Network Working Group                                       Jutta Degener
Internet Draft                                                Rand Wacker
Expires: October 2003                                          April 2003

              Sieve -- Sequential Execution of Multiple Scripts

Status of this memo

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   This document defines sieve behavior relevant when multiple
   scripts are executed sequentially on the same message.

1. Introduction

   E-mail messages frequently are processed by multiple agents
   that implement nested layers of corporate policies.

   For example, a provider may offer access to services that
   perform spam- and virus filtering; a single corporation
   may restrict e-mail to certain subdomains, or filter for
   keywords; individual divisions within a corporation may
   implement even more intrusive handling of e-mail messages,
   for example by keeping a copy of all correspondence.
   Amidst all of this, of course, users may still use sieve
   filters to presort, redirect, or notify other accounts
   as in the classic sieve use scenario.

   In this context, it is desirable to specify an execution
   model for sieve scripts when executed in series.  This
   allows each layer of the mail filtering hierarchy to use
   a separate sieve script to express its policies.

2. Conventions used.

   Conventions for notations are as in [SIEVE] section 1.1, including
   use of [KEYWORDS] and "Syntax:" label for the definition of action
   and tagged arguments syntax.

   This document defines no sieve extensions and no capability string.

3. Sequential Script Execution Model

   When multiple scripts are executed for a message, they
   are executed in a specific order.

   Within this order, this document defines that trailing
   scripts will be executed as long as the message is being
   "kept", that is, as long as either an implicit or explicit
   "keep" is in effect.

   In other words, for every script but the last, "keep"
   doesn't mean "file this message into INBOX", it means
   "process this message with the next sieve script."

4. Locality of script actions

   This document strongly limits the effects of scripts
   on each other.

   The "require" clauses at the beginning of a script
   only apply to this particular script, not to following
   ones.   Different stages in the script processing
   may support different "require" areas.  For example,
   it is conceivable that "fileinto" is not supported
   for a stage other than a user's private script.

   The "stop;" command ends the execution of its single
   containing script, not of scripts in general.

   After one script terminates, the next script is executed
   if an implicit or explicit "keep" is in effect.
   (To end all script execution, a script should execute
   "discard; stop;".)

   For sieve engines that implement the "variables" extension,
   variable state is not carried over between scripts.

   For sieve engines that implement the "notify" extension,
   the "denotify" action in one script can only cancel
   "notify" actions from that same script.

   However, if a sequence of script executions results in
   the same message sent to the same recipient, or filed to
   the same destination folder, more than once, an implementation
   MAY only produce a single message, even if the commands
   executed stem from multiple scripts.

   If a sieve implementation enforces the incompatibility
   of "reject" with other actions (a SHOULD in [SIEVE]),
   it MUST only enforce it within one script; an action
   in a preceding script MUST be compatible with a "reject"
   in a later script.

5. Script Errors

   When an error occurs during processing of any of the
   scripts, all message processing stops, and the message
   is treated as if the final script had executed a "keep;".

6. Security Considerations

   A script executed before another script can prevent the
   trailing script from running (by executing "discard; stop;"
   or by encountering an error.)  This is deliberate.

   Corporate filtering of employee e-mail may violate the
   privacy expectations of employees.  However, since these
   instances are the ones running the software that handles
   employee e-mail to begin with, they already have the
   technical capability to do this.

7. Acknowledgments

   Thanks to Eric Allman, Will Lee, Dowson Tong, and Chris Markle for comments.

8. Authors' Addresses

   Jutta Degener
   Sendmail, Inc.
   6425 Christie Ave, 4th Floor
   Emeryville, CA 94608

   Rand Wacker
   Sendmail, Inc.
   6425 Christie Ave, 4th Floor
   Emeryville, CA 94608

9. Discussion

   This section will be removed when this document leaves the
   Internet-Draft stage.

   This draft is intended as an extension to the Sieve mail filtering
   language.  Sieve extensions are discussed on the MTA Filters mailing
   list at <>.  Subscription  requests can
   be sent to <> (send an email
   message with the word "subscribe" in the body).

   More information on the mailing list along with a WWW archive of
   back messages is available at <>.

9.1 Comparison to draft-daboo-sieve-include-00

   The "include" sieve extension describes a mechanism for
   naming and combining multiple scripts.  This document doesn't
   do that; how the sequence of scripts to be executed on a
   message is determined is left up to the environment and likely
   out of control of the script owner.

   The "include" sieve extension creates a hierarchical
   tree of nested scripts; this extension describes sequential,
   not hierarchical execution.

   The "include" sieve extension defines the semantics of
   "stop" to stop all sieve execution, not just that of the
   innermost containing script.  It adds a new "return" command
   to explicitly end execution of a single script.
   This document specifies that "stop" just stops a single script,
   and uses the redefined meaning of "keep" to regulate the
   flow of messages through scripts.

9.2 "require" keyword

   This draft started out with a "require" keyword, "multiscript",
   but since what it describes lies outside the domain of strict
   sieve language behavior, I'm not sure it needs one.


Appendix A.  References

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

   [SIEVE]      Showalter, T.,  "Sieve: A Mail Filtering Language", RFC 3028,
                January 2001.

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