Network Working Group                                           C. Daboo
Internet-Draft                                                  A. Stone
Expires: January 30, December 28, 2010                                 June 28, 2010                                  July 29, 2009

                Sieve Email Filtering: Include Extension
                      draft-ietf-sieve-include-03
                      draft-ietf-sieve-include-04

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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, 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.

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

   Changes from ietf-03 to ietf-04:

   a.  No changes.

   Changes from ietf-02 to ietf-03:

   a.  Setting a variable then calling global on it is an error
       (something like 'use strict').

   b.  Specify that the 'global' keyword is only available when
       'variables' has also been required.

   c.  Uploading a script that includes a nonexistent script is not an
       error at upload time.

   Changes from ietf-01 to ietf-02:

   a.  Require that script names must be constant strings, not subject
       to variable expansion.

   b.  Try the phrase immediate script instead of current script.

   c.  Clarify that "global 'varname'" and "global.varname" refer to the
       same variable.

   d.  Drop the requirement the global keywords come after require and
       before anything else.

   Changes from ietf-00 to ietf-01:

   a.  Replaced import/export with global.

   b.  Added :once modifier to include.

   c.  Added global namespace to see if it holds water.

   Changes from daboo-06 to ietf-00:

   a.  None

   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.

Table of Contents

   1.  Introduction and Overview  . . . . . . . . . . . . . . . . . .  5
   2.  Conventions Used in This Document  . . . . . . . . . . . . . .  5
   3.  Include Extension  . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  General Considerations . . . . . . . . . . . . . . . . . .  5
     3.2.  Control Structure include  . . . . . . . . . . . . . . . .  6
     3.3.  Control Structure return . . . . . . . . . . . . . . . . . 10
     3.4.  Interaction with Variables . . . . . . . . . . . . . . . . 10
       3.4.1.  Control Structure global . . . . . . . . . . . . . . . 10
       3.4.2.  Variables Namespace global . . . . . . . . . . . . . . 12
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
     5.1.  "include" Extension Registration . . . . . . . . . . . . . 13
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     6.1.  Normative References . . . . . . . . . . . . . . . . . . . 13
     6.2.  Informative References . . . . . . . . . . . . . . . . . . 13
   Appendix A.  Acknowledgments . . . . . . . . . . . . . . . . . . . 13
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13

1.  Introduction and Overview

   It's 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 latter 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

   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].

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

   The following key phrases are used to describe scripts and script
   execution:

   script
      a valid Sieve script.

   script execution
      an instance of a Sieve interpreter invoked for a given message
      delivery, starting with the user's active script and continuing
      through any included scripts until the message is delivered.

   immediate script
      the individual Sieve script file being executed.

   including script
      the individual Sieve script file that had an include statement
      which included the immediate script.

3.  Include Extension

3.1.  General Considerations

   Sieve implementations that implement the "include", "return", and
   "global" commands described below have an identifier of "include" for
   use with the capability mechanism.  If any of the "include",
   "return", or "global" 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
   possible.  For example, 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 generate an error at execution time if an
   included script does not exist.  Implementations MUST NOT generate
   errors for scripts missing at upload time, as this would force an
   upload ordering requirement upon script authors / generators.

   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 "global" command defined
   there.

3.2.  Control Structure include

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

               LOCATION = ":personal" / ":global"

               ONCE = ":once"

   The "include" command takes an optional "location" parameter, an
   optional ":once" parameter, and a single string argument representing
   the name of the script to include for processing at that point.  It
   is RECOMMENDED that implementations restrict script names according
   to [I-D.ietf-sieve-managesieve] Section 1.7.  Implementations MUST
   NOT allow variables to be expanded into the names of Sieve scripts;
   in other words, the value MUST be a constant string as defined in
   VARIABLES [RFC5229], Section 3.

   The "location" parameter MUST default 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 ":once" parameter tells the interpreter only to include the named
   script if it has not already been included at any other point during
   script execution.  If the script has already been included,
   processing continues immediately following the include command.
   Implementations MUST NOT generate an error if an "include :once"
   command names a script whose inclusion would be recursive; in this
   case, the script MUST be considered previously included and therefore
   "include :once" will not include it again.

   Note: It is RECOMMENDED that script authors / generators use this
   parameter only when including a script that performs general duties
   such as declaring global variables and making sanity checks of the
   environment.

   The included script MUST be a valid Sieve script, including having
   necessary "require" statements for all optional capabilities used by
   the script.  The scope of a "require" statement in an included script
   is for the immediate script only, not the including script.  For
   example, 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
   immediate one.  The "return" command (described below) stops
   processing of the immediate 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 "spam_tests";
      include :personal "mailing_lists";

   Personal script "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;
      }

   Personal script "spam_tests"

      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;

      }

   Personal script "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:

   Site script "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.

   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 immediately 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 the immediate 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
   command "global" to declare that a variable is shared among scripts.
   Effectively, two namespaces are defined: one local to the immediate
   script, and another shared among all scripts.  Implementations MUST
   allow a non-global variable to have the same name as a global
   variable but have no interaction between them.

3.4.1.  Control Structure global

      Usage:   global <value: string-list>

   The "global" command contains a string list argument that defines one
   or more names of variables to be stored in the global variable space.

   The "global" command is only available when the script has both
   "include" and "variables" in its require line.  If the "global"
   command appears when only "include" or only "variables" has been
   required, an error MUST be generated when the script is uploaded.

   If a "global" command is given the name of a variable that has
   previously been defined in the immediate script with "set", an error
   MUST be generated either when the script is uploaded or at execution
   time.

   If a "global" command lists a variable that has not been defined in
   the global namespace, the name of the variable is now marked as
   global, and any subsequent "set" command will set the value of the
   variable in global scope.

   Interpretation of a string containing a variable marked as global,
   but without any value set, SHALL behave as any other access to an
   unknown variable, as specified in VARIABLES [RFC5229], Section 3
   (i.e., evaluates to an empty string).

   Example:

      require ["variables", "include"];
      global "test";
      global "test-mailbox";

      # The included script may contain repetitive code that is
      # effectively a subroutine that can be factored out.
      set "test" "$$"
      include "spam_filter_script";

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

      # Message will be filed according to the test that matched last.
      if string :count "${test-mailbox}" "1"
      {
          fileinto "INBOX${test-mailbox}";
          stop;
      }

      # If nothing matched, the message is implicitly kept.

                               Active script

      require ["variables", "include"];
      global ["test", "test-mailbox"];

      if header :contains "Subject" "${test}"
      {
          set "test-mailbox" "spam-${test};
      }

                            spam_filter_script

3.4.2.  Variables Namespace global

   In addition to the "global" command, this document defines the
   variables namespace "global", as specified in VARIABLES [RFC5229],
   Section 3.

   Example:

      require ["variables", "include"];

      set "global.i_am_on_vacation" "1";

   Variables declared global and variables accessed via the global
   namespace MUST be one and the same.  In the following example script,
   we see the variable "i_am_on_vacation" used in a "global" command,
   and again with the "global." namespace.  Consider these as two
   syntaxes with identical meaning.

   Example:

      require ["variables", "include"];
      global "i_am_on_vacation";

      set "global.i_am_on_vacation" "1";

      if string :is "${i_am_on_vacation}" "1"
      {
          vacation "It's true, I am on vacation."
      }

4.  Security Considerations

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

   Sieve implementations MUST ensure that script names are checked for
   validity and proper permissions prior to inclusion, in order to
   prevent a malicious user from gaining acess to files accessible to
   the mail server software that should not be accessible to the user.

   Beyond these, the "include" extension does not raise any security
   considerations that are not present in the base SIEVE [RFC5228]
   document and the VARIABLES [RFC5229] extension.

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:     adds the "include" command to execute other Sieve
                    scripts, and the "global" command and "global" variables
                    namespace to access variables shared among included scripts.
   RFC number:      this RFC
   Contact address: the Sieve discussion list <ietf-mta-filters@imc.org>

6.  References

6.1.  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.

6.2.  Informative References

   [I-D.ietf-sieve-managesieve]
              Martin, T. and A. Melnikov, "A Protocol for Remotely
              Managing Sieve Scripts", draft-ietf-sieve-managesieve-09
              (work in progress), January 2009.

Appendix A.  Acknowledgments

   Thanks to Ken Murchison, Rob Siemborski, Alexey Melnikov, Marc Mutz,
   Kjetil Torgrim Homme, Stephan Bosch, Arnt Gulbrandsen, Barry Leiba,
   and Jeffrey Hutzelman for comments and corrections.

Authors' Addresses

   Cyrus Daboo

   Email: cyrus@daboo.name

   Aaron Stone

   Email: aaron@serendipity.cx