Network Working Group E. Burger

Network Working Group E. Burger
Request for Comments: 5442 Consultant
Category: Informational G. Parsons
Nortel Networks
March 2009

LEMONADE Architecture - Supporting Open Mobile Alliance (OMA)
Mobile Email (MEM) Using Internet Mail

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
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.

Abstract

This document specifies the architecture for mobile email, as
described by the Open Mobile Alliance (OMA), using Internet Mail
protocols. This architecture was an important consideration for much
of the work of the LEMONADE (Enhancements to Internet email to
Support Diverse Service Environments) working group in the IETF.
This document also describes how the LEMONADE architecture meets
OMA's requirements for their Mobile Email (MEM) service.

Burger & Parsons Informational [Page 1]

RFC 5442 LEMONADE Architecture March 2009

Table of Contents

1. Introduction ....................................................2
2. OMA Mobile Email (MEM) ..........................................2
2.1. OMA MEM Requirements .......................................2
2.2. OMA MEM Architecture .......................................3
2.2.1. OMA MEM Logical Architecture ........................3
2.2.2. OMA MEM Deployment Issues ...........................4
2.3. OMA MEM Technical Specification ............................6
3. IETF LEMONADE Architecture ......................................6
3.1. Relationship between the OMA MEM and LEMONADE Logical
Architectures ..............................................7
3.2. LEMONADE Realization of OMA MEM with
non-LEMONADE-Compliant Servers .............................9
3.2.1. LEMONADE Realization of OMA MEM with
non-LEMONADE IMAP Servers ...........................9
3.2.2. LEMONADE Realization of OMA MEM with non-IMAP
Servers ............................................10
4. Filters and Server-to-Client Notifications and LEMONADE ........11
5. Security Considerations ........................................13
6. Acknowledgements ...............................................13
7. Informative References .........................................13

1. Introduction

This document describes the architecture of OMA Mobile Email (MEM)
using Internet Mail protocols defined by the IETF. The LEMONADE
working group has enhanced many of these protocols for use in the
mobile environment. The LEMONADE profile [PROFILE] and its revision,
[PROFILE-bis], summarize such protocols and protocol use. This
document shows how the OMA MEM Requirements document [MEM-req], OMA
MEM Architecture [MEM-arch], and OMA MEM Technical Specification
[MEM-ts] relate to the work of LEMONADE in the IETF.

2. OMA Mobile Email (MEM)

The OMA Mobile Email (MEM) sub-working group has spent some time
studying the requirements and architecture of mobile email. IETF
LEMONADE has been liaising with them and has based much of its
Internet Mail enhancements on their input. This section summarizes
the output of the OMA.

2.1. OMA MEM Requirements

The OMA MEM activity collected a set of use cases and derived
requirements for a Mobile Email (MEM) enabler. The OMA MEM
Requirements document [MEM-req] summarizes this work. Some
requirements relate to email protocols, some involve other OMA

Burger & Parsons Informational [Page 2]

RFC 5442 LEMONADE Architecture March 2009

technologies outside the scope of the IETF, and some relate to
implementations and normative interoperability statements for clients
and servers.

2.2. OMA MEM Architecture

This section introduces the OMA MEM Architecture.

2.2.1. OMA MEM Logical Architecture

The OMA MEM activity has derived a logical architecture from the
requirements and use cases described in [MEM-req]. A simplification
for illustrative purposes is shown in Figure 1, where arrows indicate
content flows.

__________
| Other |
+---| Mobile |<--+
| | Enablers | |
| |__________| |
|ME-4 |ME-3
_v____ ___v____ ________
| |ME-1 | | | |
| MEM |-------->| MEM | I2 | Email |
|Client| ME-2| Server |<---->| Server |
|______|<--------|________| |________|
^
|ME-5
|

Figure 1: Basic OMA MEM Logical Architecture

Figure 1 identifies the following elements:

o The MEM client that implements the client-side functionality of
the OMA Mobile Email enabler. It is also responsible for
providing the mobile email user experience and interface to the
user and storing the email and data to be sent to the MEM server
when not connected.

o The MEM server that implements the server-side functionality of
the OMA Mobile Email (MEM) enabler.

o The MEM protocol between the MEM client and MEM server. It is
responsible for all the in-band data exchanges that take place
between the MEM client and server in order to update the MEM

Burger & Parsons Informational [Page 3]

RFC 5442 LEMONADE Architecture March 2009

client with email server changes and the email server with changes
in the MEM client, and in order to send new email from the email
server.

o Other OMA enablers that are needed to directly support the Mobile
Email enabler. They are out of the scope of the IETF but may
include support for:

* Client provisioning and management for over-the-air
installation of the MEM client on the device, provisioning of
the client settings, and revocation of client privileges.

* Messaging enablers for out-of-band notification, where out-of-
band notifications that are server-to-client event exchanges
are not transported by the MEM protocol but via other channels.

* Billing, charging, and so on.

OMA identifies different interfaces:

o ME-1: MEM client interface to interact via the MEM protocol with
the MEM server.

o ME-2: Corresponding interface of the MEM server.

o ME-3: Out-of-band MEM server interfaces; for example, to support
generation of server-to-client notifications.

o ME-4: Out-of-band MEM client interfaces (e.g., to receive server-
to-client notifications).

o ME-5: Interface for management of MEM enabler server settings,
user preferences, and filters, globally and per account.

The MEM server enables an email server. In a particular
implementation, the email server may be packaged with (internal to
it) the MEM server or be a separate component. In such cases,
interfaces to the email server are out of scope of the OMA MEM
specifications. In the present document, we focus on the case where
the backend consists of IETF IMAP and SUBMIT servers. However, we
also discuss the relationship to other cases. The I2 interface is an
OMA notation to designate protocol / interfaces that are not
specified by the MEM enabler but may be standardized elsewhere.

2.2.2. OMA MEM Deployment Issues

The OMA MEM Architecture document [MEM-arch] further identifies
deployment models.

Burger & Parsons Informational [Page 4]

RFC 5442 LEMONADE Architecture March 2009

2.2.2.1. OMA MEM Proxy

The OMA MEM Architecture document [MEM-arch] identifies OMA MEM
server proxies as server components that may be deployed ahead of
firewalls to facilitate firewall traversal.

2.2.2.2. OMA MEM Deployment Cases

OMA MEM identifies that each component (MEM client, MEM servers,
other enablers, and the email server) may be deployed in different
domains, possibly separated by firewalls and other network
intermediaries. MEM proxies may be involved in front of a firewall
that protects the MEM server domain.

OMA MEM targets support of configurations where:

o All components are within the same domain, such as in a mobile
operator.

o The MEM client and other enablers are in the mobile operator
domain, there is a MEM proxy, and the MEM server and email server
are in the domain of the email service provider.

o The MEM client and other enablers as well as a MEM proxy are in
the mobile operator domain, and the MEM server and email server
are in the domain of the email service provider.

o The MEM client and other enablers are in the mobile operator
domain, a MEM proxy is in a third-party service provider domain,
and the MEM server and email server are in the domain of the email
service provider.

o The MEM client, other enabler, and MEM server are in the mobile
operator domain, and the email server is in the domain of the
email service provider.

o The MEM client and other enablers are in the mobile operator
domain, the MEM server is in a third-party service provider
domain, and the email server is in the domain of the email service
provider.

The email service provider can be a third-party service provider, a
network service provider, or an enterprise email service.

Burger & Parsons Informational [Page 5]

RFC 5442 LEMONADE Architecture March 2009

2.3. OMA MEM Technical Specification

The OMA MEM activity will conclude with a specification for a Mobile
Email (MEM) enabler. The ongoing work is in the OMA MEM Technical
Specification [MEM-ts]. LEMONADE is a basis for the mechanism.
However, some additional details that are outside the scope of the
IETF will also be included.

OMA provides ways to perform provisioning via OMA client provisioning
and device management. Other provisioning specifications are
available (e.g., SMS based).

OMA provides enablers to support out-of-band notification mechanisms,
filter specifications (such as XDM), and remote deactivate devices,
and to perform other non-Internet activities.

3. IETF LEMONADE Architecture

This section introduces the LEMONADE Architecture.

The IETF LEMONADE activity has derived a LEMONADE profile
[PROFILE-bis] with the logical architecture represented in Figure 2,
where arrows indicate content flows.

______________
| |
_________| Notification |
| | Mechanism |
| |______________|
|Notif. ^
|Protocol |
| ___|______
| | | _____
__v__ IMAP | LEMONADE | ESMTP | |
| |<----------->| IMAP |<---------------| MTA |
| MUA |- | Store | |_____|
|_____| \ |__________|
\ |
\ |URLAUTH
\SUBMIT |
\ ____v_____
\ | | _____
\ | LEMONADE | ESMTP | |
---->| Submit |--------------->| MTA |
| Server | |_____|
|__________|

Figure 2: LEMONADE logical architecture

Burger & Parsons Informational [Page 6]

RFC 5442 LEMONADE Architecture March 2009

The LEMONADE profile [PROFILE] assumes:

o IMAP protocol [RFC3501], including LEMONADE profile extensions
[PROFILE].

o SUBMIT protocol [RFC4409], including LEMONADE profile extensions.

o LEMONADE profile compliant IMAP store connected to an MTA (Mail
Transfer Agent) via the ESMTP [EMAIL].

o LEMONADE profile compliant submit server connected to an MTA,
often via the ESMTP.

o Out-of-band server-to-client notifications relying on external
notification mechanisms (and notification protocols) that may be
out of the scope of the LEMONADE profile.

o LEMONADE-aware MUA (Mail User Agent). While use of out-of-band
notification is described in the LEMONADE profile, support for the
underlying notifications mechanisms/protocols is out of the scope
of the LEMONADE specifications.

Further details on the IETF email protocol stack and architecture can
be found in [MAIL].

3.1. Relationship between the OMA MEM and LEMONADE Logical
Architectures

Figure 3 illustrates the mapping of the IETF LEMONADE logical
architecture on the OMA MEM logical architecture.

Burger & Parsons Informational [Page 7]

RFC 5442 LEMONADE Architecture March 2009

_____________________
| Other_Mob. Enablers |
| |--------------| |
_________| Notification | |
| | | Mechanism | |
| | |______________| |
|Notif. |____________^________|
|Protocol ______|__________
ME-4 | | ___|_ME-3_ |
___|____ | | | | _____
| __v__ | IMAP | | LEMONADE | | ESMTP | |
|| |<----------->| IMAP |<-----------| MTA |
|| MUA || ME-2a | | Store | | |_____|
||_____||\ME-1 | |__________| |
| MEM | \ | | |
| Client| \ | |URLAUTH |
|_______| \SUBMIT | |
\ | ____v_____ |
\ | | | | _____
\ | | LEMONADE | | ESMTP | |
---->| Submit |----------->| MTA |
ME-2b | | Server | | |_____|
| |__________| |
|MEM Email |
|Server Server|
|_________________|
^
|ME-5
|

Figure 3: Mapping of LEMONADE Logical Architecture
onto the OMA MEM Logical Architecture

As described in Section 3, the LEMONADE profile assumes LEMONADE
profile compliant IMAP stores and SUBMIT servers. Because the
LEMONADE profile extends the IMAP store and the SUBMIT server, the
mobile enablement of email provided by the LEMONADE profile is
directly provided in these servers. Mapping to the OMA MEM logical
architecture for the case considered and specified by the LEMONADE
profile, we logically combine the MEM server and email server.
However, in LEMONADE we split them logically into a distinct LEMONADE
message store and a LEMONADE SUBMIT server. ME-2 consists of two
interfaces. ME-2a is IMAP extended according to the LEMONADE
profile. ME-2b is SUBMIT extended according to the LEMONADE profile.

The MUA is part of the MEM client.

Burger & Parsons Informational [Page 8]

RFC 5442 LEMONADE Architecture March 2009

The external notifications mechanism is part of the OMA enablers
specified by the OMA.

3.2. LEMONADE Realization of OMA MEM with non-LEMONADE-Compliant
Servers

The OMA MEM activity is not limited to enabling LEMONADE-compliant
servers. It explicitly identifies the need to support other
backends. This is, of course, outside the scope of the IETF LEMONADE
activity.

3.2.1. LEMONADE Realization of OMA MEM with non-LEMONADE IMAP Servers

Figure 4 illustrates the case of IMAP servers that are not LEMONADE-
compliant. In such case, the I2 interface between the MEM server
components and the IMAP store and SUBMIT server are IMAP and SUBMIT
without LEMONADE extensions.

It is important to note the realizations are of a schematic nature
and do not dictate actual implementation. For example, one could
envision collocating the LEMONADE MEM enabler server and the submit
server shown in Figure 4 in a single instantiation of the
implementation. Likewise, we consciously label the LEMONADE MEM
enabler as neither an IMAP proxy nor an IMAP back-to-back user agent.
LEMONADE leaves the actual implementation to the developer.

Burger & Parsons Informational [Page 9]

RFC 5442 LEMONADE Architecture March 2009

______________
| |
_________| Notification |
| | Mechanism |
| |______________|
|Notif. ^
|Protocol |
| ___|______ _____________
| | LEMONADE | | | _____
__v__ IMAP | MEM | IMAP |NON-LEMONADE | ESMTP | |
| |<--------->|Enabler |<------>|IMAP |<----->| MTA |
| MUA |\ ME-2a | Server | |Store | |_____|
|_____| \ |__________| |_____________|
\ |
\ |URLAUTH
\SUBMIT |
\ ____v_____ _____________
\ | | | | _____
\ | LEMONADE | SUBMIT |NON-LEMONADE | ESMTP | |
-->| MEM | |Submit | | |
| Enabler |------->|Server |------>| MTA |
ME-2b | Server | | | |_____|
|__________| |_____________|

Figure 4: Architecture to Support Non-LEMONADE IMAP Servers
with a LEMONADE Realization of an OMA MEM Enabler

3.2.2. LEMONADE Realization of OMA MEM with non-IMAP Servers

Figure 5 illustrates the cases where the message store and submit
servers are not IMAP store or submit servers. They may be Post
Office Protocol (POP3) servers or other proprietary message stores.

Burger & Parsons Informational [Page 10]

RFC 5442 LEMONADE Architecture March 2009

______________
| |
_________| Notification |
| | Mechanism |
| |______________|
|Notif. ^
|Protocol |
| ___|______ _____________
| | LEMONADE | | | _____
__v__ IMAP | MEM | I2 |Proprietary | ESMTP | |
| |<--------->|Enabler |<------>|Message |<----->| MTA |
| MUA |\ ME-2a | Server | |Store | |_____|
|_____| \ |__________| |_____________|
\ |
\ |URLAUTH
\SUBMIT |
\ ____v_____ _____________
\ | | | | _____
\ | LEMONADE | I2 |Proprietary | ESMTP | |
-->| MEM | |Submit | | |
| Enabler |------->|Server |------>| MTA |
ME-2b | Server | | | |_____|
|__________| |_____________|

Figure 5: Architecture to Support Non-IMAP Servers with a LEMONADE
Realization of OMA MEM Enabler

I2 designates proprietary adapters to the backends.

4. Filters and Server-to-Client Notifications and LEMONADE

OMA MEM Requirements [MEM-req] and Architecture [MEM-arch] emphasize
the need to provide mechanisms for server-to-client notifications of
email events and filtering. Figure 6 illustrates how notification
and filtering works in the LEMONADE profile [PROFILE].

Burger & Parsons Informational [Page 11]

RFC 5442 LEMONADE Architecture March 2009

______________
| |
_________| Notification |
| | Mechanism |
| |______________|
|Notif. ^
|Protocol -------\ _|__
| ______| ___\>|NF|____
| | | ---- | _____
__v__| IMAP |__ LEMONADE |___ ESMTP __| |
| |<-------->|VF| IMAP |DF |<--------|AF| MTA |
| MUA |\ ME-2a |-- Store |--- --|_____|
|_____| \ |_____________| ^
\_\_______________|_______|
\ |URLAUTH
\SUBMIT |
\ ____v_____
\ | | _____
\ | LEMONADE | ESMTP | |
---->| Submit |--------------->| MTA |
ME-2b | Server | |_____|
|__________|

Figure 6: Filtering Mechanism Defined in LEMONADE Architecture

In Figure 6, we define four categories of filters:

o AF: Administrative Filters - The email service provider usually
sets administrative filters. The user typically does not
configure AF. AF applies policies covering content filtering,
virus protection, spam filtering, etc.

o DF: Deposit Filters - Filters that are executed on deposit of new
emails. They can be defined as SIEVE filters [SIEVE]. They can
include vacation notices [RFC5230]. As SIEVE filters, one can
administer them using the SIEVE management protocol [MANAGESIEVE].

o VF: View Filters - Filters that define which emails are visible to
the MUA. View filters can be performed via IMAP using the
facilities described in [NOTIFICATIONS].

o NF: Notification Filters - Filters that define for what email
server event an out-of-band notification is sent to the client, as
described in [NOTIFICATIONS].

Refer to the aforementioned references for implementation and
management of the respective filters.

Burger & Parsons Informational [Page 12]

RFC 5442 LEMONADE Architecture March 2009

5. Security Considerations

We note there are security risks associated with:

o Out-of-band notifications

o Server configuration by client

o Client configuration by server

o Presence of MEM proxy servers

o Presence of MEM servers as intermediaries

o Measures to address the need to traverse firewalls

We refer the reader to the relevant Internet Mail, IMAP, SUBMIT, and
Lemonade documents for how we address these issues.

6. Acknowledgements

The authors acknowledge and appreciate the work and comments of the
IETF LEMONADE working group and the OMA MEM working group. We
extracted the contents of this document from sections of
[PROFILE-bis] by Stephane Maes, Alexey Melnikov, and Dave Cridland,
as well as sections of [NOTIFICATIONS] by Stephane Maes and Ray
Cromwell.

7. Informative References

[EMAIL] Klensin, J., "Simple Mail Transfer Protocol",
RFC 5321, October 2008.

[MAIL] Crocker, D., "Internet Mail Architecture", Work
in Progress, October 2008.

[MANAGESIEVE] Melnikov, A. and T. Martin, "A Protocol for Remotely
Managing Sieve Scripts", Work in Progress,
January 2009.

[MEM-arch] Open Mobile Alliance, "Mobile Email Architecture
Document", OMA,
http://member.openmobilealliance.org/ftp/
public_documents/mwg/MEM/Permanent_documents/
OMA-AD-Mobile_Email-V1_0_0-20070614-D.zip,
June 2007.

Burger & Parsons Informational [Page 13]

RFC 5442 LEMONADE Architecture March 2009

[MEM-req] Open Mobile Alliance, "Mobile Email Requirements
Document", OMA, http://www.openmobilealliance.org/,
Oct 2005.

[MEM-ts] Open Mobile Alliance, "Mobile Email Technical
Specification", OMA, Work in Progress,
http://www.openmobilealliance.org/, Oct 2007.

[NOTIFICATIONS] Gellens, R. and S. Maes, "Lemonade Notifications
Architecture", Work in Progress, July 2008.

[PROFILE] Maes, S. and A. Melnikov, "Internet Email to Support
Diverse Service Environments (Lemonade) Profile",
RFC 4550, June 2006.

[PROFILE-bis] Cridland, D., Melnikov, A., and S. Maes, "The
Lemonade Profile", Work in Progress, September 2008.

[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL -
VERSION 4rev1", RFC 3501, March 2003.

[RFC4409] Gellens, R. and J. Klensin, "Message Submission for
Mail", RFC 4409, April 2006.

[RFC5230] Showalter, T. and N. Freed, "Sieve Email Filtering:
Vacation Extension", RFC 5230, January 2008.

[SIEVE] Guenther, P. and T. Showalter, "Seive: An Email
Filtering Language", RFC 5228, January 2008.

Burger & Parsons Informational [Page 14]

RFC 5442 LEMONADE Architecture March 2009

Authors' Addresses

Eric W. Burger
Consultant
New Hampshire
USA

Phone:
Fax: +1 530-267-7447
EMail: [email protected]
URI: http://www.standardstrack.com

Glenn Parsons
Nortel Networks
3500 Carling Avenue
Ottawa, ON K2H 8E9
Canada

Phone: +1 613 763 7582
EMail: [email protected]

Burger & Parsons Informational [Page 15]