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................................................... 31.1 Structure of this document ................................. 41.2 Document terminology and conventions ....................... 41.2.1 Terminology............................................ 41.2.2 Design goals........................................... 51.2.3 Other document conventions............................. 52. Background and goals........................................... 52.1 Background ................................................. 52.1.1 Fax and email.......................................... 52.1.2 Current facilities in Internet Fax..................... 62.2 Closing the loop ........................................... 6
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2.3 Goals for content negotiation .............................. 83. Framework for content negotiation..............................103.1 Send data with an indication of alternatives ...............113.1.1 Choice of default data format..........................123.1.2 MDN request indicating alternate data formats..........123.1.3 Information about alternative data formats.............133.2 Receiver options ...........................................143.2.1 Alternatives not recognized............................143.2.2 Alternative not desired................................143.2.3 Alternative preferred..................................143.3 Send alternative message data ..............................163.4 Confirm receipt of resent message data .....................174. The Content-alternative header.................................185. The Original-Message-ID message header.........................186. MDN extension for alternative data.............................196.1 Indicating readiness to send alternative data ..............196.2 Indicating a preference for alternative data ...............206.3 Indicating alternative data is no longer available .........216.4 Indicating loss of original data ...........................226.5 Automatic sending of MDN responses .........................227. Internet Fax Considerations....................................228. Examples.......................................................238.1 Sending enhanced Internet Fax image ........................238.2 Internet fax with initial data usable ......................278.3 Negotiate to lower receiver capability .....................288.4 Sending an alternative content type ........................329. IANA Considerations............................................369.1 New message headers ........................................369.2 MDN extensions .............................................369.2.1 Notification option 'Alternative-available'............369.2.2 Notification option 'Alternative-not-available'........369.2.3 Disposition modifier 'Alternative-preferred'...........379.2.4 Disposition modifier 'Original-lost'...................3710. Internationalization considerations...........................3711. Security Considerations.......................................3712. Acknowledgements..............................................3813. References....................................................38
Appendix A: Implementation issues.................................40A.1 Receiver state .............................................40A.2 Receiver buffering of message data .........................41A.3 Sender state ...............................................42A.4 Timeout of offer of alternatives ...........................42A.5 Timeout of receiver capabilities ...........................42A.6 Relationship to timely delivery ............................43A.7 Ephemeral capabilities .....................................43A.8 Situations where MDNs must not be auto-generated ...........44
Appendix B: Candidates for further enhancements...................44
Authors' Addresses................................................45
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Full Copyright Statement..........................................46
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.
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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.
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.
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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.
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.
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.
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.
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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.
"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.
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.
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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.
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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
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.
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(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.
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(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.)
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.
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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].
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
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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.
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).
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.
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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.
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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.
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.
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.
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].
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
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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
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(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.
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.
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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.
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
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.
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.
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].
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].
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.
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
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).
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.
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.
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"
--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]
--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"
--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.
--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) )
--RAA14128.773615769/example.org--
Klyne, et. al. Standards Track [Page 28]
RFC 3297 Content Negotiation for Messaging Services July 2002
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"
--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]
--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"
--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.
--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) )
--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"
--RAA14128.773615768/ example.com
Content-type: image/tiff
Content-Transfer-Encoding: base64
[TIFF-FX profile-S message goes here]
--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"
--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.
--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) )
--RAA14128.773615769/example.org--
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"
--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]
--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"
--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
--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) ) )
--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"
--RAA14128.773615768/ example.com
Content-type: application/pdf
Content-Transfer-Encoding: base64
[PDF data goes here]
--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"
--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.
--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) ) )
--RAA14128.773615769/example.org--
Klyne, et. al. Standards Track [Page 35]
RFC 3297 Content Negotiation for Messaging Services July 2002
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.
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.
(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)
(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
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.
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
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.
[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.
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]
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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.
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.)
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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.
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.
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.
It should not be assumed that receiver capabilities declared during
negotiation are available indefinitely.
Klyne, et. al. Standards Track [Page 42]
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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.
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.
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]
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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]
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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
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Klyne, et. al. Standards Track [Page 46]