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Network Working Group E. Burger Request for Comments: 3458 SnowShore Networks Category: Standards Track E. Candell

                                                              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.


 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.

Burger, et al. Standards Track [Page 1] RFC 3458 Message Context for Internet Mail January 2003

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".

Burger, et al. Standards Track [Page 2] RFC 3458 Message Context for Internet Mail January 2003

 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
 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

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
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 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

Burger, et al. Standards Track [Page 3] RFC 3458 Message Context for Internet Mail January 2003

 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?

Burger, et al. Standards Track [Page 4] RFC 3458 Message Context for Internet Mail January 2003

 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

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.

Burger, et al. Standards Track [Page 5] RFC 3458 Message Context for Internet Mail January 2003

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.

Burger, et al. Standards Track [Page 6] RFC 3458 Message Context for Internet Mail January 2003

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
 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.

Burger, et al. Standards Track [Page 7] RFC 3458 Message Context for Internet Mail January 2003

    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
 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"

Burger, et al. Standards Track [Page 8] RFC 3458 Message Context for Internet Mail January 2003

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

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.

Burger, et al. Standards Track [Page 9] RFC 3458 Message Context for Internet Mail January 2003

 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:
 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

Burger, et al. Standards Track [Page 10] RFC 3458 Message Context for Internet Mail January 2003

8.3. Message-Context Registration

 Subject: Registration of New RFC 2822 Header
 RFC 2822 Header Name:
 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
 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

Burger, et al. Standards Track [Page 11] RFC 3458 Message Context for Internet Mail January 2003

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
 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
 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-
 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

Burger, et al. Standards Track [Page 12] RFC 3458 Message Context for Internet Mail January 2003

 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"
 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
 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).

Burger, et al. Standards Track [Page 13] RFC 3458 Message Context for Internet Mail January 2003

 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-
 Voice mail data is not generally suitable for further processing by

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.

Burger, et al. Standards Track [Page 14] RFC 3458 Message Context for Internet Mail January 2003

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
 [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
 We'd also like to thank Keith Moore for helping us tighten-up our
 In the last round, we got some rather good advise from Caleb Clausen
 and Dave Aronson.

Burger, et al. Standards Track [Page 15] RFC 3458 Message Context for Internet Mail January 2003

 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
 Phone: +1 978 367 8403
 Emily Candell
 Comverse Network Systems
 200 Quannapowitt Pkwy.
 Wakefield, MA  01880
 Phone: +1 781 213 2324
 Graham Klyne
 Nine by Nine
 United Kingdom
 Charles Eliot
 Microsoft Corporation
 One Microsoft Way
 Redmond WA 98052
 Phone: +1 425 706 9760

Burger, et al. Standards Track [Page 16] RFC 3458 Message Context for Internet Mail January 2003

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
 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


 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Burger, et al. Standards Track [Page 17]

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