Movatterモバイル変換

Network Working Group                                          E. BurgerRequest for Comments: 3458                            SnowShore NetworksCategory: Standards Track                                     E. Candell                                                                Comverse                                                                C. Eliot                                                   Microsoft Corporation                                                                G. Klyne                                                            Nine by Nine                                                            January 2003Message Context for Internet MailStatus 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 newRFC 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 2003Table of Contents1. Introduction....................................................22. Conventions used in this document...............................33. Motivation......................................................34. Functional Requirements.........................................55. Determining the Message Context.................................66. Message-Context Reference Field.................................76.1. Message-Context Syntax......................................76.2. message-context-class Syntax................................76.2.1. voice-message...........................................86.2.2. fax-message.............................................86.2.3. pager-message...........................................86.2.4. multimedia-message......................................86.2.5. text-message............................................86.2.6. none....................................................87. Security Considerations.........................................98. IANA Considerations.............................................98.1. Message Content Type Registrations..........................98.2. Registration Template......................................108.3. Message-Context Registration...............................119. APPENDIX: Some messaging scenarios.............................129.1. Internet e-mail............................................129.2. Pager service..............................................129.3. Facsimile..................................................139.4. Voice mail.................................................149.5. Multimedia message.........................................1410. References....................................................1510.1 Normative References.......................................1510.2 Informative References.....................................1511. Acknowledgments...............................................1512. Authors' Addresses............................................1613. Full Copyright Statement......................................171. 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 aRFC 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   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 inBCP 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 computerBurger, 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   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.Burger, et al.              Standards Track                     [Page 5]

RFC 3458           Message Context for Internet Mail        January 20035. 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 20036. 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 CRLF6.2. 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   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.Burger, et al.              Standards Track                     [Page 8]

RFC 3458           Message Context for Internet Mail        January 20037. 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 ConsiderationsSection 8.3 is a registration for a new top-levelRFC 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 inSection 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.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 RFC8.2. 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-mailBurger, et al.              Standards Track                    [Page 10]

RFC 3458           Message Context for Internet Mail        January 20038.3. Message-Context Registration   To: iana@iana.org   Subject: Registration of NewRFC 2822 HeaderRFC 2822 Header Name:   Message-Context   Allowable values for this parameter:   Please create a new registry for Primary Context Class   registrations.  Seesection 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.orgBurger, et al.              Standards Track                    [Page 11]

RFC 3458           Message Context for Internet Mail        January 20039. 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.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"   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).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-   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.Burger, et al.              Standards Track                    [Page 14]

RFC 3458           Message Context for Internet Mail        January 200310. References10.1 Normative References   [1]  Bradner, S., "The Internet Standards Process -- Revision 3",BCP9,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",RFC2387, 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.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 pullingRFC2557 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.comBurger, et al.              Standards Track                    [Page 16]

RFC 3458           Message Context for Internet Mail        January 200313.  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]