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Sieve Email Filtering: Detecting Duplicate Deliveries

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 7352.
Author Stephan Bosch
Last updated 2013-05-22
Replaces draft-bosch-sieve-duplicate
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state WG Document
Document shepherd Ned Freed
IESG IESG state Became RFC 7352 (Proposed Standard)
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APPSAWG                                                         S. Bosch
Internet-Draft                                              May 22, 2013
Intended status: Standards Track
Expires: November 23, 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.

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

   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 November 23, 2013.

Copyright Notice

   Copyright (c) 2013 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
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect

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   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.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions Used in This Document  . . . . . . . . . . . . . .  3
   3.  Test "duplicate" . . . . . . . . . . . . . . . . . . . . . . .  3
   4.  Sieve Capability Strings . . . . . . . . . . . . . . . . . . .  6
   5.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     5.1.  Example 1  . . . . . . . . . . . . . . . . . . . . . . . .  6
     5.2.  Example 2  . . . . . . . . . . . . . . . . . . . . . . . .  7
     5.3.  Example 3  . . . . . . . . . . . . . . . . . . . . . . . .  7
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     8.1.  Normative References . . . . . . . . . . . . . . . . . . .  9
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 10
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10

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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 unique 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> /
                          ":uniqueid" <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 during an earlier Sieve execution.
   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 not.

   As a side-effect, the "duplicate" test adds the message ID to an
   internal duplicate tracking list once the Sieve execution finishes
   successfully.  This way, the same test will evaluate to "true" during
   the next Sieve execution.  Implementations MUST prevent making any
   definitive modifications to the internal duplicate tracking list
   until the Sieve script execution finishes successfully.  If failing
   script executions would add the message ID 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.

   However, deferring the definitive modification of the tracking list
   to the end of a successful Sieve script execution is not without
   problems.  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.

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

   Implementations SHOULD limit the number of entries in the duplicate
   tracking list.  When limiting the number of entries, implementations
   SHOULD discard the oldest ones first.

   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,
   which is always specified in seconds.  If the ":seconds" argument is
   omitted, an appropriate default value MUST be used.  A default
   expiration time of around 7 days is deemed to be appropriate.  Sites
   SHOULD impose a maximum limit on the expiration time.  If that limit
   is exceeded, the maximum value MUST silently be substituted;

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   exceeding the limit MUST NOT produce an error.  If the ":seconds"
   argument is zero, the "duplicate" test MUST yield "false"

   By default, the content of the message's Message-ID header field is
   used as the unique ID for duplicate tracking.  For more complex
   applications, the "duplicate" test can also be used to detect
   duplicate deliveries based on other message text.  Then, the tracked
   unique ID 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 field can be used as
   the tracked unique ID instead of the default Message-ID header.
   Alternatively, the tracked unique ID can be specified explicitly
   using the ":uniqueid" argument.  The ":header" and ":uniqueid"
   arguments are mutually exclusive and specifying both for a single
   "duplicate" test command MUST trigger an error.

   If the tracked unique ID value is extracted directly from a message
   header field, i.e. when the ":uniqueid" 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 ":uniqueid" argument is used, such
   normalization concerns are the responsibility of the user.

   If the header field specified using the ":header" argument exists
   multiple times in the message, only the first occurrence MUST be used
   for duplicate tracking.  If the specified header field is not present
   in the message, the "duplicate" test MUST yield "false"
   unconditionally.  In that case the duplicate tracking list is left
   unmodified by this test, since no unique ID value is available.  The
   same rules apply with respect to the Message-ID header field for the
   basic "duplicate" test without a ":header" or ":uniqueid" argument,
   since that header field could also be missing or occurring multiple

   The string parameter of the ":uniqueid" 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.

   Note that the "duplicate" test does not support either the "index"
   [DATE-INDEX], or "mime" [SIEVE-MIME] extensions directly, meaning
   that none of the ":index", ":mime:" or associated arguments are added
   to the "duplicate" test when these extensions are active.  The
   ":uniqueid" argument can be used in combination with the "variables"
   [VARIABLES] extension to achieve the same result indirectly.

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   The tracked unique ID value MUST be matched case-sensitively,
   irrespective of whether it originates from a header or is specified
   explicitly using the ":uniqueid" argument.  To achieve case-
   insensitive behavior, the "set" command added by the "variables"
   [VARIABLES] extension can be used in combination with the ":uniqueid"
   argument to normalize the tracked unique ID value to upper or lower

   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 unique ID was seen before in an
   earlier script execution by a "duplicate" test with the same
   ":handle" argument.

   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 unique ID 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

5.1.  Example 1

   In this basic example message duplicates are detected by 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";

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5.2.  Example 2

   This 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 :uniqueid "${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.

5.3.  Example 3

   This example shows how the "duplicate" test can be used to limit the
   frequency of notifications sent using the "enotify" [NOTIFY]
   extension.  Consider the following scenario: a mail user receives
   XMPP notifications [NOTIFY-XMPP] about new mail through Sieve, but
   sometimes a single contact sends many messages in a short period of
   time.  Now the user wants to prevent being notified of all of those
   messages.  The user wants to be notified about messages from each
   person at most once per 30 minutes and writes the following script:

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   require ["variables", "envelope", "enotify", "duplicate"];

   if envelope :matches "from" "*" { set "sender" "${1}"; }
   if header :matches "subject" "*" { set "subject" "${1}"; }

   if not duplicate :seconds 1800 :uniqueid "${sender}")
     notify :message "[SIEVE] ${sender}: ${subject}"

   The example shown above uses the message envelope sender rather than
   the Message-ID header as the unique ID for duplicate tracking.

   The example can be extended to allow multiple messages from the same
   sender in close succession as long as the discussed subject is
   different.  This can be achieved as follows:

   require ["variables", "envelope", "enotify", "duplicate"];

   if envelope :matches "from" "*" { set "sender" "${1}"; }
   if header :matches "subject" "*" { set "subject" "${1}"; }

   # account for 'Re:' prefix
   if string :comparator "i;ascii-casemap"
     :matches "${subject}" "Re:*"
     set "subject" "${1}";
   if not duplicate :seconds 1800
     :uniqueid "${sender} ${subject}")
     notify :message "[SIEVE] ${sender}: ${subject}"

   This uses a combination of the message envelope sender and the
   subject of the message as the unique ID for duplicate tracking.

6.  Security Considerations

   A flood of unique messages could cause the list of tracked message ID
   values to grow indefinitely.  Implementations therefore SHOULD
   implement limits on the number and lifespan of entries in that list.

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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

              Freed, N., "Sieve Email Filtering: Date and Index
              Extensions", RFC 5260, July 2008.

   [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.

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

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

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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.

   [NOTIFY]   Melnikov, A., Leiba, B., Segmuller, W., and T. Martin,
              "Sieve Email Filtering: Extension for Notifications",
              RFC 5435, January 2009.

              Saint-Andre, P. and A. Melnikov, "Sieve Notification
              Mechanism: Extensible Messaging and Presence Protocol
              (XMPP)", RFC 5437, January 2009.

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

Author's Address

   Stephan Bosch


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