Sieve Email Filtering: Detecting Duplicate Deliveries
EnschedeNLstephan@rename-it.nl
General
sieveduplicate deliveriesThis document defines a new test command "duplicate" for the "Sieve" email
filtering language. It can be used to test whether a particular string value
is a duplicate, i.e. whether it was seen before by the delivery agent that is
executing the Sieve script. The main application for this new test is detecting
duplicate message deliveries commonly caused by mailing list subscriptions or
redirected mail addresses.This is an extension to the Sieve filtering language defined by
RFC 5228. It adds a test to determine whether a
certain string value 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
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. Also, other
applications of this new test command are possible, as long as the tracked
value is a string.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 .Conventions for notations are as in Section 1.1,
including use of the "Usage:" label for the definition of action and tagged
arguments syntax.The "duplicate" test keeps track of which values were seen before by this
test in an earlier execution of this Sieve script. In its basic form, the tested
value is the content of the Message-ID header of the message. This way, this
test can be used to detect duplicate deliveries of the same message. It can also
detect duplicate deliveries based on other message header fields if requested
and it can even use a user-provided string value, e.g. as composed from text
extracted from the message using the "variables"
extension.The "duplicate" test evaluates to "true" when the provided value was seen
before in an earlier Sieve execution for a previous message delivery. If the
value was not seen earlier, the test evaluates to "false".As a side-effect, the "duplicate" test adds the evaluated value 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 value is encountered.
Note that the "duplicate" test MUST only check for duplicates amongst values
encountered in previous executions of the Sieve script; it MUST NOT consider
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"
extension, MUST yield the same result if the arguments
are identical.Implementations MUST prevent adding values to the internal duplicate tracking
list when the Sieve script execution fails. For example, this can be implemented
by deferring the definitive modification of the tracking list to the end of the
Sieve script execution. If failed script executions would add values to the
duplicate tracking list, all "duplicate" tests 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.Implementations SHOULD limit the number of values (and thereby messages) that
are tracked. Also, implementations SHOULD let entries in the value 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 advanced purposes, the content of another header can be
chosen for tracking by specifying the ":header" argument. The tracked string
value can also be specified explicitly using the ":value" argument. 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
value is extracted from a header, i.e. when the ":value" argument is not used,
leading and trailing whitespace (see Section 2.2 of
RFC 5228) MUST first be trimmed from the value
before performing the actual duplicate verification.Using the ":handle" argument, the duplicate test can be employed for multiple
independent purposes. Only when the tracked value was seen before in an earlier
script execution by a "duplicate" test with the same ":handle" argument, it is
recognized as a duplicate.
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. 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" argument.A Sieve implementation that defines the "duplicate" test command
will advertise the capability string "duplicate".
In the following 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" 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.
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: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" 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"
extension. All alert messages are put into the
"Alerts" mailbox irrespective of whether those messages are duplicates or not.
A flood of unique messages could cause the list of tracked values to
grow indefinitely. Implementations therefore SHOULD implement limits on the
number and lifespan of entries in that list.The following template specifies the IANA registration of the Sieve
extension specified in this document:This information should be added to the list of sieve extensions
given on http://www.iana.org/assignments/sieve-extensions.Key words for use in RFCs to Indicate Requirement LevelsHarvard University1350 Mass. Ave.CambridgeMA 02138- +1 617 495 3864sob@harvard.edu
General
keywordIn many standards track documents several words are used to signify
the requirements in the specification. These words are often
capitalized. This document defines these words as they should be
interpreted in IETF documents. Authors who follow these guidelines
should incorporate this phrase near the beginning of their 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
RFC 2119.
Note that the force of these words is modified by the requirement
level of the document in which they are used.
Sieve: An Email Filtering LanguageThis document describes a language for filtering email messages at time of final delivery.
It is designed to be implementable on either a mail client or mail server. It is meant to
be extensible, simple, and independent of access protocol, mail architecture, and operating
system. It is suitable for running on a mail server where users may not be allowed to
execute arbitrary programs, such as on black box Internet Message Access Protocol (IMAP)
servers, as the base language has no variables, loops, or ability to shell out to external
programs. [STANDARDS-TRACK]Sieve Email Filtering: Include ExtensionThe Sieve Email Filtering "include" extension permits users to include one
Sieve script inside another. This can make managing large scripts or multiple
sets of scripts much easier, and allows a site and its users to build up
libraries of scripts. Users are able to include their own personal scripts or
site-wide scripts. [STANDARDS-TRACK]Sieve Email Filtering: Imap4flags ExtensionRecent discussions have shown that it is desirable to set different IMAP (RFC 3501)
flags on message delivery. This can be done, for example, by a Sieve interpreter that
works as a part of a Mail Delivery Agent.</t><t> This document describes an
extension to the Sieve mail filtering language for setting IMAP flags. The extension
allows setting of both IMAP system flags and IMAP keywords. [STANDARDS-TRACK]The Sieve Mail-Filtering Language -- Extensions for Checking Mailbox Status and Accessing Mailbox MetadataThis memo defines an extension to the Sieve mail filtering language (RFC 5228) for accessing mailbox and server
annotations, checking for mailbox existence, and controlling mailbox creation on "fileinto" action. [STANDARDS-TRACK]Sieve Email Filtering: Vacation ExtensionThis document describes an extension to the Sieve email filtering language
for an autoresponder similar to that of the Unix "vacation" command for replying
to messages. Various safety features are included to prevent problems such as
message loops. [STANDARDS-TRACK]Sieve Email Filtering: Variables ExtensionIn advanced mail filtering rule sets, it is useful to keep state or configuration
details across rules. This document updates the Sieve filtering language (RFC 5228)
with an extension to support variables. The extension changes the interpretation of
strings, adds an action to store data in variables, and supplies a new test so that
the value of a string can be examined. [STANDARDS-TRACK]