Network Working Group                                           C. Daboo
Internet-Draft                                                  A. Stone
Expires: September 10, 2009                                March 9, 2009


                Sieve Email Filtering: Include Extension
                      draft-daboo-sieve-include-06

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   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
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Abstract

   The 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, as well as
   supporting common 'libraries' of scripts.  Users are able to include
   their own personal scripts or site-wide scripts provided by the local
   Sieve implementation.

Change History (to be removed prior to publication as an RFC)

   Changes from -05 to -06:
   a.  Aaron Stone joins as author.
   b.  Removed | characters from the script examples.
   c.  Updated draft references to published RFCs.

   Changes from -04 to -05:
   a.  Fixed examples.
   b.  Relaxed requirement that imported/exported variables be set
       before being used.

   Changes from -03 to -04:
   a.  Fixed missing 2119 definitions.
   b.  Defined interaction with variables through use of import and
       export commands.

   Changes from -02 to -03:
   a.  Refreshing expired draft (updated for nits).
   b.  Syntax -> Usage.
   c.  Updated to 3028bis reference.

   Changes from -01 to -02:
   a.  Minor formatting changes only - refreshing expired draft.

   Changes from -00 to -01:
   a.  Added IPR boiler plate.
   b.  Re-ordered sections at start to conform to RFC style.
   c.  Moved recursion comment into General Considerations section.
   d.  Switched to using optional parameter to indicate personal vs
       global.
   e.  Explicitly state that an error occurs when a missing script is
       included.





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Open Issues (to be resolved prior to publication as an RFC)

   a.  Interaction with variables (scoping).  Should variables be
       carried over between scripts that are included?  Or should
       variables defined in an included script be local to that script
       only?


Table of Contents

   1.  Introduction and Overview  . . . . . . . . . . . . . . . . . .  4
   2.  Conventions Used in This Document  . . . . . . . . . . . . . .  4
   3.  Include Extension  . . . . . . . . . . . . . . . . . . . . . .  4
     3.1.  General Considerations . . . . . . . . . . . . . . . . . .  4
     3.2.  Control Structure Include  . . . . . . . . . . . . . . . .  5
     3.3.  Control Structure Return . . . . . . . . . . . . . . . . .  8
     3.4.  Interaction with Variables . . . . . . . . . . . . . . . .  8
       3.4.1.  Control Structure Import . . . . . . . . . . . . . . .  8
       3.4.2.  Control Structure Export . . . . . . . . . . . . . . .  9
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
     5.1.  "include" Extension Registration . . . . . . . . . . . . . 10
   6.  Normative References . . . . . . . . . . . . . . . . . . . . . 11
   Appendix A.  Acknowledgments . . . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11


























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1.  Introduction and Overview

   Its convenient to be able to break SIEVE [RFC5228] scripts down into
   smaller components which can be reused in a variety of different
   circumstances.  For example, users may want to have a default script
   and a special 'vacation' script, the later being activated when the
   user goes on vacation.  In that case the default actions should
   continue to be run, but a vacation command should be executed first.
   One option is to edit the default script to add or remove the
   vacation command as needed.  Another is to have a vacation script
   that simply has a vacation command and then includes the default
   script.


2.  Conventions Used in This Document

   Conventions for notations are as in SIEVE [RFC5228] Section 1.1.

   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 [RFC2119].


3.  Include Extension

3.1.  General Considerations

   Sieve implementations that implement the "include", "return",
   "export" and "import" commands described below have an identifier of
   "include" for use with the capability mechanism.  If any of the
   "include", "return", "export" or "import" commands are used in a
   script, the "include" capability MUST be listed in the "require"
   statement in that script.

   Sieve implementations must track the use of actions in included
   scripts so that implicit "keep" behavior can be properly determined
   based on whether any actions have executed in any script.

   Sieve implementations are allowed to limit the total number of nested
   included scripts, but MUST provide for a total of at least three
   levels of nested scripts including the top-level script.  An error
   MUST be generated either when the script is uploaded to the Sieve
   repository, or when the script is executed, if any nesting limit is
   exceeded.  If such an error is detected whilst processing a Sieve
   script, an implicit "keep" action MUST be executed to prevent loss of
   any messages.

   Sieve implementations MUST ensure that recursive includes are not



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   possible. i.e. if script "A" includes script "B", and script "B"
   includes script "A" an error MUST be generated either when the script
   is uploaded to the Sieve repository, or when the script is executed.
   If such an error is detected whilst processing a Sieve script, an
   implicit "keep" action MUST be executed to prevent loss of any
   messages.

   Sieve implementations MUST handle missing scripts being referenced
   via an includes in an existing script.  An error MUST be generated
   when a missing included script is discovered during execution.  If
   such an error is detected an implicit "keep" action MUST be executed
   to prevent loss of any messages.

   If the Sieve "variables" extension [RFC5229] is present, an issue
   arises with the "scope" of variables defined in scripts that may
   include each other.  For example, if a script defines the variable
   "${status}" with one particular meaning or usage, and another defines
   "${status}" with a different meaning, then if one script includes the
   other there is an issue as to which "${status}" is being referenced.
   To solve this problem, Sieve implementations MUST follow the scoping
   rules defined in Section 3.4 and support the "import" and "export"
   commands defined there.

3.2.  Control Structure Include

           Usage:   include [<location: LOCATION>] <value: string>

           LOCATION = ":personal" / ":global"

   The "include" command includes an optional "location" parameter, and
   a single string argument representing the name of the script to
   include in the main script at that point.

   The "location" parameter defaults to ":personal" if not specified.
   The "location" has the following meanings:

      :personal

         Indicates that the named script is stored in the user's own
         personal (private) Sieve repository.

      :global

         Indicates that the named script is stored in a site-wide Sieve
         repository, accessible to all users of the Sieve system.

   The included script MUST be a valid Sieve script, including having
   necessary "require" statements for all optional capabilities used by



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   the script.  The scope of a "require" statement in an included script
   is for that script only, not the including script. e.g. if script "A"
   includes script "B", and script "B" uses the "fileinto" extension,
   script "B" must have a "require" statement for "fileinto",
   irrespective of whether script "A" has one.  In addition, if script
   "A" does not have a "require" statement for "fileinto", "fileinto"
   cannot be used anywhere in script "A", even after inclusion of script
   "B".

   A "stop" command in an included script MUST stop all script
   processing, including the processing of the scripts that include the
   current one.  The "return" command (described below) stops processing
   of the current script only, and allows the scripts that include it to
   continue.

   Examples:

   The user has four scripts stored in their personal repository:

   "default"

      This is the default active script that includes several others.

               require ["include"];

               include :personal "always_allow";
               include :global "spam_tests";
               include :personal "my_spam_tests";
               include :personal "mailing_lists";

   "always_allow"

      This script special cases some correspondent email addresses and
      makes sure any message containing those addresses are always kept.

               if header :is "From" "boss@example.com"
               {
                   keep;
               }
               elsif header :is "From" "ceo@example.com"
               {
                   keep;
               }

   "my_spam_tests"






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      This script does some user-specific spam tests to catch spam
      messages not caught by the site-wide spam tests.

               require ["reject"];

               if header :contains "Subject" "XXXX"
               {
                   reject;
               }
               elsif header :is "From" "money@example.com"
               {
                   reject;
               }

   "mailing_lists"

      This script looks for messages from different mailing lists and
      files each into a mailbox specific to the mailing list.

               require ["fileinto"];

               if header :is "Sender" "owner-ietf-mta-filters@imc.org"
               {
                   fileinto "lists.sieve";
               }
               elsif header :is "Sender" "owner-ietf-imapext@imc.org"
               {
                   fileinto "lists.imapext";
               }

   There is one script stored in the global repository:

   "spam_tests"

      This script does some site-wide spam tests which any user at the
      site can include in their own scripts at a suitable point.  The
      script content is kept up to date by the site administrator.

               require ["reject"];

               if anyof (header :contains "Subject" "$$",
                         header :contains "Subject" "Make money")
               {
                   reject;
               }

   The "include" command may appear anywhere in the script where a
   control structure is legal.



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   Example:

               require ["include"];

               if anyof (header :contains "Subject" "$$",
                         header :contains "Subject" "Make money")
               {
                   include "my_reject_script";
               }

3.3.  Control Structure Return

           Usage: return

   The "return" command stops processing of the currently included
   script only and returns processing control to the script which
   includes it.  If used in the main script (i.e. not in an included
   script), it has the same effect as the "stop" command, including the
   appropriate "keep" action if no other actions have been executed up
   to that point.

3.4.  Interaction with Variables

   In order to avoid problems of variables in an included script
   "overwriting" those from the script that includes it, this
   specification requires that all variables defined in a script MUST be
   kept "private" to that script by default - i.e. they are not
   "visible" to other scripts.  This ensures that two script authors
   cannot inadvertently cause problems by choosing the same name for a
   variable.

   However, sometimes there is a need to make a variable defined in one
   script available to others.  This specification defines the new
   commands "export" and "import" to alter the default behavior of
   variable scoping to allow variables to be "seen" by other scripts.
   The "export" command takes a list of variable names defined in a
   script and makes those available to any script that explicitly wants
   to use them.  The "import" command allows a script to gain access to
   variables that have been explicitly made available by other scripts.
   The explicit use of "export and "import", coupled with the default
   behavior of variables only having local scope, ensures that multiple
   scripts cannot inadvertently overwrite each others variables.

3.4.1.  Control Structure Import

           Usage:   import <value: string-list>

   The "import" command contains a string list argument that defines one



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   or more names of variables exported by other scripts which should be
   made available to the current script.

   The "import" command, if present, MUST be used immediately after any
   "require" commands (at least one of which will be present listing the
   "include" extension).  Multiple "import" commands are allowed.  An
   error occurs if an "import" command appears after a command other
   than "require" or "import".  Use of the "import" command makes the
   listed variables immediately available for use in the body of the
   script that uses it.

   If an "import" command lists a variable that has not been exported by
   any other script at that point during the Sieve execution process,
   then an error MUST occur.

   Example:

               require ["variables", "fileinto", "include"];
               import "test";

               if header :contains "Subject" "${test}"
               {
                   fileinto "INBOX.spam-${test};
               }

3.4.2.  Control Structure Export

           Usage:   export <value: string-list>

   The "export" command contains a string list argument that defines one
   or more names of variables defined in the current script which should
   be made available to any scripts that run during the current Sieve
   script execution process.

   The "export" command, if present, MUST be used immediately after any
   "require" or "import" commands.  Multiple "export" commands are
   allowed.  An error occurs if an "export" command appears after a
   command other than "require", "import" or "export".

   An error occurs if an "export" command lists a variable that is
   listed in an "import" command preceding it in the current script.

   Example:








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               require ["variables", "include"];
               export "test";

               set "test" "$$"
               include "spam_filter_script";

               set "test" "Make money"
               include "spam_filter_script";

   Example:

               require ["variables", "include"];
               import "test";
               export "test-mailbox";

               if header :contains "Subject" "${test}"
               {
                   set "test-mailbox" "INBOX.spam-${test};
                   include "fileinto-script"
               }


4.  Security Considerations

   Sieve implementations MUST ensure adequate security for the global
   script repository to prevent unauthorized changes to global scripts.

   Beyond that, the "include" extension does not raise any security
   considerations that are not present in the base Sieve protocol, and
   these issues are discussed in Sieve.


5.  IANA Considerations

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

5.1.  "include" Extension Registration

   Capability name: include
   Description:     add the "include" command to execute other Sieve
                    scripts.
   RFC number:      this RFC
   Contact address: the Sieve discussion list <ietf-mta-filters@imc.org>







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6.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5228]  Guenther, P. and T. Showalter, "Sieve: An Email Filtering
              Language", RFC 5228, January 2008.

   [RFC5229]  Homme, K., "Sieve Email Filtering: Variables Extension",
              RFC 5229, January 2008.


Appendix A.  Acknowledgments

   Thanks to Ken Murchison, Rob Siemborski, Alexey Melnikov, Marc Mutz
   and Kjetil Torgrim Homme for comments and corrections.


Authors' Addresses

   Cyrus Daboo

   Email: cyrus@daboo.name


   Aaron Stone

   Email: aaron@serendipity.palo-alto.ca.us























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