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rfc:rfc3297

Network Working Group G. Klyne Request for Comments: 3297 Clearswift Corporation Category: Standards Track R. Iwazaki

                                                           Toshiba TEC
                                                            D. Crocker
                                           Brandenburg InternetWorking
                                                             July 2002
     Content Negotiation for Messaging Services based on Email

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

 This memo describes a content negotiation mechanism for facsimile,
 voice and other messaging services that use Internet email.
 Services such as facsimile and voice messaging need to cope with new
 message content formats, yet need to ensure that the content of any
 given message is renderable by the receiving agent.  The mechanism
 described here aims to meet these needs in a fashion that is fully
 compatible with the current behaviour and expectations of Internet
 email.

Table of Contents

 1. Introduction................................................... 3
   1.1 Structure of this document ................................. 4
   1.2 Document terminology and conventions ....................... 4
      1.2.1 Terminology............................................ 4
      1.2.2 Design goals........................................... 5
      1.2.3 Other document conventions............................. 5
 2. Background and goals........................................... 5
   2.1 Background ................................................. 5
      2.1.1 Fax and email.......................................... 5
      2.1.2 Current facilities in Internet Fax..................... 6
   2.2 Closing the loop ........................................... 6

Klyne, et. al. Standards Track [Page 1] RFC 3297 Content Negotiation for Messaging Services July 2002

   2.3 Goals for content negotiation .............................. 8
 3. Framework for content negotiation..............................10
   3.1 Send data with an indication of alternatives ...............11
      3.1.1 Choice of default data format..........................12
      3.1.2 MDN request indicating alternate data formats..........12
      3.1.3 Information about alternative data formats.............13
   3.2 Receiver options ...........................................14
      3.2.1 Alternatives not recognized............................14
      3.2.2 Alternative not desired................................14
      3.2.3 Alternative preferred..................................14
   3.3 Send alternative message data ..............................16
   3.4 Confirm receipt of resent message data .....................17
 4. The Content-alternative header.................................18
 5. The Original-Message-ID message header.........................18
 6. MDN extension for alternative data.............................19
   6.1 Indicating readiness to send alternative data ..............19
   6.2 Indicating a preference for alternative data ...............20
   6.3 Indicating alternative data is no longer available .........21
   6.4 Indicating loss of original data ...........................22
   6.5 Automatic sending of MDN responses .........................22
 7. Internet Fax Considerations....................................22
 8. Examples.......................................................23
   8.1 Sending enhanced Internet Fax image ........................23
   8.2 Internet fax with initial data usable ......................27
   8.3 Negotiate to lower receiver capability .....................28
   8.4 Sending an alternative content type ........................32
 9. IANA Considerations............................................36
   9.1 New message headers ........................................36
   9.2 MDN extensions .............................................36
      9.2.1 Notification option 'Alternative-available'............36
      9.2.2 Notification option 'Alternative-not-available'........36
      9.2.3 Disposition modifier 'Alternative-preferred'...........37
      9.2.4 Disposition modifier 'Original-lost'...................37
 10. Internationalization considerations...........................37
 11. Security Considerations.......................................37
 12. Acknowledgements..............................................38
 13. References....................................................38
 Appendix A: Implementation issues.................................40
   A.1 Receiver state .............................................40
   A.2 Receiver buffering of message data .........................41
   A.3 Sender state ...............................................42
   A.4 Timeout of offer of alternatives ...........................42
   A.5 Timeout of receiver capabilities ...........................42
   A.6 Relationship to timely delivery ............................43
   A.7 Ephemeral capabilities .....................................43
   A.8 Situations where MDNs must not be auto-generated ...........44
 Appendix B: Candidates for further enhancements...................44
 Authors' Addresses................................................45

Klyne, et. al. Standards Track [Page 2] RFC 3297 Content Negotiation for Messaging Services July 2002

 Full Copyright Statement..........................................46

1. Introduction

 This memo describes a mechanism for email based content negotiation
 which provides an Internet fax facility comparable to that of
 traditional facsimile, which may be used by other messaging services
 that need similar facilities.
 "Extended Facsimile using Internet Mail" [1] specifies the transfer
 of image data using Internet email protocols.  "Indicating Supported
 Media Features Using Extensions to DSN and MDN" [2] describes a
 mechanism for providing the sender with the details of a receiver's
 capabilities.  The capability information thus provided, if stored by
 the sender, can be used in subsequent transfers between the same
 sender and receiver.
 Many communications are one-off or infrequent transfers between a
 given sender and receiver, and cannot benefit from this "do better
 next time" approach.
 An alternative facility available in email (though not widely
 implemented) is for the sender to use 'multipart/alternative' [15] to
 send a message in several different formats, and allow the receiver
 to choose.  Apart from the obvious drawback of network bandwidth use,
 this approach does not of itself allow the sender to truly tailor its
 message to a given receiver, or to obtain confirmation that any of
 the alternatives sent was usable by the receiver.
 This memo describes a mechanism that allows better-than-baseline data
 formats to be sent in the first communication between a sender and
 receiver.  The same mechanism can also achieve a usable message
 transfer when the sender has based the initial transmission on
 incorrect information about the receiver's capabilities.  It allows
 the sender of a message to indicate availability of alternative
 formats, and the receiver to indicate that an alternative format
 should be provided to replace the message data originally
 transmitted.
 When the sender does not have the correct information about a
 receiver's capabilities, the mechanism described here may incur an
 additional message round trip.  An important goal of this mechanism
 is to allow enough information to be provided to determine whether or
 not the extra round trip is required.

Klyne, et. al. Standards Track [Page 3] RFC 3297 Content Negotiation for Messaging Services July 2002

1.1 Structure of this document

 The main part of this memo addresses the following areas:
 Section 2 describes some of the background, and sets out some
 specific goals that are addressed in this specification.
 Section 3 describes the proposed content negotiation framework,
 indicating the flow of information between a sender and receiver.
 Section 4 contains a detailed description of the 'Content-
 alternative' header that is used to convey information about
 alternative available formats.  This description is intended to stand
 independently of the rest of this specification, with a view to being
 usable in conjunction with other content negotiation protocols.
 Section 5 describes a new mail message header, 'Original-Message-ID',
 which is used to correlate alternative data sent during negotiation
 with the original message data, and to distinguish the continuation
 of an old message transaction from the start of a new transaction.
 Section 6 describes extensions to the Message Disposition
 Notification (MDN) framework [4] that support negotiation between the
 communicating parties.

1.2 Document terminology and conventions

1.2.1 Terminology

 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 [22].
 Capability exchange
    An exchange of information between communicating parties
    indicating the kinds of information they can generate or consume.
 Capability identification
    Provision of information by the a receiving agent that indicates
    the kinds of message data that it can accept for presentation to a
    user.
 Content negotiation
    An exchange of information (negotiation metadata) which leads to
    selection of the appropriate representation (variant) when
    transferring a data resource.

Klyne, et. al. Standards Track [Page 4] RFC 3297 Content Negotiation for Messaging Services July 2002

 Message transaction
    A sequence of exchanges between a message sender and receiver that
    accomplish the transfer of message data.
 RFC 2703 [17] introduces several other terms related to content
 negotiation.

1.2.2 Design goals

 In discussing the goals for content negotiation, {1}, {2}, {3}
 notation is used, per RFC 2542, "Terminology and Goals for Internet
 Fax" [3].  The meanings associated with these notations are:
 {1}   there is general agreement that this is a critical
       characteristic of any definition of content negotiation for
       Internet Fax.
 {2}   most believe that this is an important characteristic of
       content negotiation for Internet Fax.
 {3}   there is general belief that this is a useful feature of
       content negotiation for Internet Fax, but that other factors
       might override;  a definition that does not provide this
       element is acceptable.

1.2.3 Other document conventions

 NOTE:  Comments like this provide additional nonessential information
 about the rationale behind this document.  Such information is not
 needed for building a conformant implementation, but may help those
 who wish to understand the design in greater depth.

2. Background and goals

2.1 Background

2.1.1 Fax and email

 One of the goals of the work to define a facsimile service using
 Internet mail has been to deliver benefits of the traditional Group 3
 Fax service in an email environment.  Traditional Group 3 Fax leans
 heavily on the idea that an online exchange of information discloses
 a receiver's capabilities to the sender before any message data is
 transmitted.

Klyne, et. al. Standards Track [Page 5] RFC 3297 Content Negotiation for Messaging Services July 2002

 By contrast, Internet mail has been developed to operate in a
 different fashion, without any expectation that the sender and
 receiver will exchange information prior to message transfer.  One
 consequence of this is that all mail messages must contain some kind
 of meaningful message data:  messages that are sent simply to elicit
 information from a receiving message handling agent are not generally
 acceptable in the Internet mail environment.
 To guarantee some level of interoperability, Group 3 Fax and Internet
 mail rely on all receivers being able to deal with some baseline
 format (i.e., a basic image format or plain ASCII text,
 respectively).  The role of capability exchange or content
 negotiation is to permit better-than baseline capabilities to be
 employed where available.
 One of the challenges addressed by this specification is how to adapt
 the email environment to provide a fax-like service.  A sender must
 not make any a priori assumption that the receiver can recognize
 anything other than a simple email message.  There are some important
 uses of email that are fundamentally incompatible with the fax model
 of message passing and content negotiation (notably mailing lists).
 So we need to have a way of recognizing when content negotiation is
 possible, without breaking the existing email model.

2.1.2 Current facilities in Internet Fax

 "Extended Facsimile using Internet Mail" [1] provides for a limited
 provision of receiver capability information to the sender of a
 message, using an extension to Message Disposition Notifications
 [2,4], employing media feature tags [5] and media feature expressions
 [6].
 This mechanism provides for receiver capabilities to be disclosed
 after a message has been received and processed.  This information
 can be used for subsequent transmissions to the same receiver.  But
 many communications are one-off messages from a given sender to a
 given receiver, and cannot benefit from this.

2.2 Closing the loop

 Classic Internet mail is an "open loop" process:  no information is
 returned back to the point from which the message is sent.  This has
 been unkindly --but accurately-- characterized as "send and pray",
 since it lacks confirmation.
 Sending a message and obtaining confirmation that the message has
 been received is a "closed loop" process:  the confirmation sent back
 to the sender creates a loop around which information is passed.

Klyne, et. al. Standards Track [Page 6] RFC 3297 Content Negotiation for Messaging Services July 2002

 Many Internet email agents are not designed to participate in a
 closed loop process, and thus have no responsibility to respond to
 receipt of a message.  Later additions to Internet standards, notably
 Delivery Service Notification [18] and Message Disposition
 Notification [4], specify means for certain confirmation responses to
 be sent back to the sender, thereby closing the loop.  However
 conformance to these enhancements is optional and full deployment is
 in the future.
 DSN must be fully implemented by the entire infrastructure; further
 when support is lacking, the message is still sent on in open-loop
 fashion.  Sometimes, transmission and delivery should instead be
 aborted and the fact be reported to the sender.
 Due to privacy considerations for end-users, MDN usage is entirely
 voluntary.
 Content negotiation is a closed loop function (for the purposes of
 this proposal -- see section 2.3, item (f)), and requires that the
 recipient of a message make some response to the sender.  Since
 content negotiation must retro-fit a closed-loop function over
 Internet mail's voluntary and high-latency environment, a challenge
 for content negotiation in email is to establish that consenting
 parties can recognize a closed loop situation, and hence recognize
 their responsibilities to close the loop.
 Three different loops can be identified in a content negotiation:
            Sender                      Receiver
              |                             |
       Initial message ------>------------  v
              |                             |
             (1) ------------<--- Request alternative data
              |                             |
      Send alternative ------>------------ (2)
              |                             |
             (3) ------------<------ Confirm receipt
                                     of usable data
 (1)   Sender receives acknowledgement that negotiable content has
       been received
 (2)   Receiver receives confirmation that its request for data has
       been received.
 (3)   Sender receives confirmation that received data is processable,
       or has been processed.

Klyne, et. al. Standards Track [Page 7] RFC 3297 Content Negotiation for Messaging Services July 2002

 Although the content negotiation process is initiated by the sender,
 it is not established until loop (1) is closed with an indication
 that the receiver desires alternative content.
 If content sent with the original message from the sender is
 processable by the receiver, and a confirmation is sent, then the
 entire process is reduced to a simple send/confirm loop:
                Sender                      Receiver
                  |                             |
           Initial message ------>------------  v
                  |                             |
                 (3) ------------<------ Confirm receipt
                                         of usable data

2.3 Goals for content negotiation

 The primary goal {1} is to provide a mechanism that allows arbitrary
 enhanced content features to be used with Internet fax systems.  The
 mechanism should {2} support introduction of new features over time,
 particularly those that are adopted for Group 3 fax.
 Further goals are:
 (a)   Must {1} interwork with existing simple mode Internet fax
       systems.
 (b)   Must {1} interwork with existing email clients.
       The term "interwork" used above means that the mechanism must
       be introduced in a way that may be ignored by existing systems,
       and systems enhanced to use the negotiation mechanisms will
       behave in a fashion that is expected by existing systems.
       (I.e., existing clients are not expected in any way to
       participate in or be aware of content negotiation.)
 (c)   Must {1} avoid transmission of "administrative non messages".
       (I.e., only messages that contain meaningful content for the
       end user may be sent unless it is known that the receiving
       system will interpret them, and not attempt to display them.)
       This requirement has been stated very strongly by the email
       community.
       This means that a sender must not assume that a receiver can
       understand the capability exchange protocol elements, so must
       always start by sending some meaningful message data.

Klyne, et. al. Standards Track [Page 8] RFC 3297 Content Negotiation for Messaging Services July 2002

 (d)   Avoid {1} multiple renderings of a message.  In situations
       where multiple versions of a message are transferred, the
       receiver must be able to reliably decide on a single version to
       be displayed.
 (e)   Minimize {2} round trips needed to complete a transmission.
       Ideally {3} every enhanced transmission will result in simply
       sending data that the recipient can process, and receiving a
       confirmation response.
 (f)   The solution adopted should not {3} transmit multiple versions
       of the same data.  In particular, it must not {1} rely on
       routinely sending multiple instances of the same data in a
       single message.
       This does not prohibit sending multiple versions of the same
       data, but it must not be a requirement to do so.  A sender may
       choose to send multiple versions together (e.g., plain text and
       some other format), but the capability exchange mechanism
       selected must not depend on such behaviour.
 (g)   The solution adopted should {2} be consistent with and
       applicable to other Internet email based applications; e.g.,
       regular email, voice messaging, unified messaging, etc.
 (h)   Allow for a graceful recovery from stale cache information.  A
       sender might use historic information to send non-baseline data
       with an initial message.  If this turns out to be unusable by
       the recipient, it should still be possible {3} for the baseline
       data, or some other acceptable format, to be selected and
       transferred.
 (i)   The mechanism defined should {2} operate cleanly in conjunction
       with the mechanisms already defined for extended mode Internet
       fax (extended DSN and MDN [2], etc.).
 (j)   As much as possible, existing email mechanisms should {3} be
       used rather than inventing new ones.  (It is clear that some
       new mechanisms will be needed, but they should be defined
       cautiously.)
 (k)   The mechanism should {2} be implementable in low memory
       devices.  That is, it should not depend on any party being able
       to buffer arbitrary amounts of message data.

Klyne, et. al. Standards Track [Page 9] RFC 3297 Content Negotiation for Messaging Services July 2002

       (It may not be possible to completely satisfy this goal in a
       sending system.  But if the sender does not have enough memory
       to buffer some given message, it can choose to not offer
       content negotiation.)

3. Framework for content negotiation

 This section starts with an outline of the negotiation process, and
 provides greater detail about each stage in following sub-sections.
 1. Sender sends initial message data with an indication of
    alternative formats available (section 3.1).  Initial data MAY be
    a baseline or some other guess of what the recipient can handle.
 2. The receiver has three main options:
    (a)   Does not recognize the optional alternative formats, and
          passively accepts the data as sent (section 3.2.1).
    (b)   Does recognize the alternatives offered, and actively
          accepts the data as sent (section 3.2.2).
    (c)   Recognizes the alternatives offered, and determines that it
          prefers to receive an alternative format.  An MDN response
          is sent (i) indicating that the original data was not
          processed, and (ii) containing receiver capability
          information so that the sender may select a suitable
          alternative (section 3.2.3).
          Note that only recipients named in 'to:', 'cc:' or 'bcc:'
          headers in the original message may request alternative data
          formats in this way.  Recipients not named in the original
          message headers MUST NOT attempt to initiate content
          negotiation.
          NOTE: the prohibition on initiation of negotiation by
          recipients other than those explicitly addressed is to avoid
          the sender from having to deal with negotiation requests
          from unexpected parties.
 3. On receipt of an MDN response indicating preference for an
    alternative data format, the sender MUST select and transmit
    message data matched to the receiver's declared capabilities, or
    send an indication that the receiver's request cannot be honoured.
    When sending alternative data, the sender suppresses the
    indication that alternative data is available, so the negotiation
    process cannot loop.

Klyne, et. al. Standards Track [Page 10] RFC 3297 Content Negotiation for Messaging Services July 2002

 4. On receipt of final data from the sender, the receiver sends an
    MDN response indicating acceptance (or otherwise) of the data
    received.
       NOTE:  the receiver does not choose the particular data format
       to be received;  that choice rests with the sender.  We find
       that this approach is simpler than having the receiver choose
       an alternative, because it builds upon existing mechanisms in
       email, and follows the same pattern as a traditional Group 3
       fax.  Further, it deals with situations where the range of
       alternatives may be difficult to describe.
       This approach is similar to server driven negotiation in HTTP
       using "Accept" headers [13].  This is distinct to the agent-
       driven style of negotiation provided for HTTP as part of
       Transparent Content Negotiation [14], or which might be
       constructed in email using "multipart/alternative" and
       "message/external-body" MIME types [15].

3.1 Send data with an indication of alternatives

 A sender that is prepared to provide alternative message data formats
 MUST send the following message elements:
 (a)   a default message data format,
 (b)   message identification, in the form of a Message-ID header.
 (c)   appropriate 'Content-features' header(s) [7] describing the
       default message data sent,
 (d)   a request for Message Disposition Notification [4],
 (e)   an indication that it is prepared to send different message
       data, using an 'Alternative-available' MDN option field [9],
       and
 (f)   an indication of the alternative data formats available, in the
       form of 'Content-alternative' header(s) [8].  Note:  more than
       one Content-alternative' header MAY be specified; see section
       3.1.3 for more information.
 Having indicated the availability of alternative data formats, the
 sender is expected to hold the necessary information for some time,
 allowing the receiver an opportunity to request such data.  But,
 unless it so indicates (see [9]), the sender is not expected to hold
 this information indefinitely;  the exact length of time such
 information should be held is not specified here.  Thus, the

Klyne, et. al. Standards Track [Page 11] RFC 3297 Content Negotiation for Messaging Services July 2002

 possibility exists that a request for alternative information may
 arrive too late, and the sender will then send an indication that the
 data is no longer available.  If message transference is being
 completed within a predetermined time interval (e.g., using [21]),
 then the sender should normally maintain the data for at least that
 period.

3.1.1 Choice of default data format

 The normal default format is text/plain.  This is the format sent
 unless the sender has prior knowledge or expectation of other content
 formats supported by the recipient.  Some uses of email presume some
 other default format (e.g. Intenet fax [1] has TIFF profile S [11] as
 its default format;  see section 7 of this document).
 "Extended Facsimile Using Internet Mail" [1] and "Indicating
 Supported Media Features Using Extensions to DSN and MDN" [2]
 indicate a possible mechanism for a sender to have prior knowledge of
 receiver capabilities.  This specification builds upon the mechanism
 described there.
 As always, the sender may gather information about the receiver in
 other ways beyond the scope of this document (e.g., a directory
 service or the suggested RESCAP protocol).

3.1.2 MDN request indicating alternate data formats

 When a sender is indicating preparedness to send alternative message
 data, it MUST request a Message Disposition Notification (MDN) [4].
 It indicates its readiness to send alternative message data by
 including the MDN option 'Alternative-available' [9] with the MDN
 request.  Presence of this MDN request option simply indicates that
 the sender is prepared to send some different data format if it has
 more accurate or up-to-date information about the receiver's
 capabilities.  Of itself, this option does not indicate whether the
 alternatives are likely to be better or worse than the default data
 sent -- that information is provided by the "Content-alternative"
 header(s) [8].
 When using the 'Alternative-available' option in an MDN request, the
 message MUST also contain a 'Message-ID:' header with a unique
 message identifier.

Klyne, et. al. Standards Track [Page 12] RFC 3297 Content Negotiation for Messaging Services July 2002

3.1.3 Information about alternative data formats

 A sender can provide information about the alternative message data
 available by applying one or more 'Content-alternative' headers to
 message body parts for which alternative data is available, each
 indicating media features [5,6] of an available alternative.
 The purpose of this information is to allow a receiver to decide
 whether any of the available alternatives are preferable, or likely
 to be preferable, to the default message data provided.
 Not every available alternative is required to be described in this
 way, but the sender should include enough information to allow a
 receiver to determine whether or not it can expect more useful
 message data if it chooses to indicate a preference for some
 alternative that matches its capabilities.
 Alternative formats will often be variations of the content-type
 originally sent.  When different content-types can be provided, they
 should be indicated in a corresponding content-alternative header
 using the 'type' media feature tag [24].  (See example 8.4.)
    NOTE:  the sender is not necessarily expected to describe every
    single alternative format that is available -- indeed, in cases
    where content is generated on-the-fly rather than simply selected
    from an enumeration of possibilities, this may be infeasible.  The
    sender is expected to use one or more 'Content-alternative'
    headers to reasonably indicate the range of alternative formats
    available.
    The final format actually sent will always be selected by the
    sender, based on the receiver's capabilities.  The 'Content-
    alternative' headers are provided here simply to allow the
    receiver to make a reasonable decision about whether to request an
    alternative format that better matches its capabilities.
    ALSO NOTE:  this header is intended to be usable independently of
    the MDN extension that indicates the sender is prepared to send
    alternative formats.  It could be used with a different protocol
    having nothing to do with email or MDN.  Thus, the 'Content-
    alternative' header provides information about alternative data
    formats without actually indicating if or how they might be
    obtained.
    Further, the 'Content-alternative' header applies to a MIME body
    part, where the MDN 'Alternative-available' option applies to the
    message as a whole.

Klyne, et. al. Standards Track [Page 13] RFC 3297 Content Negotiation for Messaging Services July 2002

 The example sections of this memo show how the 'Content-features:'
 and 'Content-alternative:' MIME headers may be used to describe the
 content provided and available alternatives.

3.2 Receiver options

 A negotiation-aware system receiving message data without an
 indication of alternative data formats MUST process that message in
 the same way as a standard Internet fax system or email user agent.
 Given an indication of alternative data format options, the receiver
 has three primary options:
 (a)   do not recognize the alternatives:  passively accept what is
       provided,
 (b)   do not prefer the alternatives:  actively accept what is
       provided, or
 (c)   prefer some alternative format.

3.2.1 Alternatives not recognized

 This corresponds to the case that the receiver is a simple mode
 Internet fax recipient [12], or a traditional email user agent.
 The receiver does not recognize the alternatives offered, or chooses
 not to recognize them, and simply accepts the data as sent.  A
 standard MDN response [4] or an extended MDN response [2] MAY be
 generated at the receiver's option.

3.2.2 Alternative not desired

 The receiver does recognize the alternatives offered, but
 specifically chooses to accept the data originally offered.  An MDN
 response SHOULD be sent indicating acceptance of the data and also
 containing the receiver's capabilities.
 This is the same as the defined behaviour of an Extended Internet Fax
 receiver [1,2].

3.2.3 Alternative preferred

 This case extends the behaviour of Extended Internet Fax [1,2] to
 allow an alternative form of data for the current message to be
 transferred.  This option may be followed ONLY if the original
 message contains an 'Alternative-available' MDN option (alternative

Klyne, et. al. Standards Track [Page 14] RFC 3297 Content Negotiation for Messaging Services July 2002

 data re-sends may not use this option).  Further, this option may be
 followed ONLY if the recipient is explicitly addressed in the message
 headers ('to:', 'cc:' or 'bcc:').
 The receiver recognizes that alternative data is available, and based
 on the information provided determines that an alternative format
 would be preferable.  An MDN response [4] is sent, which MUST contain
 the following:
 o  an 'Alternative-preferred' disposition modifier [9] indicating
    that some data format other than that originally sent is
    preferred,
 o  an 'Original-Message-ID:' field [4] with the message identifier
    from the received message, and
 o  receiver capabilities, per RFC 2530 [2].
 On sending such an MDN response, the receiver MAY discard the message
 data provided, in the expectation that some alternative will be sent.
 But if the sender has indicated a limited lifetime for the
 alternative data, and the original data received is within the
 receiver's capability to display, the receiver SHOULD NOT discard it.
 Lacking sufficient memory to hold the original data for a period of
 time within which alternative data would reasonably be received, the
 receiver SHOULD accept and display the original data.  In the case
 that the original data is not within the receiver's capability to
 display then it SHOULD discard the original data and request an
 alternative format.
    NOTE:  the above rules are meant to ensure that the content
    negotiation framework does not result in the loss of data that
    would otherwise be received and displayed.
 Having requested alternative data and not displayed the original
 data, the receiver MUST remember this fact and be prepared to take
 corrective action if alternative data is not received within a
 reasonable time (e.g., if the MDN response or transmission of
 alternative data is lost in transit).
 Corrective action might be any of the following:
 (a)   re-send the MDN response, and continue waiting for an
       alternative,
 (b)   present the data originally supplied (if it is still
       available), or

Klyne, et. al. Standards Track [Page 15] RFC 3297 Content Negotiation for Messaging Services July 2002

 (c)   generate an error response indicating loss of data.
 On concluding that alternative data is not forthcoming, the preferred
 option is (b), but this may not be possible for receivers with
 limited memory.
 See Appendix A for further discussion of receiver behaviour options.
    NOTE:  A cache control indicator on recipient capabilities has
    been considered, but is not included in this specification.
    (Sometimes, a recipient may want to offer certain capabilities
    only under certain circumstances, and does not wish them to be
    remembered for future use; e.g., not wanting to receive colour
    images for routine communications.)
    NOTE:  the receiver does not actually get to select any specific
    data format offered by the sender.  The final choice of data
    format is always made by the sender, based on the receiver's
    declared capabilities.  This approach:
    (a)   more closely matches the style of T.30 content negotiation,
    (b)   provides for clean integration with the current extended
          mode Internet fax specification,
    (c)   builds upon existing email mechanisms in a consistent
          fashion, and
    (d)   allows for cases (e.g., dynamically generated content) where
          it is not feasible for the sender to enumerate the
          alternatives available.

3.3 Send alternative message data

 Having offered to provide alternative data by including an
 'Alternative-available' option with the original MDN request, and on
 receipt of an MDN response indicating 'Alternative-preferred', the
 sender SHOULD transmit alternative message data that best matches the
 receiver's declared capabilities.  (In the exceptional case that the
 response requesting an alternative data format does not contain
 receiver capabilities, a baseline format should be selected.)
 If any part of the best available message data matching the receiver
 capabilities is the same as that originally sent, it MUST still be
 re-transmitted because the receiver may have discarded the original
 data.  Any data sent as a result of receiving an 'Alternative-
 preferred' response should include an MDN request but SHOULD NOT
 include an 'Alternative-available' disposition notification modifier.

Klyne, et. al. Standards Track [Page 16] RFC 3297 Content Negotiation for Messaging Services July 2002

 If the sender is no longer able to send message data for any reason,
 it MUST send a message to the receiver indicating a failed transfer.
 It SHOULD also generate a report for the receiver indicating the
 failure, containing an MDN request and including an 'Alternative-
 not-available' disposition notification modifier.
 Any message sent to a receiver in response to a request for
 alternative data MUST include an 'Original-Message-ID:' header [23]
 containing the Original-message-ID value from the received
 disposition notification message (which is the 'Message-ID:' from the
 original message).  This header serves to correlate the re-send (or
 failure message) with the original message, and also to distinguish a
 re-send from an original message.

3.4 Confirm receipt of resent message data

 When resent data is received (indicated by presence of an 'original-
 message-ID:' header field), the receiver processes that data and
 generates an MDN response indicating the final disposition of the
 data received, and also indicating capabilities that may be used for
 future messages, per RFC 2530 [2] and RFC 2532 [1].
 If the re-send indicates that alternative data is no longer available
 (by including an 'Alternative-not-available' disposition notification
 modifier), and the receiver still holds the original data sent, it
 should display or process the original data and send an MDN response
 indicating the final disposition of that data.  Thus, the response to
 an 'Alternative-not-available' indication may be a successful
 disposition notification.
 If the re-send indicates that alternative data is no longer available
 (by including an 'Alternative-not-available' disposition notification
 modifier), and the receiver has discarded the original data sent, it
 SHOULD:
 (a)   display or process the failure message received, OR
 (b)   construct and display a message indicating that message data
       has been lost, preferably indicating the sender, time, subject,
       message identifier and other information that may help the
       recipient user to identify the missing message.
 and send a message disposition response indicating a final message
 disposition of "deleted".

Klyne, et. al. Standards Track [Page 17] RFC 3297 Content Negotiation for Messaging Services July 2002

4. The Content-alternative header

 The 'Content-alternative:' header is a MIME header that can be
 attached to a MIME body part to indicate availability of some
 alternative form of the data it contains.  This header does not, of
 itself, indicate how the alternative form of data may be accessed.
 Using the ABNF notation of RFC 2234 [10], the syntax of a 'Content-
 alternative' header is defined as:
    Content-alternative-header =
        "Content-alternative" ":" Alternative-feature-expression
    Alternative-feature-expression =
        <As defined for 'Filter' by RFC 2533 [6]>
 More than one 'Content-alternative:' header may be applied to a MIME
 body part, in which case each one is taken to describe a separate
 alternative data format that is available.
 A content-alternative header is used with some MIME-encapsulated
 data, and is interpreted in that context.  The intent is to indicate
 possible variations of that data, and it is not necessarily expected
 to be a complete free-standing description of a specific available
 data.  Enough information should be provided for a receiver to be
 able to decide whether or not the alternative thus described (a) is
 likely to be an improvement over the actual data provided, and (b) is
 likely to be processable by the receiver.
 Thus, when interpreting a Content-alternative header value, a
 receiver may assume that features not explicitly mentioned are not
 different in the indicated alternative from the supplied data.  For
 example, if a Content-alternative header does not mention an
 alternative MIME content-type, the receiver may assume that the
 available alternative uses the same content-type as the supplied
 data.
 See also the example in section 8.4.

5. The Original-Message-ID message header

 The 'Original-Message-ID' header is used to correlate any message
 response or re-send with the original message to which it relates
 (see also sections 3.2.3,  3.3).  A re-send is distinct from the
 original message, so it MUST have its own unique Message-ID value
 (per RFC 2822, section 3.6.4).

Klyne, et. al. Standards Track [Page 18] RFC 3297 Content Negotiation for Messaging Services July 2002

 The syntax for this header is:
    "Original-Message-ID" ":" msg-id
 where 'msg-id' is defined by RFC 2822 as:
    msg-id = "<" id-left "@" id-right ">"
 The 'msg-id' value given must be identical to that supplied in the
 Message-ID: header of the original message for which the current
 message is a response or re-send.

6. MDN extension for alternative data

 Here, we define two extensions to the Message Disposition
 Notification (MDN) protocol [4] to allow a sender to indicate
 readiness to send alternative message data formats, and to allow a
 receiver to indicate a preference for some alternative format.
 Indication of what alternatives may be available or preferred are not
 covered here.  This functionality is provided by the 'Content-
 alternative' MIME header [8] and "Indicating Supported Media Features
 Using Extensions to DSN and MDN" [2].

6.1 Indicating readiness to send alternative data

 A sender wishing to indicate its readiness to send alternative
 message data formats must request an MDN response using the MDN
 'Disposition-Notification-To:' header [4].
 The MDN request is accompanied by a 'Disposition-Notification-
 Options:' header containing the parameter 'Alternative-available'
 with an importance value of 'optional'.  (The significance of
 'optional' is that receiving agents unaware of this option do not
 generate inappropriate failure responses.)
 This specification defines a value for 'attribute' to be used in an
 MDN 'Disposition-Notification-Options:' header [4]:
    attribute =/ "Alternative-available"
 Thus, a sender includes the following headers to indicate that
 alternative message data is available:
    Disposition-Notification-To:
        <sender-address>
    Disposition-Notification-Options:
        Alternative-available=optional,<lifetime>

Klyne, et. al. Standards Track [Page 19] RFC 3297 Content Negotiation for Messaging Services July 2002

 where <lifetime> is "transient" or "permanent", indicating whether
 the alternative data will be made available for just a short while,
 or for an indefinite period.  A value of "permanent" indicates that
 the data is held on long term storage and can be expected to be
 available for at least several days, and probably weeks or months.  A
 value of "transient" indicates that the alternative data may be
 discarded at any time, though it would normally be held for the
 expected duration of a message transaction.
    NOTE: the <lifetime> parameter is provided to help low-memory
    receivers (which are unable to store received data) avoid loss of
    information through requesting an alternative data format that may
    become unavailable.
 A message sent with a request for an MDN with an 'Alternative-
 available' option MUST also contain a 'Message-ID:' header field
 [20].

6.2 Indicating a preference for alternative data

 The MDN specification [4] defines a number of message disposition
 options that may be reported by the receiver of a message:
    disposition-type = "displayed"
                     / "dispatched"
                     / "processed"
                     / "deleted"
                     / "denied"
                     / "failed"
    disposition-modifier = ( "error" / "warning" )
                         / ( "superseded" / "expired" /
                             "mailbox-terminated" )
                         / disposition-modifier-extension
 This specification defines an additional value for 'disposition-
 modifier-extension':
    disposition-modifier-extension =/
        "Alternative-preferred"
 When a receiver requests that an alternative format be sent, it sends
 a message disposition notification message containing the following
 disposition field:
    Disposition:
      <action-mode>/<sending-mode>,
      deleted/alternative-preferred

Klyne, et. al. Standards Track [Page 20] RFC 3297 Content Negotiation for Messaging Services July 2002

 For example, an automatically generated response might contain:
    Disposition:
      automatic-action/MDN-sent-automatically,
      deleted/alternative-preferred
 An MDN response containing an 'alternative-preferred' disposition
 modifier MUST also contain an 'Original-message-ID:' field [4] with
 the 'Message-ID:' value from the original message.
 An MDN response containing an 'alternative-preferred' disposition
 modifier SHOULD also contain a 'Media-accept-features:' field [2]
 indicating the capabilities that the sender should use in selecting
 an alternative form of message data.  If this field is not supplied,
 the sender should assume some baseline feature capabilities.
 Receiver capabilities supplied with an alternative-preferred
 disposition notification MUST NOT be cached:  they may apply to the
 current transaction only.

6.3 Indicating alternative data is no longer available

 A sender that receives a request for alternative data that is no
 longer available, or is unable to provide alternative data matching
 the receiver's capabilities, MUST respond with an indication of this
 fact, sending a message containing data describing the failure.
 Such a message MUST specify the MDN 'Disposition-Notification-To:'
 header [4], accompanied by a 'Disposition-Notification-Options:'
 header containing the parameter 'Alternative-not-available' with an
 importance value of 'required'.
 This specification defines a value for 'attribute' to be used in an
 MDN 'Disposition-Notification-Options:' header [4]:
    attribute =/ "Alternative-not-available"
 Thus, a sender includes the following headers to indicate that the
 alternative message data previously offered is no longer available:
    Disposition-Notification-To:
        <sender-address>
    Disposition-Notification-Options:
        Alternative-not-available=required,(TRUE)
 A message sent with a request for an MDN with an 'Alternative-not-
 available' option MUST also contain an 'Original-message-ID:'  header
 [23] containing the value from the 'Message-ID:' header of the
 original message.

Klyne, et. al. Standards Track [Page 21] RFC 3297 Content Negotiation for Messaging Services July 2002

6.4 Indicating loss of original data

 This specification defines an additional value for 'disposition-
 modifier-extension':
    disposition-modifier-extension =/
        "original-lost"
 When a receiver loses message data because it lacks memory to store
 the original while waiting for an alternative to be sent, it sends a
 message disposition notification containing the following field:
    Disposition:
      <action-mode>/<sending-mode>,
      deleted/original-lost
 For example, an automatically generated response might contain:
    Disposition:
      automatic-action/MDN-sent-automatically,
      deleted/original-lost
 An MDN response containing an 'original-lost' disposition modifier
 MUST also contain an 'Original-message-ID:' field [4] with the
 'Message-ID:' value from the resent message, or from the original
 message (if no re-send has been received).

6.5 Automatic sending of MDN responses

 In sending an MDN response that requests alternative data, the
 security concerns stated in RFC 2298 [4] (sections 2.1 and 6.2)
 regarding automatic MDN responses must be respected.  In particular,
 a system capable of performing content negotiation MUST have an
 option for its user to disable negotiation responses, either
 generally, on a per-message basis, or both.

7. Internet Fax Considerations

 Internet fax is an application that uses email to exchange document
 images (see RFC RFC 2305 [12] and RFC 2532 [1]).
 Both sender and receiver parts of this specification involve the use
 of media feature expressions.  In the context of Internet fax, any
 such expressions SHOULD employ feature tags defined by "Content
 feature schema for Internet fax" [16].  In a wider email context, any
 valid media features MAY be used.

Klyne, et. al. Standards Track [Page 22] RFC 3297 Content Negotiation for Messaging Services July 2002

 For Internet fax [12], "image/tiff" is the assumed content-type for
 message data.  In particular, all Internet fax devices are presumed
 to be capable of sending and receiving the TIFF profile S
 capabilities (Section 3 of [11]).  When communication is between
 Internet fax devices, this capability may be assumed.  But when
 dealing with devices that go beyond these capabilities defined for
 Internet fax (e.g. generic email agents with fax capabilities) it
 would be better not to assume fax capabilities, and for the
 negotiating parties to be explicit with respect to all their
 capabilities.
 It would be better if even Internet fax devices do not assume that
 they are communicating with other such devices.  When using Internet
 email, there is no reliable way to establish this fact.  Therefore,
 for any Internet fax device that may reasonably be expected to
 exchange messages with any other email agent, it is RECOMMENDED that
 Internet fax capabilities (such as image/tiff baseline format
 handling) are not assumed but stated explicitly.
 In particular, the 'Media-Accept-Features:' header in receiver MDN
 responses SHOULD explicitly indicate (type="image/tiff") and baseline
 TIFF capabilities, rather than just assuming that they are
 understood.

8. Examples

8.1 Sending enhanced Internet Fax image

 An Internet fax sender has a profile-F (A4, 400x400dpi, MMR) image to
 send to a receiver.  The baseline for Internet fax is 200x200dpi and
 MH image compression.
 Sender's initial message:
    Date: Wed,20 Sep 1995 00:18:00 (EDT)-0400
    From: Jane Sender <Jane_Sender@example.com>
    Message-Id: <199509200019.12345@example.com>
    Subject: Internet FAX Full Mode Content Negotiation
    To: Tom Recipient <Tom_Recipient@example.org>
    Disposition-Notification-To: Jane_Sender@example.com
    Disposition-Notification-Options:
        Alternative-available=optional,permanent
    MIME-Version: 1.0
    Content-Type: multipart/mixed;
                  boundary="RAA14128.773615765/ example.com"

Klyne, et. al. Standards Track [Page 23] RFC 3297 Content Negotiation for Messaging Services July 2002

  1. -RAA14128.773615765/ example.com

Content-type: image/tiff

    Content-Transfer-Encoding: base64
    Content-features:
        (& (color=Binary)
           (image-file-structure=TIFF-minimal)
           (dpi=200)
           (dpi-xyratio=1)
           (paper-size=A4)
           (image-coding=MH)
           (MRC-mode=0)
           (ua-media=stationery) )
    Content-alternative:
        (& (color=Binary)
           (image-file-structure=TIFF-limited)
           (dpi=400)
           (dpi-xyratio=1)
           (paper-size=A4)
           (image-coding=MMR)
           (MRC-mode=0)
           (ua-media=stationery) )
    [TIFF-FX Profile-S message goes here]
  1. -RAA14128.773615765/ example.com–
 Receiver sends MDN response to initial message:
    Date: Wed,20 Sep 1995 00:19:00 (EDT)-0400
    From: Tom Recipient <Tom_Recipient@example.org>
    Message-Id: <199509200020.12345@example.org>
    Subject: Re: Internet FAX Full Mode Content Negotiation
    To: Jane Sender <Jane_Sender@example.com>
    MIME-Version: 1.0
    Content-Type: multipart/report;
                  report-type=disposition-notification;
                  boundary="RAA14128.773615766/example.org"
  1. -RAA14128.773615766/example.org
    The message sent on 1995 Sep 20 at 00:18:00 (EDT) -0400 to
    Tom Recipient <Tom_Recipient@example.org> with subject "Internet
    FAX Full Mode Content Negotiation" has been received.  An
    alternative form of the message data is requested.

Klyne, et. al. Standards Track [Page 24] RFC 3297 Content Negotiation for Messaging Services July 2002

  1. -RAA14128.773615766/example.org

Content-Type: message/disposition-notification

    Reporting-UA: Toms-pc.cs.example.org; IFAX-FullMode
    Original-Recipient: rfc822;Tom-Recipient@example.org
    Final-Recipient: rfc822;Tom-Recipient@example.org
    Original-Message-ID: <199509200019.12345@example.com>
    Disposition: automatic-action/MDN-sent-automatically;
                 deleted/alternative-preferred
    Media-Accept-Features:
        (& (type="image/tiff")
           (color=Binary)
           (image-file-structure=TIFF)
           (| (& (dpi=200) (dpi-xyratio=200/100) )
              (& (dpi=200) (dpi-xyratio=1) )
              (& (dpi=400) (dpi-xyratio=1) ) )
           (| (image-coding=[MH,MR,MMR])
              (& (image-coding=JBIG)
                 (image-coding-constraint=JBIG-T85)
                 (JBIG-stripe-size=128) ) )
           (MRC-mode=0)
           (paper-size=[A4,B4])
           (ua-media=stationery) )
  1. -RAA14128.773615766/example.org–
 Sender's message with enhanced content:
    Date: Wed,20 Sep 1995 00:21:00 (EDT)-0400
    From: Jane Sender <Jane_Sender@example.com>
    Message-Id: <199509200021.12345@example.com>
    Original-Message-Id: <199509200019.12345@example.com>
    Subject: Internet FAX Full Mode Image Transmission
    To: Tom Recipient <Tom_Recipient@example.org>
    Disposition-Notification-To: Jane_Sender@example.com
    MIME-Version: 1.0
    Content-Type: multipart/mixed;
                  boundary="RAA14128.773615768/ example.com"
  1. -RAA14128.773615768/ example.com

Content-type: image/tiff

    Content-Transfer-Encoding: base64
    [TIFF-FX profile-F message goes here]
  1. -RAA14128.773615768/ example.com–

Klyne, et. al. Standards Track [Page 25] RFC 3297 Content Negotiation for Messaging Services July 2002

 Receiver sends MDN confirmation of enhanced message content:
    Date: Wed,20 Sep 1995 00:22:00 (EDT)-0400
    From: Tom Recipient <Tom_Recipient@example.org>
    Message-Id: <199509200022.12345@example.org>
    Subject: Re: Internet FAX Full Mode Image Transmission
    To: Jane Sender <Jane_Sender@example.com>
    MIME-Version: 1.0
    Content-Type: multipart/report;
                  report-type=disposition-notification;
                  boundary="RAA14128.773615769/example.org"
  1. -RAA14128.773615769/example.org
    The message sent on 1995 Sep 20 at 00:21:00 (EDT) -0400 to Tom
    Recipient <Tom_Recipient@example.org> with subject " Internet FAX
    Full Mode Image Transmission" has been processed in Internet FAX
    Full Mode.
  1. -RAA14128.773615769/example.org

Content-Type: message/disposition-notification

    Reporting-UA: Toms-pc.cs.example.org; IFAX-FullMode
    Original-Recipient: rfc822;Tom-Recipient@example.org
    Final-Recipient: rfc822;Tom-Recipient@example.org
    Original-Message-ID: <199509200021.12345@example.com>
    Disposition: automatic-action/MDN-sent-automatically; processed
    Media-Accept-Features:
        (& (type="image/tiff")
           (color=Binary)
           (image-file-structure=TIFF)
           (| (& (dpi=200) (dpi-xyratio=200/100) )
              (& (dpi=200) (dpi-xyratio=1) )
              (& (dpi=400) (dpi-xyratio=1) ) )
           (| (image-coding=[MH,MR,MMR])
              (& (image-coding=JBIG)
                 (image-coding-constraint=JBIG-T85)
                 (JBIG-stripe-size=128) ) )
           (MRC-mode=0)
           (paper-size=[A4,B4])
           (ua-media=stationery) )
  1. -RAA14128.773615769/example.org–

Klyne, et. al. Standards Track [Page 26] RFC 3297 Content Negotiation for Messaging Services July 2002

8.2 Internet fax with initial data usable

 This example shows how the second and subsequent transfers between
 the systems in the previous example might be conducted.  Using
 knowledge gained from the previous exchange, the sender includes
 profile-F data with its first contact.
 Sender's initial message:
    Date: Wed,20 Sep 1995 00:19:00 (EDT)-0400
    From: Jane Sender <Jane_Sender@example.com>
    Message-Id: <199509200019.12345@example.com>
    Subject: Internet FAX Full Mode Content Negotiation
    To: Tom Recipient <Tom_Recipient@example.org>
    Disposition-Notification-To: Jane_Sender@example.com
    Disposition-Notification-Options:
        Alternative-available=optional,permanent
    MIME-Version: 1.0
    Content-Type: multipart/mixed;
                  boundary="RAA14128.773615765/ example.com"
  1. -RAA14128.773615765/ example.com

Content-type: image/tiff

    Content-Transfer-Encoding: base64
    Content-features:
        (& (color=Binary)
           (image-file-structure=TIFF-limited)
           (dpi=400)
           (dpi-xyratio=1)
           (paper-size=A4)
           (image-coding=MMR)
           (MRC-mode=0)
           (ua-media=stationery) )
    Content-alternative:
        (& (color=Binary)
           (image-file-structure=TIFF-minimal)
           (dpi=200)
           (dpi-xyratio=1)
           (paper-size=A4)
           (image-coding=MH)
           (MRC-mode=0)
           (ua-media=stationery) )
    [TIFF-FX Profile-F message goes here]
  1. -RAA14128.773615765/ example.com–

Klyne, et. al. Standards Track [Page 27] RFC 3297 Content Negotiation for Messaging Services July 2002

 Receiver sends MDN confirmation of received message content:
    Date: Wed,20 Sep 1995 00:22:00 (EDT)-0400
    From: Tom Recipient <Tom_Recipient@example.org>
    Message-Id: <199509200022.12345@example.org>
    Subject: Re: Internet FAX Full Mode Image Transmission
    To: Jane Sender <Jane_Sender@example.com>
    MIME-Version: 1.0
    Content-Type: multipart/report;
                  report-type=disposition-notification;
                  boundary="RAA14128.773615769/example.org"
  1. -RAA14128.773615769/example.org
    The message sent on 1995 Sep 20 at 00:19:00 (EDT) -0400 to Tom
    Recipient <Tom_Recipient@example.org> with subject "Internet FAX
    Full Mode Image Transmission" has been processed in Internet FAX
    Full Mode.
  1. -RAA14128.773615769/example.org

Content-Type: message/disposition-notification

    Reporting-UA: Toms-pc.cs.example.org; IFAX-FullMode
    Original-Recipient: rfc822;Tom-Recipient@example.org
    Final-Recipient: rfc822;Tom-Recipient@example.org
    Original-Message-ID: <199509200021.12345@example.com>
    Disposition: automatic-action/MDN-sent-automatically; processed
    Media-Accept-Features:
        (& (type="image/tiff")
           (color=Binary)
           (image-file-structure=TIFF)
           (| (& (dpi=200) (dpi-xyratio=200/100) )
              (& (dpi=200) (dpi-xyratio=1) )
              (& (dpi=400) (dpi-xyratio=1) ) )
           (| (image-coding=[MH,MR,MMR])
              (& (image-coding=JBIG)
                 (image-coding-constraint=JBIG-T85)
                 (JBIG-stripe-size=128) ) )
           (MRC-mode=0)
           (paper-size=[A4,B4])
           (ua-media=stationery) )
  1. -RAA14128.773615769/example.org–

Klyne, et. al. Standards Track [Page 28] RFC 3297 Content Negotiation for Messaging Services July 2002

8.3 Negotiate to lower receiver capability

 In this example, the sender has incorrectly assumed that the receiver
 has a higher capability, and must re-send lower capability data in
 response to the receiver's response showing lesser capability.
 An Internet fax sends a profile-F (A4, 400x400dpi, MMR) image.  When
 the receiver cannot handle this, it falls back to baseline profile-S.
 As this is a baseline format, it is not necessary to declare that
 capability with the original message.  When a receiver is faced with
 data it cannot process from a negotiating sender, it can do no better
 than to respond with a description of its actual capabilities and let
 the sender determine the outcome.
 Sender's initial message:
    Date: Wed, 20 Sep 1995 00:18:00 (EDT)-0400
    From: Jane Sender <Jane_Sender@example.com>
    Message-Id: <199509200019.12345@example.com>
    Subject: Internet FAX Full Mode Negotiate Down
    To: Tom Recipient <Tom_Recipient@example.org>
    Disposition-Notification-To: Jane_Sender@example.com
    Disposition-Notification-Options:
        Alternative-available=optional,permanent
    MIME-Version: 1.0
    Content-Type: multipart/mixed;
                  boundary="RAA14128.773615765/ example.com"
  1. -RAA14128.773615765/ example.com

Content-type: image/tiff

    Content-Transfer-Encoding: base64
    Content-features:
        (& (color=Binary)
           (image-file-structure=TIFF-limited)
           (dpi=400)
           (dpi-xyratio=1)
           (paper-size=A4)
           (image-coding=MMR)
           (MRC-mode=0)
           (ua-media=stationery) )
    [TIFF-FX Profile-F message goes here]
  1. -RAA14128.773615765/ example.com–

Klyne, et. al. Standards Track [Page 29] RFC 3297 Content Negotiation for Messaging Services July 2002

 Receiver sends MDN response to initial message:
    Date: Wed,20 Sep 1995 00:19:00 (EDT)-0400
    From: Tom Recipient <Tom_Recipient@example.org>
    Message-Id: <199509200020.12345@example.org>
    Subject: Re: Internet FAX Full Mode Negotiate Down
    To: Jane Sender <Jane_Sender@example.com>
    MIME-Version: 1.0
    Content-Type: multipart/report;
                  report-type=disposition-notification;
                  boundary="RAA14128.773615766/example.org"
  1. -RAA14128.773615766/example.org
    The message sent on 1995 Sep 20 at 00:18:00 (EDT) -0400 to
    Tom Recipient <Tom_Recipient@example.org> with subject "Internet
    FAX Full Mode Content Negotiation" has been received.  An
    alternative form of the message data is requested.
  1. -RAA14128.773615766/example.org

Content-Type: message/disposition-notification

    Reporting-UA: Toms-pc.cs.example.org; IFAX-FullMode
    Original-Recipient: rfc822;Tom-Recipient@example.org
    Final-Recipient: rfc822;Tom-Recipient@example.org
    Original-Message-ID: <199509200019.12345@example.com>
    Disposition: automatic-action/MDN-sent-automatically;
                 deleted/alternative-preferred
    Media-Accept-Features:
        (& (type="image/tiff")
           (color=Binary)
           (image-file-structure=TIFF-minimal)
           (dpi=200)
           (dpi-xyratio=1)
           (paper-size=A4)
           (image-coding=MH)
           (MRC-mode=0)
           (ua-media=stationery) )
  1. -RAA14128.773615766/example.org–

Klyne, et. al. Standards Track [Page 30] RFC 3297 Content Negotiation for Messaging Services July 2002

 Sender's message with baseline content:
    Date: Wed,20 Sep 1995 00:21:00 (EDT)-0400
    From: Jane Sender <Jane_Sender@example.com>
    Message-Id: <199509200021.12345@example.com>
    Original-Message-Id: <199509200019.12345@example.com>
    Subject: Internet FAX Full Mode Image Transmission
    To: Tom Recipient <Tom_Recipient@example.org>
    Disposition-Notification-To: Jane_Sender@example.com
    MIME-Version: 1.0
    Content-Type: multipart/mixed;
                  boundary="RAA14128.773615768/ example.com"
  1. -RAA14128.773615768/ example.com

Content-type: image/tiff

    Content-Transfer-Encoding: base64
    [TIFF-FX profile-S message goes here]
  1. -RAA14128.773615768/ example.com–
 Receiver sends MDN confirmation of impoverished message content:
    Date: Wed,20 Sep 1995 00:22:00 (EDT)-0400
    From: Tom Recipient <Tom_Recipient@example.org>
    Message-Id: <199509200022.12345@example.org>
    Subject: Re: Internet FAX Full Mode Image Transmission
    To: Jane Sender <Jane_Sender@example.com>
    MIME-Version: 1.0
    Content-Type: multipart/report;
                  report-type=disposition-notification;
                  boundary="RAA14128.773615769/example.org"
  1. -RAA14128.773615769/example.org
    The message sent on 1995 Sep 20 at 00:21:00 (EDT) -0400 to Tom
    Recipient <Tom_Recipient@example.org> with subject " Internet FAX
    Full Mode Image Transmission" has been processed in Internet FAX
    Full Mode.
  1. -RAA14128.773615769/example.org

Content-Type: message/disposition-notification

Klyne, et. al. Standards Track [Page 31] RFC 3297 Content Negotiation for Messaging Services July 2002

    Reporting-UA: Toms-pc.cs.example.org; IFAX-FullMode
    Original-Recipient: rfc822;Tom-Recipient@example.org
    Final-Recipient: rfc822;Tom-Recipient@example.org
    Original-Message-ID: <199509200021.12345@example.com>
    Disposition: automatic-action/MDN-sent-automatically; processed
    Media-Accept-Features:
        (& (color=Binary)
           (image-file-structure=TIFF-minimal)
           (dpi=200)
           (dpi-xyratio=1)
           (paper-size=A4)
           (image-coding=MH)
           (MRC-mode=0)
           (ua-media=stationery) )
  1. -RAA14128.773615769/example.org–

8.4 Sending an alternative content type

 As noted in section 4, the sender can offer the data using a
 different MIME content-type.  This example shows a profile-F (A4,
 400x400dpi, MMR) image and a limited-colour PDF document offered as
 alternatives to a baseline image/TIFF.
 Sender's initial message:
      (Note that the MIME content type is not specified for the
      image/tiff alternative, being the same as that provided, but
      is mentioned for the application/pdf alternative.)
    Date: Wed,20 Sep 1995 00:18:00 (EDT)-0400
    From: Jane Sender <Jane_Sender@example.com>
    Message-Id: <199509200019.12345@example.com>
    Subject: Internet FAX Full Mode Content Negotiation
    To: Tom Recipient <Tom_Recipient@example.org>
    Disposition-Notification-To: Jane_Sender@example.com
    Disposition-Notification-Options:
        Alternative-available=optional,permanent
    MIME-Version: 1.0
    Content-Type: multipart/mixed;
                  boundary="RAA14128.773615765/ example.com"
  1. -RAA14128.773615765/ example.com

Content-type: image/tiff

    Content-Transfer-Encoding: base64
    Content-features:
        (& (color=Binary)
           (image-file-structure=TIFF-minimal)

Klyne, et. al. Standards Track [Page 32] RFC 3297 Content Negotiation for Messaging Services July 2002

           (dpi=200)
           (dpi-xyratio=1)
           (paper-size=A4)
           (image-coding=MH)
           (MRC-mode=0)
           (ua-media=stationery) )
    Content-alternative:
        (& (color=Binary)
           (image-file-structure=TIFF-limited)
           (dpi=400)
           (dpi-xyratio=1)
           (paper-size=A4)
           (image-coding=MMR)
           (MRC-mode=0)
           (ua-media=stationery) )
    Content-alternative:
        (& (type="application/pdf")
           (color=Limited)
           (dpi=400)
           (paper-size=A4)
           (ua-media=stationery) )
    [TIFF-FX Profile-S message goes here]
  1. -RAA14128.773615765/ example.com–
 Receiver sends MDN response to initial message:
      (Note that this response indicates an ability to handle the
      PDF MIME content-types, but with only binary colour.)
    Date: Wed,20 Sep 1995 00:19:00 (EDT)-0400
    From: Tom Recipient <Tom_Recipient@example.org>
    Message-Id: <199509200020.12345@example.org>
    Subject: Re: Internet FAX Full Mode Content Negotiation
    To: Jane Sender <Jane_Sender@example.com>
    MIME-Version: 1.0
    Content-Type: multipart/report;
                  report-type=disposition-notification;
                  boundary="RAA14128.773615766/example.org"
  1. -RAA14128.773615766/example.org
    The message sent on 1995 Sep 20 at 00:18:00 (EDT) -0400 to
    Tom Recipient <Tom_Recipient@example.org> with subject "Internet
    FAX Full Mode Content Negotiation" has been received.  An
    alternative form of the message data is requested.

Klyne, et. al. Standards Track [Page 33] RFC 3297 Content Negotiation for Messaging Services July 2002

  1. -RAA14128.773615766/example.org

Content-Type: message/disposition-notification

    Reporting-UA: Toms-pc.cs.example.org; IFAX-FullMode
    Original-Recipient: rfc822;Tom-Recipient@example.org
    Final-Recipient: rfc822;Tom-Recipient@example.org
    Original-Message-ID: <199509200019.12345@example.com>
    Disposition: automatic-action/MDN-sent-automatically;
                 deleted/alternative-preferred
    Media-Accept-Features:
        (| (& (type="image/tiff")
              (color=Binary)
              (image-file-structure=TIFF-minimal)
              (dpi=200)
              (dpi-xyratio=1)
              (image-coding=MH)
              (MRC-mode=0)
              (paper-size=A4)
              (ua-media=stationery) )
           (& (type="application/pdf")
              (color=Binary)
              (dpi-xyratio=1)
              (dpi=[200,400])
              (paper-size=[A4,B4])
              (ua-media=stationery) ) )
  1. -RAA14128.773615766/example.org–
 Resend with alternative content-type:
    Date: Wed,20 Sep 1995 00:21:00 (EDT)-0400
    From: Jane Sender <Jane_Sender@example.com>
    Message-Id: <199509200021.12345@example.com>
    Original-Message-Id: <199509200019.12345@example.com>
    Subject: Internet FAX Full Mode Image Transmission
    To: Tom Recipient <Tom_Recipient@example.org>
    Disposition-Notification-To: Jane_Sender@example.com
    MIME-Version: 1.0
    Content-Type: multipart/mixed;
                  boundary="RAA14128.773615768/ example.com"
  1. -RAA14128.773615768/ example.com

Content-type: application/pdf

    Content-Transfer-Encoding: base64
    [PDF data goes here]
  1. -RAA14128.773615768/ example.com–

Klyne, et. al. Standards Track [Page 34] RFC 3297 Content Negotiation for Messaging Services July 2002

 Receiver sends MDN confirmation of enhanced message content:
      (Also indicating the PDF capability for future messages.)
    Date: Wed,20 Sep 1995 00:22:00 (EDT)-0400
    From: Tom Recipient <Tom_Recipient@example.org>
    Message-Id: <199509200022.12345@example.org>
    Subject: Re: Internet FAX Full Mode Image Transmission
    To: Jane Sender <Jane_Sender@example.com>
    MIME-Version: 1.0
    Content-Type: multipart/report;
                  report-type=disposition-notification;
                  boundary="RAA14128.773615769/example.org"
  1. -RAA14128.773615769/example.org
    The message sent on 1995 Sep 20 at 00:21:00 (EDT) -0400 to Tom
    Recipient <Tom_Recipient@example.org> with subject " Internet FAX
    Full Mode Image Transmission" has been processed in Internet FAX
    Full Mode.
  1. -RAA14128.773615769/example.org

Content-Type: message/disposition-notification

    Reporting-UA: Toms-pc.cs.example.org; IFAX-FullMode
    Original-Recipient: rfc822;Tom-Recipient@example.org
    Final-Recipient: rfc822;Tom-Recipient@example.org
    Original-Message-ID: <199509200021.12345@example.com>
    Disposition: automatic-action/MDN-sent-automatically; processed
    Media-Accept-Features:
        (| (& (type="image/tiff")
              (color=Binary)
              (image-file-structure=TIFF-minimal)
              (dpi=200)
              (dpi-xyratio=1)
              (image-coding=MH)
              (MRC-mode=0)
              (paper-size=A4)
              (ua-media=stationery) )
           (& (type="application/pdf")
              (color=Binary)
              (dpi-xyratio=1)
              (dpi=[200,400])
              (paper-size=[A4,B4])
              (ua-media=stationery) ) )
  1. -RAA14128.773615769/example.org–

Klyne, et. al. Standards Track [Page 35] RFC 3297 Content Negotiation for Messaging Services July 2002

9. IANA Considerations

9.1 New message headers

 This specification defines new email/MIME message headers:
    Content-alternative
    Original-Message-ID
 As such, there being no registry of email headers, it is an update to
 the specifications of RFC 2822 and RFC 2045.

9.2 MDN extensions

 This specification defines extensions to the Message Disposition
 Notification (MDN) protocol.  The sections below are the registration
 templates for these extensions, as required by RFC 2298 [4], section
 10.

9.2.1 Notification option 'Alternative-available'

 (a)   Disposition-notification-option name:
       Alternative-available
 (b)   Syntax:
       (see this document, section 6.1)
 (c)   Character-encoding:
       US-ASCII characters only are used
 (d)   Semantics:
       (see this document, section 6.1)

9.2.2 Notification option 'Alternative-not-available'

 (a)   Disposition-notification-option name:
       Alternative-not-available
 (b)   Syntax:
       (see this document, section 6.1)
 (c)   Character-encoding:
       US-ASCII characters only are used
 (d)   Semantics
       (see this document, section 6.3)

Klyne, et. al. Standards Track [Page 36] RFC 3297 Content Negotiation for Messaging Services July 2002

9.2.3 Disposition modifier 'Alternative-preferred'

 (a)   Disposition-modifier name:
       Alternative-preferred
 (b)   Semantics:
       (see this document, section 6.2)

9.2.4 Disposition modifier 'Original-lost'

 (a)   Disposition-modifier name:
       Original-lost
 (b)   Semantics:
       (see this document, section 6.4)

10. Internationalization considerations

 This specification deals with protocol exchanges between mail user
 agents, and as such does not deal primarily with human readable text.
 But not all user agents may automatically handle the protocol
 elements defined here, and may attempt to display text from the
 protocol elements to the user.
 The main candidate for this treatment is the text accompanying a
 disposition notification response that requests alternative
 information.  In normal use, the protocol design ensures that the
 recipient can process this response automatically; exceptionally, a
 receiving agent may display it to a user.

11. Security Considerations

 Security considerations of this specification can be divided into two
 main areas:
 o  Privacy concerns with automated MDN response generation:  see
    section 6.5 of this document, and the security considerations
    section of RFC 2298 [4].
 o  Risks of negotiation:  see the security considerations section
    transaction.  If alternative data arrives subsequently, it may be
    ignored or possibly also displayed or printed.  A successful
    completion MDN may be sent to the sender.

Klyne, et. al. Standards Track [Page 37] RFC 3297 Content Negotiation for Messaging Services July 2002

12. Acknowledgements

 The basic structure of the negotiation described here was first
 documented in a draft by Mr. Toru Maeda of Canon.
 Helpful comments on earlier drafts were provided by Mr Hiroshi
 Tamura, Ted Hardie and Larry Masinter.

13. References

 [1]   Masinter, L. and D. Wing, "Extended Facsimile using Internet
       Mail", RFC 2532, March 1999.
 [2]   Wing, D., "Indicating Supported Media Features Using Extensions
       to DSN and MDN", RFC 2530, March 1999.
 [3]   Masinter, L., "Terminology and Goals for Internet Fax", RFC
       2542, March 1999.
 [4]   Fajman, R., "An Extensible Message Format for Message
       Disposition Notifications", RFC 2298, March 1998.
 [5]   Holtman, K., Mutz, A. and T. Hardie, "Media Feature Tag
       Registration Procedure", RFC 2506, March 1999.
 [6]   Klyne, G., "A syntax for describing media feature sets", BCP
       31, RFC 2533, March 1999.
 [7]   Klyne, G., "Indicating media features for MIME content", RFC
       2938, September 2000.
 [8]  'Content-alternative' header (this memo, section 4)
 [9]   MDN extension for alternative data (this memo, section 6)
 [10]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
       Specifications: ABNF", RFC 2234, November 1997.
 [11]  McIntyre, L.,  Buckley, R., Venable, D., Zilles, S., Parsons,
       G. and J. Rafferty, "File format for Internet fax", RFC 2301,
       March 1998.
 [12]  Toyoda K., Ohno H., Murai, J. and D. Wing, "A Simple Mode of
       Facsimile Using Internet Mail", RFC 2305, March 1998.
 [13]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
       Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol --
       HTTP/1.1", RFC 2616, June 1999.

Klyne, et. al. Standards Track [Page 38] RFC 3297 Content Negotiation for Messaging Services July 2002

 [14]  Holtman, K. and A. Mutz, "Transparent Content Negotiation in
       HTTP", RFC 2295, March 1998.
 [15]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
       Extensions (MIME) Part 2: Media types", RFC 2046, November
       1996.
 [16]  Klyne, G. and L. McIntyre, "Content feature schema for Internet
       fax V2", RFC 2879, August 2000.
 [17]  Klyne, G., "Protocol-independent Content Negotiation
       Framework", RFC 2703, September 1999.
 [18]  Moore, K., "SMTP Service Extension for Delivery Status
       Notifications", RFC 1891, January 1996.
 [19]  Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, April
       2001.
 [20]  Resnick, P., "Internet Message Format", RFC 2822, April 2001.
 [21]  Klyne, G. and D. Crocker, "Timely Delivery for Facsimile Using
       Internet Mail", Work in Progress.
 [22]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
       Levels", BCP 14, RFC 2119, March 1997.
 [23] 'Original-Message-ID' header for mail messages (this memo,
       section 5)
 [24]  Klyne, G., "MIME Content Types in Media Feature Expressions",
       RFC 2913, September 2000.

Klyne, et. al. Standards Track [Page 39] RFC 3297 Content Negotiation for Messaging Services July 2002

Appendix A: Implementation issues

 This section is not a normative part of this specification.  Rather,
 it discusses some of the issues that were considered during its
 design in a way that we hope will be useful to implementers.

A.1 Receiver state

 Probably the biggest implication for implementers of this proposal
 compared with standard mail user agents is the need to maintain some
 kind of state information at the receiver while content is being
 negotiated.
 By "receiver state", we mean that a receiver needs to remember that
 it has received an initial message AND that it has requested an
 alternative form of data.  Without this, when a receiver responds
 with a request for an alternative data format there is a possibility
 (if the response does not reach the sender) that the message will be
 silently lost, despite its having been delivered to the receiving
 MTA.
 The matter of maintaining receiver state is particularly germane
 because of the requirement to allow low-memory systems to participate
 in the content negotiation.  Unlike traditional T.30 facsimile, where
 the negotiation takes place within the duration of a single
 connection, an extended time may be taken to complete a negotiation
 in email.  State information must be maintained for all negotiations
 outstanding at any time, and there is no theoretical upper bound on
 how many there may be.
 Keeping receiver state is probably not a problem for systems with
 high capacity storage devices to hold message data and state
 information.  The remainder of this section discusses strategies that
 small-system designers might employ to place an upper bound on memory
 that must be reserved for this information.  When a receiver is
 really memory constrained then message loss remains a possibility,
 but the mechanisms described here should ensure that it never happens
 silently.
 So what is this "receiver state"?  It must contain, as a minimum:
 o  the fact that message data was received, and alternative data has
    been requested,
 o  a unique message identifier, and
 o  the time at which an alternative format request was sent.

Klyne, et. al. Standards Track [Page 40] RFC 3297 Content Negotiation for Messaging Services July 2002

 This allows the receiver to re-issue a request, or to report an
 error, if requested alternative data does not arrive in a reasonable
 time.
 Receiver state may also include:
 o  a copy of the data originally received.  This allows the receiver
    to display the original data if an alternative is not received.
 o  details of the data format supplied, and alternatives offered.
    This permits improved diagnostics if alternative data is not
    received.
 If a receiver of a message with alternative content available does
 not have enough memory to hold new negotiation state information, it
 may fall back to non-negotiation behaviour, accept the data received
 and send an MDN indicating disposition of that data (see sections
 3.2.1, 3.2.2).
 If a receiving system runs low on memory after entering into a
 negotiation, a number of options may be possible:
 o  display or print buffered data, if available, and complete the
    transaction.  If alternative data arrives subsequently, it may be
    ignored or possibly also displayed or printed.  A successful
    completion MDN may be sent to the sender.
 o  discard any buffered data, and continue waiting for alternative
    data.  If alternative data does not subsequently arrive, a message
    transfer failure should be declared.
 o  abort the transfer and declare a message transfer failure:  a
    diagnostic message must be displayed to the local user, and a
    failure notification sent to the sender.

A.2 Receiver buffering of message data

 If a receiver is capable of buffering received message data while
 waiting for an alternative, this is to be preferred because it
 retains the option to display that data if an alternative is not
 received (see above).
 Partial message data should not be buffered for this purpose:
 displaying part of the original message is not an allowable
 substitute for displaying all of the received data.  (There may be
 some value in keeping some of the original message data for
 diagnostic purposes.)

Klyne, et. al. Standards Track [Page 41] RFC 3297 Content Negotiation for Messaging Services July 2002

 If a receiver starts to buffer message data pending negotiation, then
 finds that the entire message is too large to buffer, it may choose
 to fall back to "extended mode" and display the incoming data as it
 is received.
 When a sender indicates availability of alternative data, it also
 indicates whether it is permanently or transiently available.  The
 intent of this is that if alternative data is transient, a receiver
 should not discard original data received.  If necessary, it should
 simply display the original data without requesting an alternative.

A.3 Sender state

 When a sender indicates that it can offer an alternative format of
 message content, it accepts some responsibility for trying to ensure
 that alternative is available if requested.  Thus, the message
 content (both original and any alternative) should be stored for a
 reasonable period, together with any corresponding Message-ID
 value(s).
 A request for retransmission must be accompanied by an Original-
 Message-ID value that the sender can use to correlate with the
 message data originally sent.

A.4 Timeout of offer of alternatives

 If the sender is operating with a high capacity message storage
 device (e.g., a disk drive), and normally holds the data for extended
 periods (several days or weeks) then it should indicate that the
 alternative data is permanently available (see 6.1):  a recipient
 seeing this may discard the original data, assuming that the sender
 will most likely be able to re-transmit.
 If the sender has limited memory capacity, and is likely to be able
 to hold the data for no more than a few minutes or hours, it should
 indicate that the alternative data is transiently available (see
 6.1).  If there is doubt about a sender's ability to keep the message
 content, it should indicate that availability of any alternative is
 transient.

A.5 Timeout of receiver capabilities

 It should not be assumed that receiver capabilities declared during
 negotiation are available indefinitely.

Klyne, et. al. Standards Track [Page 42] RFC 3297 Content Negotiation for Messaging Services July 2002

 In particular, any receiver capabilities declared on a final message
 confirmation should be regarded as definitive, even if they differ
 from the capabilities associated with the message just accepted.
 These may be stored for future use.
 Any receiver capabilities declared when requesting an alternative
 format should not be stored for future use, as the receiver might be
 selective about the purposes for which those capabilities may be
 used.

A.6 Relationship to timely delivery

 Some of the issues of sender state maintenance may be simplified if
 content negotiation is used in conjunction with a facility for timely
 delivery (e.g., [21]).  If there is a known time window within which
 a response should be received, the sender may be less conservative
 about keeping information about outstanding offers of alternative
 data for extended periods.  A sender that exploits timely delivery in
 this way should indicate that the alternative is transiently
 available.

A.7 Ephemeral capabilities

 Ephemeral capabilities may present some special problems.  Consider
 the case of selection of a particular content variant that may depend
 on an ephemeral setting.
 Imagine someone sending a basic fax to a color fax machine,
 indicating that a color alternative is available.  The color fax
 discards the content and sends an MDN which says
 "deleted/alternative-preferred" to the originator.  It then runs out
 of colored ink.  The originating fax then sends a new message which
 the colored fax cannot print.
 Or consider an the email client in a phone with sound on/off as a
 related problem.  When sound is ON, the phone may be able to accept
 voice messages by email.
 This negotiation framework has not been designed with ephemeral
 capabilities in mind, but, with care, may be adaptable to deal with
 them.

Klyne, et. al. Standards Track [Page 43] RFC 3297 Content Negotiation for Messaging Services July 2002

A.8 Situations where MDNs must not be auto-generated

 Bearing in mind privacy concerns, implementers should be careful that
 systems do not automatically enter into a negotiation exchange in a
 way that may disclose the recipient's whereabouts without first
 having obtained explicit permission.  For example, if receiving a
 message depends in any way on the user's physical presence, automatic
 negotiation should not be performed.
 While it may be OK for an unattended fax machine to perform automated
 negotiation, it is not OK for a PC software package to do so without
 the users explicit permission as the PC may be switched on only when
 the user is present.  This suggests that default settings in this
 regard should take account of the type of system.

Appendix B: Candidates for further enhancements

 This appendix lists some possible features of content negotiation
 that were considered, but not included in the current specification.
 In most cases the reasons for exclusion were (a) that they could
 introduce unanticipated additional complexities, and (b) no
 compelling requirement was recognized.
 o  Cache control indicator for recipient capabilities.  This would
    instruct the sender, or other message system component, that
    capability information in the current message is for the current
    transaction only, and should NOT be remembered for future
    transactions.  E.g., a recipient may not wish colour capability to
    be used for routine communications.  (See also section A.5 above.)
 o  Use of q-values [6] in media feature expressions for indicating
    preference among alternatives available and/or receiver
    preferences.
 o  Partial re-sends.  There are proposals being developed for
    "partial MDN" responses that can indicate disposition status on a
    per-message-part basis.  This opens the possibility of partial
    re-sends when alternative formats are requested for only some of
    the message body parts.  The current specification assumes that
    either none or all of message is re-sent when content negotiation
    is used.
 o  Allow negotiation with parties other than originally addressed
    recipients of a message.
 o  Negotiation response might indicate different receiver endpoint
    with different capabilities.

Klyne, et. al. Standards Track [Page 44] RFC 3297 Content Negotiation for Messaging Services July 2002

Authors' Addresses

 Graham Klyne (editor)
 Clearswift Corporation,
 1310 Waterside,
 Arlington Business Park
 Theale
 Reading, RG7 4SA
 United Kingdom
 Phone: +44 11 8903 8903
 Fax:   +44 11 8903 9000
 EMail: GK@ACM.ORG
 Ryuji Iwazaki
 TOSHIBA TEC CORPORATION
 2-4-1, Shibakoen, Minato-ku,
 Tokyo, 105-8524 Japan
 Phone: +81 3 3438 6866
 Fax:   +81 3 5402 6355
 EMail:  iwa@rdl.toshibatec.co.jp
 Dave Crocker
 Brandenburg InternetWorking
 675 Spruce Dr.
 Sunnyvale, CA 94086 USA
 Phone: +1 408 246 8253
 Fax:   +1 408 249 6205
 EMail: dcrocker@brandenburg.com

Klyne, et. al. Standards Track [Page 45] RFC 3297 Content Negotiation for Messaging Services July 2002

Full Copyright Statement

 Copyright (C) The Internet Society (2002).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

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

Klyne, et. al. Standards Track [Page 46]

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