Network Working Group K. Toyoda

Network Working Group K. Toyoda
Request for Comments: 4141 PCC
Category: Standards Track D. Crocker
Brandenburg
November 2005

SMTP and MIME Extensions for Content Conversion

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 (2005).

Abstract

A message originator sometimes sends content in a form the recipient
cannot process or would prefer not to process a form of lower quality
than is preferred. Such content needs to be converted to an
acceptable form, with the same information or constrained information
(e.g., changing from color to black and white). In a store-and-
forward environment, it may be convenient to have this conversion
performed by an intermediary. This specification integrates two
ESMTP extensions and three MIME content header fields, which defines
a cooperative service that permits authorized, accountable content
form conversion by intermediaries.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Background. . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Overview. . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3. Notational Conventions. . . . . . . . . . . . . . . . . . 5
2. Applicability. . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Service Specification. . . . . . . . . . . . . . . . . . . . . 5
3.1. Sending Permission. . . . . . . . . . . . . . . . . . . . 9
3.2. Returning Capabilities. . . . . . . . . . . . . . . . . . 10
3.3. Next-Hop Non-Support of Service . . . . . . . . . . . . . 12
4. Content Conversion Permission SMTP Extension . . . . . . . . . 12
4.1. Content Conversion Permission Service Extension
Definition. . . . . . . . . . . . . . . . . . . . . . . . 12
4.2. CONPERM Parameter to Mail-From. . . . . . . . . . . . . . 13
4.3. Syntax. . . . . . . . . . . . . . . . . . . . . . . . . . 13
5. Content Negotiation SMTP Extension . . . . . . . . . . . . . . 13
5.1. Content Negotiation Service Extension Definition. . . . . 13
5.2. CONNEG Parameter to RCPT-TO . . . . . . . . . . . . . . . 14
5.3. Syntax. . . . . . . . . . . . . . . . . . . . . . . . . . 15
6. MIME Content-Features Header Field . . . . . . . . . . . . . . 16
7. MIME Content-Convert Header Field. . . . . . . . . . . . . . . 16
8. MIME Content-Previous Header Field . . . . . . . . . . . . . . 16
9. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
9.1. CONPERM Negotiation . . . . . . . . . . . . . . . . . . . 17
9.2. Example CONNEG Negotiation. . . . . . . . . . . . . . . . 18
9.3. Content-Previous. . . . . . . . . . . . . . . . . . . . . 19
10. Security Considerations. . . . . . . . . . . . . . . . . . . . 19
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Appendix A. CONNEG with Direct SMTP. . . . . . . . . . . . . . . . 22
Appendix B. USING Combinations of the Extensions . . . . . . . . . 23
Appendix C. MIME Content-Type Registrations. . . . . . . . . . . . 24

1. Introduction

Internet specifications typically define common capabilities for a
particular service that are supported by all participants. This
permits the sending of basic data without knowing which additional
capabilities individual recipients support. However, knowing those
capabilities permits the sending of additional types of data and data
of enhanced richness. Otherwise, a message originator will send
content in a form the recipient cannot process or will send multiple
forms of data. This specification extends the work of [CONMSG],
which permits a recipient to solicit alternative content forms from
the originator. The current specification enables MIME content
conversion by intermediaries, on behalf of a message originator and a
message recipient.

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1.1. Background

MIME enables the distinguishing and labeling of different types of
content [IMF, MEDTYP]. However, an email originator cannot know
whether a recipient is able to support (interpret) a particular data
type. To permit the basic use of MIME, a minimum set of data types
is specified as its support base. How will an originator know
whether a recipient can support any other data types?

A mechanism for describing MIME types is specified in [FEAT].
[CONMSG] specifies a mechanism that permits an originator to query a
recipient about the types it supports using email messages for the
control exchange. This permits a recipient to propagate information
about its capabilities back to an originator. For the control
exchange, using end-to-end email messages introduces considerable
latency and some unreliability.

An alternative approach is for an originator to use the "best" form
of data that it can, and to include the same types of permitted
representation information used in [CONMSG]. Hopefully, the
recipient, or an intermediary, can translate this into a form
supported by a limited recipient. This specification defines such a
mechanism. It defines a means of matching message content form to
the capabilities of a recipient device or system, by using MIME
content descriptors and the optional use of an SMTP-based negotiation
mechanism [ESMTP1, ESMTP2].

1.2. Overview

An originator describes desirable content forms in MIME content
descriptors. It may give "permission", to any intermediary or the
recipient, to convert the content to one of those forms. Separately,
an SMTP server may report the target's content capabilities back to
the SMTP client. The client is then able to convert the message
content into a form that is both supported by the target system and
acceptable to the originator.

A conversion service needs to balance between directions provided by
the originator, directions provided on behalf of the recipient, and
capabilities of the intermediary that performs the conversions. This
is complicated by the need to determine whether the directions are
advisory or whether they are intended to be requirements.
Conversions specified as advisory are performed if possible, but they
do not alter message delivery. In contrast, conversion
specifications that are treated as a requirement will prohibit
delivery if the recipient will not be able to process the content.

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These possibilities interact to form different processing scenarios,
in the event that the intermediary cannot satisfy the desires of both
the originator and the recipient:

Table 1: FAILURE HANDLING

\ RECEIVER| | |
+-------+ | Advise | Require |
ORIGINATOR\| | |
-----------+----------+----------+
| Deliver | Deliver |
Advise | original | original |
| content | content |
-----------+----------+----------+
| Return | Return |
Require | w/out | w/out |
| delivery | delivery |
-----------+----------+----------+

This table reflects a policy that determines failure handling solely
based on the direction provided by the originator. Thus, information
on behalf of the recipient is used to guide the details of
conversion, but not delivery of the message.

This is intended to continue the existing email practice of
delivering content that a recipient might not be able to process.
Clearly, the above table could be modified to reflect a different
policy. However, that would limit backward compatibility experienced
by users.

This specification provides mechanisms to support a controlled,
transit-time mail content conversion service, through a series of
mechanisms. These include:

* an optional ESMTP hop-by-hop service that uses the CONPERM SMTP
service extensions, issued by the originator,

* an optional ESMTP hop-by-hop service that uses the CONNEG SMTP
service extensions, issued on behalf of the recipient, and

* three MIME Content header fields (Content-Convert, Content-
Previous and * Content-Features) that specify appropriate
content header fields and record conversions that have been
performed.

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Figure 1: EXAMPLE RELAY ENVIRONMENT

+------------+ +-----------+
| Originator | | Recipient |
+------------+ +-----------+
||Posting Delivering/\
\/ ||
+--------+ +-----------------+ +--------+
| SMTP | | SMTP Relay | | SMTP |
| Client |--->| Server | Client |--->| Server |
+--------+ +--------+--------+ +--------+

1.3. Notational Conventions

In examples, "C:" and "S:" indicate lines sent by the client and
server respectively.

The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT" and "MAY" in
this document are to be interpreted as defined in "Key words for use
in RFCs to Indicate Requirement Levels" [KEYWORDS].

2. Applicability

This specification defines a cooperative mechanism that facilitates
early transformation of content. The mechanism can be used to save
bandwidth and to permit rendering on recipient devices that have
limited capabilities. In the first case, the assumption is that
conversion will produce smaller content. In the latter case, the
assumption is that the recipient device can render content in a form
derived from the original, but cannot render the original form.

The mechanism can impose significant resource requirements on
intermediaries performing conversions. Further, the intermediary
accepts responsibility for conversion prior to knowing whether it can
perform the conversion. Also note that conversion is not possible
for content that has been digitally signed or encrypted, unless the
converting intermediary can decode and re-code the content.

3. Service Specification

This service integrates two ESMTP extensions and three MIME content
header fields, in order to permit authorized, accountable content
form conversion by intermediaries. Intermediaries are ESMTP hosts
(clients and servers) along the transmission path between an
originator and a recipient.

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An originator specifies preferred content-types through the Content-
Convert MIME content header field. The content header fields occur
in each MIME body-part to which they apply. That is, each MIME
body-part contains its own record of conversion guidance and history.

The originator's preferences are raised to the level of requirement
through the ESMTP CONPERM service extension. The CONPERM mechanism
is only needed when an originator requires that conversion
limitations be enforced by the mail transfer service. If an
acceptable content type cannot be delivered, then no delivery is to
take place.

Target system capabilities are communicated in SMTP sessions through
the ESMTP CONNEG service extension. This information is used to
restrict the range of conversions that may be performed, but does not
affect delivery.

When CONPERM is used, conversions are performed by the first ESMTP
host that can obtain both the originator's permission and information
about the capabilities supported by the recipient. If a relay or
client is unable to transmit the message to a next-hop that supports
CONPERM or to perform appropriate conversion, then it terminates
message transmission and returns a [DSNSMTP, DSNFMT, SYSCOD] to the
originator, with status code 5.6.3 (Conversion required but not
supported).

When an SMTP relay or server performs content conversion, it records
which specific conversions are made into Content-Previous and
Content-Features MIME header fields associated with each converted
MIME body-part.

If a message is protected by strong content authentication or privacy
techniques, then an intermediary that converts message content MUST
ensure that the results of its processing are similarly protected.
Otherwise it MUST NOT perform conversion.

Originator Action:

An originator specifies desired conversion results through
the MIME Content-Convert header field. If the originator includes
a Content-Convert header field, then it must also include a
Content-Feature header field, to indicate the current form of the
content. Intermediaries MAY interpret the presence of this header
field as authorization to perform conversions. When Content-
Convert header fields are the sole means for guiding conversions
by intermediaries, then they serve only as advisories. Failure to
satisfy the guidance of these header fields does not affect final
delivery.

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When posting a new message, the originator MAY specify
transit-service enforcement of conversion limitations by using the
ESMTP CONPERM service extension. In each of the MIME body-parts
for which conversion is authorized, conversions MUST be limited to
those specified in MIME Content-Convert header fields. If
conversion is needed, but an authorized conversion cannot be
performed, then the message will be returned to the originator.
If CONPERM is not used, then failure to perform an authorized
conversion will not affect normal delivery handling.

Figure 2: CONPERM USAGE

+------------+
| Originator |
+------------+
SMTP ||
or || CONPERM
SUBMIT \/
+--------+ +----------------+
| SMTP | SMTP | SMTP Relay |
| Client |----------->| Server | |
+--------+ CONPERM +--------+-------+

Recipient Action:

With the ESMTP mail transfer service, capabilities that can
be supported on behalf of the recipient SHOULD be communicated to
intermediaries by the ESMTP CONNEG service extension.

Figure 3: CONNEG USAGE

+-----------+
| Recipient |
+-----------+
Capabilities||
\/
+----------------+ +--------+
| SMTP Relay | CONNEG | SMTP |
| | Client |<--------| Server |
+-------+--------+ +--------+

Intermediary Actions:

An intermediary MAY be given CONPERM direction when receiving
a message, and MAY be given CONNEG guidance before sending the
message. CONPERM and CONNEG operate on a per-message basis and
are issued through the ESMTP MAIL-FROM request. CONNEG response

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information is provided on a per-recipient basis, through the
response to ESMTP RCPT-TO.

Conversion MUST be performed by the first CONPERM
intermediary that obtains the CONNEG capability information. The
MIME Content-Type MUST conform to the result of the converted
content, as per [MEDTYP]. When an intermediary obtains different
capability information for different recipients of the same
message, it MAY either:

* Create a single, converted copy of the content that can be
supported by all of the recipients, or

* Create multiple converted copies, matching the capabilities
of subsets of the recipients. Each version is then sent
separately to an appropriate subset of the recipients, using
separate, standard SMTP sessions with separate, standard
RFC2821.Rcpt-To lists of addresses.

A record of conversions is placed into MIME Content-Previous
header fields. The current form of the content is described in
MIME Content-Features header fields.

A special case of differential capabilities occurs when an
intermediary receives capability information about some
recipients, but no information about others. An example of this
scenario can occur when sending a message to some recipients
within one's own organization, along with recipients located
elsewhere. The intermediary might have capability information
about the local recipients, but will not have any for distant
recipients. This is treated as a variation of the handling that
is required for situations in which the permissible conversions
are the null set -- that is, no valid conversions are possible for
a recipient.

Rather than simply failing transmission to the recipients for
which there is no capability information, the intermediary MAY
choose to split the list of addressees into subsets of separate,
standard RFC2821.Rcpt-To lists and separate, standard SMTP
sessions, and then continue the transmission of the original
content to those recipients via the continued use of the CONPERM
mechanism. Hence, the handling for such recipients is performed
as if no CONNEG transaction took place.

Once an intermediary has performed conversion, it MAY
terminate use of CONPERM. However, some relay environments, such
as those re-directing mail to a new target device, will benefit
from further conversion. Intermediaries MAY continue to use

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CONPERM or MAY re-initiate CONPERM use when they have knowledge of
possible variations in a target device.

NOTE: A new, transformed version of content may have less
information than the earlier version. Of course, a sequence of
transformations may lose additional information at each step.
Perhaps surprisingly, this can result in more loss than might
be necessary. For example, transformation x could change
content form A to content form B; then transformation y changes
B to C. However, it is possible that transformation y might
have accepted form A directly and produced form D, which has
more of the original information than C.

NOTE: An originator MAY validate any conversions that are made
by requesting a positive [DSNSMTP]. If the DSN request
includes the "RET" parameter, the delivery agent SHOULD return
an exact copy of the delivered (converted) message content.
This will permit the originator to inspect the results of any
conversion(s).

3.1. Sending Permission

A message originator that permits content conversion by
intermediaries MAY use the CONPERM ESMTP service extension and
Content-Convert MIME header fields to indicate what conversions are
permitted by intermediaries. Other mechanisms, by which a message
originator communicates this permission to the SMTP message transfer
service, are outside the scope of this specification.

NOTE: This option requires that a server make an open-ended
commitment to ensure that acceptable conversions are performed.
In particular, it is possible that an intermediary will be
required to perform conversion, but be unable to do so. The
result will be that the intermediary will be required to
perform conversion, but it will be performed in undelivered
mail.

When an ESMTP client is authorized to participate in the CONPERM
service, it MUST interact with the next SMTP hop server about:

* The server's ability to enforce authorized conversions, through
ESMTP CONPERM

* The capabilities supported for the target device or system,
through ESMTP CONNEG

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Successful use of CONPERM does not require that conversion take place
along the message transfer path. Rather, it requires that conversion
take place when a next-hop server reports capabilities that can be
supported on behalf of the recipient (through CONNEG) and that those
capabilities do not include support for the current representation of
the content.

NOTE: It is acceptable to have every SMTP server --
including the last-hop server -- support CONPERM, with none
offering CONNEG. In this case, the message is delivered to
the recipient in its original form. Any possible
conversions to be performed are left to the recipient.
Thus, the recipient is given the original form of the
content, along with an explicit list of conversions deemed
acceptable by the originator.

An SMTP server MAY offer ESMTP CONPERM, without being able to perform
conversions, if it knows conversions can be performed along the
remainder of the transfer path, or by the target device or system.

3.2. Returning Capabilities

A target recipient device or system arranges announcements of its
content form capabilities to the SMTP service through a means outside
the scope of this specification. Note that enabling a server to
issue CONNEG information on behalf of the recipient may require a
substantial mechanism between the recipient and server. When an
ESMTP server knows a target's capabilities, it MAY offer the CONNEG
ESMTP service extension.

NOTE: One aspect of that mechanism, between the recipient
and an ESMTP server offering the CONNEG ESMTP service
extension could include offering capabilities beyond those
directly supported by the recipient. In particular, the
server -- or other intermediaries between the server and the
recipient -- could support capabilities that they can
convert to a recipient's capability. As long as the result
is acceptable to the set specified in the relevant Content-
Convert header fields of the message being converted, the
details of these conversions are part of the
recipient/server mechanism, and fall outside the scope of
the current specification.

If a next-hop ESMTP server responds that it supports CONNEG when a
message is being processed according to the CONPERM mechanism, then
the SMTP client:

1) MUST request CONNEG information

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2) MUST perform the requisite conversions, if possible, before
sending the message to the next-hop SMTP server

3) MUST fail message processing, if any conversion for the message
fails, and MUST return a failure DSN to the originator with
status code 5.6.5 (Conversion failed).

When performing conversions, as specified in Content-Convert MIME
header fields, the Client MUST:

1) Add a Content-Previous header field and a Content-Features
header field to each MIME body-part that has been converted,
removing any existing Content-Features header fields.

2) Either:

* Send a single copy to the next-hop SMTP server, using the
best capabilities supported by all recipients along that
path, or

* Separate the transfers into multiple, standard
RFC2821.Rcpt-To and ESMTP sessions, in order to provide
the best conversions possible for subsets of the
recipients.

If the transfers are to be separated, then the current session MUST
be terminated, and new sessions conducted for each subset.

The conversions to be performed are determined by the intersection of
three lists:

* Conversions permitted by the originator

* Content capabilities of the target

* Conversions that can be performed by the SMTP client host

Failed Conversion

If the result of this intersection is the null set of
representations, for an addressee, then delivery to that addressee
MUST be handled as a conversion failure.

If handling is subject to the CONPERM mechanism and:

* the next-hop SMTP host does not indicate that it can
represent the target's capabilities through CONNEG, but

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* does respond that it can support CONPERM, then the client
SMTP MUST send the existing content, if all other SMTP
transmission requirements are satisfied.

If handling is not subject to the CONPERM mechanism, then
conversion failures do not affect message delivery.

3.3. Next-Hop Non-Support of Service

If a Client is participating in the CONPERM mechanism, but the next-
hop SMTP server does not support CONPERM or CONNEG, then the SMTP
client

1) MUST terminate the session to the next-hop SMTP server, without
sending the message

2) MUST return a DSN notification to the originator, with status
code 5.6.3 (Conversion required but not supported). [DSNSMTP,
DSNFMT, SYSCOD]

If a Client is participating in the CONPERM mechanism and the next-
hop SMTP server supports CONNEG, but provides no capabilities for an
individual RCPT-TO addressee, then the SMTP client's processing for
that recipient MUST be either to:

1) Treat the addressee as a conversion failure, or

2) Separate the addressee from the address list that is processed
according to CONNEG, and continue to process the addressee
according to CONPERM.

4. Content Conversion Permission SMTP Extension

4.1. Content Conversion Permission Service Extension Definition

1) The name of the SMTP service extension is

"Content-Conversion-Permission"

2) The EHLO keyword value associated with this extension is

"CONPERM"

3) A parameter using the keyword "CONPERM" is added to the MAIL-FROM
command.

4) The server responds with acceptance or rejection of support for
CONPERM, for this message.

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4.2. CONPERM Parameter to Mail-From

Parameter:

CONPERM

Arguments:

There are no arguments. Specification of permitted
conversions is located in a Content-Convert header field for each
MIME body-part in which conversion is permitted.

Client Action:

If the server issued a 250-CONPERM as part of its EHLO
response for the current session, and the client is participating
in the CONPERM service for this message -- such as by having
received the message with a CONPERM requirement -- then the client
MUST issue the CONPERM parameter in the MAIL-FROM. If the server
does not issue 250-CONPERM, and the client is participating in the
CONPERM service for this message, then the client MUST treat the
transmission as permanently rejected.

Server Action:

If the client specifies CONPERM in the MAIL-FROM, but the
server does not support the CONPERM parameter, the server MUST
reject the MAIL-FROM command with a 504-CONPERM reply.

If the client issues the CONPERM parameter in the MAIL-FROM,
then the server MUST conform to this specification. Either it
MUST relay the message according to CONPERM, or it MUST convert
the message according to CONNEG information.

4.3. Syntax

Content-Conversion-Permission = "CONPERM"

5. Content Negotiation SMTP Extension

5.1. Content Negotiation Service Extension Definition

1) The name of the SMTP service extension is:

"Content-Negotiation"

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2) The EHLO keyword value associated with this extension is:

"CONNEG"

3) A parameter, using the keyword "CONNEG", is added to the RCPT-TO
command.

4) The server responds with a report indicating the content
capabilities that can be received on behalf of the recipient
device or system, associated with the target RCPT-TO address.

5.2. CONNEG Parameter to RCPT-TO

Parameter:

CONNEG

Arguments:

There are no arguments.

Client Action:

If a message is subject to CONPERM requirements and the
server issues a 250-CONNEG, as part of its EHLO response for the
current session, the client MUST issue the CONNEG parameter in the
RCPT-TO request. If the message is not subject to CONPERM
requirements, and the server issues a 250-CONNEG, the client MAY
issue the CONNEG parameter with RCPT-TO.

If the client issues the CONNEG parameter with RCPT-TO, then
it MUST honor the capabilities returned in the CONNEG RCPT-TO
replies for that message. In addition, it MUST convert the
message content, if the current form of the content is not
included in the capabilities listed, on behalf of the recipient.

The conversions that are performed are determined by the
intersection of the:

* Conversions permitted by the originator

* Content capabilities of the target

* Conversions that can be performed by the SMTP client host

If the result of this intersection is the null set of
representations, then the Client processing depends upon whether
the next-hop server has offered CONPERM, as well as CONNEG:

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1) If the message will be subject to CONPERM at the next hop,
the Client MAY transmit the original content to the next hop
and continue CONPERM requirements.

2) Otherwise, the Client MUST treat the conversion as failed.

If the result of the intersection is not null, the client
SHOULD convert the data to the "highest" level of capability of
the server. Determination of the level that is highest is left to
the discretion of the host performing the conversion.

Each converted MIME body-part MUST have a Content-Previous
header field that indicates the previous body-part form and a
Content-Features header field, indicating the new body-part form.

Server Action:

If the client specifies CONNEG in the RCPT-TO, but the server
does not support the CONNEG parameter, the server MUST reject the
RCPT-TO addressees with 504 replies.

If the server does support the CONNEG parameter, and it knows
the capabilities of the target device or system, then it MUST
provide that information through CONNEG. The server MAY provide a
broader list than is supported by the recipient if the server can
ensure that the form of content delivered can be processed by the
recipient, while still satisfying the constraints of the author's
Content-Convert specification(s).

The response to a CONNEG RCPT-TO request will be multi-line
RCPT-TO replies. For successful (250) responses, at least the
first line of the response must contain RCPT-TO information other
than CONNEG. Additional response lines are for CONNEG. To avoid
problems due to variations in line buffer sizes, the total
parametric listing must be provided as a series of lines, each
beginning with "250-CONNEG", except for the last line, which is
"250 CONNEG".

The contents of the capability listing MUST conform to the
specifications in [SYN] and cover the same range of specifications
permitted in [CONMSG].

5.3. Syntax

Content-Negotiation = "CONNEG"
Capability = { <filter> specification,
as per [SYN] }

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6. MIME Content-Features Header Field

The Content-Features header field describes the characteristics of
the current version of the content for the MIME body-part in which
the header field occurs. There is a separate Content-Features header
field for each MIME body-part. The specification for this header
field is contained in [FEAT].

7. MIME Content-Convert Header Field

Content-Convert is a header field that specifies preferred
conversions for the associated content. It MAY be used without the
other mechanisms defined in this document. If present, this header
field MUST be carried unmodified and delivered to the recipient. In
its absence, the content originator provides no guidance about
content conversions, and intermediaries SHOULD NOT perform content
conversion.

In the extended ABNF notation, the Content-Convert header field is
defined as follows:

Convert = "Content-convert" ":"
permitted

Permitted = "ANY" / "NONE" / permitted-list

permitted-list = { explicit list of permitted
final forms, using <filter>
syntax in [SYN] }

If the permitted conversions are specified as "ANY", then the
intermediary may perform any conversions it deems appropriate.

If the permitted conversions are specified as "NONE", then the
intermediary SHOULD NOT perform any conversions to this MIME body-
part, even when the target device or system does not support the
original form of the content.

If a Content-Convert header field is present, then a Content-Features
header field MUST also be present to describe the current form of the
Content.

8. MIME Content-Previous Header Field

When an intermediary has performed conversion of the associated
content, the intermediary MUST record details of the previous
representation, from which the conversion was performed. This
information is placed in a Content-Previous header field that is part

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of the MIME body-part with the converted content. There is a
separate header field for each converted MIME body-part.

When an intermediary has performed conversion, the intermediary MUST
record details of the result of the conversion by creating or
revising the Content-Features header field for the converted MIME
body-part.

In the extended [ABNF] notation, the Content-Previous header field is
defined as follows:

previous = "Content-Previous" [CFWS] ":"
[CFWS]
date by type

date = "Date " [CFWS] date-time [CFWS] ";"
[CFWS]

by = "By " [CFWS] domain [CFWS] ";"
[CFWS]

type = { content characteristics, using
<filter> syntax in [SYN] }

The Date field specifies the date and time at which the indicated
representation was converted into a newer representation.

The By field specifies the domain name of the intermediary that
performed the conversion.

An intermediary MAY choose to derive the Content-Previous header
field, for a body-part, from an already-existing Content-Features
header field in that body-part, before that header field is replaced
with the description of the current representation.

9. Examples

9.1. CONPERM Negotiation

S: 220 example.com IFAX
C: EHLO example.com
S: 250- example.com
S: 250-DSN
S: 250 CONPERM
C: MAIL FROM:[email protected] CONPERM
S: 250 <[email protected]> originator ok
C: RCPT TO:<[email protected]>
S: 250-<[email protected]> recipient ok

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C: DATA
S: 354 okay, send data
C: <<RFC 2822 message with MIME Content-Type:TIFF-FX
Per:
( image-file-structure=TIFF-minimal
dpi=400
image-coding=JBIG
size-x=2150/254
paper-size=letter
)
with MIME body-parts including:
Content-Convert:
(&(image-file-structure=TIFF-minimal)
(MRC-mode=0)
(color=Binary)
(|(&(dpi=204)
(dpi-xyratio=[204/98,204/196]) )
(&(dpi=200)
(dpi-xyratio=[200/100,1]) )
(&(dpi=400)
(dpi-xyratio=1) ) )
(|(image-coding=[MH,MR,MMR])
(&(image-coding=JBIG)
(image-coding-constraint=JBIG-T85)
(JBIG-stripe-size=128) ) )
(size-x<=2150/254)
(paper-size=[letter,A4])
(ua-media=stationery) )
>>
S: 250 message accepted
C: QUIT
S: 221 goodbye

9.2. Example CONNEG Negotiation

S: 220 example.com IFAX
C: EHLO example.com
S: 250- example.com
S: 250-DSN
S: 250 CONNEG
C: MAIL FROM:<[email protected]>
S: 250 <[email protected]> originator ok
C: RCPT TO:<[email protected]> CONNEG
S: 250-<[email protected]> recipient ok
S: 250-CONNEG (&(image-file-structure=TIFF-minimal)
S: 250-CONNEG (MRC-mode=0)
S: 250-CONNEG (color=Binary)
S: 250-CONNEG (|(&(dpi=204)

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S: 250-CONNEG (dpi-xyratio=[204/98,204/196]) )
S: 250-CONNEG (&(dpi=200)
S: 250-CONNEG (dpi-xyratio=[200/100,1]) ) )
S: 250-CONNEG (image-coding=[MH,MR,MMR])
S: 250-CONNEG (size-x<=2150/254)
S: 250-CONNEG (paper-size=[letter,A4])
S: 250 CONNEG (ua-media=stationery) )
C: DATA
S: 354 okay, send data
C: <<RFC 2822 message with MIME Content-Type:TIFF-FX
Per:
( image-file-structure=TIFF-minimal
dpi=400
image-coding=JBIG
size-x=2150/254
paper-size=letter
)
>>
S: 250 message accepted
C: QUIT
S: 221 goodbye

9.3. Content-Previous

Content-Previous:
Date Tue, 1 Jul 2001 10:52:37 +0200;
By relay.example.com;
(&(image-file-structure=TIFF-minimal)
(MRC-mode=0)
(color=Binary)
(&(dpi=400)
(dpi-xyratio=1) )
(&(image-coding=JBIG)
(image-coding-constraint=JBIG-T85)
(JBIG-stripe-size=128) )
(size-x=2150/254)
(paper-size=A4)
(ua-media=stationery) )

10. Security Considerations

This service calls for disclosure of capabilities, on behalf of
recipients. Mechanisms for determining the requestor's and the
respondent's authenticated identity are outside the scope of this
specification. These mechanisms are intended to permit disclosure of
information that is safe for public distribution; hence, there is no
inherent need for security measures.

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RFC 4141 SMTP & MIME Extensions for Content Conversion November 2005

Information that should have restricted distribution is still able to
be disclosed. Therefore, it is the responsibility of the disclosing
ESMTP server or disclosing ESMTP client to determine whether
additional security measures should be applied to the use of this
ESMTP option.

Use of the ESMTP CONNEG option permits content transformation by an
intermediary, along the mail transfer path. When the contents are
encrypted, the intermediary cannot perform the conversion, because it
is not expected to have access to the relevant secret keying
material. When the contents are signed, but not encrypted,
conversion will invalidate the signature. This specification
provides for potentially unbounded computation by intermediary MTAs,
depending on the nature and amount of conversion required. Further,
this computation burden might provide an opportunity for denial-of-
service attacks, given that Internet mail typically permits
intermediaries to receive messages from all Internet sources.

This specification provides for content conversion by unspecified
intermediaries. Use of this mechanism carries significant risk.
Although intermediaries always have the ability to perform damaging
transformations, use of this specification could result in more
exploration of that potential and, therefore, more misbehavior. Use
of intermediaries is discussed in [RFC3238].

CONPERM/CONNEG provide a cooperative mechanism, rather than enabling
intermediary actions that were not previously possible.
Intermediaries already make conversions on their own initiative.
Hence, the mechanism introduces essentially no security concerns,
other than divulging recipient preferences.

11. Acknowledgements

Graham Klyne and Eric Burger provided extensive, diligent reviews and
suggestions. Keith Moore, Giat Hana, and Joel Halpern provided
feedback that resulted in improving the specification's integration
into established email practice.

12. References

12.1. Normative References

[ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.

[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

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RFC 4141 SMTP & MIME Extensions for Content Conversion November 2005

[CONMSG] Klyne, G., Iwazaki, R., and D. Crocker, "Content
Negotiation for Messaging Services based on Email", RFC
3297, July 2002.

[DSNSMTP] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
Extension for Delivery Status Notifications (DSNs)", RFC
3461, January 2003.

[DSNFMT] Moore, K. and G. Vaudreuil, "An Extensible Message Format
for Delivery Status Notifications", RFC 3464, January
2003.

[SYSCOD] Vaudreuil, G., "Enhanced Mail System Status Codes", RFC
3463, January 2003.

[ESMTP1] Klensin, J., Freed, N., Rose, M., Stefferud, E., and D.
Crocker, "SMTP Service Extensions", STD 10, RFC 1869,
November 1995.

[ESMTP2] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001.

[FEAT] Klyne, G., "Indicating Media Features for MIME Content",
RFC 2912, September 2000.

[IMF] Resnick, P., "Internet Message Format", RFC 2822, April
2001.

[SYN] Klyne, G., "A Syntax for Describing Media Feature Sets",
RFC 2533, March 1999.

[MEDTYP] IANA, "MIME Media Types",
<http://www.iana.org/assignments/media-types>

[CFWS] Alvestrand, H., "Content Language Headers", RFC 3282, May
2002.

12.2. Informative References

[RFC3238] Floyd, S. and L. Daigle, "IAB Architectural and Policy
Considerations for Open Pluggable Edge Services", RFC
3238, January 2002.

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Appendix A. CONNEG with Direct SMTP

This Appendix is descriptive. It only provides discussion of usage
issues permitted or required by the normative text

In some configurations, it is possible to have direct, email-based
interactions, where the originator's system conducts a direct,
interactive TCP connection with the recipient's system. This
configuration permits a use of the content form negotiation service
that conforms to the specification here, but permits some
simplifications. This single SMTP session does not have the
complexity of multiple, relaying sessions and therefore does not have
the requirement for propagating permissions to intermediaries.

The Originator's system provides user-level functions for the
originator, and it contains the SMTP Client for sending messages.
Hence, the formal step of email "posting" is a process that is
internal or virtual, within the Originating system. The recipient's
service contains the user-level functions for the recipient, and
contains the SMTP server for receiving messages. Hence, the formal
steps of email "delivering" and "receipt" are internal or virtual,
within the Receiving system.

Figure 4: DIRECT CONNEG

Originating system Receiving system
+------------------+ +------------------+
| +------------+ | | +-----------+ |
| | Originator | | | | Recipient | |
| +------------+ | | +-----------+ |
| ||Posting | | /\Receiving |
| \/ | | || |
| +---------+ | | +--------+ |
| | SMTP |<---|-------|----| SMTP | |
| | Client |----|-------|--->| Server | |
| +---------+ | | +--------+ |
+------------------+ +------------------+

In this case, CONPERM is not needed because the SMTP Client is part
of the originating system and already has the necessary permission.
Similarly, the SMTP server will be certain to know the recipient's
capabilities, because the server is part of the receiving system.

Therefore, Direct Mode email transmission can achieve content
capability and form matching by having:

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* Originating systems that conform to this specification and a
communication process between originator and recipient that is
the same as would take place between a last-hop SMTP Relay and
the Delivering SMTP server to which it is connected.

* That is, the Client and Server MUST employ CONNEG and the
Client MUST perform any requisite conversions.

Appendix B. Using Combinations of the Extensions

This specification defines a number of mechanisms. It is not
required that they all be used together. For example, the difference
between listing preferred conversions -- versus specifying enforced
limitations to conversions -- is discussed in the Introduction. This
Appendix further describes scenarios that might call for using fewer
than the complete set defined in this specification. It also
summarizes the conditions which mandate that an intermediary perform
conversion.

This Appendix is descriptive. It only provides discussion of usage
issues permitted or required by the normative text

The available mechanisms are:

1. CONPERM Parameter to Mail-From
2. CONNEG parameter to RCPT-TO
3. MIME Content-Convert Header Field
4. MIME Content-Previous Header Field
5. MIME Content-Features Header Field

B.1. Specifying Suggested Conversion Constraints

Use of the MIME Content-Convert header field specifies the
originator's preferences, should conversion be performed. This does
not impose any requirements on the conversion; it is merely advisory.

B.2. Specifying Required Conversion Constraints

When the MIME Content-Convert specification is coupled with the ESMTP
CONPERM option, then the originator's specification of preferred
conversions rises to the level of requirement. No other conversions
are permitted, except those specified in the Content-Convert header
field.

Note that the presence of both mechanisms does not require that
conversions be performed. Rather, it constrains conversions,
should they occur.

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B.3. Accepting All Forms of Content

Although it is unlikely that any device will always able to process
every type of existing content, some devices can be upgraded easily
(e.g., adding plug-in). Hence, such a device is able to process all
content effectively.

For such devices, it is better to refrain from issuing a CONNEG
assertion. Instead, the CONPERM request should be propagated to the
target device.

B.4. When Conversion is Required

A node is required to perform conversion when:

1. At least one MIME Content-Covert header field is present in the
message,

2. ESMTP CONPERM is in force at the node processing the message,

3. ESMTP CONNEG is also in force at the same node,

4. The current content form is not cited in the CONNEG list,

5. At least one content form is present, both in the Content-
Convert list and the CONNEG list, and

6. The intermediary is able to convert from the current form to
one of the forms listed in both Content-Convert and CONNEG.

Appendix C. MIME Content-Type Registrations

C.1. Content-Convert

Header field name:
Content-Convert

Applicable protocol:
Mail (RFC 2822)

Status:
Proposed Standard

Author/Change controller:
IETF

Specification document(s):
RFC 4141.

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Related information:
None.

C.2. Content-Previous

Header field name:
Content-Previous

Applicable protocol:
Mail (RFC 2822)

Status:
Proposed Standard

Author/Change controller:
IETF

Specification document(s):
RFC 4141, Section 8

Related information:
None.

Authors' Addresses

Kiyoshi Toyoda
Panasonic Communications Co., Ltd.
4-1-62 Minoshima Hakata-ku, Fukuoka 812-8531 Japan

EMail: [email protected]

Dave Crocker
Brandenburg InternetWorking
675 Spruce Drive
Sunnyvale, CA 94086 USA

Phone: +1.408.246.8253
EMail: [email protected]

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Full Copyright Statement

Copyright (C) The Internet Society (2005).

This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.

This document and the information contained herein are provided on an
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OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM 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.

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Acknowledgement

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

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