Internet Engineering Task Force (IETF)
Request for Comments: 6203
Category: Standards Track
ISSN: 2070-1721
T. Sirainen
March 2011

IMAP4 Extension for Fuzzy Search


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

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

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Copyright © 2011 IETF Trust and the persons identified as the document authors. All rights reserved.

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1. 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.

2. 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 [KEYWORDS].

3. The FUZZY Search Key

The FUZZY search key takes another search key as its argument. The 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 a subject of "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:

S: * SEARCH 1 4
S: A2 OK Search completed.

How the server handles multiple separate FUZZY search keys is implementation-defined.

Fuzzy search algorithms might change, or the results of the algorithms might be different from search to search, so that fuzzy searches with the same parameters might give different results for 1) the same user at different times, 2) different users (searches executed simultaneously), or 3) different users (searches executed at different times). For example, a fuzzy search might adapt to a user's search habits in an attempt to give more relevant results (in a "learning" manner). Such differences can also occur because of operational decisions, such as load balancing. Clients asking for "fuzzy" really are requesting search results in a not-necessarily- deterministic way and need to give the user appropriate warning about that.

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 the 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, e.g., as a percentage value.

As the name already indicates, 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 fuzzily matches 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, how the relevancy score is calculated is server-dependent.

If the 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 "B1") ALL 1,5,10 RELEVANCY (4 99 42)
S: B1 OK Search completed.

In the example above, the server would treat "hello", "help", and other similar strings as fuzzily matching the misspelled "Helo".

The RELEVANCY return option MUST NOT be used unless a 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 exactly what that means for different search keys is left for 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.


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). The server could return messages outside the specified date range, but the further away the message is, the lower the relevancy score.


These should be handled similarly 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.


If other search criteria match, the server could return messages that don't have the specified flags set, but with lower relevancy scores. SEARCH SUBJECT "xyz" FUZZY ANSWERED, for example, might be useful if the user thinks the message he is looking for has the ANSWERED flag set, but he isn't sure.

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

6. Extensions to SORT and SEARCH

If the 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 the RELEVANCY return option, RELEVANCY sort criteria MUST NOT be used unless a FUZZY search key is also given.

If the 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.

Furthermore, if the server advertises the CONTEXT=SORT (or CONTEXT=SEARCH) capability, then the client can limit the number of returned messages to a SORT (or a SEARCH) by using 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.

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 denial-of-service attacks against server resources. Servers MAY limit the resources that a single search (or a single user) may use. Additionally, implementors should be aware of the following: Fuzzy search engines are often complex with non-obvious disk space, memory, and/or CPU usage patterns. Server implementors should at least test the fuzzy- search behavior with 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 a 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 users to see all the search results, rather than hiding results below a certain score.

9. IANA Considerations

IMAP4 capabilities are registered by publishing a standards track or IESG-approved experimental RFC. The "Internet Message Access Protocol (IMAP) 4 Capabilities Registry" is available from

This document defines the SEARCH=FUZZY IMAP capability. IANA has added 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", STD 68, 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.
   [KEYWORDS]   Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14, 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
                Protocol - SORT and THREAD Extensions", RFC 5256,
                June 2008.

Author's Address

   Timo Sirainen