| IETF | N. Tomkinson |
| Internet-Draft | N. Borenstein |
| Intended status: Standards Track | Mimecast Ltd |
| Expires: January 21, 2017 | July 20, 2016 |
Multiple Language Content Type
draft-ietf-slim-multilangcontent-02
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.
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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 exponentially.
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.gellens-slim-negotiating-human-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].
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 Translation-Type parameter of the Content-Language 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
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.
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.
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.
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 parameter applied to the Content-Language 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. Each language message part SHOULD have a Subject field in the appropriate language for that language part and SHOULD NOT have a From field.
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).
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 parameter of the Content-Language 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 parameter may be ignored.
Additionally, interactive implementations MAY offer the user a choice from among the available languages.
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). While RFC 5646 provides a mechanism accommodating increasingly fine-grained distinctions, in the interest of maximum interoperability, each Content-Language value SHOULD be restricted to the largest granularity of language tags; in other words, it is RECOMMENDED to specify only a Primary-subtag and NOT to include subtags (e.g., for region or dialect) unless the languages might be mutually incomprehensible without them. Examples of this field for English, German and an instruction manual in Spanish and French, could look like the following:
Content-Language: en
Content-Language: de
Content-Language: es, fr
The Translation-Type parameter can be applied to the Content-Language field in the individual language message parts and is used 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 parameter 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 parameter include:
Content-Language: en; translation-type=original
Content-Language: fr; translation-type=human
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 part is selected) the top-level Subject header field value should be used.
Below is a minimal example of a multiple language email message. It has the multilingual preface and two language message parts.
From: Nik
To: Nathaniel
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
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--
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
To: Nathaniel
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
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--
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
To: Nathaniel
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
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"
Content-Disposition: inline
Content-Transfer-Encoding: base64
iVBORw0KGgoAAAANSUhEUgAAADAAAAAwCAYAAABXAvmHAAAKQ2lDQ1BJQ0MgUHJvZmlsZQAA
SA2dlndUU1... shortened for brevity ...7yxfd1SNsEy+OXr76qr
997zF2hvZYeDEP5ftGV6Xzx2o9MAAAAASUVORK5CYII=
--99911972530118999881--
--01189998819991197253--
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, Sean Leonard, John Levine, Alexey Melnikov, Addison Phillips, Pete Resnick, 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.
The multipart/multilingual MIME type will be registered with IANA.
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. An implementation might choose to include a From field, even though it is specified in this document that the language message parts SHOULD NOT have a From field. The incorrect 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.
| [I-D.gellens-slim-negotiating-human-language] | Gellens, R., "Negotiating Human Language in Real-Time Communications", Internet-Draft draft-gellens-slim-negotiating-human-language-02, July 2015. |