Network Working Group
Internet Draft: Sieve -- IMAP flag Extension                 A. Melnikov
Document: draft-ietf-sieve-imapflags-00.txt                Isode Limited
Expires: August 2005                                       February 2005


                      Sieve -- IMAP flag Extension


Status of this memo

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Copyright

   Copyright (C) The Internet Society 2000-2005.  All Rights Reserved.

Abstract

   Recent discussions have shown that it is desirable to set different
   [IMAP] flags on message delivery.  This can be done, for example,
   by a Sieve interpreter that works as a part of a Mail Delivery
   Agent.

   This document describes an extension to the Sieve mail filtering
   language for setting [IMAP] flags. The extension allows to set both
   [IMAP] system flags and [IMAP] keywords.


. Meta-information on this draft

   This information is intended to facilitate discussion.  It will be
   removed when this document leaves the Internet-Draft stage.

   Editorial comments are marked with << and >>.


0.1. Discussion

   This draft defines an extension to the Sieve mail filtering
   language (RFC 3028) being  discussed  on the MTA Filters
   mailing list at <ietf-mta-filters@imc.org>.  Subscription  requests
   can  be  sent  to <ietf-mta-filters-request@imc.org> (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 <http://www.imc.org/ietf-mta-filters/>.


0.2. Open issues

   1. Rob has suggested that we should keep syntax backward compatible
      with revision 03, that was widely deployed (in particular by CMU).
      Does anybody have any opinion on this?


0.3. Changes from the version submitted to the Sieve mailing list

   1. Added addflag and removeflag actions

   2. Changed the semantics of setflag (setflag is not additive any
      more)

   3. Corrected section "Interaction with Other Sieve Actions".
      Removed incorrect reference to the forward action as to an
      action that prohibits setflag.

   4. Added paragraph about the mutual order of "fileinto"/"keep" and
      "setflag"/"addflag"/"removeflag" actions.


0.4. Changes from the revision 00

   1. Corrected Capability Identifier section (Section 2)

   2. Corrected "Interaction with Other Sieve Actions" section
      (Section 4)

   3. Examples were updated to be compatible with Sieve-07 draft

   4. Added "mark" and "unmark" actions


0.5. Changes from the revision 01

   1. Some language fixes based on Tony Hansen comments

   2. Clarified that the extension allows to set both IMAP System
      Flags and Keywords


0.6. Changes from the revision 02

   1. BugFix: all backslashes must be escaped

   2. Added extended example and more detailed description of
      "addflag"/"removeflag" additivity.

   3. Minor example bugfixes


0.7. Changes from the revision 03

   1. Added second way to specify flags to be set (via optional tagged
      arguments). [Tim Showalter]

   2. Rules for using Reject with imapflags relaxed. [Randall Gellens]

   3. Removed ABNF section completely, added syntax description to
      action definition. [Tim Showalter]

   4. Cleaned up the example. [Ken Murchison]

   5. Added FM (Flag Manupulation) acronym.

   6. Clarified "mark"/"unmark" bahavior. [Randall Gellens]


0.8. Changes from the revision 04

   1. "Interaction with Other Sieve Actions" was simplified based on
      comments from Tim Showalter.  Added sentence saying that
      imapflags doesn't change an implicit keep.

   2. Several editorial comments from Tim Showalter.

0.9. Changes from the revision 05

   1. Updated copyright, author address, section numbers and references.

   2. Added dependency on Sieve "variables" extension.

   3. Several editorial comments from Matthew Elvey.

   4. Removed "mark" and "unmark" actions.

   5. Added "hasflag" test.

   6. Dropped ":globalflags"

   7. An invalid flag name doesn't cause a script execution failure anymore,
      as imapflags now depends on variables and a variable can have an arbitrary
      value.

0.10. Changes from the revision 06

   1. Updated boilerplate and references.

   2. Fixed capability string in the extended example.

   3. Improved implementation using macros (section 6).


. Introduction

   This is an extension to the Sieve language defined by [SIEVE] for
   setting [IMAP] flags. It adds a new tagged argument to "keep" and
   "fileinto" that describes the list of flags that have to be set
   when the message is delivered to the specified mailbox. It also
   adds several actions to help manipulate list of flags and a test
   to check if a flag belongs to a list.

   This extension depends on the presence of "variables" extension
   [Variables].

   Sieve interpreters that don't support integration with IMAP SHOULD
   ignore this extension.

   The capability string associated with extension defined in this
   document is "imap4flags".

<<Extension name has been changed as revision 03 of this document is widely
deployed in CMU Cyrus server>>


. 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.

2.1. General requirements for flag handling

   The following notes apply to processing of Addflag and Removeflag actions,
   hasflag test and :flags tagged argument.

   A Sieve interpreter MUST ignore empty strings (i.e. "") in a list-of-flags
   parameter.

   The Sieve interpreter SHOULD check the list of flags for validity as described
   by [IMAP] ABNF. In particular non-ASCII characters are not allowed in
   flag names. However spaces MUST be always allowed and multiple spaces
   between flag names MUST be treated as a single space character. A string
   containing a space separated list of flags is equivalent to a string list
   consisting of the flags. The last requirement is to simplify amalgamation
   of multiple flag lists.

   If a flag validity check fails the flag should be silently
   dropped, but a warning message should be logged by the Sieve interpreter.


. Actions

   All actions described in this specification (setflag, addflag, removeflag)
   operate on string variables that contain a set of [IMAP] flags.
   On variable substitution a set of flags is represented as a string
   containing space separated list of flag names.
<<Should this cause an automatic duplicate elimination?>>

   The "addflag" action adds flags to an existing set. The "removeflag"
   action removes flags from an existing set. The "setflag" action
   replaces an existing set of flags with a new set.  The "set" action
   defined in [Variables] can be used to replaces an existing set of flags
   with a new set as well. The :flags tagged argument to "keep" and "fileinto"
   actions is used to associate a set of flags referenced by a variable
   with the current message.


3.1. Setflag Action

   Syntax:   setflag <variablename: string> <list-of-flags: string-list>

   Setflag is used for setting [IMAP] system flags or
   keywords. Setflag replaces any previously set flags.


   Example:  if size :over 500K {
                    setflag "flags" "\\Deleted";
                }

   A more substantial example is:

      Example:
        if header :contains "from" "boss@frobnitzm.edu" {
          setflag "flagvar" "\\Flagged";
          fileinto :flags "${flagvar}" "INBOX.From Boss";
        }

3.2. Addflag action

   Syntax:   addflag <variablename: string> <list-of-flags: string-list>

   Addflag is used to add flags to a list of [IMAP] flags. It doesn't
   replace any previously set flags. This means that multiple occurrences
   of addflag are treated additively. The order of the flags MAY NOT
   be preserved and duplicates are allowed.

   The following examples demonstrate requirements described in 2.1.
   The following two actions

      addflag "flagvar" "\\Deleted";
      addflag "flagvar" "\\Answered";

   produce the same result as the single action

      addflag "flagvar" ["\\Deleted", "\\Answered"];

   or

      addflag "flagvar" "\\Deleted \\Answered";

   or

      addflag "flagvar" "\\Answered   \\Deleted";


3.3. Removeflag Action

   Syntax:   removeflag <variablename: string> <list-of-flags: string-list>

   Removeflag is used to remove flags from a list of [IMAP] flags.
   Removeflag clears flags previously set by "set"/"addflag". Calling
   removeflag with a flag that wasn't set before is not an error and
   is ignored. Note, that if an implementation doesn't perform automatic
   duplicate elimination, it MUST remove all occurences of the flags
   specified in the second parameter to removeflag. Empty strings in the
   list-of-flags MUST be ignored. Also note, that flag names are
   case-insensitive, as described in [IMAP].
   Multiple removeflag actions are treated additively.

      Example:
        if header :contains "Disposition-Notification-To" "mel@example.com" {
            addflag "flagvar" "$MDNRequired";
        }
        if header :contains "from" "imap@cac.washington.edu" {
            removeflag "flagvar" "$MDNRequired";
            fileinto :flags "${flagvar}" "INBOX.imap-list";
        }


.  Test hasflag

   Syntax: hasflag [MATCH-TYPE]
           <variable-list: string-list> <list-of-flags: string-list>

   The "hasflag" test evaluates to true if any of the variables matches any
   flag name.  The type of match defaults to ":is".

   Flagname comparisons is always done with the "i;ascii-casemap" operator,
   i.e., case-insensitive comparisons, as defined in [IMAP].

   Note, that if an implementation automatically performs flags reordering
   and/or duplicate elimination, it MUST perform it on both variable-list
   values and flag-list values. This is required so that, when the variable
   "MyFlags" has the value "A B", the following test

    hasflag :is "MyFlags" "b A"

   evaluates to true as expected. The above test can be also written as

    hasflag "MyFlags" ["b","A"]


. Tagged argument :flags

   This specification adds a new optional tagged argument ":flags" that
   alter the behavior of actions "keep" and "fileinto".

   The :flags tagged argument specifies that the flags provided in the
   subsequent argument should be set when fileinto/keep deliver the message
   to the target mailbox/user's main mailbox. If the :flags tagged argument
   is not specified, "keep" or "fileinto" will not set any flag when they
   deliver the message to the mailbox.

   Syntax:   ":flags" <list-of-flags: string-list>

   The copy of the message filed into mailbox will have only flags
   listed after ":flags".

   The Sieve interpreter MUST ignore all flags that it can't store
   permanently. This means that the interpreter must not treat failure
   to store any flag as a runtime failure to execute the Sieve
   script. For example, if the mailbox "INBOX.From Boss" can't store any
   flags, then

     fileinto :flags "\\Deleted" "INBOX.From Boss";

   and

     fileinto "INBOX.From Boss";

   are equivalent.

   This document doesn't dictate how the Sieve interpreter will set
   the [IMAP] flags. In particular, the Sieve interpreter may work as
   an IMAP client, or may have direct access to the mailstore.


. Implementation Notes

   "Addflag <variable> <flaglist>" can be implemented as several actions
   "set <variable> "${variable} <flag>", where <flag> is a flag in flaglist.

   A script interpreter MAY reorder flags and remove duplicates from the list.
   Thus a SIEVE script writer MUST NOT assume that the order or duplicates
   will be preserved.

   A "hasflag <variable> <flag>" test can be implemented as follows:

    string :matches :comparator "i;ascii-casemap"
           " ${<variable>} " "* <flag> *"


   "setflag <variable> [<flag1>,<flag2>,...]" can be implemented as
   follows:

    set <variable> "<flag1> <flag2> ... <flagN>"


   "addflag <variable> <flag>" can be implemented as:

    if not string :matches :comparator "i;ascii-casemap"
       " ${<variable>} " "* <flag> *" {
        set <variable> "${<variable>} <flag>"
    }


   "removeflag <variable> <flag>" can be implemented as:

    if string :matches :comparator "i;ascii-casemap" " ${<variable>} "
       "* <flag> *" {
        set <variable> "${1}${2}";
    }
    /* Remove any leading space */
    if string :matches :comparator "i;ascii-casemap" "${<variable>}"
       " *" {
        set <variable> "${1}";
    }
    /* Remove any trailing space */
    if string :matches :comparator "i;ascii-casemap" "${<variable>}"
       "* " {
        set <variable> "${1}";
    }


. Interaction with Other Sieve Actions

   This extension work only on the message that is currently being
   processed by Sieve, it doesn't affect another message generated
   as a side affect of any action.

   The extension decribed in this document doesn't change an implicit
   keep (see section 2.10.2 of [SIEVE]).


. Other Considerations

   This extension intentionally doesn't allow setting [IMAP] flags on
   an arbitrary message in the [IMAP] message store.


. Security Considerations

   Security considerations are discussed in the [IMAP] and [SIEVE].
   It is belived that this extension doesn't introduce any additional
   security concerns.


. Extended example

   #
   # Example Sieve Filter
   # Declare any optional features or extension used by the script
   #
   require ["fileinto", "imap4flags", "variables"];

   #
   # Move large messages to special mailbox
   #
   if size :over 1M
           {
           addflag "MyFlags" "Big";
           if header :is "From" "boss@company.com"
                      {
   # The message will be marked as "\Flagged Big" when filed into
   # mailbox "Big messages"
                      addflag "MyFlags" "\\Flagged";
                      }
           fileinto :flags "${MyFlags}" "Big messages";
           }

   if header :is "From" "grandma@example.net"
           {
           addflag "MyFlags" ["\\Answered", "$MDNSent"];
   # If the message is bigger than 1Mb it will be marked as
   # "Big \Answered $MDNSent" when filed into mailbox "grandma".
   # If the message is shorter than 1Mb it will be marked as
   # "\Answered $MDNSent"
           fileinto :flags "${MyFlags}" "GrandMa";  # move to "GrandMa" folder
           }

   #
   # Handle messages from known mailing lists
   # Move messages from IETF filter discussion list to filter folder
   #
   if header :is "Sender" "owner-ietf-mta-filters@imc.org"
           {
           set "MyFlags" "\\Flagged $Work";
   # Message will have both "\Flagged" and $Work flags
           keep :flags "${MyFlags}";
           }

   #
   # Keep all messages to or from people in my company
   #
   elsif anyof address :domain :is ["From", "To"] "company.com"
           {
           keep :flags "${MyFlags}";               # keep in "Inbox" folder
           }
   #
   # Try and catch unsolicited email.  If a message is not to me,
   # or it contains a subject known to be spam, file it away.
   #
   elsif anyof (not address :all :contains
                  ["To", "Cc"] "me@company.com",
                header :matches "subject"
                  ["*make*money*fast*", "*university*dipl*mas*"])
           {
           remove "MyFlags" "\\Flagged";
           # If message header does not contain my address,
           # it's from a list.
           fileinto :flags "${MyFlags}" "spam";   # move to "spam" folder
           }
   else
           {
           # Move all other (non-company) mail to "personal"
           # folder.
           fileinto :flags "${MyFlags}" "personal";
           }


.  Acknowledgments

    This document has been revised in part based on comments and
    discussions which took place on and off the Sieve mailing list.

    The help of those who took the time to review the draft and make
    suggestions is appreciated, especially that of Tim Showalter,
    Barry Leiba, Randall Gellens, Ken Murchison, Cyrus Daboo,
    Matthew Elvey, Jutta Degener, Ned Freed, Marc Mutz, Nigel Swinson
    and Kjetil Torgrim Homme.

    Special thanks to Tony Hansen and David Lamb for helping me
    better explain the concept.


. Author's Address

    Alexey Melnikov
    Isode Limited

    5 Castle Business Village
    Hampton, Middlesex
    United Kingdom, TW12 2BX

    Email: alexey.melnikov@isode.com


.  Normative References

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

    [ABNF] Crocker, D.,  "Augmented BNF for Syntax Specifications: ABNF",
    Internet Mail Consortium, RFC 2234, November, 1997.

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

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

    [Variables] Homme, K. T., "Sieve -- Variables Extension", University of
    Oslo, work in progress, draft-ietf-sieve-variables-XX.txt


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