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Network Working Group J. Wong, Ed.
Request for Comments: 4416 Nortel Networks
Category: Informational February 2006
Goals for Internet Email to Support Diverse Service Environments
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) The Internet Society (2006).
Abstract
This document is a history capturing the background, motivation and
thinking during the LEMONADE definition and design process.
The LEMONADE Working Group -- Internet email to support diverse
service environments -- is chartered to provide enhancements to
Internet mail to facilitate its use by more diverse clients. In
particular, by clients on hosts not only operating in environments
with high latency/bandwidth-limited unreliable links but also
constrained to limited resources. The enhanced mail must be
backwards compatible with existing Internet mail.
The primary motivation for this effort is -- by making Internet mail
protocols richer and more adaptable to varied media and environments
-- to allow mobile handheld devices tetherless access to Internet
mail using only IETF mail protocols.
The requirements for these devices drive a discussion of the possible
protocol enhancements needed to support multimedia messaging on
limited-capability hosts in diverse service environments. A list of
general principles to guide the design of the enhanced messaging
protocols is documented. Finally, additional issues of providing
seamless service between enhanced Internet mail and the existing
separate mobile messaging infrastructure are briefly listed.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Conventions Used in This Document . . . . . . . . . . . . . . 6
3. Messaging Terminology and Simple Model (Client-to-Server
Aspect Only) . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Messaging Transaction Models . . . . . . . . . . . . . . . 6
3.2. Mobile Messaging Transactions . . . . . . . . . . . . . . 7
3.2.1. Submission . . . . . . . . . . . . . . . . . . . . . . 7
3.2.2. Notification . . . . . . . . . . . . . . . . . . . . . 7
3.2.3. Retrieval . . . . . . . . . . . . . . . . . . . . . . 8
4. Profiles . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.1. Existing Profiles . . . . . . . . . . . . . . . . . . . . 8
4.1.1. Voice Messaging (VPIMv2) . . . . . . . . . . . . . . . 8
4.1.2. iFax . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.1.3. Internet Voice Mail (IVM) . . . . . . . . . . . . . . 9
4.2. Putative Client Profiles . . . . . . . . . . . . . . . . . 9
4.2.1. TUI . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.2.2. Multi-Modal Clients . . . . . . . . . . . . . . . . . 11
4.2.3. WUI . . . . . . . . . . . . . . . . . . . . . . . . . 11
5. General Principles . . . . . . . . . . . . . . . . . . . . . . 13
5.1. Protocol Conservation . . . . . . . . . . . . . . . . . . 13
5.1.1. Reuse Existing Protocols . . . . . . . . . . . . . . . 13
5.1.2. Maintain Existing Protocol Integrity . . . . . . . . . 13
5.2. Sensible Reception/Sending Context . . . . . . . . . . . . 13
5.2.1. Reception Context . . . . . . . . . . . . . . . . . . 13
5.2.2. Sending Context . . . . . . . . . . . . . . . . . . . 13
5.3. Internet Infrastructure Preservation . . . . . . . . . . . 14
5.4. Voice Requirements (Near Real-Time Delivery) . . . . . . . 14
5.5. Fax Requirements (Guaranteed Delivery) . . . . . . . . . . 14
5.6. Video Requirements (Scalable Message Size) . . . . . . . . 14
6. Issues and Requirements: TUI Subset of WUI . . . . . . . . . . 14
6.1. Requirements on the Message Retrieval Protocol . . . . . . 14
6.1.1. Performance Issues . . . . . . . . . . . . . . . . . . 15
6.1.2. Functional Issues . . . . . . . . . . . . . . . . . . 16
6.2. Requirements on the Message Submission Protocol . . . . . 18
6.2.1. Forward without Download Support . . . . . . . . . . . 18
6.2.2. Quota by Context Enforcement . . . . . . . . . . . . . 19
6.2.3. Future Delivery Support with Cancel . . . . . . . . . 19
6.2.4. Support for Committed Message Delivery . . . . . . . . 20
6.3. Requirements on Message Notification . . . . . . . . . . . 20
6.3.1. Additional Requirements on Message Notification . . . 21
7. Issues and Requirements: WUI Mobility Aspects . . . . . . . . 21
7.1. Wireless Considerations on Email . . . . . . . . . . . . . 21
7.1.1. Transport Considerations . . . . . . . . . . . . . . . 21
7.1.2. Handset-Resident Client Limitations . . . . . . . . . 22
7.1.3. Wireless Bandwidth and Network Utilization
Considerations . . . . . . . . . . . . . . . . . . . . 22
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7.1.4. Content Display Considerations . . . . . . . . . . . . 23
7.2. Requirements to Enable Wireless Device Support . . . . . . 24
7.2.1. Transport Requirements . . . . . . . . . . . . . . . . 24
7.2.2. Enhanced Mobile Email Functionality . . . . . . . . . 24
7.2.3. Client Requirements . . . . . . . . . . . . . . . . . 25
7.2.4. Bandwidth Requirements . . . . . . . . . . . . . . . . 25
7.2.5. Media Handling Requirements . . . . . . . . . . . . . 25
8. Interoperation with Existing Mobile Messaging . . . . . . . . 27
8.1. Addressing of Mobile Devices . . . . . . . . . . . . . . . 27
8.2. Push Model of Message Retrieval . . . . . . . . . . . . . 27
8.3. Message Notification . . . . . . . . . . . . . . . . . . . 27
8.4. Operator Issues . . . . . . . . . . . . . . . . . . . . . 27
8.4.1. Support for End-to-End Delivery Reports and
Message-Read Reports . . . . . . . . . . . . . . . . . 27
8.4.2. Support for Selective Downloading . . . . . . . . . . 27
8.4.3. Transactions and Operator Charging Units . . . . . . . 27
8.4.4. Network Authentication . . . . . . . . . . . . . . . . 28
8.5. LEMONADE and MMS . . . . . . . . . . . . . . . . . . . . . 28
9. Security Considerations . . . . . . . . . . . . . . . . . . . 32
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 32
10.1. Normative References . . . . . . . . . . . . . . . . . . . 32
10.2. Informative References . . . . . . . . . . . . . . . . . . 32
Appendix A. Contributors . . . . . . . . . . . . . . . . . . . . 37
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 38
Appendix C. IAB Note: Unified Notification Protocol
Considerations . . . . . . . . . . . . . . . . . . . 38
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1. Introduction
Historically, a number of separate electronic messaging systems
originated and evolved independently supporting different messaging
modes. For example:
o Internet mail systems ([4], [10], [25]) evolved to support
networked computers with messages consisting of rich text plus
attachments.
o Voice mail systems utilized a client with a telephone-based or an
answering machine style of user interface. The telephone network
was used for transport of recorded voice messages.
o Fax store-and-forward users interface with a fax machine using a
modified telephone-based interface. Fax machines use the
telephone network for transport of fax data via modems.
o SMS (Short Message Service) [58] enabled users to send short text
messages between their cellular phones using the SS7 call control
infrastructure ([60], [61], [63], [64], [65]) for transport.
In the recent past, IETF mail standards have evolved to support
additional/merged functionality:
o With MIME ([5], [6], [7], [8], [9], [28]), Internet mail transport
was enhanced to carry any kind of digital data
o Internet mail protocols were extended and profiled by VPIM ([13],
[14], [15], [34]) and iFAX ([16], [17], [18], [19], [20], [21],
[23]) so that enabled voice mail systems and fax machines could
use the common email infrastructure to carry their messages over
the Internet as an alternative to the telephone network. These
enhancements were such that the user's experience of reliability,
security, and responsiveness was not diminished by transport over
the Internet.
These successes -- making Internet mail transport the common
infrastructure supporting what were separate messaging universes --
have encouraged a new vision: to provide, over the Internet, a single
infrastructure, mailbox, and set of protocols for a user to get,
respond to, and manipulate all of his or her messages from a
collection of clients with varying capabilities, operating in diverse
environments ([46],[47]).
The LEMONADE effort -- Internet email to support diverse service
environments -- realizes this vision further by enabling Internet
mail support for mobile devices and facilitating its interoperability
with the existing mobile messaging universe.
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In the recent past, the evolution of messaging standards for
resource-limited mobile devices has been rapid:
o In the cellular space, SMS was enhanced to EMS (Extended Message
Service) [59] allowing longer text messages, images, and graphics.
With an even richer feature set, MMS (Multimedia Messaging
Service) ([43], [52], [53], [56], [57]) was developed as a
lightweight access mechanism for the transmission of pictures,
audio, and motion pictures. MMS protocols are based in part on
Internet standards (both messaging and web [24]) as well as SMS.
The cellular messaging universe is a separate infrastructure
adapted to deliver appropriate functionality in a timely and
effective manner to a special environment.
o As well, the number of different mobile clients that need to be
supported keeps proliferating. (e.g., besides cellular phones
there are wireless-enabled PDAs, tablet computers, etc.)
These resource-limited mobile devices are less powerful both in
processing speed and display capabilities than conventional
computers. They are also connected to the network by wireless links
whose bandwidth and reliability are lower, latency is longer, and
costs are higher than those of traditional wire-line links, hence the
stress on the need to support adaptation to a whole different service
environment.
This document collects a number the issues impeding Internet mail
protocols from directly supporting the mobile service environment.
Considerations arising from these issues are documented, and in some
cases possible approaches to solutions are suggested. It turns out
that the enhancements to support mobile clients also offer benefits
for some terminals in other environments. In particular, the
enhancements address the needs of the following diverse clients:
o A wireless handheld device with an email client -- a Wireless User
Interface (WUI) mode of user interaction is dictated by the
constraints of the mobile wireless handheld operating environment.
o Telephone-based voice client -- a Telephone User Interface (TUI),
this is the user mode offered by a POTS set
* This is a subset of the WUI and is useful in other contexts.
o A multi-modal messaging client providing a coordinated messaging
session using display and audio modes simultaneously. (e.g., a
system consisting of a PC with a phone, or a wireless phone with
both a voice circuit and data channel requiring coordination).
* This is also a subset of the WUI and is useful in other
contexts.
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The rest of this document is structured as follows:
o A brief survey of messaging profiles - both existing and proposed.
o A list of principles to be used to guide the design of Internet
Messaging for diverse service environments.
o Detailed discussion on enhancements to Internet mail protocols to
support WUIs.
o Some issues relating to the interoperation of enhanced Internet
mail and the existing mobile messaging services.
2. Conventions Used in This Document
This document refers generically to the sender of a message in the
masculine (he/him/his) and to the recipient of the message in the
feminine (she/her/hers). This convention is purely for convenience
and makes no assumption about the gender of a message sender or
recipient.
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 RFC2119 [1].
3. Messaging Terminology and Simple Model (Client-to-Server Aspect
Only)
In the client-server model prevalent in existing messaging
architectures, the client, also known as a "user agent", presents
messages to and accepts messages from the user. The server, also
known as a "relay/server" or a "proxy-relay", provides storage and
delivery of messages.
For a definitive description of Internet mail architecture, see [42].
3.1. Messaging Transaction Models
There are two basic transactional models. In the "pull" model, the
component, rather than the data flow, initiates the transaction. For
example, a client may initiate a connection to a server and issue
requests to the server to deliver incoming messages. Conventional
email clients, web-mail clients, and WAP-based mobile clients use the
"pull" model.
The "push" model differs in that the component initiating the
transaction does so because of some data flow affecting it. For
example, the arrival of a new message at the terminating server may
cause a notification to be sent ("pushed") to a messaging client.
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3.2. Mobile Messaging Transactions
The most common functions are: "submission", "notification", and
"retrieval". There may be other functions, such as "delivery
reports", "read-reply reports", "forwarding", "view mailbox", "store
message", etc. Each of these transactions can be implemented in
either a pull or push model. However, some transactions are more
naturally suited to one model or another.
The following figure depicts a simple client-server model (no server-
to-server interactions are shown):
(1) Message submission
(2) Message notification
(3) & (4) Message retrieval
+-------+ +------+ +-------+
|Mail |-------(1)------>| |-----------(2)-------->|Mail |
|Client | Submit msg | | Notification /|Client |
+-------+ | | / +--+----+
| | / ^
| |<----------(3)-----+ /
|Server| Retrieval request /
| | /
| | /
| |-----------(4)-------+
| | Retrieval response
| |
+------+
Simple Messaging Model
3.2.1. Submission
"Submission" is the transaction between a client and a server by
which the user of the former sends a new message to another user.
Submission is a push from client to server.
3.2.2. Notification
"Notification" is the transaction by which the server notifies the
client that it has received messages intended for that client.
Notification is a push from server to client.
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All the larger mobile messaging systems implement a push model for
the notification because data can be presented to the user without
the user's experiencing network/transport latencies, and without
tying up network resources for polling when there is no new data.
Internet mail differs in that it has not yet seen the need for a
standardized notification protocol.
3.2.3. Retrieval
"Retrieval" is the transaction between a client and a server by which
the client can obtain one or more messages from the server.
Retrieval can be push or pull.
Implemented in some mobile systems as an option, the push model has
the advantage that the user is not necessarily aware of transport or
network latencies.
The pull model, implemented in most systems (mobile or conventional),
has the advantage that the user can control what data is actually
sent to and stored by the client.
4. Profiles
Internet messaging can be made to support a variety of client and
server types other than traditional email. The clients may be
adapted for host restrictions such as limited processing power,
message store, display window size, etc. Alternatively, clients may
be adapted for different functionality (e.g., voice mail, fax, etc.).
Servers may support optional mail features that would allow better
handling of different media (e.g., voice mail, fax, video, etc.). A
number of Internet mail profiles supporting specific application
niches have been defined or proposed.
4.1. Existing Profiles
The following are examples of server-to-server profiles of SMTP and
MIME. Except for IVM, they do not address client-to-server
interactions.
4.1.1. Voice Messaging (VPIMv2)
These profiles, RFC3801 [13] to RFC3803 [15], enable the transport of
voice messages using the Internet mail system. The main driver for
this work was support of IP transport for voice mail systems. As
voice mail clients are accustomed to a higher degree of
responsiveness and certainty as to message delivery, the
functionality added by VPIMv2 includes Message Disposition
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Notification and Delivery Status Message ([12], [3]). Voice media
has also been added to multi-part message bodies.
4.1.2. iFax
This set of profiles ([16], [17], [18], [19], [20], [21]) enables the
transport of fax using Internet mail protocols. This work defined
the image/tiff MIME type. Support for fax clients also required
extensions to Message Delivery Notification.
4.1.3. Internet Voice Mail (IVM) [34]
This proposed mail enhancement (whose requirements are described in
RFC 3773 [30]) targets support for the interchange of voice messaging
between the diverse components (clients as well as servers) in
systems supporting voice mail.
4.2. Putative Client Profiles
4.2.1. TUI
It is desirable to replace proprietary protocols between telephone
user interface clients and message stores with standards-based
interfaces. The proprietary protocols were created to provide media-
aware capabilities as well as to provide the low-latency required by
some messaging applications.
An example of a TUI client is a voice mail client. Because a POTS
phone lacks any intelligence, the voice mail client functionality has
to be provided by a user agent networked to the mail server. The
main architectural difference between a conventional voice mail
system and an Internet messaging system supporting a TUI is that the
voice mail system uses a specialized message store and protocols.
The following figure depicts the architecture of current voice mail
systems implementing VPIMv2:
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|-------------|
|-------| RFC-822/MIME | |
| | |---------------------------| MTA |
| | | mail submission -> | |(E)SMTP
Telephone--|TUI|TUA| |------| |-----to
| | | Proprietary Protocol | | |another
| | |---------------------------| MS | | email
|-------| < - mail retrieval | | | server
|-------------|
mail client email server
|----------------voice messaging system -------------|
Mail client consists of: TUI (Telephone User Interface) and
TUA (Telephone User Agent)
Communication between TUI and TUA is proprietary.
Email server consists of: MS (Mail Store) and
MTA (Message Transfer Agent)
Communication between MS and MTA is proprietary.
It is proposed that the Proprietary Protocol be replaced with an IETF
standard protocol:
|-------------|
|-------| RFC-822/MIME | |
| | |---------------------------| MTA |
| | | mail submission -> | |(E)SMTP
Telephone--|TUI|TUA| |------| |-----to
| | | IETF protocol | | |another
| | |---------------------------| MS | | mail
|-------| <- mail retrieval | | | server
|-------------|
mail client email server
|- voice mail system-| |-mail server-|
Mail client consists of: TUI (Telephone User Interface) and
TUA (Telephone User Agent)
Communication between TUI and TUA is proprietary.
Email server consists of: MS (Mail Store) and
MTA (Message Transfer Agent)
Communication between MS and MTA is proprietary.
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4.2.2. Multi-Modal Clients
Multi-modal clients offer the advantage of coordinated voice and data
modes of user interaction. Architecturally, the multi-modal client
can be considered the union two user agent components -- one a TUI
client, the other a simple GUI client. See the next figure. The
Graphical User Agent (GUA) helps maintain the text display while the
Telephone User Agent (TUA) acts on behalf of the TUI functionality.
This model is the norm with cellular devices supporting data access
because historically they evolved from cell phones to which a data
channel was added. The presentation of multiple complementary modes
of interaction gives end-users their choice of the most convenient
and natural working mode for a particular task. There are other
situations where a multi-modal model is appropriate. (For example, a
telephone sales unit needs to provide a voice (telephone) mode and
conventional desktop PC mode of interaction at the same time in an
integrated manner.)
A major issue in the design of multi-modal clients -- the need to
synchronize the component user agents making up a client -- is only
addressed by LEMONADE to a limited extent in Section 6.3.
4.2.3. WUI
The Wireless User Interface is functionally equivalent to a
conventional email client on a personal workstation, but is optimized
for clients on handheld tetherless devices. Factors needing
consideration include limited memory and processing power. Limited
bandwidth is also relatively high cost. As already alluded to above,
in many cases (e.g., cellular devices), the mobile client is
multi-modal. So WUIs can be modeled as resource-and-link-limited
multi-modal clients.
These terminals require the use of protocols that minimize the number
of over-the-air transactions and reduce the amount of data that need
be transmitted over the air overall. Such reduction in over-the-air
transmission is a combination of more efficient protocol interaction
and richer message presentation choices, whereby a user may more
intelligently select what should be downloaded and what should remain
on the server.
Although not an explicit goal, providing equivalent or superior
functionality to the wireless MMS service [43] (as defined by 3GPP,
3GPP2, and the OMA) is desirable.
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Proposed Wireless User Interface (WUI)/Multi-modal Clients
|wireless GUI client| email server
(E)SMTP (client-server) |-------------|
|-------| RFC-822/MIME | |
| | |---------------------------| |
| | | mail submission -> | |(E)SMTP
-|GUI|GUA| | |-----to
| | | | IETF standard protocol |------------ |another
| | | |----------------------------to MS below| | mail
| |-------| <- mail retrieval |------------ | server
| | | |
Handheld | | | |
Device WUI | | MTA |
| | | |
| | | |
| |-------| RFC-822/MIME | |
| | | |---------------------------| |
| | | | mail submission -> | |
-|TUI|TUA| |------| |
| | | IETF standard protocol | | |
| | |---------------------------| MS | |
|-------| <- mail retrieval | | |
|-------------|
TUI client voice mail server
|----------------voice messaging system ----------------|
|------WUI-----| |---mail server---|
Wireless GUI client consists of: GUI (Graphical User Interface) and
GUA (Graphical User Agent)
Communication between UI and UA is proprietary.
TUI client consists of: TUI (Telephone User Interface) and
TUA (Telephone User Agent)
Communication between TUI and TUA is proprietary.
Communication between GUA and TUA is proprietary.
Mail (email and voice mail) server consists of:
MS (Mail Store) and
MTA (Message Transfer Agent)
Communication between MS and MTA is proprietary.
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5. General Principles
This is a list of principles to guide the design of extensions for
Internet Messaging systems and protocols to support diverse
endpoints.
5.1. Protocol Conservation
5.1.1. Reuse Existing Protocols
To the extent feasible, the enhanced messaging framework SHOULD use
existing protocols whenever possible.
5.1.2. Maintain Existing Protocol Integrity
In meeting the requirement "Reuse Existing Protocols"
(Section 5.1.1), the enhanced messaging framework MUST NOT redefine
the semantics of an existing protocol.
Extensions, based on capability declaration by the server, will be
used to introduce new functionality where required.
Said differently, we will not break existing protocols.
5.2. Sensible Reception/Sending Context
5.2.1. Reception Context
When the user receives a message, that message SHOULD receive the
treatment expected by the sender. For example, if the sender
believes he is sending a voice message, voice message semantics
should prevail to the extent that the receiving client can support
such treatment.
5.2.2. Sending Context
When the user sends a message, he SHOULD be able to specify the
message context. That is, whether the network should treat the
message as an text message, voice message, video message, etc.
Again, this can only be complied with to the extent that the
infrastructure and receiving client can provide such treatment. In
practice, this would imply that the message should be in the form
desired by the sender up to delivery to the receiving client.
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5.3. Internet Infrastructure Preservation
The infrastructure SHOULD change only where required for new
functionality. Existing functionality MUST be preserved on the
existing infrastructure; that is, all extensions must be backward
compatible to allow for the gradual introduction of the enhancements.
Messages created in an enhanced messaging context MUST NOT require
changes to existing mail clients. However, there may be a
degradation in functionality in certain circumstances.
The enhanced messaging framework MUST be able to handle messages
created in a non-enhanced messaging context; for example, a simple,
RFC822 [2] text message.
5.4. Voice Requirements (Near Real-Time Delivery)
On the retrieval side, there are significant real-time requirements
for retrieving a message for voice playback. More than any other
media type, including video, voice is extremely sensitive to
variations in playback latency. The enhanced messaging framework
MUST address the real-time needs of voice.
5.5. Fax Requirements (Guaranteed Delivery)
Fax users have a particular expectation that is a challenge for
enhanced Internet messaging. A person who sends a fax expects the
recipient to receive the fax upon successful transmission. This
clearly is not the case for Internet Mail.
Addressing this need is not in the scope of LEMONADE.
5.6. Video Requirements (Scalable Message Size)
Video mail has one outstanding feature: Video messages are
potentially large! The enhanced messaging framework MUST scale for
very large messages. Streaming from the server to the client, in
both directions, MUST be supported.
6. Issues and Requirements: TUI Subset of WUI
6.1. Requirements on the Message Retrieval Protocol
IMAP [10] is the Internet protocol for rich message retrieval and
manipulation. The project MUST limit itself to extending IMAP where
necessary and MUST not create a new protocol.
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6.1.1. Performance Issues
6.1.1.1. Real-Time Playback
The real-time playback of a voice message MUST be supported so that
the user experience does not differ noticeably from that of a
conventional voice messaging system.
Possible solutions for this include making use of the existing
incremental download capability of the IMAP protocol, or utilizing a
companion streaming protocol.
The IMAP protocol itself does not provide streaming by the strict
definition of the term. It does provide for the incremental download
of content in blocks. Most IMAP clients do not support this behavior
and instead download the entire contents into a temporary file to be
passed to the application.
There are several approaches to achieve real-time playback. The
first approach is to implement an IMAP client that can pass data
incrementally to the application as it is received from the network.
The application can then read bytes from the network as needed to
maintain a play buffer. Thus, it would not require the full download
of contents. This approach may require server-side development to
support partial download efficiently (i.e., to avoid re-opening files
and positioning to the requested location).
Alternatively, the client can use the proposed IMAP channel extension
[32] to request that the server make the selected content available
via an alternate transport mechanism. A client can then ask the
server to make the voice data available to the client via a streaming
media protocol such as RTSP. This requires support on the client and
server of a common streaming protocol.
6.1.1.2. Avoid Content-Transfer-Encoding Data Inflation
Another important performance optimization is enabling the transport
of data using more efficient native coding rather than text-like
content-transfer encodings such as "base 64".
Standard IMAP4 uses a text-based data representation scheme where all
data is represented in a form that looks like text; that is, voice
data must be encoded using "base 64" into a transport encoding that
adds 30% to the size of a message. Downloading or appending large
messages to the server already uses substantial bandwidth.
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Possible Solutions:
Where IMAP channel is appropriate, the external channel may be binary
capable; that is, the external access may not require re-encoding.
Mechanisms such as HTTP [24], FTP, or RTSP are available for this
download.
The IMAP binary extension standards proposal [31] extends the IMAP
fetch command to retrieve data in the binary form. This is
especially useful for large attachments and other binary components.
Binary in conjunction with a streaming client implementation may be
an attractive alternative to the channel extension.
6.1.2. Functional Issues
6.1.2.1. Mailbox Summary Support
The common TUI prompt, "you have two new voice messages, six unheard
messages, and one new fax message", requires more information than is
conveniently made available by current message retrieval protocols.
The existing IMAP protocol's mailbox status command does not include
a count by message context [26] [27]. A possible solution is for the
mail server to keep track of these current counters and provide a
status command that returns an arbitrary mailbox summary. The IMAP
status command provides a count of new and total messages with
standardized attributes extracted from the message headers. This
predetermined information does not currently include information
about the message type. Without additional conventions to the status
command, a client would have to download the header for each message
to determine its type, a prohibitive cost where latency or bandwidth
constraints exist.
6.1.2.2. Sort by Message Context Support
This functionality is required to present new voice messages first
and then new fax messages within a single logical queue as voice
mailboxes commonly do. Again, this is a question of convenience and
performance. Adequate performance may only be possible if the mail
server provides a sort by context or maintains a set of virtual
mailboxes (folders) corresponding to message types as for "Mailbox
Summary Support", Section 6.1.2.1.
IMAP does not support this directly. A straightforward solution is
to define an extensible sort mechanism for sorting on arbitrary
header contents.
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6.1.2.3. Status of Multiple Mailboxes Support
Extension mailbox support requires the ability to efficiently status
a mailbox other than the one currently logged into. This facility is
required to support sub-mailboxes, where a common feature is to check
whether other sub-mailboxes in the same family group have new
messages.
Current mechanisms are limited to logging into each of set of
mailboxes, checking status, logging out, and repeating until all
sub-mailboxes are processed.
6.1.2.4. Specialized Mailbox Support
Applications that provide features such as check receipt, deleted
message recovery, resave, and others, require the ability to access
messages in predetermined mailboxes with specific behaviors (e.g.,
Outbox, Sent Items, Deleted Items, Expired Items, Drafts).
IMAP provides only a single standardized folder, the inbox. This
functionality does not require new protocol additions per se, but
standardized usage and naming conventions are necessary for
interoperability. This functionality requires that the server
provide the underlying logic to support these special folders,
including automatic insertion, scheduled copying, and periodic
deletion.
6.1.2.5. CLID Restriction Indication/Preservation
Many calling features are dependent on collected caller-ID
information. Clients -- such as the TUI and other service supporting
user agents (e.g., WEB and WAP servers) -- may need trusted access to
restricted caller-ID information for such purposes as callback.
Untrusted clients must not be permitted to receive this information.
A mechanism for establishing "trust" between appropriate clients and
the server is required to restrict delivery of this information to
the end-user only as allowed.
Further, when messages are sent between servers within a network, a
means of communicating trust is needed so that the identity of the
sender can be preserved for record-keeping and certain features while
ensuring that the identity is not disclosed to the recipient in an
inappropriate way.
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6.1.2.6. Support for Multiple Access to Mailbox
If the telephone answering application client uses IMAP4 for greeting
access and message deposit, it is essential that the server provide
support for simultaneous login. It is common in voice mail for an
incoming call to be serviced by the telephone answering application
client at the same time the subscriber is logged into her mailbox.
Further, new applications such as WEB and WAP access to voice mail
may entail simultaneous login sessions, one from the TUI client and
one from the visual client.
The existing standard does not preclude multiple accesses to a
mailbox, but it does not explicitly require support of the practice.
The lack of explicit support requires the server and client to adhere
to a common set of practices and behaviors to avoid undesirable and
unpredictable behaviors. RFC2180 [29] describes a candidate set of
conventions necessary to support this multiple-access technique. It
or some other method MUST be standardized as part of LEMONADE.
6.2. Requirements on the Message Submission Protocol [22]
6.2.1. Forward without Download Support
It is common to forward messages or to reply to messages with a copy
of their attached content. Today such forwarding requires the sender
to download a complete copy of the original message, attach it to the
reply or forward message, and resubmit the result. For large
messages, this represents a substantial amount of bandwidth and
processing. For clients connected via long-thin pipes, alternatives
are required.
One approach is to define an extension to message submission to
request the submission server to resolve embedded URLs within a
message before relaying the message to the final destination. This
approach is referred to as the pull approach because the message
submission server must pull data from the IMAP server.
Another approach is to add a limited message assembly and submission
capability to the IMAP server. This approach muddies the distinction
between the message submission protocol and that for message storage
and retrieval (IMAP) because now message submission may be a side
effect of message store commands. This approach is referred to as
the push approach because in this case the IMAP server pushes data to
the message submission server.
A detailed analysis of which of the two approaches is preferable as
well as implementation details of both can be found in references
[36], [37], [38], [39], [40], and [41].
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6.2.2. Quota by Context Enforcement
It is common in a unified messaging system to offer separate quotas
[11] for each of several message contexts to avoid the condition
where a flood of email fills the mailbox and prevents the subscriber
from receiving voice messages via the telephone. It is necessary to
extend the protocols to support the reporting of the "mailbox full"
status based on the context of the submitted message.
An obvious security issue needing consideration is the prevention of
the deliberate misidentification of a message context with the
intention of overflowing a subscriber's mailbox. It is envisioned
that the message submission protocol will require the authentication
of trusted submission agents allowing only those so authorized to
submit distinguished messages.
Voice mail system mailboxes commonly contain voice and fax messages.
Sometimes, such systems also support email messages (text, text with
attachments, and multimedia messages) in addition to voice messages.
Similar to the required sort by message context, quota management is
also required per message context.
One possible use case is the prevention of multiple (large) messages
of one type (e.g., email messages) from consuming all available
quota. Consumption of all quota by one type prevents the delivery of
other types (e.g., voice or fax messages) to the mailbox.
One possible approach is to define a mechanism whereby a trusted
client can declare the context of a message for the purpose of
utilizing a protected quota. This may be by extensions to the
SMTP-submit or LMTP[35] protocols.
6.2.3. Future Delivery Support with Cancel
Traditionally messages sent with "future delivery" are held in the
recipient's client "outbox" or its equivalent until the appointed
submission time. Thin clients used with TUIs do not have such
persistent storage or may be intermittently connected and must rely
upon server-based outbox queues.
Such support requires extensions to message submission protocols to
identify a message as requiring queuing for future delivery.
Extensions to IMAP4 or SMTP are required for viewing and manipulating
the outbound queue, for such purposes as canceling a future message.
Server support for managing such a queue is required so that messages
are sent when they are intended.
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Some of the architectural issues here are the same as those in
"Forward without Download Support" (Section 6.2.1).
6.2.4. Support for Committed Message Delivery
Voice messaging service has provided a high degree of reliability and
performance for telephone answering messages. The expectation is
that once the caller has hung up, the message is in the mailbox and
available for review. The traditional Internet mail architecture
suggests these messages should be sent to the mailbox via SMTP. This
approach has two limitations. The first and most manageable is that
the message forwarding may take more time than is tolerable by the
subscriber. The second is that the message may fail to be delivered
to the mailbox. Because there is no way to return notice to the
caller, the message is "lost".
The standards community is working on an alternative to SMTP called
Local Message Transport Protocol (LMTP[35]). This protocol addresses
a number of limitations in SMTP when used to provide atomic delivery
to a mailbox. The failure modes in this proposal are carefully
controlled, as are issues of per-message quota enforcement and
message storage quota-override for designated administrative
messages.
An alternative approach is to misuse the IMAP protocol and use an
IMAP-based submission mechanism to deposit a message directly into
the recipient's inbox. This append must be done by a special
super-user with write permissions into the recipient mailbox.
Further, the message store must be able to trigger notification
events upon insertion of a message into the mailbox via the Append
command. The historic limitation on using IMAP4 for message sending
involves the inability of IMAP to communicate a full SMTP envelope.
For telephone answering, these limitations are not significant.
However, the architectural issues raised by this approach are
significant. See "Forward without Download Support" (Section 6.2.1).
6.3. Requirements on Message Notification
Clients keep local information about the IMAP store. This
information must be kept synchronized with the state of the store.
For example, voice mail systems traditionally notify subscribers of
certain events happening in their mailbox. It is common to send an
SMS or a pager notification for each message arrival event, message
read event, mailbox full event, etc.
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When implemented over IMAP-based message stores, the voice mail
client needs to be notified about these events. Furthermore, when
other applications access/manipulate the store, these events need to
be communicated to the mail client. In some cases, the client needs
to notify the user immediately. In most cases, it is a question of
maintaining client/application consistency. In the case of a
multimodal client, it is especially important to provide a means of
coordinating the client's different modal views of the state of the
store.
Email systems have traditionally polled to update this information.
There may be advantages to an event-driven approach in some cases.
The standards community is working on a standard for bulk
server-to-client status notification. An example of such work is the
Simple Notification and Alarm Protocol (SNAP) [45], which defines the
expected behavior of the message store for various events, many of
them triggered by IMAP commands.
6.3.1. Additional Requirements on Message Notification
A format for message notification for servers reporting status
information to other servers (e.g., IMAP4 server to SMS or pager
server) MUST be defined. The method for delivery of these
notifications MUST also be specified.
The design for this MUST take into account the IAB note: "Unified
Notification Protocol Considerations" (Appendix C).
7. Issues and Requirements: WUI Mobility Aspects
7.1. Wireless Considerations on Email
7.1.1. Transport Considerations
Compared to a LAN/WAN configuration or even to a wire-line dial-up
connection, the probability of an interruption to a wireless
connection is very high.
Interruptions can be due to handoff, signal fading, or stepping
beyond cell coverage.
In addition, because the mobile handset is also used for other types
of communications, there is a relatively high probability that the
data session will be interrupted either by incoming voice calls or by
"pushed" messages from services such as SMS, MMS, and WAP.
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It is also common in these environments that the device's IP address
change within a session.
7.1.2. Handset-Resident Client Limitations
Although the capabilities of wireless handsets are rapidly improving,
the wireless handset remains limited in its capability to host email
clients. Currently, email access is restricted to only high-end
wireless handsets.
These limitations include:
o Client size
Handset-resident clients are limited in size because either the
handset has limited storage space or the handset vendor/network
operator has set a limit on the size of client application that
can reside on the handset.
o Runtime memory
Wireless handsets have limited runtime memory for the use of
the mobile email client.
o CPU Speed
Wireless handsets have CPUs that are inferior to those in
conventional systems (PCs) that run email clients.
o User Interface
Handsets have very limited input and output capabilities. Most
of them have only a rudimentary keyboard (a keypad) and a
rudimentary pointing device (a text cursor).
7.1.3. Wireless Bandwidth and Network Utilization Considerations
7.1.3.1. Low Bandwidth
2G mobile networks enabled wireless data communications, but only at
very low bandwidths using circuit-switched data. 2.5G and 3G networks
improve on this. However, existing email clients require very large
files (up to several MBs) -- encountered in multi-media attachments
such as presentations, images, voice, and video -- to be downloaded
even though mobiles cannot exploit most of the data (because of color
depth and screen size limitations). Transferring such large files
over the air is of questionable value even when higher wireless
bandwidth is available.
7.1.3.2. Price Sensitivity
In many cases, users of mobile data services are charged by the
amount of data (e.g., kilobytes) downloaded to the handset. Most
users currently experience a higher per-kilobyte data charge with a
wireless service than they do over a wire-line service. Users are
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sensitive to the premium for wireless service. This results in an
unwillingness to download large amounts of unnecessary data to the
handset and the desire to be able to download only selected content.
7.1.3.3. File Size Limitations
In some cases, the size of file that can be transmitted over the air
to the handset is limited. This is a consequence of handset
limitations (Section 7.1.2), wireless media and bandwidth issues
(Section 7.1.1 and Section 7.1.3.1), and price sensitivity
(Section 7.1.3.2).
7.1.4. Content Display Considerations
7.1.4.1. Display Size and Capabilities
Wireless terminals are currently limited in their display size, color
depth, and ability to present multimedia elements (i.e., if multiple
pictures are sent, the mobile can usually present only one reduced-
sized picture element at a time rather than the several picture
elements at once in the same display that a conventional PC email
client would be able to show). Therefore, many email attachments
destined for a mobile may require changes in size, color depth, and
presentation method in order to be suitably displayed.
7.1.4.2. Supported Media Formats
Wireless handsets can only display a limited set of media format
types. Although PC clients support a large variety of document types
(and allow on-demand "codec"/player download), mobiles have very
limited support. (For example, most only support WAV audio and
cannot play other formats such as AU, MP3 and AIFF.) Furthermore,
although almost all new handsets sold today can display images and
sound in some advanced format, support for displaying other media or
application-specific formats, such as MS Office (TM), is not expected
to be widespread in the near future.
7.1.4.3. Handset Type Variety
As mentioned above, there are many handset types available in the
market, and each has different display capabilities, screen
characteristics, and processing capabilities. The mobile email
service should be able to support as many handset types as possible.
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7.1.4.4. Specific Attachment Display Scenarios
Handsets are unsuitable for perusing entire lengthy documents or
presentations. Rather than go through the whole document, a mobile
user is more likely to look at several pages of a document or several
slides of a presentation and then take action accordingly (e.g.,
forward the email message to another recipient, print it, or leave
the document for later retrieval from another device).
Therefore, there is a need to enable users to download not the entire
attachment but rather just a selected part of it. For example, users
should be able to download the "Table of Contents" of a document; to
search within a document; to download the first slide of a
presentation; the next slide of this presentation or a range of
slides, etc.
7.2. Requirements to Enable Wireless Device Support
The following requirements are derived from the considerations
mentioned above.
7.2.1. Transport Requirements
The mobile email protocol must anticipate transient losses of
connectivity and allow clients to recover (restore state) from
interrupted connections quickly and easily.
IMAP4 Context
An IMAP4 connection requires the communication socket to remain up
continuously during an email session. In case of transient loss of
communications, the connection must be reestablished. It is up to
the client to reconnect to the server and return to an equivalent
state in the session. This overhead of restoring connections is very
costly in response time and additional data transmission.
7.2.2. Enhanced Mobile Email Functionality
7.2.2.1. Forward without Fetch
To minimize the downloading of data over the air, the user MUST be
able to forward a message without initially downloading it entirely
or at all to the handset.
The mobile email protocol MUST support the ability to forward a
message without retrieving it.
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This requirement is identical to the TUI requirement described in
"Forward Without Download Support" (Section 6.2.1).
7.2.2.2. Media Streaming
The mobile email protocol MUST provide a solution that will enable
media streaming to the wireless handset.
This requirement is similar to the TUI requirement described in
"Real-Time Playback" (Section 6.1.1.1).
7.2.3. Client Requirements
IMAP4 clients are large because IMAP4 already consists of a complex
set of functions (e.g., parsing of a broad variety of MIME formats).
The mobile email client should be:
o Small in size
o Efficient in CPU consumption
o Efficient in runtime memory consumption
To enable such extremely thin clients, in developing the mobile email
protocol we should consider simplifying the IMAP functionality that
handsets need to support. However, any such simplification MUST NOT
limit interoperability with full IMAP servers.
7.2.4. Bandwidth Requirements
The mobile email solution should minimize the amount of data
transmitted over the air. There are several ways of pursuing this
goal that can be used in conjunction.
One way is the use of content transcoding and media adaptation by the
server before message retrieval in order to optimize the message for
the capabilities of the receiving handset.
Another possible optimization is to make the mobile email protocol
itself simple, containing as little overhead as possible.
A third approach is to minimize the bandwidth usage as described in
"Avoid Content-Transfer-Encoding Data Inflation" (Section 6.1.1.2).
7.2.5. Media Handling Requirements
As described above, wireless devices have limited ability to handle
media. Therefore, the server may be have to perform media
manipulation activities to enable the terminal to display the data
usefully.
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7.2.5.1. Device Capabilities Negotiation
In order to support the different characteristics and capabilities of
the various handset types available in the market correctly, the
mobile email protocol must include provision for email content
adaptation. For example, the choice of supported file formats, color
depth, and screen size. Work on ESMTP transcoding (CONNEG[33]) may
address this issue.
7.2.5.2. Adjusting Message Attachments for Handset Abilities
To support wireless handsets, the server could transcode the message
attachments into a representation that is more suitable for that
device. This behavior should be based on the device capabilities
negotiation as described in "Device Capabilities Negotiation"
(Section 7.2.5.1). For example, a device that cannot display GIF
format, and can only display WBMP, should get a WBMP image. Devices
that cannot display a PDF file should get a text version of the file.
The handset should control what transcoding, if any, is desired. It
should be able to retrieve the original attachment without any
changes. In addition, the device should be able to choose between
"flavors" of the transcoding. ("Present the content as thumbnail
image" is an example of such a specific media manipulation.)
Again, work on ESMTP transcoding (CONNEG[33]) may address this issue.
7.2.5.3. Handling Attachment Parts
A desirable feature (but out of scope for the current LEMONADE
charter) is to enable users the choice of retrieving parts of an
attachment file, not just the entire attachment. The mobile email
protocol should include the ability for the retrieving client to
specify selected elements of an attachment for download. Such
elements can be, for example, specific pages of a document, the
"table of contents" of a document, or specific slides of a
presentation.
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8. Interoperation with Existing Mobile Messaging
LEMONADE's charter includes the specification of how enhanced
Internet mail will interoperate with existing mobile messaging
services (e.g., MMS) to deliver messages to mobile clients.
8.1. Addressing of Mobile Devices
E.164 addressing [62] is prevalent in mobile messaging services to
address recipient mobiles. Consideration should be given to
supporting E.164 addressing for mobile devices in addition to RFC822
addressing.
8.2. Push Model of Message Retrieval [49] [50] [51]
MMS provides a "push" option for message retrieval. The option hides
network latencies and reduces the need for user-handheld interaction.
If a level of support for mobiles comparable to that of MMS is
desired, this mode of operation should be considered.
8.3. Message Notification [44] [55]
Message notification was alluded to in "Requirements on Message
Notification" (Section 6.3). Internet mail has not so far
standardized a server-to-client notification protocol although most
existing wireless mail systems use notification to avoid needless
polling. Client-to-server notification is not within the LEMONADE
charter.
8.4. Operator Issues
8.4.1. Support for End-to-End Delivery Reports and Message-Read Reports
Support for committed delivery is described in Section 6.2.4, but
this is different.
8.4.2. Support for Selective Downloading
If a push model of message retrieval is supported, the need for
selective downloading and SPAM control is especially important.
8.4.3. Transactions and Operator Charging Units
Mobile network providers often operate on a "pay for use" service
model. This brings in requirements for clearly delineated service
transactions that can be reported to billing systems, and for
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positive end-to-end acknowledgement of delivery or non-delivery of
messages already mentioned in Section 8.4.1. Note that billing is
specifically outside the scope of the IETF.
8.4.4. Network Authentication
Some mobile networks require network authentication as well as
application authentication.
8.5. LEMONADE and MMS
The 3GPP MMS Reference Architecture ([48] [54]) defines seven
interfaces labelled MM1 to MM7, as below:
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3GPP MMS Reference Architecture (subset)
|---------| |------------|
wireless ||-------|| | |
device || MMS || | |<- MM2 ->
|| USER |---------------------------| |---------
|| AGENT |<- MM1 ->| | to
||-------|| | | another
|---------| | | MMS
| | relay/
|--------| | | server
e.g., | | | |
Email, |EXTERNAL| | |
Fax, or| SERVER |--------------------------| |
UMS | |<- MM3 ->| |
|--------| | |
| |
|---------| | |
|"FOREIGN"| | |
| MMS |-------------------------| |
| relay/ |<- MM4 ->| |
| server | | |
|---------| | |
| MMS |
|-------| |relay/server|
| | | |
| HLR |---------------------------| |
| |<- MM5 ->| |
|-------| | |
| |
|-------| | |
| MMS | | |
| USER |---------------------------| |
| DBs |<- MM6 ->| |
|-------| | |
| |
|-------| | |
| MMS | | |
| VAS |---------------------------| |
| APPs |<- MM7 ->| |
|-------| |------------|
MMS - Multimedia Messaging Service
UMS - Unified Messaging Service
HLR - Home Location Register
DB - Data Base
VAS - Value Added Service
APP - Application
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The LEMONADE profile provides an enhanced IMAP mail retrieval
protocol suitable for use at interfaces MM1 and MM3.
In addition, if the wireless device uses a LEMONADE-enhanced IMAP
user agent, the enhanced IMAP protocol can be used to access Internet
mail directly, as below.
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3GPP MMS Reference Architecture (subset)
|---------| |------------|
wireless ||-------|| | |
device || IMAP || | |<- MM2 ->
|| USER || | |---------
|| AGENT || | | to
||---^---|| | | another
|----|---|| | | MMS
| LEMONADE Enhanced IMAP and | | relay/
|---V----| SMTP | | server
e.g., | | | |
Email, |EXTERNAL| | |
Fax, or| SERVER |--------------------------| |
UMS | |<- MM3 ->| |
|--------| | |
| |
|---------| | |
|"FOREIGN"| | |
| MMS |-------------------------| |
| relay/ |<- MM4 ->| |
| server | | |
|---------| | |
| MMS |
|-------| |relay/server|
| | | |
| HLR |---------------------------| |
| |<- MM5 ->| |
|-------| | |
| |
|-------| | |
| MMS | | |
| USER |---------------------------| |
| DBs |<- MM6 ->| |
|-------| | |
| |
|-------| | |
| MMS | | |
| VAS |---------------------------| |
| APPs |<- MM7 ->| |
|-------| |------------|
MMS - Multimedia Messaging Service
UMS - Unified Messaging Service
HLR - Home Location Register
DB - Data Base
VAS - Value Added Service
APP - Application
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9. Security Considerations
Security will be a very important part of enhanced messaging. The
goal, wherever possible, is to preserve the semantics of existing
messaging systems and to meet the (existing) expectations of users
with respect to security and reliability.
10. References
10.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
10.2. Informative References
[2] Crocker, D., "Standard for the format of ARPA Internet text
messages", STD 11, RFC 822, August 1982.
[3] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
Extension for Delivery Status Notifications (DSNs)", RFC 3461,
January 2003.
[4] Myers, J. and M. Rose, "Post Office Protocol - Version 3", STD
53, RFC 1939, May 1996.
[5] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies",
RFC 2045, November 1996.
[6] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046, November
1996.
[7] Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part
Three: Message Header Extensions for Non-ASCII Text ", RFC
2047, November 1996.
[8] Freed, N., Klensin, J., and J. Postel, "Multipurpose Internet
Mail Extensions (MIME) Part Four: Registration Procedures", BCP
13, RFC 2048, November 1996.
[9] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Five: Conformance Criteria and
Examples", RFC 2049, November 1996.
[10] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, March 2003.
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[11] Myers, J., "IMAP4 QUOTA extension", RFC 2087, January 1997.
[12] Hansen, T. and G. Vaudreuil, "Message Disposition
Notification", RFC 3798, May 2004.
[13] Vaudreuil, G. and G. Parsons, "Voice Profile for Internet Mail
- version 2 (VPIMv2)", RFC 3801, June 2004.
[14] Vaudreuil, G. and G. Parsons, "Toll Quality Voice - 32 kbit/s
Adaptive Differential Pulse Code Modulation (ADPCM) MIME Sub-
type Registration", RFC 3802, June 2004.
[15] Vaudreuil, G. and G. Parsons, "Content Duration MIME Header
Definition", RFC 3803, June 2004.
[16] Buckley, R., Venable, D., McIntyre, L., Parsons, G., and J.
Rafferty, "File Format for Internet Fax", RFC 3949, February
2005.
[17] Parsons, G. and J. Rafferty, "Tag Image File Format (TIFF) -
image/tiff MIME Sub-type Registration", RFC 3302, September
2002.
[18] Allocchio, C., "Minimal GSTN address format in Internet Mail",
RFC 3191, October 2001.
[19] Allocchio, C., "Minimal FAX address format in Internet Mail",
RFC 3192, October 2001.
[20] Toyoda, K., Ohno, H., Murai, J., and D. Wing, "A Simple Mode of
Facsimile Using Internet Mail", RFC 3965, December 2004.
[21] Parsons, G. and J. Rafferty, "Tag Image File Format (TIFF) - F
Profile for Facsimile", RFC 2306, March 1998.
[22] Gellens, R. and J. Klensin, "Message Submission", RFC 2476,
December 1998.
[23] Masinter, L. and D. Wing, " Extended Facsimile Using Internet
Mail", RFC 2532, March 1999.
[24] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol --
HTTP/1.1", RFC 2616, June 1999.
[25] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, April
2001.
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RFC 4416 LEMONADE Goals February 2006
[26] Resnick, P., "Internet Message Format", RFC 2822, April 2001.
[27] Burger, E., Candell, E., Eliot, C., and G. Klyne, "Message
Context for Internet Mail", RFC 3458, January 2003.
[28] Burger, E., "Critical Content Multi-purpose Internet Mail
Extensions (MIME) Parameter", RFC 3459, January 2003.
[29] Gahrns, M., "IMAP4 Multi-Accessed Mailbox Practice", RFC 2180,
July 1997.
[30] Candell, E., "High-Level Requirements for Internet Voice Mail",
RFC 3773, June 2004.
[31] Nerenberg, L., "IMAP4 Binary Content Extension", RFC 3516,
April 2003.
[32] Nerenberg, "IMAP4 Channel Transport Mechanism", Work in
Progress, November 2001.
[33] Toyoda, K. and D. Crocker, "SMTP Service Extensions for Fax
Content Negotiation", Work in Progress, February 2003.
[34] McRae, S. and G. Parsons, "Internet Voice Messaging (IVM)", RFC
4239, November 2005.
[35] Murchison, K. and L. Greenfield, "LMTP Service Extension for
Ignoring Recipient Quotas", Work in Progress, June 2002.
[36] Crispin, M., "Message Submission", Work in Progress,
February 2004.
[37] Newman, C., "Message Submission with Composition", Work in
Progress, February 2004.
[38] Gellens, R., "IMAP Message Submission", Work in Progress,
December 2003.
[39] Resnick, P., "Internet Message Access Protocol (IMAP) CATENATE
Extension", Work in Progress, December 2003.
[40] Crispin, M. and C. Newman, "Internet Message Access (IMAP) -
URLAUTH Extension", Work in Progress, July 2004.
[41] Newman, D., "Message Submission BURL Extension", Work in
Progress, July 2004.
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RFC 4416 LEMONADE Goals February 2006
[42] Crocker, D., "Internet Mail Architecture", Work in Progress,
July 2004.
[43] Leuca, I., "Multimedia Messaging Service", Presentation to the
VPIM WG, IETF53 Proceedings , April 2002.
[44] Mahy, R., "A Message Summary and Message Waiting Indication
Event Package for the Session Initiation Protocol (SIP)", RFC
3842, August 2004.
[45] Shapira, N. and E. Aloni, "Simple Notification and Alarm
Protocol (SNAP)", Work in Progress, December 2001.
[46] Vaudreuil, G., "Messaging profile for telephone-based Messaging
clients", Work in Progress, February 2002.
[47] Burger, E., "Internet Unified Messaging Requirements", Work in
Progress, February 2002.
[48] OMA, "Multimedia Messaging Service Architecture Overview
Version 1.1", Open Mobile Alliance (OMA) OMA-WAP-MMS-ARCH-v1_1-
20021101-C, November 2002.
[49] OMA, "Push Architectural Overview", Open Mobile Alliance
(OMA) WAP-250-PushArchOverview-20010703-a, July 2001.
[50] OMA, "Push Access Protocol Specification", Open Mobile Alliance
(OMA) WAP-247-PAP-20010429-a, April 2001.
[51] OMA, "Push Proxy Gateway Service Specification", Open Mobile
Alliance (OMA) WAP-249-PPGService-20010713a, July 2001.
[52] OMA, "Multimedia Messaging Service; Client Transactions Version
1.1", Open Mobile Alliance
(OMA) OMA-WAP-MMS-CTR-v1_1-20021031-C, October 2002.
[53] OMA, "Multimedia Messaging Service; Encapsulation Protocol
Version 1.1", Open Mobile Alliance (OMA) OMA-MMS-ENC-v1_1-
20021030-C, October 2002.
[54] OMA, "User Agent Profile, Version 1.1", Open Mobile Alliance
(OMA) OMA-UAProf-v1_1-20021212-C, December 2002.
[55] OMA, "Email Notification Version 1.0", Open Mobile Alliance
(OMA) OMA-EMN-v1_0-20021031-C, October 2002.
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RFC 4416 LEMONADE Goals February 2006
[56] 3GPP, "Third Generation Partnership Project; Technical
Specification Group Services and System Aspects; Service
aspects; Functional description; Stage 1 Multimedia Messaging
Service", 3GPP TS 22.140, 2001.
[57] 3GPP, "Third Generation Partnership Project; Technical
Specification Group Terminals; Multimedia Messaging Service
(MMS); Functional description; Stage 2", 3GPP TS 23.140, 2001.
[58] 3GPP2, "Short Message Service (SMS)", 3GPP2 TSG C.S0015-0,
December 1999.
[59] 3GPP2, "Enhanced Message Service (EMS) Stage 1 Description",
3GPP2 TSG S.R0051-0 v1.0, July 2001.
[60] CCITT, "Recommendations Q.700-Q.716: Specifications of
Signalling System No. 7", CCITT White Book, Volume VI,
Fascicle VI.7.
[61] CCITT, "Recommendations Q.721-Q.766: Specifications of
Signalling System No.7", CCITT White Book, Volume VI,
Fascicle VI.8.
[62] ITU, "E.164: The international public telecommunication
numbering plan", ITU-T Recommendations Series E, May 1997.
[63] ITU, "Specifications of Signalling System Number 7", ITU White
Book, ITU-T Recommendation Q.763.
[64] ITU, "Interface between Data Terminal Equipment (DTE) and Data
Circuit-terminating Equipment (DCE) for terminals operating in
the packet mode and connected to public data networks by
dedicated circuit", ITU-T Recommendation X.25, October 1996.
[65] BELLCORE, "Specifications of Signalling System Number 7", GR-
246-CORE Issue 1, December 1994.
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Appendix A. Contributors
Eric Burger
Brooktrout Technology, Inc.
18 Keewaydin Dr.
Salem, MA 03079
USA
Phone: +1 603 890-7587
EMail: eburger@brooktrout.com
Yair Grosu
Comverse
29 Habarzel St.
Tel-Aviv 69710
Israel
EMail: Yair.Grosu@comverse.com
Glenn Parsons
Nortel Networks
P.O. Box 3511 Station C
Ottawa, ON K1Y 4H7
Canada
Phone: +1 613 763-7582
EMail: gparsons@nortelnetworks.com
Milt Roselinsky
Openwave Systems, Inc.
530 E. Montecito St.
Santa Barbara, CA 93103
USA
Phone: +1 805 884-6207
EMail: milt.roselinsky@openwave.com
Dan Shoshani
Comverse
29 Habarzel St.
Tel-Aviv 69710
Israel
EMail: Dan.Shoshani@comverse.com
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Alan K. Stebbens
Openwave Systems, Inc.
530 E. Montecito St.
Santa Barbara, CA 93103
USA
Phone: +1 805 884-3162
EMail: alan.stebbens@openwave.com
Gregory M. Vaudreuil
Lucent Technologies
7291 Williamson Rd.
Dallas, TX 75214
USA
Phone: +1 214 823-9325
EMail: GregV@ieee.org
Appendix B. Acknowledgements
Ari Erev and Noam Shapira (both from Comverse) contributed
substantial requirements for IMAP to support a telephone-based (TUI)
messaging client. Meir Mendelovich (Comverse) helped in merging the
wireless requirements section. Benjamin Ellsworth (Openwave)
contributed to mobile messaging architectures and requirements.
Yaacov (Jerry) Weingarten (Comverse) and Stephane Maes (Oracle)
provided detailed comments on the final document.
Appendix C. IAB Note: Unified Notification Protocol Considerations
Note: dated July 10, 2003
This note was formulated in response to an informal IESG request to
look at the architectural issues surrounding a unified notification
protocol. The following materials were used as reference:
* draft-dusseault-s2s-event-reqs-00.txt (notification
requirements)
* meeting notes for the LEMONADE WG from IETF 56.
* draft-shapira-snap-05.txt (protocol design for SNAP which has
some aspects of a generic notification protocol)
* the LEMONADE WG charter
* Recent email on the Lemonade list
* A few presentations from the 1998 UCI workshop on Internet-wide
notification
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* The Web pages for KnowHow, a company founded by Rohit Khare
which has a proprietary Internet-wide notification system.
Thanks to Lisa Dusseault for providing these references.
Note that this opinion does not represent IAB concensus, it is just
the opinion of the author after having reviewed the references.
After the reviewing the material, it seemed that the same kinds of
functionality are being asked from a generic notification protocol as
are asked of desktop application integration mechanisms, like OLAY/
COM on Windows or like Tooltalk was on Solaris, but at the level of
messaging across the Internet. The desire is that various
distributed applications with different application specific
mechanisms should be able to interoperate without having an n x n
problem of having each application interact with each other
application. The cannonical example, which is in a presentation by
Lisa Dusseault to LEMONADE from IETF 56, is sending a notification
from one application, like XMPP Instant Messaging, and having it
delivered on whatever device the recipient happened to be using at
the time, like SMS on a cell phone.
The usual problem with application intergration mechanisms on the
desktop is how to get the various applications to actually use the
mechanism. For Windows, this is relatively easy, since most
application developers see major value-added in their applications
being able to play nicely with Microsoft Office. For Tooltalk,
unfortunatly, Solaris developers didn't see the 10x improvement, and
so it was not used outside of Sun's internally maintained
applications and a few flagship applications like Framemaker. If the
generic notification mechanism requires application developers and
other notification protocol designers to make a major effort to
utilize it, including modifying their applications or protocols in
some way, the protocol could become "just another notification
mechanism" rather than a unifying device, because most application
developers and other protocol designers could ignore it.
So the first architectural consideration is how do clients of a
particular protocol (and the word "client" is used here to mean "any
entity using the protocol", they may peers or they may be
client/server) actually utilize the generic notification protocol?
Is there some code change required in the client or can a legacy
client interoperate without change?
If you look at Fig. 1 in draft-shapira-snap-05.txt, the answer seems
to be that the notifying client uses the generic protocol, SNAP in
this case, to a functional entity (server? module on the receiving
client?) called the "Notification Service" that processes the generic
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notification into an application specific notification and sends that
notification to the client. From this figure it looks as if the
notifying client would require modification but the receiving client
wouldn't.
Another characteristic of application integration mechansims is that
they typically focus on very simple operations, the semantics of
which are shared between different applications. Examples are
"here's a rectangle, display yourself in it" or "put this styled text
object into the clipboard", and applications agree on what styled
text means. More complicated semantics are hard to share because
each application has its own particular twist on the meaning of a
particular sequence of operations on a collection of objects. The
result is a "least common denominator" collection of integration
mechanisms, primarily focussed on display integration and, to a
lesser extent, cut and paste integration.
In the context of a generic notification protocol, this raises
several possible issues. One is addressing, which is identified
draft-dusseault-s2s-event-reqs-00.txt, but in a sense this is the
easiest to resolve, by using existing and perhaps newly defined URIs.
A more complex problem is matching the semantics of what
preconditions constitute the trigger for an event across different
application notification mechanisms. This is of course necessary for
translating notifications between the different event notification
mechanisms and the generic mechanism, but, more problematically, it
is also required for a subscription service whereby subscriptions can
be made to filter events using the generic notification mechanism and
the subscriptions can be translated to different application specific
mechanisms. Any language for expressing generic subscriptions is
unlikely to support expressing the fine points in the different
application notification semantics. Note that SNAP does not seem to
support a subscription service so perhaps this isn't an issue for
SNAP.
Another architectural issue, which was discussed earlier this year on
the LEMONADE list w.r.t. some other topics, is gatewaying. The
cannonical example above (message sent using XMPP and arriving via
SMS on a cell phone) is actually a gateway example, because it would
require translation between an IP-based messaging mechanism (XMPP) to
a PSTN based mechanism (SMS). The problem with using a unified
notification mechanism for this purpose is that if there are other
functions common between the two, it is likely that a gateway will be
built anyway. In fact, one of the work items for LEMONADE is to
investigate such gateways. The value of a generic notification
mechanism therefore needs to be assessed in the light of this.
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These are the primary architectural issues, but there are a few
others that need consideration in any major system development
effort. End to end security is one,
draft-dusseault-s2s-event-reqs-00.txt talks about this quite
extensively, so it won't be repeated here. The major issue is how to
ensure that the end to end security properties are maintained in the
face of movement of the notification through the generic intermediary
protocol. Another issue is scalability. Peer to peer v.s. server
based mechanisms have implications for how scalable the notification
mechanism would be, and this needs consideration. Extensibility
needs careful consideration. What is required to integrate a new
application? Ideally, with time, application developers will stop
"rolling their own" notification service and simply use the generic
service, but this ideal may be extremely hard to achieve, and may
depend to a large extent on market acceptance.
Finally, there are some considerations that aren't architectural but
may impact the ultimate success of a generic notification protocol,
in the sense that the protocol becomes widely deployed and used. The
author's experience is that IETF has not had particular success in
introducing mechanisms that unify or supplant existing proprietary
mechanisms unless strong vendor and service provider by-in is there.
Two examples are instant messaging and service discovery. With
instant messaging, it seems that a standarized, unified instant
messaging protocol has been delayed by the lack of committment from
major service providers. With service discovery, weak commitment
from vendors has resulted in the continued introduction of vendor
specific service discovery solutions even after an IETF standard is
in place. The situation with service discovery (with which the
author is most familiar) resulted from a lack of major vendor
committment during the end phases of the standarization process.
Applying these lessions to a generic notification protocol, having
important players with proprietary notification protocols on board
and committed until the conclusion of the design process will be
crucial. Major committment is needed from various application
notification protocols before a generic mechanism could succeed.
Given the amount of time and effort required in any IETF
standardization work, assessing these with an objective eye is
critical, otherwise, regardless of how technically well designed the
protocol is, deployment success may be lacking. Having an elegently
design solution that nobody deploys is an outcome that might be wise
to avoid.
James Kempf
July 2003
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Author's Address
Jin Kue Wong (Editor)
Nortel Networks
P.O. Box 3511 Station C
Ottawa, ON K1Y 4H7
Canada
Phone: +1 613 763-2515
EMail: j.k.wong@sympatico.ca
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