaningless for a
   particular key, and falling back to exact matching.  Some suggestions
   are given below.

   Dates: A typical example could be when a user wants to find a message
   "from Dave about a week ago".  A client could perform this search
   using SEARCH FUZZY (FROM "Dave" SINCE 21-Jan-2009 BEFORE 24-Jan-
   2009).  Server could return messages outside the specified date

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   range, but the further away the message is, the lower the relevancy

   Sizes: These should be handled similar to dates.  If a user wants to
   search for "about 1 MB attachments", the client could do this by
   sending SEARCH FUZZY (LARGER 900000 SMALLER 1100000).  Again the
   further away the message size is from the specified range, the lower
   the relevancy score.

   Flags: Server could return messages that don't have the specified
   flags, but with a lower relevancy score.

   UIDs, sequences, modification sequences: These are examples of keys
   for which exact matching is probably what makes sense.
   Alternatively, a server might choose, for instance, to expand a UID
   range by 5% on each side.

6.  Extensions to SORT

   If server also advertises the SORT capability as defined by [SORT],
   the results can be sorted by the new RELEVANCY sort criteria:

   S: * SORT 5 10 1
   S: C1 OK Sort completed.

   The message with the highest score is returned first.  As with
   RELEVANCY return option, RELEVANCY sort criteria MUST NOT be used
   unless FUZZY search key is also given.

   If server also advertises the ESORT capability as defined by
   [CONTEXT], the relevancy scores can be retrieved using the new
   RELEVANCY return option for SORT:

   S: * ESEARCH (TAG "C2") ALL 5,10,1 RELEVANCY (99 42 4)
   S: C2 OK Sort completed.

   To limit the number of returned messages, use the PARTIAL return
   option.  For example this returns the 10 most relevant messages:

   S: * ESEARCH (TAG "C3") PARTIAL (1:10 42,9,34,13,15,4,2,7,23,82)
   S: C3 OK Sort completed.

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7.  Formal Syntax

   The following syntax specification uses the augmented Backus-Naur
   Form (BNF) as described in [ABNF].  It includes definitions from
   [RFC3501], [IMAP-ABNF] and [SORT].

       capability         =/ "SEARCH=FUZZY"

       score              = 1*3DIGIT
          ;; (1 <= n <= 100)

       score-list         = "(" [score *(SP score)] ")"

       search-key         =/ "FUZZY" SP search-key

       search-return-data =/ "RELEVANCY" SP score-list
          ;; Conforms to <search-return-data>, from [IMAP-ABNF]

       search-return-opt  =/ "RELEVANCY"
          ;; Conforms to <search-return-opt>, from [IMAP-ABNF]

       sort-key           =/ "RELEVANCY"

8.  Security Considerations

   Implementation of this extension might enable a denial-of-service
   attack if the implementation isn't careful to prevent them.  Fuzzy
   search engines are often complex with non-obvious disk space, memory
   and/or CPU usage patterns.  Implementors should test at least the
   behavior of large messages that contain very long words and/or unique
   random strings.  Also very long search keys might cause excessive
   memory or CPU usage.

   Invalid input may also be problematic.  For example if the search
   engine takes UTF-8 stream as input, it might fail more or less badly
   when illegal UTF-8 sequences are fed to it from a message whose
   character set was claimed to be UTF-8.  This could be avoided by
   validating all the input and, for example, replacing illegal UTF-8
   sequences with the Unicode replacement character (U+FFFD).

   Search relevancy rankings might be susceptible to "poisoning" by
   smart attackers using certain keywords or hidden markup (e.g.  HTML)
   in their messages to boost the rankings.  This can't be fully
   prevented by servers, so clients should prepare for it by at least
   allowing user to see all the search results, rather than hide results
   below a certain score.

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9.  IANA Considerations

   IMAP4 capabilities are registered by publishing a standards track or
   IESG approved experimental RFC.  The registry is currently located


   This document defines the X-DRAFT-I03-SEARCH=FUZZY [[anchor7: Note to
   RFC Editor: fix before publication]] IMAP capability.  IANA is
   requested to add it to the registry.

10.  Acknowledgements

   Alexey Melnikov, Zoltan Ordogh, Barry Leiba, Cyrus Daboo and Dave
   Cridland have helped with this document.

11.  Normative References

   [ABNF]     Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 5234, January 2008.

   [CONTEXT]  Cridland, D. and C. King, "Contexts for IMAP4", RFC 5267,
              July 2008.

   [ESEARCH]  Melnikov, A. and D. Cridland, "IMAP4 Extension to SEARCH
              Command for Controlling What Kind of Information Is
              Returned", RFC 4731, November 2006.

              Melnikov, A. and C. Daboo, "Collected Extensions to IMAP4
              ABNF", RFC 4466, April 2006.

   [Kwds]     Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", RFC 2119, March 1997.

              4rev1", RFC 3501, March 2003.

   [RFC5255]  Newman, C., Gulbrandsen, A., and A. Melnikov, "Internet
              Message Access Protocol Internationalization", RFC 5255,
              June 2008.

   [SORT]     Crispin, M. and K. Murchison, "Internet Message Access

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              Protocol - SORT and THREAD Extensions", RFC 5256,
              June 2008.

Author's Address

   Timo Sirainen

   Email: tss@iki.fi

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