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rfc:rfc6133

Internet Engineering Task Force (IETF) R. George Request for Comments: 6133 B. Leiba Category: Informational Huawei Technologies ISSN: 2070-1721 A. Melnikov

                                                         Isode Limited
                                                             July 2011
                       Sieve Email Filtering:
   Use of Presence Information with Auto-Responder Functionality

Abstract

 This document describes how the Sieve email filtering language, along
 with some extensions, can be used to create automatic replies to
 incoming electronic mail messages based on the address book and
 presence information of the recipient.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for informational purposes.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Not all documents
 approved by the IESG are a candidate for any level of Internet
 Standard; see Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6133.

Copyright Notice

 Copyright (c) 2011 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

George, et al. Informational [Page 1] RFC 6133 Auto Response July 2011

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 2
 2.  How To Create Auto-Replies  . . . . . . . . . . . . . . . . . . 3
 3.  Example Use Cases for Auto-Replies  . . . . . . . . . . . . . . 3
 4.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
 5.  Normative References  . . . . . . . . . . . . . . . . . . . . . 8

1. Introduction

 This document describes how the Sieve email filtering language
 [RFC5228], along with some extensions [RFC5230] [RFC5435] [RFC6134]
 [RFC6132] [RFC6131], can be used to generate automatic replies to
 incoming electronic mail messages based on the presence information
 of the recipient.  This can be used, for example, to inform the
 sender that messages will not be answered immediately because the
 recipient is busy or away.
 The auto-reply message can additionally be based on information about
 the sender from the recipient's address book, sub-lists therefrom, or
 other lists available to the recipient, so that different senders
 might get different responses.  The recipient can create separate
 rules for friends, family members, colleagues, and so on.
 This can be used in mail filtering software, email-based information
 services, and other automatic responder situations.  There are many
 programs currently in use that automatically respond to email.  Some
 of them send many useless or unwanted responses, or send responses to
 inappropriate addresses.  The mechanism described herein will help
 avoid those problems (but see the discussion in Section 4).
 Implementations need to take care of tracking previous messages
 received from the same sender, and they will start or stop sending
 responses as the presence status of the recipient changes.
 An important note, though: users of any auto-reply mechanism should
 really think about whether automatic replies are necessary, and at
 what interval they make sense when they are.  Email is not Instant
 Messaging, and senders generally expect that replies might take a
 while.  Consider whether it's truly important to tell people that
 you'll read their mail in an hour or so, or whether that can just be
 taken as how email works.  There are times when this makes sense, but
 let's not use it to exacerbate information overload.  Judicious use
 of appropriate presence information might serve to mitigate these
 issues.
 Implementors, therefore, need to consider this with respect to the
 features they expose to users, and the potential for inappropriate
 use those features represent.  The ability to create auto responders

George, et al. Informational [Page 2] RFC 6133 Auto Response July 2011

 might be hidden behind an "advanced" button, and users might be
 warned of the consequences and advised of the considerations in the
 previous paragraph.

2. How To Create Auto-Replies

 When an email message arrives, the Sieve script can use the
 notify_method_capability of the Notify extension [RFC5435] to check
 the recipient's presence information.  The Notify-presence extension
 [RFC6132] makes additional presence, such as "away" and "do not
 disturb" status, available.  The script can use the External-lists
 extension [RFC6134] to look the sender up in the recipient's address
 book or other list.  If the information retrieved warrants an auto-
 reply message, the message can then be composed based on that
 information.
 The Vacation extension [RFC5230] provides an easy way to send the
 auto-reply message to the sender, as it automatically keeps track of
 the automatic replies and attempts to avoid excessive messages and
 mail loops.  The Vacation-seconds extension [RFC6131] allows auto-
 replies to be sent this way more frequently than once per day, when
 that's appropriate.  (Alternatively, the script can use the Notify
 extension [RFC5435] to send a notification by a means other than
 email.)
 Personal and Group Responders can refuse to generate responses except
 to known correspondents or addresses otherwise known to the
 recipient.  Such responders can also generate different kinds of
 responses for "trusted" vs. "untrusted" addresses.  This might be
 useful, for instance, to avoid inappropriate disclosure of personal
 or confidential information to arbitrary addresses.

3. Example Use Cases for Auto-Replies

 1.  In this example, we check that the envelope "from" is in the
     recipient's address book [RFC6134] and that the recipient's
     presence shows "extended away" [RFC6132].  If both of those are
     true, the "vacation" action [RFC5230] is used to send an auto-
     reply, making sure we don't reply to the same sender more than
     once every half hour [RFC6131].  The variables extension
     [RFC5229] is used to extract the value of the recipient's
     natural-language presence status message, which will be used as
     the response to the sender.

George, et al. Informational [Page 3] RFC 6133 Auto Response July 2011

 require ["envelope", "extlists", "enotify", "variables",
          "vacation-seconds"];
 if allof (
     envelope :list "from" ":addrbook:default",
     notify_method_capability "xmpp:me@example.com" "show" "xa"
   ) {
     # :matches "*" is used here to extract the value
     if notify_method_capability :matches
         "xmpp:myjid@example.com" "status" "*" {
       set "resp_msg" "${1}";
     } else {
       set "resp_msg" "Away for a while, without access to email.";
     }
     vacation :handle "ext-away" :seconds 1800 "${resp_msg}";
   }
 2.  In the next example, we'll check for the recipient's personal
     assistant, and give very detailed information about the
     recipient's status to that sender.  For other senders in the
     "family" and "friends" lists, we'll also send an auto-reply.
     Other senders will be considered less important, and don't need
     auto-replies.
 require ["envelope", "extlists", "enotify", "vacation-seconds"];
 if envelope :is "from" "assistant@example.com"
   {
     if notify_method_capability "xmpp:me@example.com" "show" "away"
       {
         vacation :handle "away" :seconds 600
             "I'm away for now, but I'll be back soon.";
       }
     elsif notify_method_capability "xmpp:me@example.com" "show" "dnd"
       {
         vacation :handle "dnd" :seconds 1800
             "I'm not to be disturbed.  I'll check mail later.";
       }
     elsif notify_method_capability "xmpp:me@example.com" "show" "xa"
       {
         vacation :handle "ext-away" :seconds 3600
             "I'm away for a while, without access to email.";
       }
     elsif notify_method_capability "xmpp:me@example.com" "busy" "yes"
       {
         vacation :handle "busy" :seconds 1800
             "I'm very busy, but might check email now and then.";
       }

George, et al. Informational [Page 4] RFC 6133 Auto Response July 2011

   }
 elsif envelope :list "from" [":addrbook:family",
                              ":addrbook:friends"]
   {
     if notify_method_capability "xmpp:me@example.com" "show"
            ["away", "dnd", "xa"]
       {
         vacation :handle "away" :seconds 3600
             "I'm not available to respond to email.";
       }
   }
 else
   { # We could respond as below, making it only once a day
     # for less important senders.  Better to just omit
     # that, though (see the end of the Introduction section).
     #
     # vacation :handle "catchall" :days 1
     #     "I got your message, and might read it eventually.";
   }
 3.  For this example, if the sender is a work colleague and the
     recipient is on extended away status, then reply with a message
     giving alternative contact information.  The message might also
     include details about the reason for the absence, or other
     personal or confidential information that shouldn't be shared
     with senders who aren't associated with the recipient's company.
 require ["envelope", "extlists", "enotify", "vacation"];
 if envelope :list "from" ":addrbook:co-workers"
   {
     if notify_method_capability "xmpp:me@example.com" "show" "xa"
       {
         vacation :handle "bigtrip" :days 3
             "I'm on an extended business trip to Texas for the Foo
              project.  Contact my backup, Susan <susan@example.com>,
              or call my assistant on +1 666 555 1234 if you urgently
              need to contact me.";
       }
   }
 4.  This example is used to send an acknowledgment to every message
     received.  A :seconds value of zero is used to reply to every
     message, with no removal of duplicates to the same sender.  This
     requires that the Sieve engine allow an interval of zero; if it

George, et al. Informational [Page 5] RFC 6133 Auto Response July 2011

     does not, and it imposes a minimum value, not every message will
     receive an auto-reply.
 require ["envelope", "extlists", "vacation-seconds"];
 if not envelope :list "from" ":addrbook:staff"
   {
     vacation :handle "auto-resp" :seconds 0
         "Your request has been received.  A service
          representative will contact you as soon as
          possible, usually within one business day.";
   }
 5.  This example uses the same structure to automatically send a copy
     of each incoming message to the recipient's backup, if the sender
     is a customer contact or co-worker, or if the message's subject
     includes the word "urgent".
 require ["envelope", "extlists", "enotify"];
 if anyof (
     envelope :list "from" [":addrbook:customers",
                            ":addrbook:co-workers"],
     header :contains "subject" "urgent"
   ) {
     if notify_method_capability "xmpp:me@example.com" "show" "xa"
       {
         redirect "susan@example.com"; # send a copy to my backup
         keep; # also keep a copy for myself
       }
   }
 }

4. Security Considerations

 See the Security Considerations sections of the following
 specifications for discussion of security considerations not covered
 here:
 o  Sieve base specification [RFC5228]
 o  Sieve Vacation extension [RFC5230]
 o  Vacation "Seconds" parameter [RFC6131]

George, et al. Informational [Page 6] RFC 6133 Auto Response July 2011

 o  Sieve Externally Stored Lists extension [RFC6134]
 o  Sieve Notify extension [RFC5435] (and any applicable notification
    methods)
 This document describes how to set up a system that creates automatic
 replies in an intelligent way.  Despite the "intelligence", errors in
 scripts can result in too many auto-reply messages, especially when
 the reply interval is minimal (using the "notify" action, or the
 "vacation" action with a small value for ":seconds").
 Despite the "intelligence", too, errors in scripts can result in
 private information getting to senders inappropriately.  In example 3
 in Section 3, for instance, if the :list test checks the wrong list,
 or none at all, information about the recipient's business trip might
 be sent to someone who has no need to know about it, and that
 information should not have been sent.
 Even without errors in scripts, a sender who recognizes that auto-
 replies are dependent upon the recipient's presence can use that fact
 to probe the presence information.  One result of that can be that
 the sender discerns changes in the recipient's presence that the
 sender would normally not be allowed to see, making this an
 unintentional back door into the user's presence information.
 Another result is that this can create a "covert channel", allowing
 the recipient to send information to a sender by changing his
 presence information, his address book, and/or his Sieve script
 (though in this regard, the exposure is comparable to any other case
 of shared presence information).
 An auto responder can cause leaks of other pieces of information,
 including potentially providing the ability to attack cryptographic
 keying material.  For example, using the time it takes to perform a
 cryptographic operation, an attacker may obtain information about the
 secret key.  An auto responder that doesn't take timing into account
 could accidentally leak this kind of information.
 Moreover, if an auto responder script directly returns the results of
 a cryptographic operation, that could also provide an attack vector.
 For example, if a script returns the results of a decryption
 operation, an attacker can send an arbitrarily encrypted message and
 use the results as a chosen cyphertext attack to decode the
 encryption key.  Authors of scripts should be careful about what
 information they return to senders.

George, et al. Informational [Page 7] RFC 6133 Auto Response July 2011

5. Normative References

 [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.
 [RFC5230]  Showalter, T. and N. Freed, "Sieve Email Filtering:
            Vacation Extension", RFC 5230, January 2008.
 [RFC5435]  Melnikov, A., Leiba, B., Segmuller, W., and T. Martin,
            "Sieve Email Filtering: Extension for Notifications",
            RFC 5435, January 2009.
 [RFC6131]  George, R. and B. Leiba, "Sieve Vacation Extension:
            "Seconds" Parameter", RFC 6131, July 2011.
 [RFC6132]  George, R. and B. Leiba, "Sieve Notification Using
            Presence Information", RFC 6132, July 2011.
 [RFC6134]  Melnikov, A. and B. Leiba, "Sieve Extension: Externally
            Stored Lists", RFC 6134, July 2011.

George, et al. Informational [Page 8] RFC 6133 Auto Response July 2011

Authors' Addresses

 Robins George
 Huawei Technologies
 Bangalore, Karnataka  560071
 India
 Phone: +91-080-41117676
 EMail: robinsgv@gmail.com
 Barry Leiba
 Huawei Technologies
 Phone: +1 646 827 0648
 EMail: barryleiba@computer.org
 URI:   http://internetmessagingtechnology.org/
 Alexey Melnikov
 Isode Limited
 5 Castle Business Village, 36 Station Road
 Hampton, Middlesex  TW12 2BX
 UK
 EMail: Alexey.Melnikov@isode.com
 URI:   http://www.melnikov.ca/

George, et al. Informational [Page 9]

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