Message Organization Working Group T. Sirainen Internet-Draft November 16, 2010 Intended status: Standards Track Expires: May 20, 2011 IMAP4 Extension for Fuzzy Search draft-ietf-morg-fuzzy-search-03 Abstract This document describes an IMAP protocol extension enabling server to perform searches with inexact matching and assigning relevancy scores for matched messages. Note 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 morg@ietf.org. 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 Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." 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 Sirainen Expires May 20, 2011 [Page 1] Internet-Draft IMAP4 FUZZY SEARCH November 2010 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", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 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: C: A1 SEARCH FUZZY (SUBJECT "IMAP break") 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 match. This example does a fuzzy SUBJECT search, but a non-fuzzy FROM search: C: A2 SEARCH FUZZY SUBJECT work FROM user@example.com S: * SEARCH 1 4 Sirainen Expires May 20, 2011 [Page 2] Internet-Draft IMAP4 FUZZY SEARCH November 2010 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: C: B1 SEARCH RETURN (RELEVANCY ALL) FUZZY TEXT "Helo" 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 Sirainen Expires May 20, 2011 [Page 3] Internet-Draft IMAP4 FUZZY SEARCH November 2010 range, but the further away the message is, the lower the relevancy score. 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: C: C1 SORT (RELEVANCY) UTF-8 FUZZY SUBJECT "Helo" 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: C: C2 SORT RETURN (RELEVANCY ALL) (RELEVANCY) UTF-8 FUZZY TEXT "Helo" 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: C: C3 SORT RETURN (PARTIAL 1:10) (RELEVANCY) UTF-8 FUZZY TEXT "World" S: * ESEARCH (TAG "C3") PARTIAL (1:10 42,9,34,13,15,4,2,7,23,82) S: C3 OK Sort completed. Sirainen Expires May 20, 2011 [Page 4] Internet-Draft IMAP4 FUZZY SEARCH November 2010 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 , from [IMAP-ABNF] search-return-opt =/ "RELEVANCY" ;; Conforms to , 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. Sirainen Expires May 20, 2011 [Page 5] Internet-Draft IMAP4 FUZZY SEARCH November 2010 9. IANA Considerations IMAP4 capabilities are registered by publishing a standards track or IESG approved experimental RFC. The registry is currently located at: http://www.iana.org/assignments/imap4-capabilities 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. [IMAP-ABNF] 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. [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 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 Sirainen Expires May 20, 2011 [Page 6] Internet-Draft IMAP4 FUZZY SEARCH November 2010 Protocol - SORT and THREAD Extensions", RFC 5256, June 2008. Author's Address Timo Sirainen Email: tss@iki.fi Sirainen Expires May 20, 2011 [Page 7]