LEMONADE Architecture - Supporting OMA Mobile Email (MEM) using Internet Mail
draft-ietf-lemonade-architecture-00.txt

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Abstract

This document specifies the architecture for mobile email, as described by the OMA, using Internet Mail protocols. This architecture is the basis of the work of the LEMONADE WG and is a guidleine for the LEMONADE Profile.

Conventions used in this document

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 (Bradner, S., “Key words for use in RFCs to Indicate Requirements Levels,” March 1997.) [refs.RFC2119].

Table of Contents

1.  Introduction
2.  OMA Mobile Email (MEM)
    2.1.  OMA MEM Requirements
    2.2.  OMA MEM Architecture
        2.2.1.  OMA MEM logical Architecture
        2.2.2.  OMA MEM Deployment Issues
    2.3.  OMA MEM Technical Specification
3.  IETF LEMONADE Architecture
    3.1.  Relationship between the OMA MEM and LEMONADE logical architectures
    3.2.  LEMONADE realization of OMA MEM with non-LEMONADE compliant servers
        3.2.1.  LEMONADE realization of OMA MEM with non-LEMONADE IMAP servers
        3.2.2.  LEMONADE realization of OMA MEM with non-IMAP servers
4.  Filters and server to client notifications and LEMONADE
5.  Notifications objectives
6.  Security considerations
7.  IANA considerations
8.  Version history
9.  Acknowledgements
10.  References
§  Authors' Addresses
§  Intellectual Property and Copyright Statements

1.  Introduction

This document describes the architecture of OMA mobile email (MEM) using Internet Mail protocols defined by the IETF. Many of these protocols have been enhanced by the LEMONADE work group for use in the mobile environment and are summarized in the LEMONADE profile (Maes, S. and A. Melnikov, “LEMONADE profile,” .) [refs.profile] and its revision LEMONADE profile bis (Maes, S. and A. Melnikov, “LEMONADE profile bis,” .) [refs.profilebis]. This document shows how the OMA MEM Requirement document (, “Mobile Email RequirementS Document,” Oct 2005.) [refs.MEM‑req] , OMA MEM Architecture (, “Mobile Email Architecture Document,” June 2007.) [refs.MEM‑arch] and OMA MEM Technical Specification (, “Mobile Email Technical Specification,” Oct 2007.) [refs.MEM‑ts] relate to the work of LEMONADE.

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 have based much of our Internet Mail enhancements based on their input. This section summarizes the output of the OMA.

2.1.  OMA MEM Requirements

The OMA MEM activity has collected a set of use cases and derived requirements for a mobile email enabler (MEM). the resulting work is summarized in OMA MEM Requirements (, “Mobile Email RequirementS Document,” Oct 2005.) [refs.MEM‑req]. Some requirements relates to email protocols, some involve other OMA technologies outside the scope of IETF and some relate to implementations and normative interoperability statements for clients and servers.

2.2.  OMA MEM Architecture

This section gives a brief introduction to 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 [refs.MEM‑req] (, “Mobile Email RequirementS Document,” Oct 2005.). A simplification for illustrative purposes is shown inFigure 1 (Basic OMA MEM logical architecture), where arrows indicate content flows.

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

It identifies the following elements:

• The MEM client which 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.
• The MEM server which implements the server-side functionality of the OMA Mobile Email Enabler (MEM).
• The MEM protocol between the MEM Client and MEM Server. It is responsible for all the inband data exchanges that take place between the MEM client and server in order to update the MEM client with email server changes, the email server with changes in the MEM client and to send new email from the email server.
• Other OMA enablers are needed to directly support the mobile email enabler. They are out of scope of IETF but they may include support for:
• Client provisioning and management for over the air installation of the MEM client on the device, provisioning of its settings and revocation
• Messaging enablers for outband notification, where outband notifications that are server to client event exchanges not transported by the MEM protocol but via other channels.

OMA identifies different interfaces:

• ME-1: MEM client interface to interact via the MEM protocol with the MEM server
• ME-2: Corresponding interface of the MEM server
• ME-3: Outband MEM server interfaces (e.g. to support generation of server to client notifications).
• ME-4: Outband MEM client interfaces (e.g. to receive server to client notifications).
• 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 in 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, relationship to other cases are also discussed. 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 (, “Mobile Email Architecture Document,” June 2007.) [refs.MEM‑arch] further identifies deployment models.

2.2.2.1.  OMA MEM proxy

The OMA MEM Architecture document (, “Mobile Email Architecture Document,” June 2007.) [refs.MEM‑arch] identifies OMA MEM server proxies as server components that may be deployed ahead of firewalls to facilitate traversa of firewalls.

2.2.2.2.  OMA MEM deployment cases

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

OMA MEM target support of configurations where:

• All components are within a same domain (Mobile operator)
• MEM client and other enablers are in the mobile operator domain, a MEM proxy is involved and MEM server and email server are in the domain of the email service provider
• MEM client and other enablers as well as a MEM proxy are in the mobile operator domain, MEM server and email server are in the domain of the email service provider
• MEM client and other enablers are in the mobile operator domain, a MEM proxy is in a third party service provider domain and MEM server and email server are in the domain of the email service provider
• MEM client, other enabler and MEM server are in the mobile operator domain and email server is in the domain of the email service provider
• MEM client and other enablers are in the mobile operator domain, MEM server is in a third party service provider domain and the email server is in the domain of the email service provider

2.3.  OMA MEM Technical Specification

The OMA MEM activity will conclude with a specification for a mobile email enabler (MEM). The ongoing work is in OMA MEM Technical Specification (, “Mobile Email Technical Specification,” Oct 2007.) [refs.MEM‑ts]. LEMONADE is a basis for the mechanism, however, some additional details that are outside the scope of 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 outband notifications: the outband notification mechanisms. Also, OMA XDM may be considered also for outband filter changes. .

3.  IETF LEMONADE Architecture

This section gives a brief introduction to the LEMONADE Architecture.

The IETF LEMONADE activity has derived a LEMONADE profile (Maes, S. and A. Melnikov, “LEMONADE profile,” .) [refs.profile] with the logical architecture represented in Figure 2 (LEMONADE logical architecture), 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   |                |_____|
                               |__________|

The LEMONADE profile (Maes, S. and A. Melnikov, “LEMONADE profile,” .) [refs.profile] assumes:

• IMAP protocol (Crispin, M., “IMAP4, Internet Message Access Protocol Version 4 rev1,” March 2003.) [refs.RFC3501] including LEMONADE profile extensions (Maes, S. and A. Melnikov, “LEMONADE profile,” .) [refs.profile]
• SUBMIT protocol (SMTP (Klensin, J., “Simple Mail Transfer Protocol,” April 2001.) [refs.RFC2821], ...) including LEMONADE profile extensions
• LEMONADE profile compliant IMAP store connected to MTA (Mail Transfer Agent) via ESMTP (Klensin, J., “SMTP Service Extensions,” November 1995.) [refs.RFC1861]
• LEMONADE profile compliant Submit server connected to MTA via ESMTP
• Lemonade profile message store / submit server protocols (URLAUTH) (see [refs.profile] (Maes, S. and A. Melnikov, “LEMONADE profile,” .)).
• Outband server to client notifications relying on external notification mechanisms (and notification protocols) that may be out of scope of the LEMONADE profile.
• A LEMONADE aware MUA (Mail User Agent). While use of outband notification is described in the LEMONADE profile, support for the underlying notifications mechanisms/protocols is out of scope of the LEMONADE specifications.

Further details on the IETF email protcol stack and architecture can be found in [refs.internetemailarch] (Crocker, D., “Internet Mail Architecture,” .)

3.1.  Relationship between the OMA MEM and LEMONADE logical architectures

Figure 3 (Mapping of LEMONADE logical architecture onto the OMA MEM logical architecture.) illustrates the mapping of the IETF LEMONADE logical architecture on the OMA MEM logical architecture.

                       _____________________
                      | 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
                                     |

As described in Section 3 (IETF LEMONADE Architecture), 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 server. Mapped to OMA MEM logical architecture, for the case considered and specified by the LEMONADE profile, the MEM server and email server logically combined. They are however split into distinct LEMONADE message store and LEMONADE submit server. ME-2 consists of two interfaces ME-2a and ME-2b associated respectively to IMAP extended according to the LEMONADE profile and SUBMIT extended according to the LEMONADE profile.

The MUA is part of the MEM client.

External notifications mechanism can be part of the other OMA enabler specified by OMA (or other activities).

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.

3.2.1.  LEMONADE realization of OMA MEM with non-LEMONADE IMAP servers

Figure 4 (Architecture to support non-LEMONADE IMAP servers with a LEMONADE realization of OMA MEM enabler.) illustrates the case of IMAP servers that are not (yet) LEMONADE compliant. In such case, the I2 interface between the MEM server components and the IMAP store and submit server are IMAP and SUBMIT.

              ______________
             |              |
    _________| 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   |        |             |       |_____|
                  |__________|        |_____________|

3.2.2.  LEMONADE realization of OMA MEM with non-IMAP servers

Figure 5 (Architecture to support non-IMAP servers with a LEMONADE realization of OMA MEM enabler.) illustrates the cases where the message store and submit servers are not IMAP store or submit servers. They may be POP3 servers or other proprietary message stores.

              ______________
             |              |
    _________| 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   |        |             |       |_____|
                  |__________|        |_____________|

I2 designates proprietary adapters to the backends. They may invoved functions performed in the message stores or submit server as well as in the MEM enabler server.

4.  Filters and server to client notifications and LEMONADE

OMA MEM RD (, “Mobile Email RequirementS Document,” Oct 2005.) [refs.MEM‑req] and AD (, “Mobile Email Architecture Document,” June 2007.) [refs.MEM‑arch] emphasize the need to provide mechanisms for server to client notifications of email events and filtering. Figure 6 (Filtering mechanism defined in LEMONADE architecture) illustrates how notification and filterings are introduced in LEMONADE profile (Maes, S. and A. Melnikov, “LEMONADE profile,” .) [refs.profile].

                ______________
               |              |
      _________| 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   |                |_____|
                      |__________|

In Figure 6 (Filtering mechanism defined in LEMONADE architecture), four categories of filters are defined:

• AF: Administrative Filters - Set up by email service provider. AF are typically not configured by the user and set to apply policies content filtering, virus protection, spam filtering etc...
• DF: Deposit Filters - Filters that are executed on deposit of new emails. They can be defined as SIEVE filters (, “SIEVE,” .) [refs.SIEVE]. They can include vacation notices.
• 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 [refs.profilebis] (Maes, S. and A. Melnikov, “LEMONADE profile bis,” .).
• NF: Notification Filters - Filters that define for what email server event an outband notification is sent to the client, as described in [refs.profilebis] (Maes, S. and A. Melnikov, “LEMONADE profile bis,” .).

The filters are manageable from the MUA:

• NF and DF: via SIEVE management protocol

5.  Notifications objectives

According to these analyses, there is a need to support: # Mechanisms for event-based (server to client) synchronization: * Defines the relationship between notification mechanisms and the IMAP4 protocol - To minimize the latency observed for email events on the email server to be reflected in the email client. - To avoid unnecessary polling and requests from the e-mail clients: . To reduce the total amount of data to be exchanged between email server and client, e.g. by allowing the email client to select which messages to synchronize and how to synchronize. . To reduce the amount of transactions. * Needs to cope with possible lost or delayed notifications * Support in-band (within IMAP band) and out-band notifications (Exchanged via other servers / enablers). - Specified in ways that are network transport independent but may contain some bindings to particular notification channels (e.g. SMS binary, WAP Push, SIP Notification, ...) - When the email client is connected to the email server, only inband notifications is expected take place * Defines notification payload for inband and outband mechanisms. # Server-side filtering to decide which messages will be accessible by the email client. * Filtering results into the following logical types: - Type A: Messages filtered out and not accessible by the email client (no notification, no header access, no access) - Type B: Messages that are accessible by the mobile e-mail enabler client but no outband notification takes place. Inband notification might however take place if email client is already connected to email server. - Type C: Messages that are accessible by the e-mail client for which notifications (outband or inband) are always sent to the email client. # Notions of Filters: * View filters: Filters that determine which email messages are of type B and C or A * Notification filters: Filters that determine which email messages are of type C or B * Event filters: Filters that determines what events are to be notified to the client # Mechanisms to allow the user to update the filters from the email client # Mechanisms to allow configuration and exchange of settings between the client and the server in band or outband: - Server to client: e.g. server ID, account name, policies, … - Client to server: e.g. rules filters vacation notices, notification channel, ...

6.  Security considerations

This specification provides no security measures beyond those in the referenced Internet Mail and LEMONADE documents.

We note however that there are security risks associated to:

• Outband notifications
• Server configuration by client
• Client configuration by server
• Presence of MEM proxy servers
• Presence of MEM servers as intermediaries
• Measures to addess the need to traverse firewalls

7.  IANA considerations

No specific IANA considerations have been identified that are not covered by the different drafts and RFCs included in the realization described in this document.

8.  Version history

Version 0: This document was extracted from sections of draft-ietf-lemonade-profile-bis-05.txt and draft-ietf-lemonade-notifications-04.txt.

9.  Acknowledgements

The authors acknowledge and appreciate the work and comments of the IETF LEMONADE working group and the OMA MEM working group.

10. References

Authors' Addresses

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