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Versions: (draft-tomkinson-slim-multilangcontent) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 RFC 8255

IETF                                                        N. Tomkinson
Internet-Draft                                             N. Borenstein
Intended status: Standards Track                            Mimecast Ltd
Expires: April 15, 2017                                 October 12, 2016


                     Multiple Language Content Type
                  draft-ietf-slim-multilangcontent-06

Abstract

   This document defines 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 April 15, 2017.

Copyright Notice

   Copyright (c) 2016 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
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.



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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.

   The objective of this document is to define 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



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   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
   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 encoding is used for this message part.

   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.





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   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.

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 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.

3.3.  The Language Independent Message Part

   If there is language independent content intended 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 could typically be a language independent graphic.
   When this part is present, it MUST be the last part, MUST have a
   Content-Language field with a value of "zxx" (as described in BCP 47/
   RFC 5646 [RFC5646]) and SHOULD NOT have a Subject field and SHOULD
   NOT have a From field.  The part SHOULD have a Content-Type of
   message/rfc822 or message/global (to match the language message
   parts).




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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 should treat the message as if it was multipart/
   mixed and render message parts accordingly.

   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
   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 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 Translation-Type field may be ignored.

   Additionally, interactive implementations MAY offer the user a choice
   from among the available languages.

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 code
   values).

   Examples of this field for English, German and an instruction manual
   in Spanish and French, could look like the following:



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   Content-Language: en

   Content-Language: de

   Content-Language: es, fr

6.  The Translation-Type Field

   The 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.

   Examples of this field include:

   Translation-Type: original

   Translation-Type: human

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.

   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 (for example if the language independent



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   part is selected) 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 Jul 2016 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
   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



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   Content-Language: es
   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 Jul 2016 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
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
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
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: multipart/mixed;
        boundary="99911972530118999881"; charset="US-ASCII"
Content-Transfer-Encoding: 7bit
MIME-Version: 1.0

--99911972530118999881
Content-Type: image/png; name="icon.png"
Content-Disposition: inline
Content-Transfer-Encoding: base64

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

--99911972530118999881--
--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 Jul 2016 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
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
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
Content-Type: image/png; name="icon.png"



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Content-Disposition: inline
Content-Transfer-Encoding: base64

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

--99911972530118999881--
--01189998819991197253--

9.  Changes from Previous Versions

9.1.  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.

9.2.  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).

9.3.  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.





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   o  Many wording improvements and clarifications throughout the
      document.

9.4.  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.

9.5.  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.

9.6.  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.






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9.7.  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.

9.8.  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.

9.9.  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.

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

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

9.10.  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.

10.  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, Mark Davis, Doug Ewell,
   Randall Gellens, Gunnar Hellstrom, Barry Leiba, Sean Leonard, John



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   Levine, Alexey Melnikov, Addison Phillips, Julian Reschke, Pete
   Resnick, 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.

11.  IANA Considerations

11.1.  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: only "7bit", "8bit", or "binary" are
                            permitted

   Security considerations: See the Security Considerations section
                            in this document

   Interoperability considerations:
       Existing systems that do not treat unknown multipart subtypes
       as multipart/mixed may not correctly render a
       multipart/multilingual type.

   Published specification: This document

   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

11.2.  The Translation-Type field

   The 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:






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   Header field name: Translation-Type

   Applicable protocol: mail

   Status: Standard

   Author/Change controller: IETF

   Specification document(s): this document

   Related information: none

12.  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.

   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.

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>.






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   [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>.

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

   [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>.

   [RFC5646]  Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
              Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646,
              September 2009, <http://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-04 (work in progress), July 2016.

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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