Network Working Group R. Gellens, Ed.

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.

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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.

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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.

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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

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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.

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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

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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:

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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.

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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.

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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.

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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.

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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: [email protected]

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