rfc3458
Network Working Group E. Burger
Request for Comments: 3458 SnowShore Networks
Category: Standards Track E. Candell
Comverse
C. Eliot
Microsoft Corporation
G. Klyne
Nine by Nine
January 2003
Message Context for Internet Mail
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This memo describes a new RFC 2822 message header, "Message-Context".
This header provides information about the context and presentation
characteristics of a message.
A receiving user agent (UA) may use this information as a hint to
optimally present the message.
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Table of Contents
1. Introduction....................................................2
2. Conventions used in this document...............................3
3. Motivation......................................................3
4. Functional Requirements.........................................5
5. Determining the Message Context.................................6
6. Message-Context Reference Field.................................7
6.1. Message-Context Syntax......................................7
6.2. message-context-class Syntax................................7
6.2.1. voice-message...........................................8
6.2.2. fax-message.............................................8
6.2.3. pager-message...........................................8
6.2.4. multimedia-message......................................8
6.2.5. text-message............................................8
6.2.6. none....................................................8
7. Security Considerations.........................................9
8. IANA Considerations.............................................9
8.1. Message Content Type Registrations..........................9
8.2. Registration Template......................................10
8.3. Message-Context Registration...............................11
9. APPENDIX: Some messaging scenarios.............................12
9.1. Internet e-mail............................................12
9.2. Pager service..............................................12
9.3. Facsimile..................................................13
9.4. Voice mail.................................................14
9.5. Multimedia message.........................................14
10. References....................................................15
10.1 Normative References.......................................15
10.2 Informative References.....................................15
11. Acknowledgments...............................................15
12. Authors' Addresses............................................16
13. Full Copyright Statement......................................17
1. Introduction
This document describes a mechanism to allow senders of an Internet
mail message to convey the message's contextual information. Taking
account of this information, the receiving user agent (UA) can make
decisions that improve message presentation for the user in the
context the sender and receiver expects.
In this document, the "message context" conveys information about the
way the user expects to interact with the message. For example, a
message may be e-mail, voice mail, fax mail, etc. A smart UA may
have specialized behavior based on the context of the message.
This document specifies a RFC 2822 header called "Message-Context".
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The mechanism is in some ways similar to the use of the Content-
Disposition MIME entity described in [6]. Content-Disposition gives
clues to the receiving User Agent (UA) for how to display a given
body part. Message-Context can give clues to the receiving UA for
the presentation of the message. This allows the receiving UA to
present the message to the recipient, in a meaningful and helpful
way.
Typical uses for this mechanism include:
o Selecting a special viewer for a given message.
o Selecting an icon indicating the kind of message in a displayed
list of messages.
o Arranging messages in an inbox display.
o Filtering messages the UA presents when the user has limited
access.
2. Conventions used in this document
This document refers generically to the sender of a message in the
masculine (he/him/his) and 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 BCP 14, RFC 2119 [2].
FORMATTING NOTE: Notes, such at this one, provide additional
nonessential information that the reader may skip without missing
anything essential. The primary purpose of these non-essential notes
is to convey information about the rationale of this document, or to
place this document in the proper historical or evolutionary context.
Readers whose sole purpose is to construct a conformant
implementation may skip such information. However, it may be of use
to those who wish to understand why we made certain design choices.
3. Motivation
Multimedia messaging systems receive messages that a UA may present
in variety of ways. For example, traditional e-mail uses simple text
messages that the recipient displays and edits. One UA may
automatically print Fax images. Another UA may play voice messages
through a telephone handset. Likewise, a receiving desktop computer
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may process or present documents transferred over e-mail using a
local application. Emerging and future developments may deliver
other forms of information that have their own characteristics for
user presentation, such as video messages and pager messages.
An often-requested characteristic for multimedia messaging systems is
to collect received messages in a "universal inbox", and to offer
them to the user as a combined list.
In the context of "unified messaging", different message contexts may
have different implied semantics. For example, some users may
perceive voicemail to have an implicit assumption of urgency. Thus
they may wish to gather them together and process them before other
messages. This results in the end-user receiving agent needing to be
able to identify voicemail and distinguish it from other messages.
The uses of this kind of presentation characteristic for each message
are multi-fold:
o Display an indication to the user (e.g., by a suitably evocative
icon along with other summary fields),
o Auto-forward a given message type into another messaging
environment (e.g., a page to a mobile short message service),
o Prioritize and group messages in an inbox display list,
o Suggest appropriate default handling for presentation,
o Suggest appropriate default handling for reply, forward, etc.
A problem faced by multimedia messaging systems is that it is not
always easy to decide the context of a received message. For
example, consider the following scenarios.
o A message that contains audio and image data: Is this a fax
message that happens to have some voice commentary? Is it a voice
message that is accompanied by some supplementary diagrams? Is it
a fully multimedia message, in which all parts are expected to
carry equal significance?
o A message containing text and audio data: Is this e-mail with an
MP3 music attachment? Is it a voice message that happens to have
been generated with an initial text header for the benefit of
non-voice-enabled e-mail receivers?
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The message context does relate to the message media content.
However, it is not the same thing. As shown above, the media type
used in a message is not sufficient to indicate the message context.
One cannot determine a priori which media types to use in alternative
(gateway) messages. Also, what if the user cares about
distinguishing traditional e-mail text from SMS messages? They are
both the same media type, text, but they have different user
contexts.
4. Functional Requirements
The goals stated above lead to the following functional requirements.
For receivers:
o Identify a message as belonging to a message class.
o Incorrect or invalid message classification must not result in
failure to transfer or inability to present a message.
For senders:
o Specify message classes by the originating user's choice of
authoring tool or simple user interaction.
For both:
o Specify a well-defined set of message classes to make
interoperability between mail user agents (UAs) possible.
o Message classification information has to be interpretable in
reasonable fashion by many different user agent systems.
o The mechanism should be extensible to allow for the introduction
of new kinds of messages.
NOTE: We specifically do not specify user agent behavior when the
user agent forwards a message. Clearly, the user agent, being
message-context-aware, should provide a meaningful message-context.
It is obvious what to do for the easy cases. Messages that the user
simply forwards will most likely keep the context unchanged.
However, it is beyond the scope of this document to specify the user
agent behavior for any other scenario.
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5. Determining the Message Context
One method of indicating the interpretation context of a message is
to examine the media types in the message. However, this requires
the UA to scan the entire message before it can make this
determination. This approach is particularly burdensome for the
multi-media mail situation, as voice and especially video mail
objects are quite large.
We considered indicating the message context by registering a
multipart/* MIME subtype (Content-Type). For example, the VPIM Work
Group has registered multipart/voice-message to indicate that a
message is primarily voice mail [7]. However, multipart/voice-
message is identical in syntax to multipart/mixed. The only
difference is that VPIM mail transfer agents and user agents
recognize that they can perform special handling of the message based
on it being a voice mail message. Moreover, Content-Type refers to a
given MIME body part, not to the message as a whole.
We wish to avoid scanning the entire message. In addition, we wish
to avoid having to create multiple aliases for multipart/mixed every
time someone identifies a new primary content type. Multiple aliases
for multipart/mixed are not desirable as they remove the possibility
for specifying a message as multipart/alternate, multipart/parallel,
or multipart/encrypted, for example.
Since the message context is an attribute of the entire message, it
is logical to define a new top-level (RFC 2822 [3]) message
attribute. To this end, this document introduces the message
attribute "Message-Context".
Message-Context only serves to identify the message context. It does
not provide any indication of content that the UA must be capable of
delivering. It does not imply any message disposition or delivery
notification. There is a related effort to define Critical Content
of Internet Mail [8] that one might use to perform these tasks.
Message-Context is only an indicator. We do not intend for it to
convey information that is critical for presentation of the message.
One can conceive of goofy situations, such as a message marked
"voice-message" but without an audio body part. In this case, the
fact that the contents of a message don't match its context does not
mean the receiving system should generate an error report or fail to
deliver or process the message.
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6. Message-Context Reference Field
The Message-Context reference field is a top-level header inserted by
the sending UA to indicate the context of the message.
A receiving user agent MUST NOT depend on the indicated message-
context value in a way that prevents proper presentation of the
message. If the value is incorrect or does not match the message
content, the receiving user agent MUST still be capable of displaying
the message content at least as meaningfully as it would if no
Message-Context value were present.
One can envision situations where a well-formed message ends up not
including a media type one would expect from the message-context.
For example, consider a voice messaging system that records a voice
message and also performs speech-to-text processing on the message.
The message then passes through a content gateway, such as a
firewall, that removes non-critical body parts over a certain length.
The receiving user agent will receive a message in the voice-message
context that has only a text part and no audio. Even though the
message does not have audio, it is still in the voice message
context.
Said differently, the receiving UA can use the message-context to
determine whether, when, and possibly where to display a message.
However, the message-context should not affect the actual rendering
or presentation. For example, if the message is in the voice-message
context, then don't try to send it to a fax terminal. Conversely,
consider the case of a message in the voice-message context that gets
delivered to a multimedia voice terminal with a printer. However,
this message only has fax content. In this situation, the "voice-
message" context should not stop the terminal from properly rendering
the message.
6.1. Message-Context Syntax
The syntax of the Message-Context field, described using the ABNF [4]
is as follows. Note that the Message-Context header field name and
message-context-class values are not case sensitive.
"Message-Context" ":" message-context-class CRLF
6.2. message-context-class Syntax
The message-context-class indicates the context of the message. This
is an IANA registered value. Current values for message-context-
class are as follows.
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message-context-class = ( "voice-message"
/ "fax-message"
/ "pager-message"
/ "multimedia-message"
/ "text-message"
/ "none"
)
Note: The values for Message-Context MUST be IANA registered values
following the directions in the IANA Considerations section below.
6.2.1. voice-message
The voice-message class states the message is a voice mail message.
6.2.2. fax-message
The fax-message class states the message is a facsimile mail message.
6.2.3. pager-message
The pager-message class states the message is a page, such as a text
or numeric pager message or a traditional short text message service
(SMS) message.
6.2.4. multimedia-message
The multimedia-message class states the message is an aggregate
multimedia message, such as a message specified by [9]. This helps
identify a message in a multimedia context. For example, a MIME
multipart/related [10] data part and resource part looks the same as
a multimedia MHTML multipart/related. However, the semantics are
quite different.
6.2.5. text-message
The text-message class states the message is a traditional internet
mail message. Such a message consists of text, possibly richly
formatted, with or without attachments.
6.2.6. none
The none class states there is no context information for this
message.
If a message has no Message-Context reference field, a receiving user
agent MUST treat it the same as it would if the message has a "none"
value.
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7. Security Considerations
The intention for this header is to be an indicator of message
context only. One can imagine someone creating an "Application"
Message-Context. A poorly designed user agent could blindly execute
a mailed program based on the Message-Context. Don't do that!
One can envision a denial of service attack by bombing a receiver
with a message that has a Message-Context that doesn't fit the
profile of the actual body parts. This is why the receiver considers
the Message-Context to be a hint only.
8. IANA Considerations
Section 8.3 is a registration for a new top-level RFC 2822 [3]
message header, "Message-Context".
This document creates an extensible set of context types. To promote
interoperability and coherent interpretations of different types, a
central repository has been established for well-known context types.
The IANA has created a repository for context types called "Internet
Message Context Types". Following the policies outlined in [5], this
repository is "Specification Required" by RFC. Section 8.1 describes
the initial values for this registry.
To create a new message context type, you MUST publish an RFC to
document the type. In the RFC, include a copy of the registration
template found in Section 8.2 of this document. Put the template in
your IANA Considerations section, filling-in the appropriate fields.
You MUST describe any interoperability and security issues in your
document.
8.1. Message Content Type Registrations
Internet Message Content Types
==============================
Value Description Reference
----- ----------- ---------
voice-message Indicates a message whose primary This RFC
content is a voice mail message. The
primary content is audio data. The
context is usually a message recorded
from a voice telephone call.
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fax-message Indicates a message whose primary This RFC
content is a fax mail message. The
primary content is image data. The
context is usually a message recorded
from a facsimile telephone call.
pager-message Indicates a message whose primary This RFC
content is a page. The primary
content is text data. The context is
an urgent message usually of a
limited length.
multimedia-message Indicates a message whose primary This RFC
content is a multimedia message. The
primary content is multimedia, most
likely MHTML. The context is often
spam or newsletters.
text-message Indicates a classic, text-based, This RFC
Internet message.
None Indicates an unknown message context. This RFC
8.2. Registration Template
In the following template, a pipe symbol, "|", precedes instructions
or other helpful material. Be sure to replace "<classname>" with the
class name you are defining.
Message-Context class name:
<classname>
Summary of the message class:
| Include a short (no longer than 4 lines) description or summary
| Examples:
| "Palmtop devices have a 320x160 pixel display, so we can..."
| "Color fax is so different than black & white that..."
Person & email address to contact for further information:
| Name & e-mail
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8.3. Message-Context Registration
To: iana@iana.org
Subject: Registration of New RFC 2822 Header
RFC 2822 Header Name:
Message-Context
Allowable values for this parameter:
Please create a new registry for Primary Context Class
registrations. See section 8.1 of this document for the initial
values.
RFC 2822 Section 3.6 Repeat Value:
Field Min Number Max Number Notes
Message-Context 0 1
Person & email address to contact for further information:
Eric Burger
e.burger@ieee.org
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9. APPENDIX: Some messaging scenarios
This section is not a normative part of this document. We include it
here as a historical perspective on the issue of multimedia message
types.
These scenarios are neither comprehensive nor fixed. For example,
e-mails being typically text-based do not mean that they cannot
convey a voice-message. This very mutability serves to underline the
desirability of providing some explicit message context hint.
9.1. Internet e-mail
Internet e-mail carries textual information. Sometimes it conveys
computer application data of arbitrary size.
Typically, one uses e-mail for non-urgent messages, which the
recipient will retrieve and process at a time convenient to her.
The normal device for receiving and processing e-mail messages is
some kind of personal computer. Modern personal computers usually
come with a reasonably large display and an alphanumeric keyboard.
Audio, video, and printing capabilities are not necessarily
available.
One can use E-mail for communication between two parties (one-to-
one), a small number of known parties (one-to-few) or, via an e-mail
distribution list, between larger numbers of unknown parties (one-
to-many).
One of the endearing characteristics of e-mail is the way that it
allows the recipient to forward all or part of the message to another
party, with or without additional comments. It is quite common for
an e-mail to contain snippets of content from several previous
messages. Similar features apply when replying to e-mail.
9.2. Pager service
One uses a pager message to convey notifications and alerts. For the
most part, these notifications are textual information of limited
size. The typical limit is 160 characters. People use pages for
relatively urgent messages, which the sender wishes the receiver to
see and possibly respond to within a short time period. Pager
messages are often used as a way of alerting users to something
needing their attention. For example, a system can use a page to
notify a subscriber there is a voicemail message requiring her
attention.
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Example devices for sending and receiving a pager message are a
mobile telephone with a small character display or a text pager.
Personal computers and personal digital assistants (PDAs) can also
participate in pager messaging.
Currently, the most common use of pager messages are between just two
parties (one-to-one).
One delivery method for pager messages is the short text messaging
service (SMS). SMS is a facility that has evolved for use with
mobile telephones, and has an associated per-message transmission
charge. Note that the focus here is on the notification aspect of
SMS. From the beginning, SMS was envisioned to be more than a simple
pager service. Operators can use SMS to provision the phone, for
example. From the subscriber point of view, SMS has evolved
considerably from its origins as a pure pager replacement service.
For example, with mobile originate service, people can have two-way
text chat sessions using SMS and a mobile phone. In addition, there
are SMS-enabled handsets that can display pictures. However, for the
purposes of this document, there is still a need to capture the
essence of a "highly urgent, short-text, notification or alert"
service.
Users often send pager messages in isolation, rather than as part of
a longer exchange. One use for them is as a prompt or invitation to
communicate by some more convenient and content-rich method, such as
a telephone call.
9.3. Facsimile
People use facsimile to convey image information of moderate size,
typically a small number of pages. Sometimes people use facsimile
for larger documents.
Facsimile is a facility that usually uses circuit-switched telephone
circuits, with connection-time charges. Message transfer takes place
in real-time. Thus, people often use facsimile for urgent
communication.
The normal device for sending and receiving a facsimile is a self-
contained scanning and printing device connected to a telephone line
or a desktop computer.
Most facsimiles are between just two parties (one-to-one). However,
a significant portion of facsimile service is broadcast between
multiple parties (one-to-many).
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Most facsimile exchanges are in isolation, rather than as part of a
longer exchange. Facsimile data is typically not suitable for
further processing by computer.
9.4. Voice mail
People use voice mail to convey audio information, almost exclusively
human speech.
Voice mail is a facility that usually uses circuit-switched telephone
circuits, with modest connection-time charges, often used for
moderately urgent messages. A common use for them is as a prompt or
invitation to communicate by some more convenient method, such as a
telephone call. In most, but not all cases, the sender of a voice
message does not want to send a message at all. Rather, they wished
to engage in a real-time conversation.
The normal device for sending and receiving a voice mail is a
telephone handset.
Voice messages are usually sent between just two parties (one-to-
one).
Voice mail data is not generally suitable for further processing by
computer.
9.5. Multimedia message
We define a multimedia message as a message containing more than one
basic media type (text, image, audio, video, model, application).
The following are some characteristics of a multimedia message.
In some cases, a multimedia message is just e-mail with an attachment
that a multimedia display application presents. For example, I can
send you an MP3 of something I recorded in my garage today.
In other cases, a multimedia message represents a convergence between
two or more of the scenarios described above. For example, a voice
message with an accompanying diagram or a talking head video message
is a multimedia message.
The characteristics will vary somewhat with the intent of the sender.
This in turn may affect the user agent or application used to render
the message.
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10. References
10.1 Normative References
[1] Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Resnick, P., "Internet Message Format", RFC 2822, April 2001.
[4] Crocker, D. and P. Overell, Eds., "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[5] Alvestrand, H. and T. Narten, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.
10.2 Informative References
[6] Troost, R., Dorner, S. and K. Moore, "Communicating Presentation
Information in Internet Messages: The Content-Disposition Header
Field", RFC 2183, August 1997.
[7] Vaudreuil, G. and G. Parsons, "VPIM Voice Message MIME Sub-type
Registration", RFC 2423, September 1998.
[8] Burger, E., "Critical Content of Internet Mail", RFC 3459,
January 2003.
[9] Palme, J., Hopmann, A. and N. Shelness, "MIME Encapsulation of
Aggregate Documents, such as HTML (MHTML)", RFC 2557, March
1999.
[10] Levinson, E., "The MIME Multipart/Related Content-type", RFC
2387, August 1998.
11. Acknowledgments
Many of the ideas here arose originally from a discussion with Jutta
Degener.
We'd also like to thank Keith Moore for helping us tighten-up our
explanations.
In the last round, we got some rather good advise from Caleb Clausen
and Dave Aronson.
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Antti Vaha-Sipila pointed out advances in SMS, while Stuart McRae
helped distil the essence of the pager service vis a vis SMS.
We offer an extra special thanks to Greg Vaudreuil for pulling RFC
2557 out of his hat.
12. Authors' Addresses
Eric Burger
SnowShore Networks, Inc.
285 Billerica Rd.
Chelmsford, MA 01824-4120
USA
Phone: +1 978 367 8403
EMail: e.burger@ieee.org
Emily Candell
Comverse Network Systems
200 Quannapowitt Pkwy.
Wakefield, MA 01880
USA
Phone: +1 781 213 2324
EMail: emily.candell@comverse.com
Graham Klyne
Nine by Nine
United Kingdom
EMail: GK-IETF@ninebynine.org
Charles Eliot
Microsoft Corporation
One Microsoft Way
Redmond WA 98052
USA
Phone: +1 425 706 9760
EMail: charle@Microsoft.com
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13. Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Burger, et al. Standards Track [Page 17]
ERRATA
