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Versions: (draft-tomkinson-slim-multilangcontent) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14

IETF                                                        N. Tomkinson
Internet-Draft                                             N. Borenstein
Intended status: Standards Track                            Mimecast Ltd
Expires: February 19, 2018                               August 18, 2017


                     Multiple Language Content Type
                  draft-ietf-slim-multilangcontent-14

Abstract

   This document defines the multipart/multilingual content type, which
   is an addition to the Multipurpose Internet Mail Extensions (MIME)
   standard to make it possible to send one message that contains
   multiple language versions of the same information.  The translations
   would be identified by a language tag and selected by the email
   client based on a user's language settings.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on February 19, 2018.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of




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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  The Content-Type Header Field . . . . . . . . . . . . . . . .   3
   3.  The Message Parts . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  The Multilingual Preface  . . . . . . . . . . . . . . . .   4
     3.2.  The Language Message Parts  . . . . . . . . . . . . . . .   5
     3.3.  The Language Independent Message Part . . . . . . . . . .   5
   4.  Message Part Selection  . . . . . . . . . . . . . . . . . . .   5
   5.  The Content-Language Field  . . . . . . . . . . . . . . . . .   6
   6.  The Content-Translation-Type Field  . . . . . . . . . . . . .   7
   7.  The Subject Field in the Language Message parts . . . . . . .   7
   8.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     8.1.  An Example of a Simple Multiple language email message  .   8
     8.2.  An Example of a Multiple language email message with
           language independent part . . . . . . . . . . . . . . . .   9
     8.3.  An Example of a complex Multiple language email message
           with language independent part  . . . . . . . . . . . . .  11
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
     10.1.  The multipart/multilingual MIME type . . . . . . . . . .  13
     10.2.  The Content-Translation-Type Field . . . . . . . . . . .  15
     10.3.  The Content-Translation-Type Field Values  . . . . . . .  15
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  16
   12. Changes from Previous Versions  . . . . . . . . . . . . . . .  16
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  21
     13.2.  Informational References . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

1.  Introduction

   Since the invention of email and the rapid spread of the Internet,
   more and more people have been able to communicate in more and more
   countries and in more and more languages.  But during this time of
   technological evolution, email has remained a single-language
   communication tool, whether it is English to English, Spanish to
   Spanish or Japanese to Japanese.

   Also during this time, many corporations have established their
   offices in multi-cultural cities and formed departments and teams
   that span continents, cultures and languages, so the need to
   communicate efficiently with little margin for miscommunication has
   grown significantly.



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   This document defines the multipart/multilingual content type, which
   is an addition to the Multipurpose Internet Mail Extensions (MIME)
   standard, to make it possible to send a single message to a group of
   people in such a way that all of the recipients can read the email in
   their preferred language.  The methods of translation of the message
   content are beyond the scope of this document, but the structure of
   the email itself is defined herein.

   Whilst this document depends on identification of language in message
   parts for non-real-time communication, there is a companion document
   that is concerned with a similar problem for real-time communication:
   [I-D.ietf-slim-negotiating-human-language]

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  The Content-Type Header Field

   The "multipart/multilingual" MIME subtype allows the sending of a
   message in a number of different languages with the translations
   embedded in the same message.  This MIME subtype helps the receiving
   email client make sense of the message structure.

   The multipart subtype "multipart/multilingual" has similar semantics
   to "multipart/alternative" (as discussed in RFC 2046 [RFC2046]) in
   that each of the message parts is an alternative version of the same
   information.  The primary difference between "multipart/multilingual"
   and "multipart/alternative" is that when using "multipart/
   multilingual", the message part to select for rendering is chosen
   based on the values of the Content-Language field and optionally the
   Content-Translation-Type field instead of the ordering of the parts
   and the Content-Types.

   The syntax for this multipart subtype conforms to the common syntax
   for subtypes of multipart given in section 5.1.1. of RFC 2046
   [RFC2046].  An example "multipart/multilingual" Content-Type header
   field would look like this:

   Content-Type: multipart/multilingual; boundary=01189998819991197253

3.  The Message Parts

   A multipart/multilingual message will have a number of message parts:
   exactly one multilingual preface, one or more language message parts




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   and zero or one language independent message part.  The details of
   these are described below.

3.1.  The Multilingual Preface

   In order for the message to be received and displayed in non-
   conforming email clients, the message SHOULD contain an explanatory
   message part which MUST NOT be marked with a Content-Language field
   and MUST be the first of the message parts.  For maximum support in
   the most basic of non-conforming email clients, it SHOULD have a
   Content-Type of text/plain.  Because non-conforming email clients are
   expected to treat a message with an unknown multipart type as
   multipart/mixed (in accordance with sections 5.1.3 and 5.1.7 of RFC
   2046 [RFC2046]) they may show all of the message parts sequentially
   or as attachments.  Including and showing this explanatory part will
   help the message recipient understand the message structure.

   This initial message part SHOULD explain briefly to the recipient
   that the message contains multiple languages and the parts may be
   rendered sequentially or as attachments.  This SHOULD be presented in
   the same languages that are provided in the subsequent language
   message parts.

   As this explanatory section is likely to contain languages using
   scripts that require non-US-ASCII characters, it is RECOMMENDED that
   UTF-8 charset is used for this message part.  See RFC 3629 [RFC3629]
   for details of UTF-8.

   Whilst this section of the message is useful for backward
   compatibility, it will normally only be shown when rendered by a non-
   conforming email client, because conforming email clients SHOULD only
   show the single language message part identified by the user's
   preferred language and the language message part's Content-Language.

   For the correct display of the multilingual preface in a non-
   conforming email client, the sender MAY use the Content-Disposition
   field with a value of 'inline' in conformance with RFC 2183 [RFC2183]
   (which defines the Content-Disposition field).  If provided, this
   SHOULD be placed at the multipart/multilingual level and in the
   multilingual preface.  This makes it clear to a non-conforming email
   client that the multilingual preface should be displayed immediately
   to the recipient, followed by any subsequent parts marked as
   'inline'.

   For an example of a multilingual preface, see the examples in
   Section 8.





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3.2.  The Language Message Parts

   The language message parts are typically translations of the same
   message content.  These message parts SHOULD be ordered so that the
   first part after the multilingual preface is in the language believed
   to be the most likely to be recognised by the recipient as this will
   constitute the default part when language negotiation fails and there
   is no Language Independent part.  All of the language message parts
   MUST have a Content-Language field and a Content-Type field and MAY
   have a Content-Translation-Type field.

   The Content-Type for each individual language message part SHOULD be
   message/rfc822 to provide good support with non-conforming email
   clients.  However, an implementation MAY use message/global as
   support for message/global becomes more commonplace.  See RFC 6532
   [RFC6532] for details of message/global.  Each language message part
   should have a Subject field in the appropriate language for that
   language part.  If there is a From field present, its value MUST
   include the same email address as the top-level From header although
   the display name MAY be a localised version.  If there is a mismatch
   of sender email address, the top-level From header value SHOULD be
   used to show to the recipient.

3.3.  The Language Independent Message Part

   If there is language independent content for the recipient to see if
   they have a preferred language other than one of those specified in
   the language message parts and the default language message part is
   unlikely to be understood, another part MAY be provided.  This part
   could typically include one or more language independent graphics.
   When this part is present, it MUST be the last part and MUST have a
   Content-Language field with a value of "zxx" (as described in BCP 47/
   RFC 5646 [RFC5646]).  The part SHOULD have a Content-Type of message/
   rfc822 or message/global (to match the language message parts).

4.  Message Part Selection

   The logic for selecting the message part to render and present to the
   recipient is summarised in the next few paragraphs.

   Firstly, if the email client does not understand multipart/
   multilingual then it will treat the message as if it was multipart/
   mixed and render message parts accordingly (in accordance with
   sections 5.1.3 and 5.1.7 of RFC 2046 [RFC2046]).

   If the email client does understand multipart/multilingual then it
   SHOULD ignore the multilingual preface and select the best match for
   the user's preferred language from the language message parts



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   available.  Also, the user may prefer to see the original message
   content in their second language over a machine translation in their
   first language.  The Content-Translation-Type field value can be used
   for further selection based on this preference.  The selection of
   language part may be implemented in a variety of ways, although the
   matching schemes detailed in RFC 4647 [RFC4647] are RECOMMENDED as a
   starting point for an implementation.  The goal is to render the most
   appropriate translation for the user.

   If there is no match for the user's preferred language (or there is
   no preferred language information available) the email client SHOULD
   select the language independent part (if one exists) or the first
   language part (directly after the multilingual preface) if a language
   independent part does not exist.

   If there is no translation type preference information available, the
   values of the Content-Translation-Type field may be ignored.

   Additionally, interactive implementations MAY offer the user a choice
   from among the available languages or the option to see them all.

5.  The Content-Language Field

   The Content-Language field in the individual language message parts
   is used to identify the language in which the message part is
   written.  Based on the value of this field, a conforming email client
   can determine which message part to display (given the user's
   language settings).

   The Content-Language MUST comply with RFC 3282 [RFC3282] (which
   defines the Content-Language field) and BCP 47/RFC 5646 [RFC5646]
   (which defines the structure and semantics for the language tag
   values).

   Examples of this field could look like the following:

   Content-Language: en-GB

   Content-Language: de

   Content-Language: es-MX, fr

   Content-Language: sr-Cyrl








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6.  The Content-Translation-Type Field

   The Content-Translation-Type field can be used in the individual
   language message parts to identify the type of translation.  Based on
   the value of this parameter and the user's preferences, a conforming
   email client can determine which message part to display.

   This field can have one of three possible values: 'original', 'human'
   or 'automated' although other values may be added in the future.  A
   value of 'original' is given in the language message part that is in
   the original language.  A value of 'human' is used when a language
   message part is translated by a human translator or a human has
   checked and corrected an automated translation.  A value of
   'automated' is used when a language message part has been translated
   by an electronic agent without proofreading or subsequent correction.
   New values of the Content-Translation-Type header field
   ("translTypeExt" in the ABNF) are added according to procedure
   specified in Section 10.3.

   Examples of this field include:

   Content-Translation-Type: original

   Content-Translation-Type: human

   The syntax of the Content-Translation-Type field in ABNF RFC 5234
   [RFC5234] is:


   Content-Translation-Type = [FWS] translationtype

   FWS                      = <Defined in RFC 5322>
   translationtype          = "original" / "human" / "automated" /
                              translTypeExt
   translTypeExt            = 1*atext
   atext                    = <Defined in RFC 5322>

   This references RFC 5322 [RFC5322] for the pre-defined rules FWS and
   atext.

7.  The Subject Field in the Language Message parts

   On receipt of the message, conforming email clients will need to
   render the subject in the correct language for the recipient.  To
   enable this the Subject field SHOULD be provided in each language
   message part.  The value for this field should be a translation of
   the email subject.




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   US-ASCII and 'encoded-word' examples of this field include:

   Subject: A really simple email subject

   Subject: =?UTF-8?Q?Un_asunto_de_correo_electr=C3=b3nico_
           realmente_sencillo?=

   See RFC 2047 [RFC2047] for the specification of 'encoded-word'.

   The subject to be presented to the recipient SHOULD be selected from
   the message part identified during the message part selection stage.
   If no Subject field is found the top-level Subject header field value
   should be used.

8.  Examples

8.1.  An Example of a Simple Multiple language email message

   Below is a minimal example of a multiple language email message.  It
   has the multilingual preface and two language message parts.

   From: Nik@example.com
   To: Nathaniel@example.com
   Subject: Example of a message in Spanish and English
   Date: Thu, 7 Apr 2017 21:28:00 +0100
   MIME-Version: 1.0
   Content-Type: multipart/multilingual;
           boundary="01189998819991197253"

   --01189998819991197253
   Content-Type: text/plain; charset="UTF-8"
   Content-Disposition: inline
   Content-Transfer-Encoding: quoted-printable

   This is a message in multiple languages.  It says the
   same thing in each language.  If you can read it in one language,
   you can ignore the other translations. The other translations may be
   presented as attachments or grouped together.

   Este es un mensaje en varios idiomas. Dice lo mismo en
   cada idioma. Si puede leerlo en un idioma, puede ignorar las otras
   traducciones. Las otras traducciones pueden presentarse como archivos
   adjuntos o agrupados.

   --01189998819991197253
   Content-Type: message/rfc822
   Content-Language: en-GB
   Content-Translation-Type: original



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   Content-Disposition: inline

   Subject: Example of a message in Spanish and English
   Content-Type: text/plain; charset="US-ASCII"
   Content-Transfer-Encoding: 7bit
   MIME-Version: 1.0

   Hello, this message content is provided in your language.

   --01189998819991197253
   Content-Type: message/rfc822
   Content-Language: es-ES
   Content-Translation-Type: human
   Content-Disposition: inline

   Subject: =?UTF-8?Q?Ejemplo_pr=C3=A1ctico_de_mensaje_?=
    =?UTF-8?Q?en_espa=C3=B1ol_e_ingl=C3=A9s?=
   Content-Type: text/plain; charset="US-ASCII"
   Content-Transfer-Encoding: 7bit
   MIME-Version: 1.0

   Hola, el contenido de este mensaje esta disponible en su idioma.

   --01189998819991197253--

8.2.  An Example of a Multiple language email message with language
      independent part

   Below is an example of a multiple language email message that has the
   multilingual preface followed by two language message parts and then
   a language independent png image.

From: Nik@example.com
To: Nathaniel@example.com
Subject: Example of a message in Spanish and English
Date: Thu, 7 Apr 2017 21:08:00 +0100
MIME-Version: 1.0
Content-Type: multipart/multilingual;
        boundary="01189998819991197253"

--01189998819991197253
Content-Type: text/plain; charset="UTF-8"
Content-Disposition: inline
Content-Transfer-Encoding: quoted-printable

This is a message in multiple languages.  It says the
same thing in each language.  If you can read it in one language,
you can ignore the other translations. The other translations may be



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presented as attachments or grouped together.

Este es un mensaje en varios idiomas. Dice lo mismo en
cada idioma. Si puede leerlo en un idioma, puede ignorar las otras
traducciones. Las otras traducciones pueden presentarse como archivos
adjuntos o agrupados.

--01189998819991197253
Content-Type: message/rfc822
Content-Language: en-GB
Content-Translation-Type: original
Content-Disposition: inline

Subject: Example of a message in Spanish and English
Content-Type: text/plain; charset="US-ASCII"
Content-Transfer-Encoding: 7bit
MIME-Version: 1.0

Hello, this message content is provided in your language.

--01189998819991197253
Content-Type: message/rfc822
Content-Language: es-ES
Content-Translation-Type: human
Content-Disposition: inline

Subject: =?UTF-8?Q?Ejemplo_pr=C3=A1ctico_de_mensaje_?=
 =?UTF-8?Q?en_espa=C3=B1ol_e_ingl=C3=A9s?=
Content-Type: text/plain; charset="US-ASCII"
Content-Transfer-Encoding: 7bit
MIME-Version: 1.0

Hola, el contenido de este mensaje esta disponible en su idioma.

--01189998819991197253
Content-Type: message/rfc822; name="Icon"
Content-Language: zxx
Content-Disposition: inline

Content-Type: image/png; name="icon.png"
Content-Transfer-Encoding: base64

iVBORw0KGgoAAAANSUhEUgAAADAAAAAwCAYAAABXAvmHAAAKQ2lDQ1BJQ0MgUHJvZmlsZQAA
SA2dlndUU1... shortened for brevity ...7yxfd1SNsEy+OXr76qr
997zF2hvZYeDEP5ftGV6Xzx2o9MAAAAASUVORK5CYII=

--01189998819991197253--




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8.3.  An Example of a complex Multiple language email message with
      language independent part

   Below is an example of a more complex multiple language email
   message.  It has the multilingual preface and two language message
   parts and then a language independent png image.  The language
   message parts have multipart/alternative contents and would therefore
   require further processing to determine the content to display.

From: Nik@example.com
To: Nathaniel@example.com
Subject: Example of a message in Spanish and English
Date: Thu, 7 Apr 2017 20:55:00 +0100
MIME-Version: 1.0
Content-Type: multipart/multilingual;
        boundary="01189998819991197253"

--01189998819991197253
Content-Type: text/plain; charset="UTF-8"
Content-Disposition: inline
Content-Transfer-Encoding: quoted-printable

This is a message in multiple languages.  It says the
same thing in each language.  If you can read it in one language,
you can ignore the other translations. The other translations may be
presented as attachments or grouped together.

Este es un mensaje en varios idiomas. Dice lo mismo en
cada idioma. Si puede leerlo en un idioma, puede ignorar las otras
traducciones. Las otras traducciones pueden presentarse como archivos
adjuntos o agrupados.

--01189998819991197253
Content-Type: message/rfc822
Content-Language: en-GB
Content-Translation-Type: original
Content-Disposition: inline

Subject: Example of a message in Spanish and English
Content-Type: multipart/alternative;
        boundary="72530118999911999881"; charset="US-ASCII"
Content-Transfer-Encoding: 7bit
MIME-Version: 1.0

--72530118999911999881
Content-Type: text/plain; charset="US-ASCII"
Content-Transfer-Encoding: 7bit




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Hello, this message content is provided in your language.

--72530118999911999881
Content-Type: text/html; charset="US-ASCII"
Content-Transfer-Encoding: 7bit

<html><body>Hello, this message content is <b>provided</b> in
<i>your</i> language.</body></html>

--72530118999911999881--
--01189998819991197253
Content-Type: message/rfc822
Content-Language: es-ES
Content-Translation-Type: human
Content-Disposition: inline

Subject: =?UTF-8?Q?Ejemplo_pr=C3=A1ctico_de_mensaje_?=
 =?UTF-8?Q?en_espa=C3=B1ol_e_ingl=C3=A9s?=
Content-Type: multipart/alternative;
        boundary="53011899989991197281"; charset="US-ASCII"
Content-Transfer-Encoding: 7bit
MIME-Version: 1.0

--53011899989991197281
Content-Type: text/plain; charset="US-ASCII"
Content-Transfer-Encoding: 7bit

Hola, el contenido de este mensaje esta disponible en su idioma.

--53011899989991197281
Content-Type: text/html; charset="US-ASCII"
Content-Transfer-Encoding: 7bit

<html><body>Hola, el contenido de este <b>mensaje</b> <i>esta</i>
disponible en su idioma.</body></html>

--53011899989991197281--
--01189998819991197253
Content-Type: message/rfc822; name="Icon"
Content-Language: zxx
Content-Disposition: inline

Content-Type: multipart/mixed;
        boundary="99911972530118999881"; charset="US-ASCII"
Content-Transfer-Encoding: 7bit
MIME-Version: 1.0

--99911972530118999881



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Content-Type: image/png; name="icon.png"
Content-Disposition: inline
Content-Transfer-Encoding: base64

iVBORw0KGgoAAAANSUhEUgAAADAAAAAwCAYAAABXAvmHAAAKQ2lDQ1BJQ0MgUHJvZmlsZQAA
SA2dlndUU1... shortened for brevity ...7yxfd1SNsEy+OXr76qr
997zF2hvZYeDEP5ftGV6Xzx2o9MAAAAASUVORK5CYII=

--99911972530118999881--
--01189998819991197253--

9.  Acknowledgements

   The authors are grateful for the helpful input received from many
   people but would especially like to acknowledge the help of Harald
   Alvestrand, Stephane Bortzmeyer, Eric Burger, Ben Campbell, Mark
   Davis, Doug Ewell, Ned Freed, Randall Gellens, Gunnar Hellstrom,
   Mirja Kuehlewind, Barry Leiba, Sean Leonard, John Levine, Alexey
   Melnikov, Addison Phillips, Julian Reschke, Pete Resnick, Adam Roach,
   Brian Rosen, Fiona Tomkinson, Simon Tyler and Daniel Vargha.

   The authors would also like to thank Fernando Alvaro and Luis de
   Pablo for their work on the Spanish translations.

10.  IANA Considerations

10.1.  The multipart/multilingual MIME type

   The multipart/multilingual MIME type will be registered with IANA
   including a reference to this document.  This is the registration
   template:




















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   Media Type name: multipart

   Media subtype name: multilingual

   Required parameters: boundary (defined in RFC2046)

   Optional parameters: N/A

   Encoding considerations: There are no encoding considerations for
       this multipart other than that of the embedded body parts.
       The embedded body parts (typically one text/plain plus one or
       more message/*) may contain 7-bit, 8-bit or binary encodings.

   Security considerations: See the Security Considerations section
                            in RFC XXXX

   Interoperability considerations:
       Existing systems that do not treat unknown multipart subtypes
       as multipart/mixed may not correctly render a
       multipart/multilingual type. These systems would also be non-
       compliant with MIME.

   Author/Change controller: IETF

   Published specification: RFC XXXX

   Applications that use this media type:
       Mail Transfer Agents, Mail User Agents, spam detection,
       virus detection modules and message authentication modules.

   Additional information:
       Deprecated alias names for this type: N/A
       Magic number(s): N/A
       File extension(s): N/A
       Macintosh file type code(s): N/A

   Person & email address to contact for further information:
       Nik Tomkinson
       rfc.nik.tomkinson@gmail.com

       Nathaniel Borenstein
       nsb@mimecast.com

   Intended usage: Common







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10.2.  The Content-Translation-Type Field

   The Content-Translation-Type field will be added to the IANA
   "Permanent Message Header Field Names" registry.  That entry will
   reference this document.  This is the registration template:

   Header field name: Content-Translation-Type

   Applicable protocol: mime

   Status: Standard

   Author/Change controller: IETF

   Specification document(s): RFC XXXX

   Related information: none

10.3.  The Content-Translation-Type Field Values

   IANA is requested to create a new registry for Content-Translation-
   Type Header Field values.  New values must be registered using
   "Specification Required" IANA registration procedure.  Registrations
   must include translation type value, short description and a URI of
   the specification.

   This document also registers 3 initial values specified below.


   Value: original
   Description:
       Content in the original language
   Reference: RFC XXXX

   Value: human
   Description:
       Content that has been translated by a human translator
       or a human has checked and corrected an automated translation
   Reference: RFC XXXX

   Value: automated
   Description:
       Content that has been translated by an electronic agent
       without proofreading or subsequent correction
   Reference: RFC XXXX






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11.  Security Considerations

   Whilst it is intended that each language message part is a direct
   translation of the original message, this may not always be the case
   and these parts could contain undesirable content.  Therefore there
   is a possible risk that undesirable text or images could be shown to
   the recipient if the message is passed through a spam filter that
   does not check all of the message parts.  The risk should be minimal
   due to the fact that an unknown multipart subtype should be treated
   as multipart/mixed and so each message part should be subsequently
   scanned.

   If the email contains undesirable content in a language that the
   recipient cannot understand and this unknown content is assumed to be
   a direct translation of the content that the recipient can
   understand, the recipient may unintentionally forward undesirable
   content to a recipient that can understand it.  To mitigate this
   risk, an interactive implementation may allow the recipient to see
   all of the translations for comparison.

   Because the language message parts have a Content-Type of message/
   rfc822 or message/global, they might contain From fields which could
   have different values to that of the top-level From field and may not
   reflect the actual sender.  The inconsistent From field values might
   get shown to the recipient in a non-conforming email client and may
   mislead the recipient into thinking that the email came from someone
   other than the real sender.

12.  Changes from Previous Versions

12.1.  Changes from draft-tomkinson-multilangcontent-01 to draft-
       tomkinson-slim-multilangcontent-00

   o  File name and version number changed to reflect the proposed WG
      name SLIM (Selection of Language for Internet Media).

   o  Replaced the Subject-Translation field in the language message
      parts with Subject and provided US-ASCII and non-US-ASCII
      examples.

   o  Introduced the language-independent message part.

   o  Many wording improvements and clarifications throughout the
      document.







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12.2.  Changes from draft-tomkinson-slim-multilangcontent-00 to draft-
       tomkinson-slim-multilangcontent-01

   o  Added Translation-Type in each language message part to identify
      the source of the translation (original/human/automated).

12.3.  Changes from draft-tomkinson-slim-multilangcontent-01 to draft-
       tomkinson-slim-multilangcontent-02

   o  Changed Translation-Type to be a parameter for the Content-
      Language field rather than a new separate field.

   o  Added a paragraph about using Content-Disposition field to help
      non-conforming mail clients correctly render the multilingual
      preface.

   o  Recommended using a Name parameter on the language part Content-
      Type to help the recipient identify the translations in non-
      conforming mail clients.

   o  Many wording improvements and clarifications throughout the
      document.

12.4.  Changes from draft-tomkinson-slim-multilangcontent-02 to draft-
       ietf-slim-multilangcontent-00

   o  Name change to reflect the draft being accepted into SLIM as a
      working group document.

   o  Updated examples to use UTF-8 encoding where required.

   o  Removed references to 'locale' for identifying language
      preference.

   o  Recommended language matching schemes from RFC 4647 [RFC4647].

   o  Renamed the unmatched part to language independent part to
      reinforce its intended purpose.

   o  Added requirement for using Content-Language: zxx in the language
      independent part.

   o  Many wording improvements and clarifications throughout the
      document.







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12.5.  Changes from draft-ietf-slim-multilangcontent-00 to draft-ietf-
       slim-multilangcontent-01

   o  Changed the inner content type to require message/rfc822 or
      message/global.

   o  Updated the examples to reflect the new inner content types.

   o  Added to the security considerations to highlight the risk from
      insufficient spam filters.

12.6.  Changes from draft-ietf-slim-multilangcontent-01 to draft-ietf-
       slim-multilangcontent-02

   o  Restricted the use of a From field in the language message parts
      and the language independent part.

   o  Updated the security considerations to highlight the risk of an
      unmatched sender addresses that could be set in the language
      message parts.

12.7.  Changes from draft-ietf-slim-multilangcontent-02 to draft-ietf-
       slim-multilangcontent-03

   o  Relaxed the restriction on the use of the From field in the
      language message parts to allow a localised version of the
      sender's display name.

12.8.  Changes from draft-ietf-slim-multilangcontent-03 to draft-ietf-
       slim-multilangcontent-04

   o  Updated the wording of the security considerations section to
      reflect the relaxation of the use of the From field in the
      language message parts.

12.9.  Changes from draft-ietf-slim-multilangcontent-04 to draft-ietf-
       slim-multilangcontent-05

   o  Referenced the RFC for message/global in Language Message Parts
      section.

   o  Removed RFC 2119 keyword in the Message Part Selection section.

   o  Included full email addresses in all examples.

   o  Updated reference name of real-time companion document in the
      Introduction.




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   o  Removed paragraph warning of over use of language sub-tags.

   o  Changed 'exponential' to 'significantly' in Introduction.

12.10.  Changes from draft-ietf-slim-multilangcontent-05 to draft-ietf-
        slim-multilangcontent-06

   o  Changed parameter Translation-Type back to a new field to reduce
      the risk of breaking existing implementations that don't expect
      any parameters on Content-Language.

   o  Improved the IANA Considerations section to include the full
      registration template for the multipart/multilingual type and the
      new Translation-Type field.

12.11.  Changes from draft-ietf-slim-multilangcontent-06 to draft-ietf-
        slim-multilangcontent-07

   o  Updated the Encoding Considerations in the IANA Registration
      Template.

12.12.  Changes from draft-ietf-slim-multilangcontent-07 to draft-ietf-
        slim-multilangcontent-08

   o  Reordered the sections to make the Security Considerations easier
      to find.

   o  Shortened a line in one of the examples that was longer than 72
      characters.

   o  Updated the link to the real-time companion document to the latest
      version.

12.13.  Changes from draft-ietf-slim-multilangcontent-08 to draft-ietf-
        slim-multilangcontent-09

   o  Removed the phrase "and SHOULD NOT have a Subject field and SHOULD
      NOT have a From field" in section 3.3 because the language
      independent part would be message/rfc822 or message/global so it
      is likely to have From and Subject fields.

   o  For the same reason as above, the phrase "(for example if the
      language independent part is selected)" was removed from section
      7.

   o  Phrase in part 3.3 was reworded from "This could typically be a
      language independent graphic" to "This could typically include a
      language independent graphic".



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12.14.  Changes from draft-ietf-slim-multilangcontent-09 to draft-ietf-
        slim-multilangcontent-10

   o  Added Normative Reference to RFC 3629 for details of UTF-8 in the
      Multilingual Preface section.

   o  Added ABNF for Translation-Type field.

   o  Updated example 2 to contain a image/png language independent
      image directly rather than it being nested inside multipart/mixed
      content.

   o  Inserted Change Controller into IANA template for multipart/
      multilingual.

   o  Replaced references to "This document" with RFC XXXX in the IANA
      considerations.

12.15.  Changes from draft-ietf-slim-multilangcontent-10 to draft-ietf-
        slim-multilangcontent-11

   o  Updated the applicable protocol for the Translation-Type field in
      the IANA registration template to be 'mime' rather than 'mail'.

   o  Added that updated specification documents would be the source of
      new values for the Translation-Type field.

12.16.  Changes from draft-ietf-slim-multilangcontent-11 to draft-ietf-
        slim-multilangcontent-12

   o  Updated the ABNF for Translation-Type to allow for future values.

   o  Added section 10.3 to explain about the Translation-Type values
      and providing new values.

12.17.  Changes from draft-ietf-slim-multilangcontent-12 to draft-ietf-
        slim-multilangcontent-13

   o  Changed the field name Translation-Type to Content-Translation-
      Type.

   o  Explicitly specified the 3 initial values for Content-Translation-
      Type in the IANA Considerations section.








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12.18.  Changes from draft-ietf-slim-multilangcontent-13 to draft-ietf-
        slim-multilangcontent-14

   o  Added table of contents.

   o  Added notes about the risk of unwittingly forwarding undesirable
      content to the Security Considerations section.

   o  Updated 'language code' to 'language tag' in section 5 to make it
      clear that language tag values including country codes and script
      codes are allowed.

   o  Updated the examples to use language tag values that include a
      country code.

   o  Added a note into section 3.2 to specify what should happen if
      mismatched sender addresses are found in the language parts.

   o  Many wording improvements and clarifications throughout the
      document.

13.  References

13.1.  Normative References

   [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Two: Media Types", RFC 2046,
              DOI 10.17487/RFC2046, November 1996,
              <http://www.rfc-editor.org/info/rfc2046>.

   [RFC2047]  Moore, K., "MIME (Multipurpose Internet Mail Extensions)
              Part Three: Message Header Extensions for Non-ASCII Text",
              RFC 2047, DOI 10.17487/RFC2047, November 1996,
              <http://www.rfc-editor.org/info/rfc2047>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2183]  Troost, R., Dorner, S., and K. Moore, Ed., "Communicating
              Presentation Information in Internet Messages: The
              Content-Disposition Header Field", RFC 2183,
              DOI 10.17487/RFC2183, August 1997,
              <http://www.rfc-editor.org/info/rfc2183>.






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   [RFC3282]  Alvestrand, H., "Content Language Headers", RFC 3282,
              DOI 10.17487/RFC3282, May 2002,
              <http://www.rfc-editor.org/info/rfc3282>.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <https://www.rfc-editor.org/info/rfc3629>.

   [RFC4647]  Phillips, A. and M. Davis, "Matching of Language Tags",
              BCP 47, RFC 4647, DOI 10.17487/RFC4647, September 2006,
              <http://www.rfc-editor.org/info/rfc4647>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008, <https://www.rfc-
              editor.org/info/rfc5234>.

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              DOI 10.17487/RFC5322, October 2008, <https://www.rfc-
              editor.org/info/rfc5322>.

   [RFC5646]  Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
              Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646,
              September 2009, <https://www.rfc-editor.org/info/rfc5646>.

   [RFC6532]  Yang, A., Steele, S., and N. Freed, "Internationalized
              Email Headers", RFC 6532, DOI 10.17487/RFC6532, February
              2012, <http://www.rfc-editor.org/info/rfc6532>.

13.2.  Informational References

   [I-D.ietf-slim-negotiating-human-language]
              Gellens, R., "Negotiating Human Language in Real-Time
              Communications", draft-ietf-slim-negotiating-human-
              language-13 (work in progress), July 2017.

Authors' Addresses

   Nik Tomkinson
   Mimecast Ltd
   CityPoint, One Ropemaker Street
   London  EC2Y 9AW
   United Kingdom

   Email: rfc.nik.tomkinson@gmail.com






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   Nathaniel Borenstein
   Mimecast Ltd
   480 Pleasant Street
   Watertown  MA 02472
   North America

   Email: nsb@mimecast.com












































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