Multiple Language Content Type
RFC 8255

Internet Engineering Task Force (IETF)                      N. Tomkinson
Request for Comments: 8255                                 N. Borenstein
Category: Standards Track                                 Mimecast, Ltd.
ISSN: 2070-1721                                             October 2017

                     Multiple Language Content Type

Abstract

   This document defines the 'multipart/multilingual' content type,
   which is an addition to the Multipurpose Internet Mail Extensions
   (MIME) standard.  This content type makes 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 is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8255.

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
   (https://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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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  The Content-Type Header Field . . . . . . . . . . . . . . . .   3
   3.  The Message Parts . . . . . . . . . . . . . . . . . . . . . .   4
     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  . . . . . . . . . . . . . . . . . . .   6
   5.  The Content-Language Field  . . . . . . . . . . . . . . . . .   6
   6.  The Content-Translation-Type Field  . . . . . . . . . . . . .   7
   7.  The Subject Field in the Language Message Parts . . . . . . .   8
   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 a
           Language-Independent Part . . . . . . . . . . . . . . . .   9
     8.3.  An Example of a Complex Multiple-Language Email Message
           with a Language-Independent Part  . . . . . . . . . . . .  11
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
     9.1.  The 'multipart/multilingual' Media Type . . . . . . . . .  13
     9.2.  The Content-Translation-Type Field  . . . . . . . . . . .  15
     9.3.  The Content-Translation-Type Header Field Values  . . . .  15
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  16
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  16
     11.2.  Informative References . . . . . . . . . . . . . . . . .  18
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

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 multicultural cities and have formed departments and teams
   that span continents, cultures, and languages.  Thus, the need to
   communicate efficiently with little margin for miscommunication has
   grown significantly.

   This document defines the 'multipart/multilingual' content type,
   which is an addition to the Multipurpose Internet Mail Extensions
   (MIME) standard specified in [RFC2045], [RFC2046], [RFC2047],

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   [RFC4289], and [RFC6838].  This content type makes 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.

   This document depends on the identification of language in message
   parts for non-real-time communication.  [HUMAN-LANG] is concerned
   with a similar problem for real-time communication.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  The Content-Type Header Field

   The 'multipart/multilingual' Media Subtype allows the sending of a
   message in a number of different languages with the different
   language versions embedded in the same message.  This Media 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

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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 that 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 briefly explain 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
   a UTF-8 charset be 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.  This is 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

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   to the recipient, followed by any subsequent parts marked as
   'inline'.

   For examples of a multilingual preface, see 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 recognized by the recipient; 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; they 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 field,
   although the display name MAY be a localized version.  If there is a
   mismatch of sender email address, the top-level From header field
   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
   [RFC5646]).  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 summarized in the next few paragraphs.

   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
   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 the
   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 [RFC5646] (which
   defines the structure and semantics for the language tag values).

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   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

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 field 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 the procedure
   specified in Section 9.3.

   Examples of this field include:

   Content-Translation-Type: original

   Content-Translation-Type: human

   The syntax of the Content-Translation-Type field in ABNF [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 predefined rules 'folding
   white space (FWS)' and 'atext'.

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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, 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.

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   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
   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 a 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"

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   --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
   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.

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

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

   iVBORw0KGgoAAAANSUhEUgAAADAAAAAwCAYAAABXAvmHAAAKQ2lDQ1BJQ0MgUHJvZmlsZ
   QAASA2dlndUU1... shortened for brevity ...7yxfd1SNsEy+OXr76qr
   997zF2hvZYeDEP5ftGV6Xzx2o9MAAAAASUVORK5CYII=

   --01189998819991197253--

8.3.  An Example of a Complex Multiple-Language Email Message with a
      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.

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   --01189998819991197253
   Content-Type: message/rfc822
   Content-Language: en
   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

   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
   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.

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

   iVBORw0KGgoAAAANSUhEUgAAADAAAAAwCAYAAABXAvmHAAAKQ2lDQ1BJQ0MgUHJvZmlsZ
   QAASA2dlndUU1... shortened for brevity ...7yxfd1SNsEy+OXr76qr
   997zF2hvZYeDEP5ftGV6Xzx2o9MAAAAASUVORK5CYII=

   --99911972530118999881--
   --01189998819991197253--

9.  IANA Considerations

9.1.  The 'multipart/multilingual' Media Type

   The 'multipart/multilingual' Media Type has been registered with
   IANA.  This is the registration template based on the template
   specified in [RFC6838]:

   Media Type name: multipart

   Media subtype name: multilingual

   Required parameters: boundary (defined in RFC 2046)

   Optional parameters: N/A

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   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 8255

   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.

   Published specification: RFC 8255

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

   Fragment identifier considerations: N/A

   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

   Restrictions on usage: N/A

   Author/Change controller: IETF

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9.2.  The Content-Translation-Type Field

   The Content-Translation-Type field has been added to the IANA
   "Permanent Message Header Field Names" registry.  That entry
   references this document.  This registration template is below:

   Header field name: Content-Translation-Type

   Applicable protocol: MIME

   Status: standard

   Author/Change controller: IETF

   Specification document(s): RFC 8255

   Related information: none

9.3.  The Content-Translation-Type Header Field Values

   IANA has created a new registry titled "Content-Translation-Type
   Header Field Values".  New values must be registered using the
   "Specification Required" [RFC8126] IANA registration procedure.
   Registrations must include a translation type value, a short
   description, and a reference to the specification.

   This document also registers three initial values specified below.

   Value: original
   Description:
       Content in the original language
   Reference: RFC 8255

   Value: human
   Description:
       Content that has been translated by a human translator
       or a human has checked and corrected an automated translation
   Reference: RFC 8255

   Value: automated
   Description:
       Content that has been translated by an electronic agent
       without proofreading or subsequent correction
   Reference: RFC 8255

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10.  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;
   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'; thus, 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 that
   could have different values from that of the top-level From field,
   and they 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.

11.  References

11.1.  Normative References

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
              <https://www.rfc-editor.org/info/rfc2045>.

   [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Two: Media Types", RFC 2046,
              DOI 10.17487/RFC2046, November 1996,
              <https://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,
              <https://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,
              <https://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,
              <https://www.rfc-editor.org/info/rfc2183>.

   [RFC3282]  Alvestrand, H., "Content Language Headers", RFC 3282,
              DOI 10.17487/RFC3282, May 2002,
              <https://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>.

   [RFC4289]  Freed, N. and J. Klensin, "Multipurpose Internet Mail
              Extensions (MIME) Part Four: Registration Procedures",
              BCP 13, RFC 4289, DOI 10.17487/RFC4289, December 2005,
              <https://www.rfc-editor.org/info/rfc4289>.

   [RFC4647]  Phillips, A. and M. Davis, "Matching of Language Tags",
              BCP 47, RFC 4647, DOI 10.17487/RFC4647, September 2006,
              <https://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, <https://www.rfc-editor.org/info/rfc6532>.

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   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,
              <https://www.rfc-editor.org/info/rfc6838>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

11.2.  Informative References

   [HUMAN-LANG]
              Gellens, R., "Negotiating Human Language in Real-Time
              Communications", Work in Progress, draft-ietf-slim-
              negotiating-human-language-13, July 2017.

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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.

Authors' Addresses

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

   Email: rfc.nik.tomkinson@gmail.com

   Nathaniel Borenstein
   Mimecast, Ltd.
   480 Pleasant Street
   Watertown, MA  02472
   United States of America

   Email: nsb@mimecast.com

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