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rfc:rfc5442

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: eburger@standardstrack.com
 URI:   http://www.standardstrack.com
 Glenn Parsons
 Nortel Networks
 3500 Carling Avenue
 Ottawa, ON  K2H 8E9
 Canada
 Phone: +1 613 763 7582
 EMail: gparsons@nortel.com

Burger & Parsons Informational [Page 15]

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