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Borenstein 4 Intended status: Standards Track Mimecast Ltd 5 Expires: February 18, 2017 August 17, 2016 7 Multiple Language Content Type 8 draft-ietf-slim-multilangcontent-05 10 Abstract 12 This document defines an addition to the Multipurpose Internet Mail 13 Extensions (MIME) standard to make it possible to send one message 14 that contains multiple language versions of the same information. 15 The translations would be identified by a language tag and selected 16 by the email client based on a user's language settings. 18 Status of This Memo 20 This Internet-Draft is submitted in full conformance with the 21 provisions of BCP 78 and BCP 79. 23 Internet-Drafts are working documents of the Internet Engineering 24 Task Force (IETF). Note that other groups may also distribute 25 working documents as Internet-Drafts. The list of current Internet- 26 Drafts is at http://datatracker.ietf.org/drafts/current/. 28 Internet-Drafts are draft documents valid for a maximum of six months 29 and may be updated, replaced, or obsoleted by other documents at any 30 time. It is inappropriate to use Internet-Drafts as reference 31 material or to cite them other than as "work in progress." 33 This Internet-Draft will expire on February 18, 2017. 35 Copyright Notice 37 Copyright (c) 2016 IETF Trust and the persons identified as the 38 document authors. All rights reserved. 40 This document is subject to BCP 78 and the IETF Trust's Legal 41 Provisions Relating to IETF Documents 42 (http://trustee.ietf.org/license-info) in effect on the date of 43 publication of this document. Please review these documents 44 carefully, as they describe your rights and restrictions with respect 45 to this document. Code Components extracted from this document must 46 include Simplified BSD License text as described in Section 4.e of 47 the Trust Legal Provisions and are provided without warranty as 48 described in the Simplified BSD License. 50 1. Introduction 52 Since the invention of email and the rapid spread of the Internet, 53 more and more people have been able to communicate in more and more 54 countries and in more and more languages. But during this time of 55 technological evolution, email has remained a single-language 56 communication tool, whether it is English to English, Spanish to 57 Spanish or Japanese to Japanese. 59 Also during this time, many corporations have established their 60 offices in multi-cultural cities and formed departments and teams 61 that span continents, cultures and languages, so the need to 62 communicate efficiently with little margin for miscommunication has 63 grown significantly. 65 The objective of this document is to define an addition to the 66 Multipurpose Internet Mail Extensions (MIME) standard, to make it 67 possible to send a single message to a group of people in such a way 68 that all of the recipients can read the email in their preferred 69 language. The methods of translation of the message content are 70 beyond the scope of this document, but the structure of the email 71 itself is defined herein. 73 Whilst this document depends on identification of language in message 74 parts for non-real-time communication, there is a companion document 75 that is concerned with a similar problem for real-time communication: 76 [I-D.ietf-slim-negotiating-human-language] 78 1.1. Requirements Language 80 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 81 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 82 document are to be interpreted as described in RFC 2119 [RFC2119]. 84 2. The Content-Type Header Field 86 The "multipart/multilingual" MIME subtype allows the sending of a 87 message in a number of different languages with the translations 88 embedded in the same message. This MIME subtype helps the receiving 89 email client make sense of the message structure. 91 The multipart subtype "multipart/multilingual" has similar semantics 92 to "multipart/alternative" (as discussed in RFC 2046 [RFC2046]) in 93 that each of the message parts is an alternative version of the same 94 information. The primary difference between "multipart/multilingual" 95 and "multipart/alternative" is that when using "multipart/ 96 multilingual", the message part to select for rendering is chosen 97 based on the values of the Content-Language field and optionally the 98 Translation-Type parameter of the Content-Language field instead of 99 the ordering of the parts and the Content-Types. 101 The syntax for this multipart subtype conforms to the common syntax 102 for subtypes of multipart given in section 5.1.1. of RFC 2046 103 [RFC2046]. An example "multipart/multilingual" Content-Type header 104 field would look like this: 106 Content-Type: multipart/multilingual; boundary=01189998819991197253 108 3. The Message Parts 110 A multipart/multilingual message will have a number of message parts: 111 exactly one multilingual preface, one or more language message parts 112 and zero or one language independent message part. The details of 113 these are described below. 115 3.1. The Multilingual Preface 117 In order for the message to be received and displayed in non- 118 conforming email clients, the message SHOULD contain an explanatory 119 message part which MUST NOT be marked with a Content-Language field 120 and MUST be the first of the message parts. For maximum support in 121 the most basic of non-conforming email clients, it SHOULD have a 122 Content-Type of text/plain. Because non-conforming email clients are 123 expected to treat a message with an unknown multipart type as 124 multipart/mixed (in accordance with sections 5.1.3 and 5.1.7 of RFC 125 2046 [RFC2046]) they may show all of the message parts sequentially 126 or as attachments. Including and showing this explanatory part will 127 help the message recipient understand the message structure. 129 This initial message part SHOULD explain briefly to the recipient 130 that the message contains multiple languages and the parts may be 131 rendered sequentially or as attachments. This SHOULD be presented in 132 the same languages that are provided in the subsequent language 133 message parts. 135 As this explanatory section is likely to contain languages using 136 scripts that require non-US-ASCII characters, it is RECOMMENDED that 137 UTF-8 encoding is used for this message part. 139 Whilst this section of the message is useful for backward 140 compatibility, it will normally only be shown when rendered by a non- 141 conforming email client, because conforming email clients SHOULD only 142 show the single language message part identified by the user's 143 preferred language and the language message part's Content-Language. 145 For the correct display of the multilingual preface in a non- 146 conforming email client, the sender MAY use the Content-Disposition 147 field with a value of 'inline' in conformance with RFC 2183 [RFC2183] 148 (which defines the Content-Disposition field). If provided, this 149 SHOULD be placed at the multipart/multilingual level and in the 150 multilingual preface. This makes it clear to a non-conforming email 151 client that the multilingual preface should be displayed immediately 152 to the recipient, followed by any subsequent parts marked as 153 'inline'. 155 For an example of a multilingual preface, see the examples in 156 Section 8. 158 3.2. The Language Message Parts 160 The language message parts are typically translations of the same 161 message content. These message parts SHOULD be ordered so that the 162 first part after the multilingual preface is in the language believed 163 to be the most likely to be recognised by the recipient as this will 164 constitute the default part when language negotiation fails and there 165 is no Language Independent part. All of the language message parts 166 MUST have a Content-Language field and a Content-Type field and MAY 167 have a Translation-Type parameter applied to the Content-Language 168 field. 170 The Content-Type for each individual language message part SHOULD be 171 message/rfc822 to provide good support with non-conforming email 172 clients. However, an implementation MAY use message/global as 173 support for message/global becomes more commonplace. See RFC 6532 174 [RFC6532] for details of message/global. Each language message part 175 SHOULD have a Subject field in the appropriate language for that 176 language part. If there is a From field present, its value MUST 177 include the same email address as the top-level From header although 178 the display name MAY be a localised version. 180 3.3. The Language Independent Message Part 182 If there is language independent content intended for the recipient 183 to see if they have a preferred language other than one of those 184 specified in the language message parts and the default language 185 message part is unlikely to be understood, another part MAY be 186 provided. This could typically be a language independent graphic. 187 When this part is present, it MUST be the last part, MUST have a 188 Content-Language field with a value of "zxx" (as described in BCP 47/ 189 RFC 5646 [RFC5646]) and SHOULD NOT have a Subject field and SHOULD 190 NOT have a From field. The part SHOULD have a Content-Type of 191 message/rfc822 or message/global (to match the language message 192 parts). 194 4. Message Part Selection 196 The logic for selecting the message part to render and present to the 197 recipient is summarised in the next few paragraphs. 199 Firstly, if the email client does not understand multipart/ 200 multilingual then it should treat the message as if it was multipart/ 201 mixed and render message parts accordingly. 203 If the email client does understand multipart/multilingual then it 204 SHOULD ignore the multilingual preface and select the best match for 205 the user's preferred language from the language message parts 206 available. Also, the user may prefer to see the original message 207 content in their second language over a machine translation in their 208 first language. The Translation-Type parameter of the Content- 209 Language field value can be used for further selection based on this 210 preference. The selection of language part may be implemented in a 211 variety of ways, although the matching schemes detailed in RFC 4647 212 [RFC4647] are RECOMMENDED as a starting point for an implementation. 213 The goal is to render the most appropriate translation for the user. 215 If there is no match for the user's preferred language (or there is 216 no preferred language information available) the email client SHOULD 217 select the language independent part (if one exists) or the first 218 language part (directly after the multilingual preface) if a language 219 independent part does not exist. 221 If there is no translation type preference information available, the 222 values of the Translation-Type parameter may be ignored. 224 Additionally, interactive implementations MAY offer the user a choice 225 from among the available languages. 227 5. The Content-Language Field 229 The Content-Language field in the individual language message parts 230 is used to identify the language in which the message part is 231 written. Based on the value of this field, a conforming email client 232 can determine which message part to display (given the user's 233 language settings). 235 The Content-Language MUST comply with RFC 3282 [RFC3282] (which 236 defines the Content-Language field) and BCP 47/RFC 5646 [RFC5646] 237 (which defines the structure and semantics for the language code 238 values). 240 Examples of this field for English, German and an instruction manual 241 in Spanish and French, could look like the following: 243 Content-Language: en 245 Content-Language: de 247 Content-Language: es, fr 249 6. The Translation-Type Parameter 251 The Translation-Type parameter can be applied to the Content-Language 252 field in the individual language message parts and is used to 253 identify the type of translation. Based on the value of this 254 parameter and the user's preferences, a conforming email client can 255 determine which message part to display. 257 This parameter can have one of three possible values: 'original', 258 'human' or 'automated' although other values may be added in the 259 future. A value of 'original' is given in the language message part 260 that is in the original language. A value of 'human' is used when a 261 language message part is translated by a human translator or a human 262 has checked and corrected an automated translation. A value of 263 'automated' is used when a language message part has been translated 264 by an electronic agent without proofreading or subsequent correction. 266 Examples of this parameter include: 268 Content-Language: en; translation-type=original 270 Content-Language: fr; translation-type=human 272 7. The Subject Field in the Language Message parts 274 On receipt of the message, conforming email clients will need to 275 render the subject in the correct language for the recipient. To 276 enable this the Subject field SHOULD be provided in each language 277 message part. The value for this field should be a translation of 278 the email subject. 280 US-ASCII and 'encoded-word' examples of this field include: 282 Subject: A really simple email subject 284 Subject: =?UTF-8?Q?Un_asunto_de_correo_electr=C3=b3nico_ 285 realmente_sencillo?= 287 See RFC 2047 [RFC2047] for the specification of 'encoded-word'. 289 The subject to be presented to the recipient should be selected from 290 the message part identified during the message part selection stage. 292 If no Subject field is found (for example if the language independent 293 part is selected) the top-level Subject header field value should be 294 used. 296 8. Examples 298 8.1. An Example of a Simple Multiple language email message 300 Below is a minimal example of a multiple language email message. It 301 has the multilingual preface and two language message parts. 303 From: Nik@example.com 304 To: Nathaniel@example.com 305 Subject: Example of a message in Spanish and English 306 Date: Thu, 7 Jul 2016 21:28:00 +0100 307 MIME-Version: 1.0 308 Content-Type: multipart/multilingual; 309 boundary="01189998819991197253" 311 --01189998819991197253 312 Content-Type: text/plain; charset="UTF-8" 313 Content-Disposition: inline 314 Content-Transfer-Encoding: quoted-printable 316 This is a message in multiple languages. It says the 317 same thing in each language. If you can read it in one language, 318 you can ignore the other translations. The other translations may be 319 presented as attachments or grouped together. 321 Este es un mensaje en varios idiomas. Dice lo mismo en 322 cada idioma. Si puede leerlo en un idioma, puede ignorar las otras 323 traducciones. Las otras traducciones pueden presentarse como archivos 324 adjuntos o agrupados. 326 --01189998819991197253 327 Content-Type: message/rfc822 328 Content-Language: en; translation-type=original 329 Content-Disposition: inline 331 Subject: Example of a message in Spanish and English 332 Content-Type: text/plain; charset="US-ASCII" 333 Content-Transfer-Encoding: 7bit 334 MIME-Version: 1.0 336 Hello, this message content is provided in your language. 338 --01189998819991197253 339 Content-Type: message/rfc822 340 Content-Language: es; translation-type=human 341 Content-Disposition: inline 343 Subject: =?UTF-8?Q?Ejemplo_pr=C3=A1ctico_de_mensaje_?= 344 =?UTF-8?Q?en_espa=C3=B1ol_e_ingl=C3=A9s?= 345 Content-Type: text/plain; charset="US-ASCII" 346 Content-Transfer-Encoding: 7bit 347 MIME-Version: 1.0 349 Hola, el contenido de este mensaje esta disponible en su idioma. 351 --01189998819991197253-- 353 8.2. An Example of a Multiple language email message with language 354 independent part 356 Below is an example of a multiple language email message that has the 357 multilingual preface followed by two language message parts and then 358 a language independent png image. 360 From: Nik@example.com 361 To: Nathaniel@example.com 362 Subject: Example of a message in Spanish and English 363 Date: Thu, 7 Jul 2016 21:08:00 +0100 364 MIME-Version: 1.0 365 Content-Type: multipart/multilingual; 366 boundary="01189998819991197253" 368 --01189998819991197253 369 Content-Type: text/plain; charset="UTF-8" 370 Content-Disposition: inline 371 Content-Transfer-Encoding: quoted-printable 373 This is a message in multiple languages. It says the 374 same thing in each language. If you can read it in one language, 375 you can ignore the other translations. The other translations may be 376 presented as attachments or grouped together. 378 Este es un mensaje en varios idiomas. Dice lo mismo en 379 cada idioma. Si puede leerlo en un idioma, puede ignorar las otras 380 traducciones. Las otras traducciones pueden presentarse como archivos 381 adjuntos o agrupados. 383 --01189998819991197253 384 Content-Type: message/rfc822 385 Content-Language: en; translation-type=original 386 Content-Disposition: inline 387 Subject: Example of a message in Spanish and English 388 Content-Type: text/plain; charset="US-ASCII" 389 Content-Transfer-Encoding: 7bit 390 MIME-Version: 1.0 392 Hello, this message content is provided in your language. 394 --01189998819991197253 395 Content-Type: message/rfc822 396 Content-Language: es; translation-type=human 397 Content-Disposition: inline 399 Subject: =?UTF-8?Q?Ejemplo_pr=C3=A1ctico_de_mensaje_?= 400 =?UTF-8?Q?en_espa=C3=B1ol_e_ingl=C3=A9s?= 401 Content-Type: text/plain; charset="US-ASCII" 402 Content-Transfer-Encoding: 7bit 403 MIME-Version: 1.0 405 Hola, el contenido de este mensaje esta disponible en su idioma. 407 --01189998819991197253 408 Content-Type: message/rfc822; name="Icon" 409 Content-Language: zxx 410 Content-Disposition: inline 412 Content-Type: multipart/mixed; 413 boundary="99911972530118999881"; charset="US-ASCII" 414 Content-Transfer-Encoding: 7bit 415 MIME-Version: 1.0 417 --99911972530118999881 418 Content-Type: image/png; name="icon.png" 419 Content-Disposition: inline 420 Content-Transfer-Encoding: base64 422 iVBORw0KGgoAAAANSUhEUgAAADAAAAAwCAYAAABXAvmHAAAKQ2lDQ1BJQ0MgUHJvZmlsZQAA 423 SA2dlndUU1... shortened for brevity ...7yxfd1SNsEy+OXr76qr 424 997zF2hvZYeDEP5ftGV6Xzx2o9MAAAAASUVORK5CYII= 426 --99911972530118999881-- 427 --01189998819991197253-- 429 8.3. An Example of a complex Multiple language email message with 430 language independent part 432 Below is an example of a more complex multiple language email 433 message. It has the multilingual preface and two language message 434 parts and then a language independent png image. The language 435 message parts have multipart/alternative contents and would therefore 436 require further processing to determine the content to display. 438 From: Nik@example.com 439 To: Nathaniel@example.com 440 Subject: Example of a message in Spanish and English 441 Date: Thu, 7 Jul 2016 20:55:00 +0100 442 MIME-Version: 1.0 443 Content-Type: multipart/multilingual; 444 boundary="01189998819991197253" 446 --01189998819991197253 447 Content-Type: text/plain; charset="UTF-8" 448 Content-Disposition: inline 449 Content-Transfer-Encoding: quoted-printable 451 This is a message in multiple languages. It says the 452 same thing in each language. If you can read it in one language, 453 you can ignore the other translations. The other translations may be 454 presented as attachments or grouped together. 456 Este es un mensaje en varios idiomas. Dice lo mismo en 457 cada idioma. Si puede leerlo en un idioma, puede ignorar las otras 458 traducciones. Las otras traducciones pueden presentarse como archivos 459 adjuntos o agrupados. 461 --01189998819991197253 462 Content-Type: message/rfc822 463 Content-Language: en; translation-type=original 464 Content-Disposition: inline 466 Subject: Example of a message in Spanish and English 467 Content-Type: multipart/alternative; 468 boundary="72530118999911999881"; charset="US-ASCII" 469 Content-Transfer-Encoding: 7bit 470 MIME-Version: 1.0 472 --72530118999911999881 473 Content-Type: text/plain; charset="US-ASCII" 474 Content-Transfer-Encoding: 7bit 476 Hello, this message content is provided in your language. 478 --72530118999911999881 479 Content-Type: text/html; charset="US-ASCII" 480 Content-Transfer-Encoding: 7bit 482 Hello, this message content is provided in your language. 483 --72530118999911999881-- 484 --01189998819991197253 485 Content-Type: message/rfc822 486 Content-Language: es; translation-type=human 487 Content-Disposition: inline 489 Subject: =?UTF-8?Q?Ejemplo_pr=C3=A1ctico_de_mensaje_?= 490 =?UTF-8?Q?en_espa=C3=B1ol_e_ingl=C3=A9s?= 491 Content-Type: multipart/alternative; 492 boundary="53011899989991197281"; charset="US-ASCII" 493 Content-Transfer-Encoding: 7bit 494 MIME-Version: 1.0 496 --53011899989991197281 497 Content-Type: text/plain; charset="US-ASCII" 498 Content-Transfer-Encoding: 7bit 500 Hola, el contenido de este mensaje esta disponible en su idioma. 502 --53011899989991197281 503 Content-Type: text/html; charset="US-ASCII" 504 Content-Transfer-Encoding: 7bit 506 Hola, el contenido de este mensaje esta 507 disponible en su idioma. 509 --53011899989991197281-- 510 --01189998819991197253 511 Content-Type: message/rfc822; name="Icon" 512 Content-Language: zxx 513 Content-Disposition: inline 515 Content-Type: multipart/mixed; 516 boundary="99911972530118999881"; charset="US-ASCII" 517 Content-Transfer-Encoding: 7bit 518 MIME-Version: 1.0 520 --99911972530118999881 521 Content-Type: image/png; name="icon.png" 522 Content-Disposition: inline 523 Content-Transfer-Encoding: base64 525 iVBORw0KGgoAAAANSUhEUgAAADAAAAAwCAYAAABXAvmHAAAKQ2lDQ1BJQ0MgUHJvZmlsZQAA 526 SA2dlndUU1... shortened for brevity ...7yxfd1SNsEy+OXr76qr 527 997zF2hvZYeDEP5ftGV6Xzx2o9MAAAAASUVORK5CYII= 529 --99911972530118999881-- 530 --01189998819991197253-- 531 9. Changes from Previous Versions 533 9.1. Changes from draft-tomkinson-multilangcontent-01 to draft- 534 tomkinson-slim-multilangcontent-00 536 o File name and version number changed to reflect the proposed WG 537 name SLIM (Selection of Language for Internet Media). 539 o Replaced the Subject-Translation field in the language message 540 parts with Subject and provided US-ASCII and non-US-ASCII 541 examples. 543 o Introduced the language-independent message part. 545 o Many wording improvements and clarifications throughout the 546 document. 548 9.2. Changes from draft-tomkinson-slim-multilangcontent-00 to draft- 549 tomkinson-slim-multilangcontent-01 551 o Added Translation-Type in each language message part to identify 552 the source of the translation (original/human/automated). 554 9.3. Changes from draft-tomkinson-slim-multilangcontent-01 to draft- 555 tomkinson-slim-multilangcontent-02 557 o Changed Translation-Type to be a parameter for the Content- 558 Language field rather than a new separate field. 560 o Added a paragraph about using Content-Disposition field to help 561 non-conforming mail clients correctly render the multilingual 562 preface. 564 o Recommended using a Name parameter on the language part Content- 565 Type to help the recipient identify the translations in non- 566 conforming mail clients. 568 o Many wording improvements and clarifications throughout the 569 document. 571 9.4. Changes from draft-tomkinson-slim-multilangcontent-02 to draft- 572 ietf-slim-multilangcontent-00 574 o Name change to reflect the draft being accepted into SLIM as a 575 working group document. 577 o Updated examples to use UTF-8 encoding where required. 579 o Removed references to 'locale' for identifying language 580 preference. 582 o Recommended language matching schemes from RFC 4647 [RFC4647]. 584 o Renamed the unmatched part to language independent part to 585 reinforce its intended purpose. 587 o Added requirement for using Content-Language: zxx in the language 588 independent part. 590 o Many wording improvements and clarifications throughout the 591 document. 593 9.5. Changes from draft-ietf-slim-multilangcontent-00 to draft-ietf- 594 slim-multilangcontent-01 596 o Changed the inner content type to require message/rfc822 or 597 message/global. 599 o Updated the examples to reflect the new inner content types. 601 o Added to the security considerations to highlight the risk from 602 insufficient spam filters. 604 9.6. Changes from draft-ietf-slim-multilangcontent-01 to draft-ietf- 605 slim-multilangcontent-02 607 o Restricted the use of a From field in the language message parts 608 and the language independent part. 610 o Updated the security considerations to highlight the risk of an 611 unmatched sender addresses that could be set in the language 612 message parts. 614 9.7. Changes from draft-ietf-slim-multilangcontent-02 to draft-ietf- 615 slim-multilangcontent-03 617 o Relaxed the restriction on the use of the From field in the 618 language message parts to allow a localised version of the 619 sender's display name. 621 9.8. Changes from draft-ietf-slim-multilangcontent-03 to draft-ietf- 622 slim-multilangcontent-04 624 o Updated the wording of the security considerations section to 625 reflect the relaxation of the use of the From field in the 626 language message parts. 628 9.9. Changes from draft-ietf-slim-multilangcontent-04 to draft-ietf- 629 slim-multilangcontent-05 631 o Referenced the RFC for message/global in Language Message Parts 632 section. 634 o Removed RFC 2119 keyword in the Message Part Selection section. 636 o Included full email addresses in all examples. 638 o Updated reference name of real-time companion document in the 639 Introduction. 641 o Removed paragraph warning of over use of language sub-tags. 643 o Changed 'exponential' to 'significantly' in Introduction. 645 10. Acknowledgements 647 The authors are grateful for the helpful input received from many 648 people but would especially like to acknowledge the help of Harald 649 Alvestrand, Stephane Bortzmeyer, Eric Burger, Mark Davis, Doug Ewell, 650 Randall Gellens, Gunnar Hellstrom, Sean Leonard, John Levine, Alexey 651 Melnikov, Addison Phillips, Pete Resnick, Brian Rosen, Fiona 652 Tomkinson, Simon Tyler and Daniel Vargha. 654 The authors would also like to thank Fernando Alvaro and Luis de 655 Pablo for their work on the Spanish translations. 657 11. IANA Considerations 659 The multipart/multilingual MIME type will be registered with IANA. 661 12. Security Considerations 663 Whilst it is intended that each language message part is a direct 664 translation of the original message, this may not always be the case 665 and these parts could contain undesirable content. Therefore there 666 is a possible risk that undesirable text or images could be shown to 667 the recipient if the message is passed through a spam filter that 668 does not check all of the message parts. The risk should be minimal 669 due to the fact that an unknown multipart subtype should be treated 670 as multipart/mixed and so each message part should be subsequently 671 scanned. 673 Because the language message parts have a Content-Type of message/ 674 rfc822 or message/global, they might contain From fields which could 675 have different values to that of the top-level From field and may not 676 reflect the actual sender. The inconsistent From field values might 677 get shown to the recipient in a non-conforming email client and may 678 mislead the recipient into thinking that the email came from someone 679 other than the real sender. 681 13. References 683 13.1. Normative References 685 [RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail 686 Extensions (MIME) Part Two: Media Types", RFC 2046, 687 DOI 10.17487/RFC2046, November 1996, 688 . 690 [RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions) 691 Part Three: Message Header Extensions for Non-ASCII Text", 692 RFC 2047, DOI 10.17487/RFC2047, November 1996, 693 . 695 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 696 Requirement Levels", BCP 14, RFC 2119, 697 DOI 10.17487/RFC2119, March 1997, 698 . 700 [RFC2183] Troost, R., Dorner, S., and K. Moore, Ed., "Communicating 701 Presentation Information in Internet Messages: The 702 Content-Disposition Header Field", RFC 2183, 703 DOI 10.17487/RFC2183, August 1997, 704 . 706 [RFC3282] Alvestrand, H., "Content Language Headers", RFC 3282, 707 DOI 10.17487/RFC3282, May 2002, 708 . 710 [RFC4647] Phillips, A. and M. Davis, "Matching of Language Tags", 711 BCP 47, RFC 4647, DOI 10.17487/RFC4647, September 2006, 712 . 714 [RFC5646] Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying 715 Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646, 716 September 2009, . 718 [RFC6532] Yang, A., Steele, S., and N. Freed, "Internationalized 719 Email Headers", RFC 6532, DOI 10.17487/RFC6532, February 720 2012, . 722 13.2. Informational References 724 [I-D.ietf-slim-negotiating-human-language] 725 Gellens, R., "Negotiating Human Language in Real-Time 726 Communications", draft-ietf-slim-negotiating-human- 727 language-04 (work in progress), July 2016. 729 Authors' Addresses 731 Nik Tomkinson 732 Mimecast Ltd 733 CityPoint, One Ropemaker Street 734 London EC2Y 9AW 735 United Kingdom 737 Email: rfc.nik.tomkinson@gmail.com 739 Nathaniel Borenstein 740 Mimecast Ltd 741 480 Pleasant Street 742 Watertown MA 02472 743 North America 745 Email: nsb@mimecast.com