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

Lemonade Notifications Architecture


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.

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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.

                     |              |....
           +=========| 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.
   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

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.

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 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:

             +--------+                                 +--------+
      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.

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.

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.

7. References

7.1. Normative References

                  VERSION 4rev1", RFC 3501, March 2003.


                  Cridland, D., Ed., Melnikov, A., Ed., and S. Maes,
                  Ed., "The Internet Email to Support Diverse Service
                  Environments (Lemonade) Profile", RFC 5550, August

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
   [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.


                  Burger, E. and G. Parsons, "LEMONADE Architecture -
                  Supporting Open Mobile Alliance (OMA) Mobile Email
                  (MEM) Using Internet Mail", RFC 5442, March 2009.
   [OMA-ME-RD]    Open Mobile Alliance Mobile Email Requirement
                  Document, (Work in progress).
   [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.


                  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