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rfc:rfc4141

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.

Toyoda & Crocker Standards Track [Page 1] RFC 4141 SMTP & MIME Extensions for Content Conversion November 2005

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.

Toyoda & Crocker Standards Track [Page 2] RFC 4141 SMTP & MIME Extensions for Content Conversion November 2005

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.

Toyoda & Crocker Standards Track [Page 3] RFC 4141 SMTP & MIME Extensions for Content Conversion November 2005

 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.

Toyoda & Crocker Standards Track [Page 4] RFC 4141 SMTP & MIME Extensions for Content Conversion November 2005

            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.

Toyoda & Crocker Standards Track [Page 5] RFC 4141 SMTP & MIME Extensions for Content Conversion November 2005

 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.

Toyoda & Crocker Standards Track [Page 6] RFC 4141 SMTP & MIME Extensions for Content Conversion November 2005

         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

Toyoda & Crocker Standards Track [Page 7] RFC 4141 SMTP & MIME Extensions for Content Conversion November 2005

    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

Toyoda & Crocker Standards Track [Page 8] RFC 4141 SMTP & MIME Extensions for Content Conversion November 2005

    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

Toyoda & Crocker Standards Track [Page 9] RFC 4141 SMTP & MIME Extensions for Content Conversion November 2005

 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

Toyoda & Crocker Standards Track [Page 10] RFC 4141 SMTP & MIME Extensions for Content Conversion November 2005

    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

Toyoda & Crocker Standards Track [Page 11] RFC 4141 SMTP & MIME Extensions for Content Conversion November 2005

  • 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.

Toyoda & Crocker Standards Track [Page 12] RFC 4141 SMTP & MIME Extensions for Content Conversion November 2005

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"

Toyoda & Crocker Standards Track [Page 13] RFC 4141 SMTP & MIME Extensions for Content Conversion November 2005

 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:

Toyoda & Crocker Standards Track [Page 14] RFC 4141 SMTP & MIME Extensions for Content Conversion November 2005

       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] }

Toyoda & Crocker Standards Track [Page 15] RFC 4141 SMTP & MIME Extensions for Content Conversion November 2005

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

Toyoda & Crocker Standards Track [Page 16] RFC 4141 SMTP & MIME Extensions for Content Conversion November 2005

 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:May@some.example.com CONPERM
 S: 250 <May@some.example.com> originator ok
 C: RCPT TO:<June@some.example.com>
 S: 250-<June@some.example.com> recipient ok

Toyoda & Crocker Standards Track [Page 17] RFC 4141 SMTP & MIME Extensions for Content Conversion November 2005

 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:<May@some.example.com>
 S: 250 <May@some.example.com> originator ok
 C: RCPT TO:<June@ifax1.jp> CONNEG
 S: 250-<June@some.example.com> 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)

Toyoda & Crocker Standards Track [Page 18] RFC 4141 SMTP & MIME Extensions for Content Conversion November 2005

 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.

Toyoda & Crocker Standards Track [Page 19] 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.

Toyoda & Crocker Standards Track [Page 20] 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.

Toyoda & Crocker Standards Track [Page 21] RFC 4141 SMTP & MIME Extensions for Content Conversion November 2005

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:

Toyoda & Crocker Standards Track [Page 22] RFC 4141 SMTP & MIME Extensions for Content Conversion November 2005

  • 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.

Toyoda & Crocker Standards Track [Page 23] RFC 4141 SMTP & MIME Extensions for Content Conversion November 2005

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.

Toyoda & Crocker Standards Track [Page 24] RFC 4141 SMTP & MIME Extensions for Content Conversion November 2005

 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: toyoda.kiyoshi@jp.panasonic.com
 Dave Crocker
 Brandenburg InternetWorking
 675 Spruce Drive
 Sunnyvale, CA  94086  USA
 Phone: +1.408.246.8253
 EMail: dcrocker@bbiw.net

Toyoda & Crocker Standards Track [Page 25] RFC 4141 SMTP & MIME Extensions for Content Conversion November 2005

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
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 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.

Intellectual Property

 The IETF takes no position regarding the validity or scope of any
 Intellectual Property Rights or other rights that might be claimed to
 pertain to the implementation or use of the technology described in
 this document or the extent to which any license under such rights
 might or might not be available; nor does it represent that it has
 made any independent effort to identify any such rights.  Information
 on the procedures with respect to rights in RFC documents can be
 found in BCP 78 and BCP 79.
 Copies of IPR disclosures made to the IETF Secretariat and any
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 attempt made to obtain a general license or permission for the use of
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 http://www.ietf.org/ipr.
 The IETF invites any interested party to bring to its attention any
 copyrights, patents or patent applications, or other proprietary
 rights that may cover technology that may be required to implement
 this standard.  Please address the information to the IETF at ietf-
 ipr@ietf.org.

Acknowledgement

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

Toyoda & Crocker Standards Track [Page 26]

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