Network Working Group E. Burger Internet Draft SnowShore Networks Document: draft-ietf-vpim-cc-02.txt E. Candell Category: Standards Track Comverse Network Systems Expires May 2001 November 24, 2000 Critical Content of 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." One can access the list of current Internet-Drafts at http://www.ietf.org/ietf/1id-abstracts.txt One can access the list of Internet-Draft Shadow Directories at http://www.ietf.org/shadow.html. This document is a work product of the IETF Voice Profile for Internet Mail (vpim) Work Group. The URL for the VPIM website is <http://www.vpim.org>. 1. Abstract This document describes a mechanism for identifying the body parts that the sender deems critical in a multi-part Internet mail message. By knowing what parts of a message the sender deems critical, a MTA or UA can intelligently handle multi-part messages when gatewaying (MTA) or presenting (UA) to systems of lesser capability. Critical content can help a smart gateway decide what parts to forward. It can indicate how hard a gateway should try to deliver a body part. It can help an MTA to select body parts to silently delete when a system of lesser capability receives a message. In addition, critical content can help indicate the notification strategy of the receiving system. Expires 5/24/01 [Page 1]
Critical Content of Internet Mail November 2000 Table of Contents 1. ABSTRACT .........................................................1 2. CONVENTIONS USED IN THIS DOCUMENT ................................2 3. INTRODUCTION .....................................................3 4. CONTENT-CRITICALITY ENTITY .......................................5 4.1. CRITICAL .......................................................6 4.2. IGNORE .........................................................6 4.3. Other Values ...................................................6 4.4. Summary ........................................................7 5. STATUS CODE ......................................................7 6. BACKWARD COMPATIBILITY CONSIDERATIONS ............................8 7. MIME INTERACTIONS ................................................8 7.1. multipart/alternative ..........................................8 7.2. multipart/related ..............................................9 7.3. message/rfc822 .................................................9 8. IMPLEMENTATION EXAMPLES ..........................................9 8.1. Content Gateways ...............................................9 8.2. Non-Traditional UA ............................................10 9. SECURITY CONSIDERATIONS .........................................11 10. COLLECTED SYNTAX ................................................11 11. REFERENCES ......................................................11 12. ACKNOWLEDGMENTS .................................................12 13. AUTHOR'S ADDRESSES ..............................................13 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 [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. Burger and Candell Expires 5/24/01 [Page 2]
Critical Content of Internet Mail November 2000 3. Introduction This document describes the Critical Content identification for multi-part Internet mail. The need for a critical content identification mechanism comes about because of the internetworking of Internet mail systems with legacy messaging systems that do not fulfil all of the semantics of Internet mail. Such legacy systems have a limited ability to render all parts of a given message. This document will use the case of an Internet mail system exchanging electronic messages with a legacy voice messaging system for illustrative purposes. Electronic mail has historically been text-centric. Extensions such as MIME [3] enable the desktop to send and receive multi-part, multimedia messages. Popular multimedia data types include binary word processing documents, binary business presentation graphics, voice, and video. Voice mail has historically been audio-centric. Many voice- messaging systems only render voice. Extensions such as fax enable the voice mail system to send and receive fax images as well as create multi-part voice and fax messages. A few voice mail systems can render text using text-to-speech or text-to-fax technology. Although theoretically possible, none can today render video. An important aspect of the interchange between voice messaging services and desktop e-mail client applications is that the rendering capability of the voice-messaging platform is often much less than the rendering capability of a desktop e-mail client. In the e-mail case, the sender has the expectation that the recipient receives all components of a multimedia message. This is so even if the recipient cannot render all body parts. In most cases, the recipient can either find the appropriate rendering tool or tell the sender that she cannot read the particular attachment. This is an important issue. By definition, a MIME-enabled user agent, conforming to [4] will present or make available all of the body parts to the recipient. However, a voice mail system may not be capable of storing non-voice objects. Moreover, the voice mail system may not be capable of notifying the recipient that there were undeliverable message parts. The inability of the receiving system to render a body part is usually a permanent failure. Retransmission of the message will not improve the likelihood of a future successful delivery. Contrast this to the case with normal data delivery. Traditional message failures, such as a garbled message or disabled link will benefit from retransmission. This situation is fundamentally different from normal Internet mail. In the Internet mail case, either the system delivered the message, Burger and Candell Expires 5/24/01 [Page 3]
Critical Content of Internet Mail November 2000 or it didn't. There is no concept of a system partially delivering a message. In addition, the sender would not mind if the system did not deliver non-critical parts of a message. In fact, the sender's user agent may be silently adding body parts to a message unbeknownst to the sender. For example, take Microsoft Outlook as a user agent. Outlook often will attach a TNEF section or other body parts. If the receiving system rejected the message because it could not render TNEF, the sender would be understandably confused and upset. Thus, there is a need for a method of indicating to a Mail Transfer Agent (MTA) or User Agent (UA) that the sender considers parts of a message to be critical. From the sender's perspective, he would not consider the message delivered if the system did not deliver the critical parts. One method of indicating critical content of a message is to define a profile. The profile defines discard rules based on knowledge of the user population for silently deleting body parts. Citing the example above, a voice profile can easily declare that MTAs or UAs can silently delete TNEF data and yet consider the message successfully delivered. This is, in fact, the approach taken by VPIMv2 [5]. Since one aspect of the issue is deciding when to notify the sender that the system cannot deliver part of a message, one could use a partial non-delivery notification mechanism [6] to indicate a problem with delivering a given body part. However, this requires the user request a MDN. Moreover, the sender will receive PNDN failures for objects the sender may not be aware he is sending. An example would be the TNEF part. Summarizing the needs, we need a mechanism that will let the sender or sender's UA mark body parts he considers critical to the message that the system must deliver. The mechanism MUST NOT burden the sender with failure notifications for non-critical body parts. The mechanism MUST conform to the general notification status request mechanism for positive or negative notification. When requested, the mechanism MUST indicate to the sender when a receiving system cannot deliver a critical body part. In short, we need a method of letting the sender indicate what body- parts he considers to be critical. This document describes a Critical Content marking mechanism that satisfies these needs. Following the format for Internet message bodies [3], this document introduces the Content-Criticality body part header. Values for this header are CRITICAL, NOTIFY, or IGNORE. The receiving MTA or UA will generate a delivery status notification (DSN) [7] or Message Disposition Notification (MDN) [8] at delivery time or reading time if it receives a request for Burger and Candell Expires 5/24/01 [Page 4]
Critical Content of Internet Mail November 2000 notification and the (non-)delivery status of the parts marked NOTIFY meet the criteria for notification. NOTE: A straightforward alternative implementation method for marking a body part critical is to use a content-disposition parameter called "criticality". NOTE: We could have made the critical content indicator a parameter to content-disposition. This has the benefit of being very easy for IMAP servers to implement. In particular, IMAP servers automatically present the content-disposition entity when a UA requests information on a message. On the other hand, the UA must explicitly request the presence and value of different entities, such as content-criticality. However, implementing the critical content indicator as a parameter to content-disposition overloads the meaning of the entity. Moreover, IMAP servers can present, in the future, content-criticality by default. Lastly, most UA's that have interest in content-criticality will explicitly request the header in any event. 4. Content-Criticality Entity The Content-Criticality field is a MIME body part header inserted by the sending UA to indicate to the receiving MTA or UA whether to consider this body part critical. A CRITICAL body part is one the sender requires the receiving system to deliver for him to consider the message delivered. An IGNORE body part is one the sender doesn't care whether the receiving system delivers it or not. The receiving system can silently delete such body parts if the receiving system cannot deliver the part. The terms "entity" and "body part" have the meanings defined in [3]. One obvious application of critical content is generating a (non-)delivery message. If the value of the field is IGNORE, the receiving MTA or UA MUST NOT generate a notification. If the value of the field is CRTITICAL, the receiving MTA or UA will generate a notification, based on the normal notification request mechanisms. Normal notification request mechanisms include the SMTP RCPT NOTIFY command [9] and the Disposition-Notification-To header [8]. For example, if the sending system requests a notification, and a CRITICAL part fails, the receiving system will generate a NDN for the whole message. Conversely, if only an IGNORE part fails, the receiving system will not generate a NDN. The next sections examine the actions taken by an MTA or UA given the different values of Content-Criticality. Burger and Candell Expires 5/24/01 [Page 5]
Critical Content of Internet Mail November 2000 NOTE: This implies that the MTA must examine the entire message on receipt to determine whether it needs to generate a notification. However, the MTA need not examine the message if it knows it can store and forward all media types. Said differently, Internet e- mail MTA can, by default, handle any arbitrary MIME-encapsulated type. Some voice mail systems, on the other hand, cannot store binary attachments at all, such as application/ms-word. The voice mail MTA, in this example, would be scanning for non-renderable body parts in any event. 4.1. CRITICAL "Content-Criticality: CRITICAL" signifies that this body part is critical to the sender. If the receiving system cannot render or store a body part marked CRITICAL, then the entire message has failed. In this case, the receiving system MUST take the appropriate failure action. NOTE: We say "appropriate action", because the sender may have suppressed all notifications. In this case, the appropriate action is to simply discard the message. 4.2. IGNORE "Content-Criticality: IGNORE" signifies that the sender does not care about notification reports for this body part. If the receiving system cannot render or store a body part marked IGNORE, the receiving system may silently delete the body part. The receiving system MUST NOT return a delivery failure, unless parts marked IMPORTANT or CRITICAL have also failed. 4.3. Other Values The receiving system MUST treat unrecognized values as CRITICAL. This is to provide backward compatibility with future uses of the Content-Criticality entity. The most likely new value is IMPORTANT. An IMPORTANT body part is something the sender wants the receiver to get, but would not want the message rejected outright if the IMPORTANT body part fails. A receiving system that does not understand IMPORTANT MUST take the default value of CRITICAL. In this case, the MTA or UA MUST take the conservative action of rejecting the message. Burger and Candell Expires 5/24/01 [Page 6]
Critical Content of Internet Mail November 2000 4.4. Summary The following table summarizes the actions expected of a conforming MTA or UA. NOTE: This section is normative: it suggests what to put into the DSN or MDN. +--------------------------------------+ | Sending UA Has Marked Body Part | |---------------------+----------------| | CRITICAL | IGNORE | +--------------------+---------------------+----------------+ | Body Part is | | | | Deliverable/read | Appropriate Action | ignore | +--------------------+---------------------+----------------+ | Body Part is | | | | Undeliverable / | | | | Unreadable | Fail Entire Message | ignore | +--------------------+--------------------------------------+ The distinction between deliverable/read is as follows. A MTA can determine if a body part is deliverable. If the body part is not deliverable and is critical, the MTA could generate a DSN. An example of such a MTA is a content-converting gateway. An UA can determine if a body part is readable. If the body part is not readable and is critical the UA could generate a MDN. The "Appropriate Action" is the action the MTA or UA would take given the context of execution. For example, if a sender requests return receipt and the receiver reads a CRITICAL body part, the receiving UA must generate the appropriate MDN (following the rules for MDN). Likewise, if the MTA or UA cannot deliver the body part and the body part is critical, the MTA or UA MUST generate the appropriate DSN or MDN. "Ignore" means the MTA or UA MUST ignore the operation on the body part. The MTA or UA MUST treat the message as if the body part was not present in the message. 5. Status Code The critical content indication, in itself, does not guarantee any notification. Notification follows the rules described in [7] and [8]. NOTE: The content of actual DSNs or MDNs are beyond the scope of this document. This document only specifies how to mark a critical body part. On the other hand, we do envision sensible DSN and MDN contents. For example, DSNs should include the appropriate failure Burger and Candell Expires 5/24/01 [Page 7]
Critical Content of Internet Mail November 2000 code as enumerated in [10]. Likewise, MDNs should include the failure code in the MDN "Failure:" field. If the receiving system is to generate a notification based on its inability to render or store the media type, the notification MUST include the status code 5.6.1, "Media not supported", from [10]. 6. Backward Compatibility Considerations If there are no Content-Criticality entities in the message, the default value for Content-Criticality is CRITICAL. The standard notification mechanisms work for sending user agents (UA) that do not know about the content notification entity. All body parts are critical, because they have the default marking of CRITICAL. If there is at least one Content-Criticality entity in the message, the default value for unspecified body parts is IGNORE. The philosophy is that UAs, especially manually constructed messages, will explicitly mark the critical body parts. NOTE: We could choose the default value for Content-Criticality to be IGNORE. This would make VPIMv2 automatically compliant with this document, as VPIMv2 has provision to silently delete undeliverable parts. However, VPIMv2 systems should not be receiving arbitrary e- mail from the Internet. If they do, they should be compliant with this series of documents. By defaulting to CRITICAL, this draft is compliant with the rest of the Internet infrastructure. NOTE: Some VPIMv2 implementations can receive arbitrary e-mail from the Internet. However, these systems are really acting in the capacity of an Internet Voice Mail system. In this case, one would expect the implementation to provide Internet Voice Mail semantics to Internet Voice Mail messages. 7. MIME Interactions 7.1. multipart/alternative Content-Criticality is only in effect for the selected alternative. If the selected alternative has the critical content indicator, then the entire alternative takes on the criticality indicated. That is, if the alternative selected has Content-Criticality: IGNORE, then the receiving system MUST NOT generate any delivery notifications (MDN, NDN, return-receipt, etc.). It is unlikely for a selected alternative to fail, as the receiving UA presumably picks the alternative specifically because it can render it. Burger and Candell Expires 5/24/01 [Page 8]
Critical Content of Internet Mail November 2000 If the selected alternative is a message/rfc822 that encloses a multipart MIME message or the selected alternative is itself a multipart MIME type, the individual top-level body parts follow the Content-Criticality mechanism described in this document. 7.2. multipart/related Content-Criticality fits in rather well with the multipart/related construction. For example, consider a multipart/related message consisting of a Macintosh data fork and a Macintosh resource fork. For a Microsoft Word document, the data fork is likely to be critical. The receiving system can safely ignore the resource fork. 7.3. message/rfc822 Content-Criticality only affects the outermost level of the message or, in the case of multipart/alternative, the outermost level of the selected alternative. Specifically, the receiving system ignores Content-Criticality indicators in embedded body parts. This avoids the situation of a forwarded message triggering or suppressing undesired or desired reporting. 8. Implementation Examples This section is not a normative part of the definition of Content- Criticality. However, we hope it helps implementers to understand the mechanics of the Content-Criticality mechanism. We will examine two cases. They are how a content gateway processes a message and how a UA processes a message. 8.1. Content Gateways Content gateways examine the contents of a message from a first network before the gateway forwards the message to a second network. For the purposes of this example, we assume the second network has less capability than the first network. In particular, we expect there will be certain message body types that the gateway cannot pass onto the second network. Consider a gateway between the Internet and a text-only short message service. A message comes through the gateway containing a text part and a tnef part. The sender marks the text part CRITICAL. The gateway, knowing the capability of the short message service, silently deletes the non-critical, tnef part, passing the critical content to the shore message service network. Any subsequent notifications, such as failure notices or delivery notices, follow the normal rules for notification. Note the gateway, by silently deleting non-critical content, may affect proprietary message correlation schemes. One can envision Burger and Candell Expires 5/24/01 [Page 9]
Critical Content of Internet Mail November 2000 the sending UA inserting a body part for tracking purposes. By deleting non-critical content, the content gateway will break such a scheme. If a sending UA understands how to mark critical content, it should use Internet standard mechanisms for tracking messages, such as Message-ID [11]. What if no body parts have critical content indicators? In this case, the entire message is critical. Thus, when the gateway sees the tnef part, it will reject the entire message, generating a DSN with a status code 5.6.1, "Media not supported". Likewise, consider a three part message with a text annotation (part 1) to a voice message (part 2) with a vCard [12] (part 3). The sender marks the first two parts CRITICAL. Now, let us assume the receiving MTA (gateway) is a voice mail only system, without even the capability to store text. In this case, the gateway (receiving MTA) will reject the message, generating a DSN with a status code 5.6.1, "Media not supported". 8.2. Non-Traditional UA For this example, we will examine the processing of a three-part message. The first part is a text annotation of the second part, an audio message. The third part is the sender's vCard. The sender marks the first and second parts CRITICAL. In addition, the sender marks the message for read receipt. For the purposes of example, the telephone user interface (TUI) does not perform text-to-speech conversion. A TUI is a mail user agent (UA) that uses DTMF touch-tone digits for input and audio for output (display). The TUI is unable to render the first part of the message, the text part. In addition, it is unable to render the third part of the message, the vCard part. Since the sender did not mark the third part of the message CRITICAL, the system ignores the failure of the TUI to render the third part of the message. However, since the sender did mark the first part CRITICAL, and the TUI is unable to render text, the message fails. What happens next is implementation dependent. If the TUI is part of a unified messaging system, a reasonable action is to hold the message for the user. The user can access the message at a later time from a terminal that can render all of the critical body parts. It would be reasonable for the TUI to notify the user about the message. If the TUI is part of a voice messaging system, or if the user does not subscribe to a text-to-speech service, a reasonable action is for the TUI to return a MDN with the disposition "failed" and the failure modifier "5.6.1 (Media not supported)". Burger and Candell Expires 5/24/01 [Page 10]
Critical Content of Internet Mail November 2000 9. Security Considerations Receiving systems and users should not place any authentication value on the Content-Criticality entity. Just because a message has a particular Content-Criticality value doesn't mean that the message really originated at a given type of system. 10. Collected Syntax The format of the collected syntax is in accordance with the ABNF of [13]. Note that per RFC 2045, none of the strings are case sensitive. "Content-Criticality" ":" notification-type CRLF notification-type = "CRITICAL" / "IGNORE" 11. 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 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. 4 Freed, N. and Borenstein, N., "Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types", RFC 2046, Innosoft and First Virtual, November 1996. 5 Vaudreuil, G. and Parsons, G., "Voice Profile for Internet Mail - version 2", RFC 2321, Lucent Technologies and Nortel Networks, September 1998. 6 Burger, E., "Partial Non-Delivery Notification", Work in Progress, draft-ema-burger-pndn-01.txt, March 2000. 7 Moore, K. and Vaudreuil, G., "An Extensible Message Format for Delivery Status Notifications", RFC 1894, University of Tennessee and Octel Network Services, January 1996. Burger and Candell Expires 5/24/01 [Page 11]
Critical Content of Internet Mail November 2000 8 Fajman, R., "An Extensible Message Format for Message Disposition Notifications", RFC 2298, National Institutes of Health, March 1998. 9 Moore, K., "SMTP Service Extension for Delivery Status Notifications", RFC 1981, University of Tennessee, January 1996. 10 Vaudreuil, G., "Enhanced Mail System Status Codes", RFC 1893, Octel Network Services, January 1996. 11 Crocker, D., "Standard for the Format of ARPA Internet Text Messages", RFC 822, University of Delaware, August 1982. 12 Dawson, F. and Howes, T., "vCard MIME Directory Profile", RFC 2426, Lotus Development Corporation and Netscape Communications, September 1998. 13 Crocker, D. and Overell, P.(Editors), "Augmented BNF for Syntax Specifications: ABNF", RFC 2234, Internet Mail Consortium and Demon Internet Ltd., November 1997. 12. Acknowledgments We'd like to thank Ned Freed for pointing out that this mechanism was about criticality, not notification per se. That insight made the concept and descriptions infinitely more straightforward. If it's still confusing, it's our fault! We'd also like to thank Keith Moore for helping us tighten-up our explanations. Dropping the IMPORTANT critical content type took away one of the reasons for partial non-delivery notification. That makes Jutta Degener very happy! Harald Alvestrand and Chris Newman suggested we add implementation examples, which we did. Burger and Candell Expires 5/24/01 [Page 12]
Critical Content of Internet Mail November 2000 13. Author's Addresses Eric Burger SnowShore Networks c/o CRV 1000 Winter St. Waltham, MA 02451 USA Phone: +1 703/304-3883 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 Burger and Candell Expires 5/24/01 [Page 13]
Critical Content of Internet Mail November 2000 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 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. Copies of claims of rights made available for publication and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF Secretariat. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to practice this standard. Please address the information to the IETF Executive Director. Copyright (C) 2000 The Internet Society. 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. Burger and Candell Expires 5/24/01 [Page 14]
