Network Working Group E. Burger Internet Draft Centigram Communications Document: draft-ietf-vpim-hint-00.txt E. Candell Obsoletes: draft-burger-vpim-pc-00.txt Comverse Network Systems Category: Standards Track C. Eliot Expires in six Months Microsoft Corporation G. Klyne Content Technologies July 14, 2000 Content Hint for Internet Mail Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026 [1]. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet- Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. 1. Abstract This document describes a mechanism to allow senders of a multi-part Internet mail message to convey presentational information on the message as a whole. The document specifies a RFC 822 header called "Content-Hint". This mechanism is very similar to the use of the Content-Disposition MIME entity described in [2]. Content- Disposition gives clues to the receiving User Agent (UA) for how to display a given body part. Content-Hint gives clues to the receiving UA for the context of the message display. This allows the receiving UA to present the message in a meaningful way to the recipient. This mechanism is needed because examining the message itself is insufficient to determine the context of the message. For example, one can envision a UA that distinguishes between a voice mail message with a text annotation and a text message with an audio attachment. Content-Hint would provide the hint to the receiving UA which context to present the message. Expires 1/14/01 [Page 1] Primary Content of Internet Mail July 2000 Table of Contents 1. ABSTRACT .........................................................1 2. CONVENTIONS USED IN THIS DOCUMENT ................................2 3. MOTIVATION AND GOALS .............................................3 3.1. The problem ....................................................3 3.2. Some messaging scenarios .......................................4 3.2.1. Internet e-mail.............................................4 3.2.2. Short text messaging service................................5 3.2.3. Facsimile...................................................5 3.2.4. Voice mail..................................................6 3.2.5. Multimedia message..........................................6 3.3. The goal .......................................................7 4. FUNCTIONAL REQUIREMENTS ..........................................7 5. THE CONTENT-HINT .................................................7 6. CONTENT-HINT REFERENCE FIELD .....................................8 6.1. Content-Hint Syntax ............................................8 6.2. content-hint Syntax ............................................8 6.2.1. voice-message...............................................9 6.2.2. fax-message.................................................9 6.2.3. video-message...............................................9 6.2.4. sms-message.................................................9 6.2.5. none........................................................9 7. SECURITY CONSIDERATIONS ..........................................9 8. IANA CONSIDERATIONS .............................................10 8.1. Content-Hint Registration .....................................10 8.2. Primary Content Type Registrations ............................11 8.2.1. voice-message..............................................11 8.2.2. fax-message................................................11 8.2.3. video-message..............................................12 8.2.4. sms-message................................................13 9. REFERENCES ......................................................14 10. ACKNOWLEDGMENTS .................................................14 11. AUTHOR'S ADDRESSES ..............................................15 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 RFC-2119 [3]. Burger et. al. Expires 1/14/01 [Page 2] Primary Content of Internet Mail July 2000 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 and goals 3.1. The problem Multimedia messaging systems receive messages that may be presented in variety of ways. For example, traditional e-mail uses simple text messages that the recipient displays and edits. An UA may automatically print Fax images. Another UA may play voice messages through a telephone handset. Likewise, the receiving desktop computer may process and/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 short text 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 media 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. Thus the end-user receiving agent needs to be able to identify voicemail and distinguish it from other messages. The uses of this kind of presentation characteristic for each message is multi-fold: o display an indication to the user (e.g. by a suitably evocative icon along with other summary fields), o auto-forwarding a specific message type into another messaging environment? (e.g., short text to a mobile short message service), o prioritizing and grouping messages in an inbox display list, o suggesting appropriate default handling for presentation, Burger et. al. Expires 1/14/01 [Page 3] Primary Content of Internet Mail July 2000 o suggesting appropriate default handling for reply, forward, etc., and o filtering the message list for presentation via limited- capability user interfaces (e.g. there is no point in offering images when the user is connected by a voice-only telephone user interface). A problem faced by multimedia messaging systems is that it is not always easy to decide the presentation characteristics of a received message. For example: o a message that contains audio and image data: is this a fax message that happens to have some voice commentary, or is it a voice message that is accompanied by some supplementary diagrams, or 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 an e-mail with an MP3 music attachment, or 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? Thus, the issue of presentation characteristics may be related to message media content, but is not the same thing. The media type used in a message is not sufficient to indicate presentation characteristics. One cannot determine a priori which of multiple media types to use in a alternative message. Also what about distinguishing traditional e-mail text and SMS messages? They are the same media type, but have different presentation characteristics. 3.2. Some messaging scenarios 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 handling hint. 3.2.1. 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. Burger et. al. Expires 1/14/01 [Page 4] Primary Content of Internet Mail July 2000 Two parties can use E-mail can 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 a larger number 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 a 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 an e-mail. 3.2.2. Short text messaging service One can use a short text message to convey textual information of limited size, typically, up to 160 characters. The short text messaging service (SMS) is a facility that has evolved for use with mobile telephones, and has an associated per- message transmission charge. People use SMS for relatively urgent messages, which the sender wishes the receiver to see and possibly respond to within a short time period. The normal device for sending and receiving a short text message is a mobile telephone with a small character display and a numeric-only keyboard. Personal computers and personal digital assistants (PDAs) can also participate in short text messaging. Currently, the most common use of short text messages are between just two parties (one-to-one). Users often send short text 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. 3.2.3. 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 modest connection-time charges. Message transfer takes place in real-time. Thus, people often use facsimile for moderately 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. Burger et. al. Expires 1/14/01 [Page 5] Primary Content of Internet Mail July 2000 Most facsimiles are between just two parties (one-to-one). However, broadcast facsimile service is between multiple parties (one-to- many). 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. 3.2.4. 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. 3.2.5. Multimedia message We define a multimedia message as a message containing more than one basic media type (text, image, audio, video, model, application). These are the characteristics of a multimedia message. In some cases, it 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, it 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. The characteristics will vary somewhat with the intent of the sender. This in turn may affect the user agent or application used to create the message. Burger et. al. Expires 1/14/01 [Page 6] Primary Content of Internet Mail July 2000 3.3. The goal The goal, then, of this document is to describe a simple mechanism that provides enough information to allow a receiving user agent to make some reasonable decisions about how to present a message. The sort of decisions that a receiving agent needs to make include the following items. o what icon or class name to display for each message in a list o a default device and/or application to use for presentation of the message o whether to try and forward the message into another environment It is not a goal for this mechanism to provide detailed handling information. One may apply other techniques to provide more detailed handling information. The mechanism designed here should work with, rather than against, these other techniques. 4. Functional requirements o To identify a message as belonging to one of small number of enumerated message classes. o Specify a core set of message classes for all message user agents to recognize. o Specify message classes by the originating user's choice of authoring tool or simple user interaction. o Incorrect or invalid message labelling must not result in failure to transfer or inability to present a message. o Message labeling information has to be interpretable in reasonable fashion by many different user agent systems. o The mechanism should be extensible to allow new kinds of message to be introduced and labelled. 5. The Content-Hint One method of indicating the interpretation context of the media content of a message is to examine the media types in the message. However, this requires the UA to scan the entire message before making this determination. This is particularly burdensome for the multi-media mail situation, as voice and especially video mail objects are quite large. Burger et. al. Expires 1/14/01 [Page 7] Primary Content of Internet Mail July 2000 Another method of indicating the primary media content of a message is to register a multipart/* MIME subtype. For example, the VPIM Work Group has registered multipart/voice-message to indicate that a message is primarily voice mail [4]. 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. 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. Since the content context is an attribute of the entire message, it is logical to define a new top-level (RFC 822 [5]) message attribute. To this end, this document introduces the message attribute "Content-Hint". Content-Hint only serves to identify the content context of the message. 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. See the companion document, Critical Content of Internet Mail [6], for a mechanism to perform these tasks. Since Content-Hint is only an indicator, goofy situations, such as a message marked "voice-message" but without a voice body part, MUST NOT generate any error report. 6. Content-Hint Reference Field The Content-Hint reference field is a top-level header inserted by the sending UA to indicate the primary content type of the message. 6.1. Content-Hint Syntax The syntax of the Content-Hint field, formatted according to the ABNF [7] is as follows. Note that "Content-Hint" is not case sensitive, per RFC 822. "Content-Hint" ":" content-hint CRLF 6.2. content-hint Syntax The content-type indicates the primary media content context of the message. This is an IANA registered value. Current values for Content-Hint are as follows. Burger et. al. Expires 1/14/01 [Page 8] Primary Content of Internet Mail July 2000 content-hint = 1 *( [ "voice-message"] [ "fax-message" ] [ "video-message" ] [ "sms-message" ] [ "none" ] Extension-type ) Extension-type = token ; Defined and registered per Section 8 / x-token ; Experimental, private use token = x-token = 6.2.1. voice-message The voice-message hint states the message is a voice message, with voice messaging semantics. 6.2.2. fax-message The fax-message hint states the message is a facsimile message, with facsimile messaging semantics. 6.2.3. video-message The video-message hint states the message is a video message, with facsimile messaging sematics. 6.2.4. sms-message The sms-message hint states the message is a short text message service (SMS) message, with SMS messaging semantics. 6.2.5. none The none hint states there is no hint for this message. Clearly, if a message has no Content-Hint reference field, none MUST be the default value. 7. Security Considerations The intention for this header is to indicate media content context only. One can imagine one creating an "Application" content hint, and have a poorly designed user agent blindly execute a mailed program. Don't do that! One can envision a denial of service attack by bombing a receiver with a message with a Content-hint that doesn't fit the profile of the actual body parts. This is why the receiver MUST consider the Burger et. al. Expires 1/14/01 [Page 9] Primary Content of Internet Mail July 2000 Content-hint to be a hint only. The receiver SHOULD NOT rely on the Content-hint exclusively for presentation processing. 8. IANA Considerations NOTE: We won't send in any registrations until it looks like this will become a RFC! Following the policies outlined in [9], IANA assigns values for Content-Hint as Specification Required. 8.1. Content-Hint Registration To: ietf-types@iana.org Subject: Registration of New Top-Level Header Field Content-Hint Header name: Content-Hint Required parameters: Single 7bit text value Parameter value: The parameter value specifies the primary media content type for the message. Security considerations: The intention for this header is to indicate media content type only. One can imagine one creating an "Application" primary content type, and have a poorly designed user agent blindly execute a mailed program. Published specification: draft-ietf-vpim-hint-00.txt Applications which use this media type: Mail VPIM FPIM Additional information: none Person & email address to contact for further information: Eric Burger e.burger@ieee.org Intended usage: COMMON Burger et. al. Expires 1/14/01 [Page 10] Primary Content of Internet Mail July 2000 8.2. Primary Content Type Registrations 8.2.1. voice-message To: ietf-types@iana.org Subject: Registration of New Content-Hint type voice-message Content-Hint type name: voice-message Required parameters: none Optional parameters: none Encoding considerations: none Security considerations: none Interoperability considerations: User agents declaring the primary content to be voice-message SHOULD conform to VPIMv2. Published specification: draft-ietf-vpim-hint-00.txt RFC 2421, Voice Profile for Internet Mail - version 2 Applications which use this media type: VPIM Additional information: none Person & email address to contact for further information: Eric Burger e.burger@ieee.org Intended usage: COMMON 8.2.2. fax-message To: ietf-types@iana.org Subject: Registration of New Content-Hint type fax-message Content-Hint type name: fax-message Burger et. al. Expires 1/14/01 [Page 11] Primary Content of Internet Mail July 2000 Required parameters: none Optional parameters: none Encoding considerations: none Security considerations: none Interoperability considerations: none Published specification: draft-ietf-vpim-hint-00.txt Applications which use this media type: FPIM Additional information: none Person & email address to contact for further information: Eric Burger e.burger@ieee.org Intended usage: COMMON 8.2.3. video-message To: ietf-types@iana.org Subject: Registration of New Content-Hint type video-message Content-Hint type name: voice-message Required parameters: none Optional parameters: none Encoding considerations: none Security considerations: none Burger et. al. Expires 1/14/01 [Page 12] Primary Content of Internet Mail July 2000 Interoperability considerations: none Published specification: draft-ietf-vpim-hint-00.txt Applications which use this media type: VPIM, FPIM Additional information: none Person & email address to contact for further information: Eric Burger e.burger@ieee.org Intended usage: COMMON 8.2.4. sms-message To: ietf-types@iana.org Subject: Registration of New Content-Hint type sms-message Content-Hint type name: sms-message Required parameters: none Optional parameters: none Encoding considerations: none Security considerations: none Interoperability considerations: none Published specification: draft-ietf-vpim-hint-00.txt Applications which use this media type: VPIM, FPIM Additional information: none Burger et. al. Expires 1/14/01 [Page 13] Primary Content of Internet Mail July 2000 Person & email address to contact for further information: Eric Burger e.burger@ieee.org Intended usage: COMMON 9. References 1 Bradner, S., "The Internet Standards Process -- Revision 3", BCP 9, RFC 2026, October 1996. 2 Troost, R., Dorner, S., and Moore, K., "Communicating Presentation Information in Internet Messages: The Content- Disposition Header Field", RFC 2183, New Century Systems, QUALCOMM Incorporated, and University of Tennessee, August 1997. 3 Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. 4 Vaudreuil, G. and Parsons, G., "VPIM Voice Message MIME Sub-type Registration", RFC 2423, Lucent Technologies and Northern Telecom, September 1998. 5 Crocker, D., "Standard for the Format of ARPA Internet Text Messages", STD 11, RFC 822, August 1982. 6 Burger, E. and Candell, E., "Critical Content of Internet Mail", draft-ietf-vpim-cc-00.txt, Work in Progress. 7 Crocker, D. and Overell, P.(Editors), "Augmented BNF for Syntax Specifications: ABNF", RFC 2234, Internet Mail Consortium and Demon Internet Ltd., November 1997. 8 Freed, N. and Borenstein, N., "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, Innosoft and First Virtual, November 1996. 9 Alvestrand, H. and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, October 1998. 10. 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. Burger et. al. Expires 1/14/01 [Page 14] Primary Content of Internet Mail July 2000 11. Author's Addresses Eric Burger Centigram Communications Corporation Maryland Technology Center 1375 Piccard Dr., MS 150I Rockville, MD 20850-4311 USA Phone/Fax: +1 301/212-3320 Email: e.burger@ieee.org Emily Candell Comverse Network Systems 200 Quannapowitt Pkwy. Wakefield, MA 01880 USA Phone: +1 781/213-2324 Email: emily@comversens.com Graham Klyne Content Technologies Ltd. 1220 Parkview, Arlington Business Park Theale Reading, RG7 4SA United Kingdom. Telephone: +44 118 930 1300 Facsimile: +44 118 930 1301 E-mail: GK@ACM.ORG Charles Eliot Microsoft Corporation <<>> Telephone: E-Mail: charle@Exchange.Microsoft.com Full Copyright Statement The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in Burger et. al. Expires 1/14/01 [Page 15] Primary Content of Internet Mail July 2000 this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. 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