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Network Working Group R. Gellens, Ed. Request for Comments: 5551 Qualcomm Category: Informational August 2009

                Lemonade Notifications Architecture

Abstract

 Notification and filtering mechanisms can make email more enjoyable
 on mobile and other constrained devices (such as those with limited
 screen sizes, memory, data transfer rates, etc.).  Notifications make
 the client aware of significant events (such as the arrival of new
 mail) so it can react (such as by fetching interesting mail
 immediately).  Filtering reduces the visible mail to a set of
 messages that meet some criteria for "interesting".  This
 functionality is included in the goals of the Lemonade (Enhancements
 to Internet email to Support Diverse Service Environments) Working
 Group.
 This document also discusses the use of server-to-server
 notifications, and how server to server notifications fit into an
 architecture that provides server to client notifications.

Status of This Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (c) 2009 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 in effect on the date of
 publication of this document (http://trustee.ietf.org/license-info).
 Please review these documents carefully, as they describe your rights
 and restrictions with respect to this document.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified

Gellens Informational [Page 1] RFC 5551 Lemonade Notifications Architecture August 2009

 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Table of Contents

 1. Introduction ....................................................2
    1.1. Conventions Used in This Document ..........................2
 2. Notifications Logical Architecture and LEMONADE Profile .........2
 3. Event-Based Synchronization .....................................4
 4. Push Email ......................................................5
 5. Server-to-Server Notifications Rationale ........................5
    5.1. Notifications Discussion ...................................6
    5.2. Server to Server Notifications Scope .......................7
    5.3. Basic Operation ............................................8
    5.4. Event Order ...............................................10
    5.5. Reliability ...............................................10
 6. Security Considerations ........................................10
 7. References .....................................................11
    7.1. Normative References ......................................11
    7.2. Informative References ....................................11
 8. Contributors ...................................................12

1. Introduction

 The Lemonade work [LEMONADE-PROFILE] identified a need to provide
 notification and filtering mechanisms for use with IMAP [IMAP].
 In addition, external groups that make use of IETF work also
 expressed such requirements (see, for example, [OMA-LEMONADE-ARCH];
 Open Mobile Alliance (OMA) requirements for within-IMAP ("inband")
 and out-of-IMAP ("outband") server to client notifications are listed
 in [OMA-ME-RD]).

1.1. Conventions Used in This Document

 Within this document, the terms "Lemonade Profile" and "Lemonade"
 generally refer to the revised Lemonade Profile document, RFC 5550
 [LEMONADE-PROFILE].

2. Notifications Logical Architecture and LEMONADE Profile

 The target logical architecture for the LEMONADE Profile is described
 in the revised Lemonade Profile document [LEMONADE-PROFILE].
 Figure 1 illustrates how notification and filtering fit in the
 context of Lemonade.

Gellens Informational [Page 2] RFC 5551 Lemonade Notifications Architecture August 2009

                   +--------------+
                   |              |....
         +=========| Notification |.NF.
         !         |    Server    |....
         !         |              |^ ^               NOTE:
         !         +--------------+! !               NF is either in
   Notif-!                         ! !               Notification
 ications!       Filter Protocol   ! !               Server or IMAP
 Protocol!  !======================! !               Store, not both
         !  !                        !
         !  !    Filter Protocol   ....
         !  !=====================>.  .            +---------+
         !  !          +-----------.NF.---+        |         |
         V  !          |           ....   |        |   MTA   |
      +-----+   IMAP   |....              |  LMTP/ |....     |<==SMTP
      |     | <======> |.VF.  IMAP    ....|  SMTP  |.AF.     |
      | MUA |\   ME-2a |....  Store   .DF.|<=======|....     |
      |     | \        |              ....|        |         |
      +-----+  \       +------------------+        +---------+
                \              !
                 \             !URLAUTH
            SUBMIT\            !
                   \      +----v-----+
                    \     |          |                +-----+
                     \    | LEMONADE |       SMTP     |     |==>SMTP
                      ===>| Submit   |===============>| MTA |
                  ME-2b   | Server   |                |     |
                          |          |                +-----+
                          +----------+
               Figure 1: Filtering Mechanism Defined in
                    Lemonade Profile Architecture
 In Figure 1, four categories of filters are defined:
 1. AF:  Administrative Filters:  Created and maintained by mail
    administration.  AF are typically not configured by the user and
    are used to apply policies, content filtering, virus protection,
    spam filtering, etc.
 2. DF:  Deposit Filters:  Executed on deposit of new mail.  Can be
    defined as Sieve filters [SIEVE].
 3. VF:  View Filters:  Define which messages are important to a
    client.  May be implemented as pseudo-virtual mailboxes [CONTEXT].
    Clients may use this to restrict which messages they synchronize.

Gellens Informational [Page 3] RFC 5551 Lemonade Notifications Architecture August 2009

 4. NF:  Notification Filters:  Determine when out-of-IMAP ("outband")
    notifications are sent to the client.  These filters can be
    implemented either in the message store or in a separate
    notifications engine.
 Note that when implementing DF or NF using Sieve, the 'enotify'
 [SIEVE-NOTIFY] and likely the 'variables' [SIEVE-VARIABLES] Sieve
 extensions might be needed.
 The filters are manageable by the client as follows:
  • NF and DF: When internal to the mail store, these are typically

implemented using Sieve; hence, a Sieve management protocol is used

   for client modifications.  See [MANAGE-SIEVE] for more information.
   Per-mailbox notifications might be implemented using a combination
   of a primary Sieve script for notifications on delivery into a
   mailbox (e.g., FILEINTO) and a per-mailbox Sieve script such as
   [IMAP-SIEVE] for transfers into a mailbox.  When the NF is within a
   notification server, it is out of scope of Lemonade.
  • VF: via pseudo-virtual mailboxes as defined in [CONTEXT].
 In Figure 1, the NF are shown both as part of the mail store (for
 example, using Sieve) and as an external notification server.  Either
 approach can be used.

3. Event-Based Synchronization

 +----------------+       +---------------+            +------------+
 |    COMPLETE    | (VF)  |   VIEW        |    (NF)    |   PUSH     |
 |   REPOSITORY   | View  |  REPOSITORY   |Notification| REPOSITORY |
 |                |Filters|               |  Filters   |            |
 |   all email    |       |  email to be  |            | important  |
 | in the account |=======|synched by the |=====<?>====| email /    |
 |                |       | mobile client |      |     | events     |
 |                |       |   (CONTEXT)   |      |     |            |
 +----------------+       +---------------+      |     +------------+
                                                 |            |
                                               IDLE /         |
                                               NOTIFY    Out-of-IMAP
                                                 |      Notifications
                                                 |            |
                                                 V            V
                  Figure 2:  Filters and Repositories

Gellens Informational [Page 4] RFC 5551 Lemonade Notifications Architecture August 2009

 For in-IMAP ("inband") notifications, the Mail User Agent (MUA)
 (client) issues IDLE [IDLE], or the successor extension command
 NOTIFY [NOTIFY]; the LEMONADE IMAP server sends notifications as
 unsolicited responses to the client.
 Out-of-IMAP ("outband") notifications are messages sent to the user
 or client not through IMAP.  When directed at the user, they are
 human-consumable and intended to alert the user.  When directed at
 the client, they are machine-consumable and may be acted upon by the
 receiver in various ways, for example, fetching data from the mail
 store or resynchronizing one or more mailboxes, updating internal
 state information, and alerting the user.

4. Push Email

 A good user experience of "push email" requires that when
 "interesting" events occur in the mail store, the client is informed
 so that it can connect and resynchronize.  The Lemonade Profile
 [LEMONADE-PROFILE] contains more information, especially in Section
 5.4.2, titled "External Notifications".

5. Server-to-Server Notifications Rationale

 With server-to-server notifications, a mail system generates event
 notifications.  These notifications describe mailbox state change
 events such as arrival of a new message, mailbox full, and so forth.
 See [MSGEVENT] for a list of such events.
 These state change notifications are sent to a notification system,
 which may generate alerts or notifications for delivery to one or
 more clients or the user.
 Server-to-server notifications allow the mail system to generate end
 user or client notifications without needing to keep track of
 notification settings for users or clients; the notification system
 maintains notification preferences for clients and users.
 Using server-to-server notifications, the mail system can provide the
 end user with a unified notification experience (the same look and
 feel for accounts at all messaging systems, such as email and
 voicemail), while allowing smooth integration of additional messaging
 systems.

Gellens Informational [Page 5] RFC 5551 Lemonade Notifications Architecture August 2009

5.1. Notifications Discussion

 The POP3 and IMAP4 Internet mail protocols allow mail clients to
 access and manipulate electronic mail messages on mail systems.  By
 definition and scope, these protocols do not provide off-line methods
 to notify an end user when the mailbox state changes.  Nor does
 either protocol define a way to aggregate the status within the end
 user's various mailboxes.
 The desire for this functionality is obvious.  For example, from the
 very early days of electronic mail, various notifications mechanisms
 have been used, including login shell checks, and simple hacks such
 as [BIFF].
 To provide an end user with unified notifications and one centralized
 Message-Waiting Indicator (MWI), notification mechanisms are needed
 that aggregate the information of all the events occurring on the end
 user's different messaging systems.
 Server-to-server notifications allow the messaging system to send
 state change events to the notification system when something happens
 in or to an end user's mailbox.
 Notification systems can be broadly grouped into three general
 architectures: external smart clients, intrinsic notification, and
 separate notification mechanisms.
 External smart clients are agents independent of the mail system that
 periodically check mailbox state (or receive notifications, for
 example, via IMAP IDLE) and inform the user or the user's mail
 client.  Many such systems have been used over the years, including
 login shells that check the user's mail spool, laptop/desktop tiny
 clients that periodically poll the user's mail servers, etc.
 Intrinsic notification is any facility within a mail system that
 generates notifications, for example, the server component of [BIFF],
 or, for more modern systems, the recent Sieve extensions for
 notifications [SIEVE-NOTIFY].
 Separate notification systems decouple the state change event
 notification from the end user or client notification, allowing a
 mail system to do the former, and specialized systems (such as those
 that handle presence) to be responsible for the latter.  This
 separation is architecturally cleaner, since the mail system only
 needs to support one additional protocol (for communication to the
 notification system) instead of multiple notification delivery
 protocols, and does not need to keep track of which clients and which
 users are interested in which events.  It also allows notifications

Gellens Informational [Page 6] RFC 5551 Lemonade Notifications Architecture August 2009

 to be generated for any service, not just electronic mail.  However,
 it requires a new service (the notification system) and the mail
 system needs to support an additional protocol (to communicate with
 the notification system).
 In addition to any external notification mechanisms, Sieve can be
 used for notifications [SIEVE-NOTIFY].  Since many mail systems
 already provide Sieve support, this can be a fairly easy and quick
 deployment option to provide a useful form of notifications.

5.2. Server-to-Server Notifications Scope

 For the purposes of the Lemonade work, the scope of server-to-server
 notifications is limited to communications between the mail system
 and the notification system (the third architectural type described
 in Section 5.1).  Communication between the notification system and
 the end user or devices (which might use SMS, WAP Push, instant
 messaging, etc.) is out of scope.  Likewise, the scope generally
 presumes a security relationship between the mail system and the
 notification system.  Thus, the security relationship then becomes
 the responsibility of the notification system.  However, the
 specifics of security, trust relationships, and related issues depend
 on the specifics of both server-to-server notifications and
 notification systems.
 Figure 3 shows the context of server-to-server notifications; only
 the left side is in scope for Lemonade:

Gellens Informational [Page 7] RFC 5551 Lemonade Notifications Architecture August 2009

           +--------+                                 +--------+
    New    |        |_                                |  SMS   |
   Message |  Mail  | \                               |Gateway |
  -------> |Server 1|  \                            __|        |
           +--------+   \                          /  +--------+
                       ^ \                        /
                       |  \                      / ^
                       |   \  +--------------+  /  |  +--------+
           +--------+  |    \ |              | /   |  |  MWI   |
   Read    | Voice  |  |     -| Notification |/    |  |Gateway |
  Message  |  Mail  |-------->|    Server    |------->|        |
  -------> | Server |  | ^  __|              |\  ^ |  +--------+
           +--------+  | | /  |(out of scope)| \ | |
                       | |/   |              |  \| |
                       | / ^  +--------------+ ^ \ |
                       |/| |                \  | |\|
           +--------+  / | |                 \ | | \  +--------+
   Mailbox |        | /| | |                  \| | |\ |  WAP   |
   Full    |  Mail  |/ | | |                 ^ \ | | \|  Push  |
  -------> |Server 2|  | | |                 | |\| |  |Gateway |
           +--------+  | | |                 | | \ |  +--------+
                       | | |                 | | |\|
                       | | |                 | | | \
                       | | |                 | | | |\ +--------+
                       | | |                 | | | | \| IM     |
                       | | |                 | | | |  |Gateway |
                       | | |                 | | | |  |        |
                       | | |                 | | | |  +--------+
                       | | |                 | | | |
                       | | |                 | | | |
                     Server-to-               OTHER
                       Server               PROTOCOLS
                   Notifications          (out of scope)
                   (in scope)
           Figure 3: Scope of Server-to-Server Notifications

5.3. Basic Operation

 The mail system sends state change event notifications to the
 notification system (which in turn might notify a client or end user)
 for events that occur in the end user's mailboxes.  Each such
 notification, referring to a single mailbox event, is called a state
 change event.

Gellens Informational [Page 8] RFC 5551 Lemonade Notifications Architecture August 2009

 The state change event contains data regarding the mailbox event that
 has occurred.  The state change event describes the change, but
 normally does not specify how or if the end user or client is
 notified; this allows the end user and client notification
 preferences to be maintained only within the notification server.
 From the Lemonade viewpoint, out-of-IMAP (outband) notifications are
 usually desired only when the client is not connected to the IMAP
 server (since inband notifications are used when there is an IMAP
 connection).  Thus, it is helpful for the mail system to be able to
 inform the notification system when the user logs in or out, and
 which client is used (when this information is available).
 When Sieve is used, the Sieve engine might have access to this
 information.
 A message is generated by the message store as a result of a state
 change event.  This message may be delivered to the end user, a
 client, or to an external notification server that might deliver an
 equivalent message to the user or to a client.
 Within the context of the Lemonade Profile (Figure 1), the event is
 filtered by NF.  That is, the Notification Filters logically
 determine which state change events cause notification to the user or
 client.
 Notifications allow for a rich end user experience.  This might
 include conveying mailbox status, new message attributes, etc., to
 the user or client independent of the client's connection to the mail
 store.
 Notifications also allow for different Message Waiting Indicator
 (MWI) behaviors (e.g., turn MWI indication off after all the messages
 in all the end user's mailboxes have been read, should such an
 unlikely thing occur in the real world).
 The payload of a notification might include a URL referring to the
 message that caused the event, possibly using URLAUTH [URLAUTH].
 As state change events occur in the mail store, they are filtered,
 which is to say matched against client or user preferences.  As a
 result, a notification may or may not be generated for delivery to
 the user or client.
 In the most general case, the mail system sends bulk state change
 events to an external notification server, and it is the notification
 server that filters the events by matching against the user's or
 client's preferences.

Gellens Informational [Page 9] RFC 5551 Lemonade Notifications Architecture August 2009

 In the most mail-specific case, the mail system performs the
 filtering itself, for example, using Sieve.

5.4. Event Order

 For the Lemonade Profile, the event order is generally not important.
 By including information such as the modification sequence identifier
 (called a modseq or mod-sequence) [CONDSTORE] in notifications, the
 receiving client can quickly and easily determine if it has already
 processed the triggering event (for example, if a notification
 arrives out of order, or if the client has resynchronized since the
 event was generated).
 For generic server-to-server notifications, the order is likely to
 matter and the mail system needs to provide notifications to the
 notification system in the order that they occur.

5.5. Reliability

 For the Lemonade Profile, lost or delayed notifications to the client
 are tolerated.  A client can resynchronize its state (including that
 reported by any missing events) when it next connects to the server.
 For generic server-to-server notifications, it is assumed that the
 data in a state change event is important, and therefore a high level
 of reliability is needed between the mail system and any external
 notification systems.

6. Security Considerations

 Notification content (payload) needs to be protected against
 eavesdropping and alteration when it contains specific information
 from messages, such as the sender.
 Even when the content is trivial and does not contain privacy-
 sensitive information, guarding against denial-of-service attacks may
 require authentication or verification of the notification sender.
 Protocols that manipulate filters need mechanisms to protect against
 modification by, as well as disclosure to, unauthorized entities.
 For example, a malicious entity might try to delete notifications the
 user wants, or try to flood the target device with notifications to
 incur usage charges, or prevent normal use.  In addition, the filters
 themselves might contain sensitive information or reveal
 interpersonal or inter-organizational relationships, as well as email
 addresses.

Gellens Informational [Page 10] RFC 5551 Lemonade Notifications Architecture August 2009

7. References

7.1. Normative References

 [IMAP]         Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL -
                VERSION 4rev1", RFC 3501, March 2003.
 [LEMONADE-PROFILE]
                Cridland, D., Ed., Melnikov, A., Ed., and S. Maes,
                Ed., "The Internet Email to Support Diverse Service
                Environments (Lemonade) Profile", RFC 5550, August
                2009.

7.2. Informative References

 [BIFF]         Gellens, R., "Simple New Mail Notification", RFC 4146,
                August 2005.
 [CONTEXT]      Cridland, D. and C. King, "Contexts for IMAP4", RFC
                5267, July 2008.
 [CONDSTORE]    Melnikov, A. and S. Hole, "IMAP Extension for
                Conditional STORE Operation or Quick Flag Changes
                Resynchronization", RFC 4551, June 2006.
 [IMAP-SIEVE]   Leiba, B., "Support for Sieve in Internet Message
                Access Protocol (IMAP4)", Work in Progress, February
                2008.
 [MANAGE-SIEVE] Melnikov, A., Ed., and T. Martin, "A Protocol for
                Remotely Managing Sieve Scripts", Work in Progress,
                September 2008.
 [MSGEVENT]     Gellens, R. and C. Newman, "Internet Message Store
                Events", RFC 5423, March 2009.
 [IDLE]         Leiba, B., "IMAP4 IDLE command", RFC 2177, June 1997.
 [NOTIFY]       Gulbrandsen, A., King, C., and A. Melnikov, "The IMAP
                NOTIFY Extension", RFC 5465, February 2009.
 [OMA-LEMONADE-ARCH]
                Burger, E. and G. Parsons, "LEMONADE Architecture -
                Supporting Open Mobile Alliance (OMA) Mobile Email
                (MEM) Using Internet Mail", RFC 5442, March 2009.

Gellens Informational [Page 11] RFC 5551 Lemonade Notifications Architecture August 2009

 [OMA-ME-RD]    Open Mobile Alliance Mobile Email Requirement
                Document, (Work in progress).
                http://www.openmobilealliance.org/
 [SIEVE]        Guenther, P., Ed., and T. Showalter, Ed., "Sieve: An
                Email Filtering Language", RFC 5228, January 2008.
 [SIEVE-NOTIFY] Melnikov, A., Ed., Leiba, B., Ed., Segmuller, W., and
                T. Martin, "Sieve Email Filtering: Extension for
                Notifications", RFC 5435, January 2009.
 [SIEVE-VARIABLES]
                Homme, K., "Sieve Email Filtering: Variables
                Extension", RFC 5229, January 2008.
 [URLAUTH]      Crispin, M., "Internet Message Access Protocol (IMAP)
                - URLAUTH Extension", RFC 4467, May 2006.

8. Contributors

 The original (and longer and more detailed) version of this document
 was authored by Stephane H. Maes and Ray Cromwell of Oracle
 Corporation.
 The current and original authors want to thank all who have
 contributed key insight in notifications and filtering and have
 authored specifications or documents used in this document.
 The current and original authors want to thank the authors of the
 original work on "Server To Server Notification Protocol
 Requirements", some of whose material has been incorporated in the
 present document, in particular, Gev Decktor.

Author's Address

 Randall Gellens, Editor
 QUALCOMM Incorporated
 5775 Morehouse Drive
 San Diego, CA  92121
 USA
 EMail: rg+ietf@qualcomm.com

Gellens Informational [Page 12]

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