Sieve Email Filtering: Detecting Duplicate Deliveries
RFC 7352

Internet Engineering Task Force (IETF)                          S. Bosch
Request for Comments: 7352                                September 2014
Category: Standards Track
ISSN: 2070-1721

         Sieve Email Filtering: Detecting Duplicate Deliveries

Abstract

   This document defines a new test command, "duplicate", for the Sieve
   email filtering language.  This test adds the ability to detect
   duplications.  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 field
   or other parts of the message.

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
   http://www.rfc-editor.org/info/rfc7352.

Copyright Notice

   Copyright (c) 2014 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
   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.

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Table of Contents

   1. Introduction ....................................................2
   2. Conventions Used in This Document ...............................3
   3. Test "duplicate" ................................................3
      3.1. Arguments ":header" and ":uniqueid" ........................5
      3.2. Argument ":handle" .........................................7
      3.3. Arguments ":seconds" and ":last" ...........................8
      3.4. Interaction with Other Sieve Extensions ....................9
   4. Sieve Capability Strings ........................................9
   5. Examples ........................................................9
      5.1. Example 1 ..................................................9
      5.2. Example 2 .................................................10
      5.3. Example 3 .................................................11
      5.4. Example 4 .................................................12
   6. Security Considerations ........................................12
   7. IANA Considerations ............................................13
   8. Acknowledgements ...............................................14
   9. References .....................................................14
      9.1. Normative References ......................................14
      9.2. Informative References ....................................15

1.  Introduction

   This document specifies an extension to the Sieve filtering language
   defined by RFC 5228 [SIEVE].  It adds a test to track whether or not
   a text string 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
   often get one copy of the message directly and another through the
   mailing list.  Also, if someone crossposts over several mailing lists
   to which the user is subscribed, the user will likely 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).

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   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 criteria for
   defining what makes a message a duplicate (e.g., using 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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [KEYWORDS].

   Conventions for notations are as in Section 1.1 of [SIEVE], 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>] [":last"]

   The "duplicate" test identifies the message by a "unique ID" and,
   using that unique ID, keeps track of which messages were seen by a
   "duplicate" test during an earlier Sieve execution.  In its basic
   form, the test gets the unique ID from the content of the message's
   Message-ID header field.  The "duplicate" test evaluates to "true"
   if the message was seen before, and it evaluates to "false" if it
   was not.

   As a side effect, the "duplicate" test adds the unique ID to an
   internal duplicate-tracking list once the Sieve execution finishes
   successfully.  The first time a particular unique ID is seen, the
   message is not a duplicate, and the unique ID is added to the
   tracking list.  If a future Sieve execution sees a message whose
   unique ID appears in the tracking list, that test will evaluate to
   "true", and that message will be considered a duplicate.

   Note that this side effect is performed only when the "duplicate"
   test is actually evaluated.  If the "duplicate" test is nested in a
   control structure or if it is not the first item of an "allof" or
   "anyof" test list, its evaluation depends on the result of preceding
   tests, which may produce unexpected results.

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   Implementations MUST only update the internal duplicate-tracking list
   when the Sieve script execution finishes successfully.  If failing
   script executions add the unique 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.  This
   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.

   The "duplicate" test MUST only check for duplicates amongst unique 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.

   The Message-ID header field is assumed to be globally unique as
   required in Section 3.6.4 of RFC 5322 [IMAIL].  In practice, this
   assumption may not always prove to be true.  The "duplicate" test
   does not deal with this situation, which means that false duplicates
   may be detected in this case.  However, the user can address such
   situations by specifying an alternative means of message
   identification using the ":header" or the ":uniqueid" argument, as
   described in the next section.

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3.1.  Arguments ":header" and ":uniqueid"

   Duplicate tracking involves determining the unique ID for a
   given message and checking whether that unique ID is in the
   duplicate-tracking list.  The unique ID for a message is
   determined as follows:

   o  When neither the ":header" argument nor the ":uniqueid" argument
      is used, the unique ID is the content of the message's Message-ID
      header field.

   o  When the ":header" argument is used, the unique ID is the content
      of the specified header field in the message.  The header field
      name is not part of the resulting unique ID; it consists only of
      the field value.

   o  When the ":uniqueid" argument is used, the unique ID is the string
      parameter that is specified with the argument.

   The ":header" and ":uniqueid" arguments are mutually exclusive;
   specifying both for a single "duplicate" test command MUST trigger an
   error.

   The syntax rules for the header name parameter of the ":header"
   argument are specified in Section 2.4.2.2 of RFC 5228 [SIEVE].  Note
   that implementations MUST NOT trigger an error for an invalid header
   name.  Instead, the "duplicate" test MUST yield "false"
   unconditionally in this case.  The parameter of the ":uniqueid"
   argument can be any string.

   If the tracked unique ID value is extracted directly from a message
   header field (i.e., when the ":uniqueid" argument is not used), the
   following operations MUST be performed before the actual duplicate
   verification:

   o  Unfold the header line as described in Section 2.2.3 of RFC 5322
      [IMAIL] (see also Section 2.4.2.2 of RFC 5228 [SIEVE]).

   o  If possible, convert the header value to Unicode, encoded as UTF-8
      (see Section 2.7.2 of RFC 5228 [SIEVE]).  If conversion is not
      possible, the value is left unchanged.

   o  Trim leading and trailing whitespace from the header value (see
      Section 2.2 of RFC 5228 [SIEVE]).

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   Note that these rules also apply to the Message-ID header field used
   by the basic "duplicate" test without a ":header" or ":uniqueid"
   argument.  When the ":uniqueid" argument is used, any normalization
   needs to be done in the Sieve script itself as the unique ID is
   created.

   If the header field specified using the ":header" argument exists
   multiple times in the message, extraction of the unique ID MUST use
   only the first occurrence.  This is true whether or not multiple
   occurrences are allowed by Section 3.6 of RFC 5322 [IMAIL].  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 occur multiple times.

   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 how
   the message's unique ID is created.

   The unique ID MUST be matched case-sensitively with the contents of
   the duplicate-tracking list, irrespective of how the unique ID was
   determined.  To achieve case-insensitive behavior when the
   ":uniqueid" argument is used, the "set" command added by the
   "variables" [VARIABLES] extension can be used to normalize the unique
   ID value to upper or lower case.

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3.2.  Argument ":handle"

   The "duplicate" test MUST track a unique ID value independent of its
   source.  This means that all values in the duplicate-tracking list
   should be used for duplicate testing, regardless of whether they were
   obtained from the Message-ID header field, from an arbitrary header
   specified using the ":header" argument, or explicitly from the
   ":uniqueid" argument.  The following three examples are equivalent
   and match the same entry in the duplicate-tracking list:

   require "duplicate";
   if duplicate {
     discard;
   }

   require "duplicate";
   if duplicate :header "message-id" {
     discard;
   }

   require ["duplicate", "variables"];
   if header :matches "message-id" "*" {
     if duplicate :uniqueid "${0}" {
       discard;
     }
   }

   The ":handle" argument can be used to override this default behavior.
   The ":handle" argument separates a "duplicate" test from other
   "duplicate" tests with a different or omitted ":handle" argument.
   Using the ":handle" argument, unrelated "duplicate" tests can be
   prevented from interfering with each other: a message is only
   recognized as a duplicate 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" action [VACATION].  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" argument.

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3.3.  Arguments ":seconds" and ":last"

   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
   accepts an integer value specifying the timeout value in seconds.  If
   the ":seconds" argument is omitted, an appropriate default value MUST
   be used.  A default expiration time of around 7 days is usually
   appropriate.  Sites SHOULD impose a maximum limit on the expiration
   time.  If that limit is exceeded by the ":seconds" argument, the
   maximum value MUST be silently substituted; exceeding the limit MUST
   NOT produce an error.  If the ":seconds" argument is zero, the
   "duplicate" test MUST yield "false" unconditionally.

   When the ":last" argument is omitted, the expiration time for entries
   in the duplicate-tracking list MUST be measured relative to the
   moment at which the entry was first created (i.e., at the end of the
   successful script execution during which the "duplicate" test
   returned "false" for a message with that particular unique ID value).
   This means that subsequent duplicate messages have no influence on
   the time at which the entry in the duplicate-tracking list finally
   expires.

   In contrast, when the ":last" argument is specified, the expiration
   time MUST be measured relative to the last script execution during
   which the "duplicate" test was used to check the entry's unique ID
   value.  This effectively means that the entry in the duplicate-
   tracking list will not expire while duplicate messages with the
   corresponding unique ID keep being delivered within intervals smaller
   than the expiration time.

   It is possible to write Sieve scripts where, during a single
   execution, more than one "duplicate" test is evaluated with the same
   unique ID value and ":handle" argument but different ":seconds" or
   ":last" arguments.  The resulting behavior is left undefined by this
   specification, so such constructs should be avoided.  Implementations
   MAY choose to use the ":seconds" and ":last" arguments from the
   "duplicate" test that was evaluated last.

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3.4.  Interaction with Other Sieve Extensions

   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.

   Normally, Sieve scripts are executed at final delivery.  However,
   with the "imapsieve" [IMAPSIEVE] extension, Sieve scripts are invoked
   when the IMAP [IMAP] server performs operations on the message store
   (e.g., when messages are uploaded, flagged, or moved to another
   location).  The "duplicate" test is devised for use at final
   delivery, and the semantics in the "imapsieve" context are left
   undefined.  Therefore, implementations SHOULD NOT allow the
   "duplicate" test to be used in the context of "imapsieve".

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 field.  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.  Several notifications can be received about the same event
   from different monitoring systems.  The Message-ID header field of
   these messages is different, because these are all distinct messages
   from different senders.  To avoid being notified more than a single
   time about the same event, the user writes the following script:

   require ["duplicate", "variables", "imap4flags",
     "fileinto"];

   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 that 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 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.

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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
   Extensible Messaging and Presence Protocol (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:

   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}"
       "xmpp:user@im.example.com";
   }

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

   The example can be extended to allow more 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}"
       "xmpp:user@im.example.com";
   }

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

5.4.  Example 4

   For this example, the mail user uses the "duplicate" test for two
   separate applications: for discarding duplicate events from a
   notification system and for marking certain follow-up messages in a
   software support mailing as "seen" using the "imap4flags"
   [IMAP4FLAGS] extension.

   The two "duplicate" tests in the following example each use a
   different header to identify messages.  However, these "X-Event-ID"
   and "X-Ticket-ID" headers can have similar values in this case (e.g.,
   both based on a time stamp), meaning that one "duplicate" test can
   erroneously detect duplicates based on ID values tracked by the
   other.  Therefore, the user wants to prevent the second "duplicate"
   test from matching ID values tracked by the first "duplicate" test
   and vice versa.  This is achieved by specifying different ":handle"
   arguments for these tests.

   require ["duplicate", "imap4flags"];

   if duplicate :header "X-Event-ID" :handle "notifier" {
     discard;
   }
   if allof (
     duplicate :header "X-Ticket-ID" :handle "support",
     address "to" "support@example.com",
     header :contains "subject" "fileserver")
   {
     setflag "\\seen";
   }

6.  Security Considerations

   A flood of unique messages could cause the duplicate-tracking list to
   grow indefinitely.  Therefore, 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.

   Scripts using the "duplicate" test evaluation should be aware that
   message IDs are not necessarily unique, either through the fault of
   benign generators or attackers injecting a message with the
   properties used by the duplicate Sieve filter at some point prior to

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   the Sieve filter.  Therefore, scripts are well advised to be
   conservative with respect to actions taken when duplicate messages
   are identified only by message ID.

   The list of unique IDs used for duplicate tracking can include
   privacy-sensitive information, such as message ID values, content of
   subject lines, and content extracted from message bodies.
   Implementations SHOULD protect that information by obscuring it
   through hashing (see the note at the end of Section 3.2) and/or by
   storing it with a level of access control equivalent to that of the
   messages themselves.

   These measures will not prevent an entity that has access to the
   duplicate-tracking list from querying whether messages with certain
   unique ID values were received.  As this operation is the essence of
   the "duplicate" test, this cannot be prevented and may violate the
   expectations of the user.  For example, a user who deletes a message
   from the server may expect that no record of it remains on the
   server, but that will not be true if its message ID is persisted on
   the server in the duplicate-tracking list.

   It's notable, however, that server logs will often store the
   information present on the duplicate-tracking list anyway and
   probably would expose plaintext message IDs for a much longer period
   than this mechanism would.  Users of email services that
   intentionally delete such logs with the intent of limiting
   traceability should be made aware that use of the duplicate-tracking
   mechanism re-exposes this information for the duration of the expiry
   interval.  Therefore, a shorter default expiry interval may be
   appropriate in those situations.

7.  IANA Considerations

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

      To: iana@iana.org
      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
                        script.
      RFC number:       RFC 7352
      Contact address:  Sieve mailing list <sieve@ietf.org>

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   This information has been added to the list of Sieve extensions given
   on <http://www.iana.org/assignments/sieve-extensions>.

8.  Acknowledgements

   Thanks to Brian Carpenter, Cyrus Daboo, Arnt Gulbrandsen, Tony
   Hansen, Kristin Hubner, Barry Leiba, Alexey Melnikov, Subramanian
   Moonesamy, Tom Petch, Hector Santos, Robert Sparks, Aaron Stone, and
   Stefan Winter for reviews and suggestions.  Special thanks to Ned
   Freed for his guidance and support.

9.  References

9.1.  Normative References

   [DATE-INDEX]
              Freed, N., "Sieve Email Filtering: Date and Index
              Extensions", RFC 5260, July 2008.

   [IMAIL]    Resnick, P., Ed., "Internet Message Format", RFC 5322,
              October 2008.

   [IMAPSIEVE]
              Leiba, B., "Support for Internet Message Access Protocol
              (IMAP) Events in Sieve", RFC 6785, November 2012.

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

   [KEYWORDS]
              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.

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

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

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9.2.  Informative References

   [IMAP]     Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL -
              VERSION 4rev1", RFC 3501, March 2003.

   [IMAP4FLAGS]
              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.

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

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

Author's Address

   Stephan Bosch
   Enschede
   NL

   EMail: stephan@rename-it.nl

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