Network Working Group                                           E. Burger
Internet Draft                                         SnowShore Networks
Document: draft-ietf-vpim-cc-06.txt
Category: Standards Track
Updates: RFC 3204
Expires October 2002                                       April 11, 2002

                    Critical Content MIME Parameter

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026 [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, and other documents may
   obsolete this one at any time.  It is inappropriate to use Internet-
   Drafts as reference material or to cite them other than as "work in

   One can access the list of current Internet-Drafts at

   One can access the list of Internet-Draft Shadow Directories at

   This document is a work product of the IETF Voice Profile for
   Internet Mail (VPIM) Work Group.

1. Abstract

   This document describes the use of a mechanism for identifying body
   parts that a sender deems critical in a multi-part Internet mail
   message.  The mechanism described is a parameter to Content-
   Disposition, as described by RFC 3204.

   By knowing what parts of a message the sender deems critical, a
   content gateway can intelligently handle multi-part messages when
   providing gateway services to systems of lesser capability.
   Critical content can help a content gateway to decide what parts to
   forward.  It can indicate how hard a gateway should try to deliver a
   body part.  It can help the gateway to pick body parts that are safe
   to silently delete when a system of lesser capability receives a
   message.  In addition, critical content can help the gateway chose
   the notification strategy for the receiving system.  Likewise, if
   the sender expects the destination to do some processing on a body
   part, critical content allows the sender to mark body parts that the
   receiver must process.

                         Expires October 2002                 [Page 1]

                  Critical Content of Internet Mail        April 2002

Table of Contents

1. Abstract...........................................................1
2. Conventions used in this document..................................2
3. Introduction.......................................................3
4. Handling Parameter.................................................3
4.1. REQUIRED.........................................................4
4.2. OPTIONAL.........................................................4
4.3. Default Values...................................................4
4.4. Other Values.....................................................5
5. Collected Syntax...................................................5
6. Notification.......................................................5
6.1. DSN vs. MDN Generation...........................................6
6.2. Summary..........................................................6
7. Status Code........................................................7
8. Requirements for Critical Content..................................8
8.1. Needs............................................................8
8.2. Current Approaches...............................................9
9. The Content Gateway...............................................10
9.1. Integrated Content Gateway......................................10
9.2. Disaggregated Delivery Network..................................11
10. Backward Compatibility Considerations............................11
11. MIME Interactions................................................11
11.1. multipart/alternative..........................................11
11.2. multipart/related..............................................12
11.3. message/rfc822.................................................12
12. Implementation Examples..........................................12
12.1. Content Gateways...............................................12
12.2. Disaggregated Content Gateway..................................13
13. Security Considerations..........................................14
14. IANA Considerations..............................................14
15. References.......................................................15
16. Acknowledgments..................................................16
17. Author's Address.................................................17

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",
   this document are to be interpreted as described in RFC 2119 [2].

Burger                   Expires October 2002                 [Page 2]

                  Critical Content of Internet Mail        April 2002

   NOTE: Notes, such as 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. Introduction

   The specification of Critical Content is small and compact.  For the
   benefit of developers, the specification comes first, the rationale

   One concept that an implementer must understand is the content
   gateway.  Section Error! Reference source not found. describes the
   content gateway.  In brief, a content gateway has knowledge of the
   receiving system's capabilities.  The content gateway passes
   messages the receiving system can process, render or store.  The
   content gateway can modify a message, for example by deleting
   unrenderable or storable body parts, for delivery to the receiving
   system.  Finally, the content gateway can reject a message that the
   receiving system cannot handle.

   In this document, the "sending agent" is the originator of the
   message.  It could be a mail user agent (MUA) for an Internet
   message, or a SIP User Agent Client (UAC) for a SIP [3] message.

   NOTE: This document updates RFC 3204 to separate the Handling
   parameter from the ISUP/QSIG transport mechanism.  Future versions
   of RFC 3204 should reference this document for the Handling
   parameter, as it is orthogonal to the tunneling of signaling.

4. Handling Parameter

   The Handling parameter is a Content-Disposition [5] parameter
   inserted by the sending agent to indicate to the content gateway
   whether to consider the marked body part critical.

   A REQUIRED body part is one the sender requires the receiving system
   to deliver for him to consider the message delivered.

   An OPTIONAL body part is one the sender doesn't care whether the
   receiving system delivers it or not.  A content gateway can silently
   delete such body parts if the receiving system cannot deliver the

Burger                   Expires October 2002                 [Page 3]

                  Critical Content of Internet Mail        April 2002

   The terms "entity" and "body part" have the meanings defined in
   [Error! Bookmark not defined.].


   "Handling=REQUIRED" signifies that this body part is critical to the

   If the content gateway cannot pass a body part marked REQUIRED, then
   the entire message has failed.  In this case, the content gateway
   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 silently discard the message.  In addition, as a general MIME
   parameter, the MIME body part may not be in an Internet Mail
   message.  Moreover, in the SIP case, the appropriate notification is
   a status return code, not a delivery notification.


   "Handling=OPTIONAL" signifies that the sender does not care about
   notification reports for this body part.

   If the content gateway cannot pass a body part marked OPTIONAL, the
   receiving system may silently delete the body part.  The receiving
   system MUST NOT return a delivery failure, unless parts marked
   REQUIRED have also failed.

4.3. Default Values

   The default value for Handling for a given body part is REQUIRED.
   This enables the existing notification mechanisms to work for
   sending agents that do not know about the content notification
   entity.  All body parts are critical, because they have the default
   marking of REQUIRED.

   NOTE: In the case of Internet mail, critical content processing is a
   function of the content gateway and not the mail transfer agent
   (MTA) or user agent (UA).  Often, the entity performing content
   gateway processing is the receiving UA.  However, in this case the
   UA is acting as a content gateway.  Thus the default action for any
   Content-Disposition [Error! Bookmark not defined.]-compliant user
   agent to ignore unrecognized disposition parameters ensures that
   this mechanism is compatible with the Internet architecture.

   NOTE: This parameter is fully backwards compatible and works as
   expected for Internet mail.  It is not backwards compatible for
   RFC 2543 SIP, but does work for RFC 2543bis SIP [6].   This is
   because the sense of failure is opposite.  When a SIP UAC issues a
   request with a MIME body part marked OPTIONAL, it does not expect

Burger                   Expires October 2002                 [Page 4]

                  Critical Content of Internet Mail        April 2002

   the SIP UAS to fail the message if the UAS cannot process the body
   part.  However, only a RFC2543bis-complient UAS will understand the
   Handling parameter.  Non-RFC 3204-complient UAS will return a 415 if
   it does not understand the MIME body part.

   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.

4.4. Other Values

   The content gateway MUST treat unrecognized values as REQUIRED.
   This is to provide backward compatibility with future uses of the
   Content-Criticality entity.

   NOTE: A possible 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, but
   they do want notification of the failure.  However, as no
   implementations do IMPORTANT, it is not important to this version of
   this document.

5. Collected Syntax

   The format of the collected syntax is in accordance with the ABNF of
   [7].  Note that per RFC 2183 [Error! Bookmark not defined.], the
   HANDLING Content-Disposition parameter is not case sensitive.  In
   addition, the notification-type is not case sensitive.

        "handling" "=" notification-type CRLF

        notification-type = "REQUIRED" / "OPTIONAL" /
                            other-handling / generic-param

        other-handling    =  token

6. Notification

   One obvious application of critical content is generating a
   (non-)delivery notification in the Internet mail environment.  If
   the value of the field is OPTIONAL, the content gateway MUST NOT
   generate a notification.  If the value of the field is REQUIRED, the
   content gateway MAY generate a notification, based on the normal
   notification request mechanisms.  Normal notification request
   mechanisms include the SMTP RCPT NOTIFY command [8] and the
   Disposition-Notification-To header [Error! Bookmark not defined.].

Burger                   Expires October 2002                 [Page 5]

                  Critical Content of Internet Mail        April 2002

   In SIP, all requests have responses.  These responses provide
   notification in the status code of the response.  For the RFC 3204
   case, a content gateway generates a 415 (Unsupported Media Type)
   response if the field is REQURED.

   If the sending system requests a notification, and a REQUIRED part
   fails, the content gateway will generate a notification for the
   whole message.  Conversely, if the gateway cannot pass on a body
   part marked OPTIONAL, the gateway will not generate a notification.

   NOTE: This implies that the content gateway must examine the entire
   message to determine whether it needs to generate a notification.
   However, the content gateway need not examine the message if it
   knows it can store and forward all media types.  Said differently,
   Internet e-mail MTAs or gateways 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 content gateway, in this
   example, would be scanning for non-renderable body parts in any

6.1. DSN vs. MDN Generation

   The content gateway generates a delivery status notification (DSN)
   [9] if it operates as a gateway.  The content gateway generates a
   Message Disposition Notification (MDN) [10] if it operates as a mail
   user agent.  Section 7 describes the operating modes of a content
   gateway.  In short, if there is a MTA that "delivers" the message to
   the content gateway for processing, the MTA takes responsibility for
   DSN processing.  In this case, the only option available to the
   content gateway is to generate MDNs.  If the content gateway
   operates as a MTA, then it generates DSNs.  DSN generation is the
   preferred option.

   If the content gateway is part of a SIP endpoint, then it generates
   the appropriate success or error response code.

6.2. Summary

   The following table summarizes the actions expected of a conforming
   content gateway.

   NOTE: This section is normative: it suggests what to put into the
   DSN or MDN.

   NOTE: In the case of SIP, this section is informative.  See RFC 3204
   for the normative set of actions on failure.

Burger                   Expires October 2002                 [Page 6]

                  Critical Content of Internet Mail        April 2002

                            |    Sending UA Has Marked Body Part   |
                            |      REQUIRED       |    OPTIONAL    |
       | Body Part is       |                     |                |
       | Deliverable        | Appropriate Action  |     ignore     |
       | Body Part is       |                     |                |
       | Undeliverable      | Fail Entire Message |     ignore     |

   The "Appropriate Action" is the action the content gateway would
   take given the context of execution.  For example, if a sender
   requests return receipt and the receiver reads a HANDLING body part,
   the receiving UA must generate the appropriate MDN (following the
   rules for MDN).  Likewise, if the content gateway cannot deliver the
   body part and the body part is critical, the content gateway
   generates the appropriate DSN or MDN.

   "Optional" means the content gateway ignores the disposition of the
   body part.  The content gateway treats the message as if the body
   part was not present in the message.

7. Status Code

   The critical content indication, in itself, does not guarantee any
   notification.  Notification follows the rules described in [Error!
   Bookmark not defined.], [Error! Bookmark not defined.], and [Error!
   Bookmark not defined.].

   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
   code as enumerated in [11].  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 should
   use the status code 5.6.1, "Media not supported", from [Error!
   Bookmark not defined.].

   For the SIP case, all requests have notification provided by the
   status response message.  Per RFC 3204, a content gateway generates
   a 415 (Unsupported Media Type) response.

Burger                   Expires October 2002                 [Page 7]

                  Critical Content of Internet Mail        April 2002

8. Requirements for Critical Content

8.1. Needs

   The need for a critical content identification mechanism comes about
   because of the internetworking of Internet mail systems with
   messaging systems that do not fulfill all of the semantics of
   Internet mail.  Such legacy systems have a limited ability to render
   or store 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 [12] enable the user agents 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 [13], 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 with 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                   Expires October 2002                 [Page 8]

                  Critical Content of Internet Mail        April 2002

   or it didn't.  There is no concept of a system partially delivering
   a message.

   In addition, there are many situations where the sender would not
   mind if the system did not deliver non-critical parts of a message.
   For example, the sender's user agent may add body parts to a message
   unbeknownst to the sender.  If the receiving system rejected the
   message because it could not render a hidden body part, 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.

8.2. Current Approaches

   One method of indicating critical content of a message is to define
   a profile.  The profile defines rules for silently deleting mail
   body parts based on knowledge of the UA capabilities.  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 [14].

   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 to indicate a problem
   with delivering a given body part.  However, this requires the user
   request a delivery notification.  In addition, the sender may not be
   aware of parts added by the sending user agent.  In this case, a
   failure notice would mystify the sender.

   A straightforward alternative implementation method for marking a
   body part critical is to use a Critical-Content MIME entity.  This
   has the benefit that criticality is meta information for the body
   part.  However, IMAP servers in particular would need to either put
   Critical-Content into the BODYSTRUCTURE method or create a new
   method to retrieve arbitrary MIME entities.  Given the experience of
   trying to get Content-Location accepted by IMAP vendors, we chose
   not to go that route.

   What we need is a way of letting the sender indicate what body-parts
   he considers to be critical.  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.

Burger                   Expires October 2002                 [Page 9]

                  Critical Content of Internet Mail        April 2002

9. The Content Gateway

   In this section, we use the definition of [15] for the term

   A content gateway is a gateway that connects a first network to a
   second network.  The second network often has lesser capability than
   the first network.  The canonical topology follows.  "[MTA]", with
   square brackets, signifies an optional component.

   +---------+     +-----+   |         |     +-------+   +-----------+
   | Sending |=...=|[MTA]|===| Content |=...=| [MTA] |===| Receiving |
   |   UA    |     +-----+   | Gateway |     +-------+   |    UA     |
   +---------+               |         |                 +-----------+
          First Network                         Second Network

   The content gateway can be the last hop before the receiving MTA.
   The content gateway can be between networks, and thus not the last
   hop before the receiving MTA.  The content gateway can be the first
   MTA the sending UA contacts.  Finally, the content gateway can be an
   integrated component of the receiving MTA.

   For the SIP case, consider each MTA as a SIP Proxy, the Sending UA
   as a SIP User Agent Client, and the Receiving UA as a SIP User Agent

9.1. Integrated Content Gateway

   In this situation, the receiving user agent is integrated with the
   content gateway. The integrated content gateway knows the
   capabilities of the user agent.  The topology is as follows.

   +---------+     +-----+   |         :           |
   | Sending |=...=|[MTA]|===| Content : Receiving |
   |   UA    |     +-----+   | Gateway :    UA     |
   +---------+               |         :           |
          First Network           Second Network

   The processing of ISUP and QSIG objects, as described in [Error!
   Bookmark not defined.], is an example of an integrated gateway.

Burger                   Expires October 2002                [Page 10]

                  Critical Content of Internet Mail        April 2002

9.2. Disaggregated Delivery Network

   A degenerate case, although one that does occur, is where the
   content gateway sits behind the final MTA.  This happens when one
   implements the content gateway as a post-processing step to a normal
   delivery.  For example, one could configure a mail handling system
   to deliver the message to a queue or directory, where the content
   gateway process picks up the message.  If there were any directives
   for DSN processing, the delivering MTA would execute them.  For
   example, the message could have requested notification on successful
   delivery.  The delivering MTA, having delivered the message to the
   queue, would consider the message delivered and thus notify the
   sender of such.  However, the content gateway process could then
   discover that the receiving UA cannot render the message.  In this
   case, the content gateway generates a NDN, as it is the only option

                               |      +---------+
   +---------+     +-----+     v      |         |     +-----------+
   | Sending |=...=| MTA |--> File -->| Content |=...=| Receiving |
   |   UA    |     +-----+            | Gateway |     |    UA     |
   +---------+                        |         |     +-----------+
          First Network              Second Network

10. Backward Compatibility Considerations

   DSN requires ESMTP.  If MTAs in the path from the sending UA to the
   receiving UA do not support ESMTP, then that MTA will reject the DSN
   request.  In addition, the message will default to notification on
   delay or failure.  While not ideal, the sender will know that DSN is
   not available, and that critical content that fails will get

11. MIME Interactions

11.1. multipart/alternative

   As is true for all Content-Disposition parameters, handling 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 HANDLING=OPTIONAL, then the content gateway MUST NOT
   generate any delivery notifications.

   NOTE: This statement explicitly shows that HANDLING overrides the
   DSN and MDN request mechanisms.

Burger                   Expires October 2002                [Page 11]

                  Critical Content of Internet Mail        April 2002

   It is unlikely for a selected alternative to fail, as the content
   gateway presumably picks the alternative specifically because it can
   render it.

   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
   HANDLING mechanism described in this document.

   NOTE: This means that a forwarded message's criticality will not
   affect the forwarding agent's intentions.

11.2. multipart/related

   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.

11.3. message/rfc822

   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
   criticality indicators in embedded body parts.  This avoids the
   situation of a forwarded message triggering or suppressing undesired
   reporting.  This simply implements the procedures described in
   [Error! Bookmark not defined.].

12. Implementation Examples

   This section is not a normative part of the definition of
   Criticality.  However, we hope it helps implementers to understand
   the mechanics of the Handling mechanism.

   We will examine two cases.  They are how a content gateway processes
   a message and how a disaggregated content gateway processes a

12.1. 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

Burger                   Expires October 2002                [Page 12]

                  Critical Content of Internet Mail        April 2002

   text part and a tnef part.  The sender marks the text part REQUIRED.
   The gateway, knowing the capability of the short message service,
   silently deletes the non-critical, tnef part, passing the critical
   content to the short 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
   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 [16].

   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 [17] (part 3).  The
   sender marks the first two parts REQUIRED.  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, acting as
   the receiving MTA, will reject the message, generating a DSN with
   the status code 5.6.1, "Media not supported".

12.2. Disaggregated Content Gateway

   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 REQUIRED.  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

   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 REQUIRED, 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 REQUIRED, 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

Burger                   Expires October 2002                [Page 13]

                  Critical Content of Internet Mail        April 2002

   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
   undeliverable body part.

   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)".

13. Security Considerations

   Receiving systems and users should not place any authentication
   value on the Handling parameter.

14. IANA Considerations

   RFC 3204 already registered the Handling parameter.  It is collected
   here only for reference and as a placeholder for use both for
   further expansion in the future and as the normative reference for
   other documents that need to reference the Handling parameter.

   Per section 9 of [Error! Bookmark not defined.], here is the IANA
   registration for Handling.

   Subject: Registration of new Content-Disposition parameter

   Content-Disposition parameter name:

   Allowable values for this parameter:

   Marks the body part as required for delivery (REQUIRED) or can be
   silently discarded (OPTIONAL).  See RFC <this document> and RFC 3204.
   Per RFC 2183, the Content-Disposition parameter name is not case
   sensitive.  Per RFC <this document>, the values of the parameter are
   also not case sensitive.

Burger                   Expires October 2002                [Page 14]

                  Critical Content of Internet Mail        April 2002

15. 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  Handley, M., Schulzrinne, H., Schooler, E., and Rosenberg, J.,
      "SIP: Session Initiation Protocol", RFC 2543, March 1999.

   4  Zimmerer, E., et. al., "MIME media types for ISUP and QSIG
      Objects", RFC 3204, December 2001.

   5  Troost, R., Dorner, S., Moore, K. (ed), "Communicating
      Presentation Information in Internet Messages: The Content-
      Disposition Header Field", RFC 2183, August 1997.

   6  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
      Peterson, J., Sparks, R., Handley, M., and Schooler, E., "SIP:
      Session Initiation Protocol", draft-ietf-sip-rfc2543bis-09.txt,
      February 2002, work in progress.

   7  Crocker, D. and Overell, P.(Editors), "Augmented BNF for Syntax
      Specifications: ABNF", RFC 2234, November 1997.

   8  Moore, K., "SMTP Service Extension for Delivery Status
      Notifications", RFC 1981, January 1996.

   9  Moore, K. and Vaudreuil, G., "An Extensible Message Format for
      Delivery Status Notifications", RFC 1894, January 1996.

   10  Fajman, R., "An Extensible Message Format for Message
      Disposition Notifications", RFC 2298, March 1998.

   11 Vaudreuil, G., "Enhanced Mail System Status Codes", RFC 1893,
      January 1996.

   12 Freed, N. and Borenstein, N., "Multipurpose Internet Mail
      Extensions (MIME) Part One: Format of Internet Message Bodies",
      RFC 2045, November 1996.

   13 Freed, N. and Borenstein, N., "Multipurpose Internet Mail
      Extensions (MIME) Part Two: Media Types", RFC 2046, November

   14 Vaudreuil, G. and Parsons, G., "Voice Profile for Internet Mail -
      version 2", RFC 2421, September 1998.

Burger                   Expires October 2002                [Page 15]

                  Critical Content of Internet Mail        April 2002

   15 Kille, S. "MIXER (Mime Internet X.400 Enhanced Relay): Mapping
      between X.400 and RFC 822/MIME", RFC 2156, January 1998.

   16 Crocker, D., "Standard for the Format of ARPA Internet Text
      Messages", RFC 822, August 1982.

   17 Dawson, F. and Howes, T., "vCard MIME Directory Profile", RFC
      2426, September 1998.

16. Acknowledgments

   Emily Candell of Comverse Network Systems was instrumental in
   helping work out the base issues in the รป00 draft in Adelaide.

   Ned Freed pointed out that this mechanism was about criticality, not
   notification.  That insight made the concept and descriptions
   infinitely more straightforward.  If it's still confusing, it's my

   Keith Moore for helped tighten-up the explanations, and he approved
   of the use of Content-Disposition.

   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 some implementation

   Greg White asked THE key question that let us realize that critical
   content processing was a gateway function, and not a MTA or UA

   Jon Peterson cleared up how handling actually does work in the SIP

   An enormous thank you to Michelle S. Cotton at IANA for helping me
   craft the original IANA Considerations section in 2000, and for
   catching the functional overlap with RFC 3204 this January.

   Any errors, omissions, or silliness are my fault.

Burger                   Expires October 2002                [Page 16]

                  Critical Content of Internet Mail        April 2002

17. Author's Address

   Eric Burger
   SnowShore Networks, Inc.
   285 Billerica Rd.
   Chelmsford, MA  01824-4120

   Phone: +1 978 367 8403
   Fax:   +1 603 457 5944

Burger                   Expires October 2002                [Page 17]

                  Critical Content of Internet Mail        April 2002

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

   Copyright (C) 2000, 2001, 2002, The Internet Society.  All Rights

   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

Burger                   Expires October 2002                [Page 18]