Network Working Group                                           S. Bosch
Internet-Draft                                         February 10, 2013
Intended status: Standards Track
Expires: August 14, 2013

         Sieve Email Filtering: Detecting Duplicate Deliveries


   This document defines a new test command "duplicate" for the "Sieve"
   email filtering language.  This test adds the ability to detect
   duplicate message deliveries.  The main application for this new test
   is handling duplicate deliveries commonly caused by mailing list
   subscriptions or redirected mail addresses.  The detection is
   normally performed by matching the message ID to an internal list of
   message IDs from previously delivered messages.  For more complex
   applications, the "duplicate" test can also use the content of a
   specific header or other parts of the message.

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   to this document.  Code Components extracted from this document must
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Conventions Used in This Document . . . . . . . . . . . . . . . 3
   3.  Test "duplicate"  . . . . . . . . . . . . . . . . . . . . . . . 3
   4.  Sieve Capability Strings  . . . . . . . . . . . . . . . . . . . 5
   5.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 7
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 8

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1.  Introduction

   This is an extension to the Sieve filtering language defined by RFC
   5228 [SIEVE].  It adds a test to determine whether a certain message
   was seen before by the delivery agent in an earlier execution of the
   Sieve script.  This can be used to detect and handle duplicate
   message deliveries.

   Duplicate deliveries are a common side-effect of being subscribed to
   a mailing list.  For example, if a member of the list decides to
   reply to both the user and the mailing list itself, the user will get
   one copy of the message directly and another through mailing list.
   Also, if someone cross-posts over several mailing lists to which the
   user is subscribed, the user will receive a copy from each of those
   lists.  In another scenario, the user has several redirected mail
   addresses all pointing to his main mail account.  If one of the
   user's contacts sends the message to more than one of those
   addresses, the user will likely receive more than a single copy.
   Using the "duplicate" extension, users have the means to detect and
   handle such duplicates, e.g. by discarding them, marking them as
   "seen", or putting them in a special folder.

   Duplicate messages are normally detected using the Message-ID header
   field, which is required to be unique for each message.  However, the
   "duplicate" test is flexible enough to use different (weaker)
   criteria for defining what makes a message a duplicate, for example
   based on the subject line or parts of the message body.  Other
   applications of this new test command are also possible, as long as
   the tracked value is a string.

2.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [KEYWORDS].

   Conventions for notations are as in [SIEVE] Section 1.1, including
   use of the "Usage:" label for the definition of action and tagged
   arguments syntax.

3.  Test "duplicate"

   Usage: "duplicate" [":handle" <handle: string>]
                      [":header" <header-name: string> /
                          ":value" <value: string>]
                      [":seconds" <timeout: number>]

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   In its basic form, the "duplicate" test keeps track of which messages
   were seen before by this test in an earlier execution of the Sieve
   script.  Messages are identified by their message ID as contained in
   the Message-ID header.  The "duplicate" test evaluates to "true" when
   the message was seen before and it evaluates to "false" when it was

   As a side-effect, the "duplicate" test adds the message to an
   internal duplicate tracking list, so that the test will evaluate to
   "true" the next time the Sieve script is executed and the same
   message ID is encountered.  Implementations MUST prevent adding
   messages to the internal duplicate tracking list when the Sieve
   script execution fails.  If failed script executions would add
   messages to the duplicate tracking list, all "duplicate" tests in the
   Sieve script would erroneously yield "true" for the next delivery
   attempt of the same message, which can -- depending on the action
   taken for a duplicate -- easily lead to discarding the message
   without further notice.

   For example, this can be implemented by deferring the definitive
   modification of the tracking list to the end of a successful Sieve
   script execution.  This implementation is not without problems,
   however, as it can cause a race condition when a duplicate message is
   delivered in parallel before the tracking list is updated.  This way,
   a duplicate message could be missed by the "duplicate" test.  More
   complex implementations could use a locking mechanism to prevent this
   problem.  But irrespective of what implementation is chosen,
   situations in which the "duplicate" test erroneously yields "true"
   MUST be prevented at all costs.

   Implementations SHOULD limit the number messages that are tracked.
   Also, implementations SHOULD let entries in the tracking list expire
   after a short period of time.  The user can explicitly control the
   length of this expiration time by means of the ":seconds" argument.
   If the ":seconds" argument is omitted, an appropriate default MUST be
   used.  Sites SHOULD impose a maximum limit on the expiration time.
   If that limit is exceeded, the maximum value MUST silently be
   substituted; exceeding the limit MUST NOT produce an error.

   By default, the tracked value is the content of the message's
   Message-ID header field.  For more complex applications, the
   "duplicate" test can also be used to detect duplicate deliveries
   based on other message text.  Then, the tracked value can be an
   arbitrary string value extracted from the message.

   By adding the ":header" argument with a message header field name,
   the content of the specified header can be used as the tracked value
   instead of the default Message-ID header.  Alternatively, the tracked

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   value can be specified explicitly using the ":value" argument.  The
   string parameter of ":value" argument can be composed from arbitrary
   text extracted from the message using the "variables" [VARIABLES]
   extension.  To extract text from the message body, the "foreverypart"
   and "extracttext" [SIEVE-MIME] extensions need to be used as well.
   This provides the user with detailed control over what identifies a
   message as a duplicate.  The ":header" and ":value" arguments are
   mutually exclusive and specifying both for a single "duplicate" test
   command MUST trigger an error at compile time.

   If the tracked value is extracted directly from a header, i.e. when
   the ":value" argument is not used, leading and trailing whitespace
   (see Section 2.2 of RFC 5228 [SIEVE]) MUST first be trimmed from the
   value before performing the actual duplicate verification.  When the
   ":value" argument is used, such normalization concerns are the
   responsibility of the user.

   The "duplicate" test MUST only check for duplicates amongst tracked
   values encountered in previous executions of the Sieve script; it
   MUST NOT consider tracked values encountered earlier in the current
   Sieve script execution as potential duplicates.  This means that all
   "duplicate" tests in a Sieve scrip execution, including those located
   in scripts included using the "include" [INCLUDE] extension, MUST
   always yield the same result if the arguments are identical.

   Using the ":handle" argument, the duplicate test can be employed for
   multiple independent purposes.  The message is recognized as a
   duplicate only when the tracked value was seen before in an earlier
   script execution by a "duplicate" test with the same ":handle"

   NOTE: The necessary mechanism to track duplicate messages is very
   similar to the mechanism that is needed for tracking duplicate
   responses for the "vacation" [VACATION] action.  One way to implement
   the necessary mechanism for the "duplicate" test is therefore to
   store a hash of the tracked value and, if provided, the ":handle"

4.  Sieve Capability Strings

   A Sieve implementation that defines the "duplicate" test command will
   advertise the capability string "duplicate".

5.  Examples

   In the following basic example, message duplicates are detected by

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   tracking the Message-ID header.  Duplicate deliveries are stored in a
   special folder contained in the user's Trash folder.  If the folder
   does not exist, it is created automatically using the "mailbox"
   [MAILBOX] extension.  This way, the user has a chance to recover
   messages when necessary.  Messages that are not recognized as
   duplicates are stored in the user's inbox as normal.

   require ["duplicate", "fileinto", "mailbox"];

   if duplicate {
     fileinto :create "Trash/Duplicate";

   The next example shows a more complex use of the "duplicate" test.
   The user gets network alerts from a set of remote automated
   monitoring systems.  Multiple notifications can be received about the
   same event from different monitoring systems.  The Message-ID of
   these messages is different, because these are all distinct messages
   from different senders.  To avoid being notified multiple times about
   the same event the user writes the following script:

   require ["duplicate", "variables", "imap4flags",

   if header :matches "subject" "ALERT: *" {
     if duplicate :seconds 60 :value "${1}" {
       setflag "\\seen";
     fileinto "Alerts";

   The subjects of the notification message are structured with a
   predictable pattern which includes a description of the event.  In
   the script above the "duplicate" test is used to detect duplicate
   alert events.  The message subject is matched against a pattern and
   the event description is extracted using the "variables" [VARIABLES]
   extension.  If a message with that event in the subject was received
   before, but more than a minute ago, it is not detected as a duplicate
   due to the specified ":seconds" argument.  In the the event of a
   duplicate, the message is marked as "seen" using the "imap4flags"
   [IMAP4FLAGS] extension.  All alert messages are put into the "Alerts"
   mailbox irrespective of whether those messages are duplicates or not.

6.  Security Considerations

   A flood of unique messages could cause the list of tracked values to
   grow indefinitely.  Implementations therefore SHOULD implement limits

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   on the number and lifespan of entries in that list.

7.  IANA Considerations

   The following template specifies the IANA registration of the Sieve
   extension specified in this document:

      Subject: Registration of new Sieve extension

      Capability name: duplicate
      Description:     Adds test 'duplicate' that can be used to test
                       whether a particular message is a duplicate,
                       i.e. whether a copy of it was seen before by the
                       delivery agent that is executing the Sieve
      RFC number:      this RFC
      Contact address: Sieve mailing list <>

   This information should be added to the list of sieve extensions
   given on

8.  References

8.1.  Normative References

   [INCLUDE]  Daboo, C. and A. Stone, "Sieve Email Filtering: Include
              Extension", RFC 6609, May 2012.

              Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [SIEVE]    Guenther, P. and T. Showalter, "Sieve: An Email Filtering
              Language", RFC 5228, January 2008.

8.2.  Informative References

              Melnikov, A., "Sieve Email Filtering: Imap4flags
              Extension", RFC 5232, January 2008.

   [MAILBOX]  Melnikov, A., "The Sieve Mail-Filtering Language --
              Extensions for Checking Mailbox Status and Accessing
              Mailbox Metadata", RFC 5490, March 2009.

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              Hansen, T. and C. Daboo, "Sieve Email Filtering: MIME Part
              Tests, Iteration, Extraction, Replacement, and Enclosure",
              RFC 5703, October 2009.

              Showalter, T. and N. Freed, "Sieve Email Filtering:
              Vacation Extension", RFC 5230, January 2008.

              Homme, K., "Sieve Email Filtering: Variables Extension",
              RFC 5229, January 2008.

Author's Address

   Stephan Bosch


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