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'UNF' -- Obsolete informational reference (is this intentional?): RFC 5335 (Obsoleted by RFC 6532) Summary: 3 errors (**), 0 flaws (~~), 6 warnings (==), 6 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Email Address Internationalization A. Yang 3 (EAI) TWNIC 4 Internet-Draft S. Steele 5 Obsoletes: 5335 (if approved) Microsoft 6 Updates: 2045 (if approved) N. Freed 7 Intended status: Standards Track Oracle 8 Expires: April 23, 2012 October 21, 2011 10 Internationalized Email Headers 11 draft-ietf-eai-rfc5335bis-13 13 Abstract 15 Internet mail was originally limited to 7-bit ASCII. MIME added 16 support for the use of 8-bit character sets in body parts, and also 17 defined an encoded-word construct so other character sets could be 18 used in certain header field values. But full internationalization 19 of electronic mail requires additional enhancements to allow the use 20 of Unicode, including characters outside the ASCII repertoire, in 21 mail addresses as well as direct use of Unicode in header fields like 22 From:, To:, and Subject:, without requiring the use of complex 23 encoded-word constructs. This document specifies an enhancement to 24 the Internet Message Format and to MIME that allows use of Unicode in 25 mail addresses and most header field content. 27 This specification replaces RFC 5335. This specification also 28 updates Section 6.4 of RFC 2045 to eliminate the restriction 29 prohibiting the use of non-identity content-transfer-encodings on 30 subtypes of "message/". 32 Status of This Memo 34 This Internet-Draft is submitted in full conformance with the 35 provisions of BCP 78 and BCP 79. 37 Internet-Drafts are working documents of the Internet Engineering 38 Task Force (IETF). Note that other groups may also distribute 39 working documents as Internet-Drafts. The list of current Internet- 40 Drafts is at http://datatracker.ietf.org/drafts/current/. 42 Internet-Drafts are draft documents valid for a maximum of six months 43 and may be updated, replaced, or obsoleted by other documents at any 44 time. It is inappropriate to use Internet-Drafts as reference 45 material or to cite them other than as "work in progress." 47 This Internet-Draft will expire on April 23, 2012. 49 Copyright Notice 51 Copyright (c) 2011 IETF Trust and the persons identified as the 52 document authors. All rights reserved. 54 This document is subject to BCP 78 and the IETF Trust's Legal 55 Provisions Relating to IETF Documents 56 (http://trustee.ietf.org/license-info) in effect on the date of 57 publication of this document. Please review these documents 58 carefully, as they describe your rights and restrictions with respect 59 to this document. Code Components extracted from this document must 60 include Simplified BSD License text as described in Section 4.e of 61 the Trust Legal Provisions and are provided without warranty as 62 described in the Simplified BSD License. 64 Table of Contents 66 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 67 2. Terminology Used In This Specification . . . . . . . . . . . . 3 68 3. Changes to Message Header Fields . . . . . . . . . . . . . . . 4 69 3.1. UTF-8 Syntax and Normalization . . . . . . . . . . . . . . 4 70 3.2. Syntax Extensions to RFC 5322 . . . . . . . . . . . . . . 5 71 3.3. Use of 8-bit UTF-8 in Message-Ids . . . . . . . . . . . . 5 72 3.4. Effects on Line Length Limits . . . . . . . . . . . . . . 6 73 3.5. Changes to MIME Message Type Encoding Restrictions . . . . 6 74 3.6. Use of MIME Encoded-Words . . . . . . . . . . . . . . . . 6 75 3.7. The Message/global Media Type . . . . . . . . . . . . . . 7 76 4. Security Considerations . . . . . . . . . . . . . . . . . . . 8 77 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 78 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9 79 7. Edit history . . . . . . . . . . . . . . . . . . . . . . . . . 9 80 7.1. draft-ietf-eai-rfc5335bis-00 . . . . . . . . . . . . . . . 9 81 7.2. draft-ietf-eai-rfc5335bis-01 . . . . . . . . . . . . . . . 10 82 7.3. draft-ietf-eai-rfc5335bis-02 . . . . . . . . . . . . . . . 10 83 7.4. draft-ietf-eai-rfc5335bis-03 . . . . . . . . . . . . . . . 10 84 7.5. draft-ietf-eai-rfc5335bis-04 . . . . . . . . . . . . . . . 10 85 7.6. draft-ietf-eai-rfc5335bis-05 . . . . . . . . . . . . . . . 10 86 7.7. draft-ietf-eai-rfc5335bis-06 . . . . . . . . . . . . . . . 10 87 7.8. draft-ietf-eai-rfc5335bis-07 . . . . . . . . . . . . . . . 10 88 7.9. draft-ietf-eai-rfc5335bis-09 . . . . . . . . . . . . . . . 11 89 7.10. draft-ietf-eai-rfc5335bis-10 . . . . . . . . . . . . . . . 11 90 7.11. draft-ietf-eai-rfc5335bis-11 . . . . . . . . . . . . . . . 11 91 7.12. draft-ietf-eai-rfc5335bis-12 . . . . . . . . . . . . . . . 11 92 7.13. draft-ietf-eai-rfc5335bis-13 . . . . . . . . . . . . . . . 11 93 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12 94 8.1. Normative References . . . . . . . . . . . . . . . . . . . 12 95 8.2. Informative References . . . . . . . . . . . . . . . . . . 13 97 1. Introduction 99 Internet mail distinguishes a message from its transport and further 100 divides a message between a header and a body [RFC5322]. Internet 101 mail header field values contain a variety of strings that are 102 intended to be user-visible. The range of supported characters for 103 these strings was originally limited to 7-bit [ASCII]. MIME 104 [RFC2045] [RFC2046] [RFC2047] provides the ability to use additional 105 character sets, but this support is limited to body part data and to 106 special encoded-word constructs that were only allowed in a limited 107 number of places in header field values. 109 Globalization of the Internet requires support of the much larger set 110 of characters provided by Unicode [RFC5198] in both mail addresses 111 and most header field values. Additionally, complex encoding schemes 112 like encoded-words introduce inefficiencies as well as significant 113 opportunities for processing errors. And finally, native support for 114 the UTF-8 charset is now available on most systems. Hence it is 115 strongly desirable for Internet mail to support UTF-8 [RFC3629] 116 directly. 118 This document specifies an enhancement to the Internet Message Format 119 [RFC5322] and to MIME that permits the direct use of UTF-8, rather 120 than only ASCII, in header field values, including mail addresses. A 121 new media type, message/global, is defined for messages that use this 122 extended format. This specification also lifts the MIME restriction 123 on having non-identity content-transfer-encodings on any subtype of 124 the message top-level type so that message/global parts can be safely 125 transmitted across existing mail infrastructure. 127 This specification is based on a model of native, end-to-end support 128 for UTF-8, which depends on having an "8-bit clean" environment 129 assured by the transport system. Support for carriage across legacy, 130 7-bit infrastructure and for processing by 7-bit receivers requires 131 additional mechanisms that are not provided by these specifications. 133 This specification is a revision of and replacement for [RFC5335]. 134 Section 6 of [I-D.ietf-eai-frmwrk-4952bis] describes the change in 135 approach between this specification and the previous version. 137 2. Terminology Used In This Specification 139 A plain ASCII string is fully compatible with [RFC5321] and 140 [RFC5322]. In this document, non-ASCII strings are UTF-8 strings if 141 they are in header field values which contain at least one (see Section 3.1). 144 Unless otherwise noted, all terms used here are defined in [RFC5321], 146 [RFC5322], [I-D.ietf-eai-frmwrk-4952bis], or 147 [I-D.ietf-eai-rfc5336bis]. 149 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 150 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 151 document are to be interpreted as described in [RFC2119]. 153 The term "8-bit" means octets are present in the data with values 154 above 0x7F. 156 3. Changes to Message Header Fields 158 To permit Unicode characters in field values, the header definition 159 in [RFC5322] is extended to support the new format. The following 160 sections specify the necessary changes to RFC 5322's ABNF. 162 The syntax rules not mentioned below remain defined as in [RFC5322]. 164 Note that this protocol does not change RFC 5322 rules for defining 165 header field names. The bodies of header fields are allowed to 166 contain Unicode characters, but the header field names themselves 167 must contain only ASCII characters. 169 Also note that messages in this format require the use of the 170 UTF8SMTPbis extension [I-D.ietf-eai-rfc5336bis] to be transferred via 171 SMTP. 173 3.1. UTF-8 Syntax and Normalization 175 UTF-8 characters can be defined in terms of octets using the 176 following ABNF [RFC5234], taken from [RFC3629]: 178 UTF8-non-ascii = UTF8-2 / UTF8-3 / UTF8-4 180 UTF8-2 = 182 UTF8-3 = 184 UTF8-4 = 186 See [RFC5198] for a discussion of Unicode normalization; 187 normalization form NFC [UNF] SHOULD be used. Actually, if one is 188 going to do internationalization properly, one of the most often- 189 cited goals is to permit people to spell their names correctly. 190 Since many mailbox local parts reflect personal names, that principle 191 applies to mailboxes as well. The NFKC normalization form [UNF] 192 SHOULD NOT be used because it may lose information that is needed to 193 correctly spell some names in some unusual circumstances. 195 3.2. Syntax Extensions to RFC 5322 197 The following rules extend the ABNF syntax defined in [RFC5322] and 198 [RFC5234] in order to allow UTF-8 content. 200 VCHAR =/ UTF8-non-ascii 202 ctext =/ UTF8-non-ascii 204 atext =/ UTF8-non-ascii 206 qtext =/ UTF8-non-ascii 208 text =/ UTF8-non-ascii 209 ; note that this upgrades the body to UTF-8 211 dtext =/ UTF8-non-ascii 213 The preceding changes mean that the following constructs now allow 214 UTF-8: 216 1. Unstructured text, used in header fields like Subject: or 217 Content-description:. 219 2. Any construct that uses atoms, including but not limited to the 220 local parts of addresses and message-ids. This includes 221 addresses in the "for" clauses of Received: header fields. 223 3. Quoted strings. 225 4. Domains. 227 Note that header field names are not on this list; these are still 228 restricted to ASCII. 230 3.3. Use of 8-bit UTF-8 in Message-Ids 232 Implementers of message-id generation algorithms MAY prefer to 233 restrain their output to ASCII since that has some advantages, such 234 as when constructing In-reply-to: and References: header fields in 235 mailing-list threads where some senders use EAI and others not. 237 3.4. Effects on Line Length Limits 239 Section 2.1.1 of [RFC5322] limits lines to 998 characters and 240 recommends that the lines be restricted to only 78 characters. This 241 specification changes the former limit to 998 octets. (Note that in 242 ASCII octets and characters are effectively the same but this is not 243 true in UTF-8.) The 78 character limit remains defined in terms of 244 characters, not octets, since it is intended to address display width 245 issues, not line length issues. 247 3.5. Changes to MIME Message Type Encoding Restrictions 249 This specification updates Section 6.4 of [RFC2045]. [RFC2045] 250 prohibits applying a content-transfer-encoding to any subtypes of 251 "message/". This specification relaxes that rule -- it allows newly 252 defined MIME types to permit content-transfer-encoding, and it allows 253 content-transfer-encoding for message/global (see Section 3.7). 255 Background: Normally, transfer of message/global will be done in 256 8-bit-clean channels, and body parts will have "identity" encodings, 257 that is, no decoding is necessary. 259 But in the case where a message containing a message/global is 260 downgraded from 8-bit to 7-bit as described in [RFC6152], an encoding 261 might have to be applied to the message; if the message travels 262 multiple times between a 7-bit environment and an environment 263 implementing these extensions, multiple levels of encoding may occur. 264 This is expected to be rarely seen in practice, and the potential 265 complexity of other ways of dealing with the issue is thought to be 266 larger than the complexity of allowing nested encodings where 267 necessary. 269 3.6. Use of MIME Encoded-Words 271 The MIME encoded-words facility [RFC2047] provides the ability to 272 place non-ASCII text, but only in a subset of the places allowed by 273 this extension. Additionally, encoded-words are substantially more 274 complex since they allow the use of arbitrary charsets. Accordingly, 275 encoded-words SHOULD NOT be used when generating header fields for 276 messages employing this extension. Agents MAY, when incorporating 277 material from another message, convert encoded-word use to direct use 278 of UTF-8. 280 Note that care must be taken when decoding encoded-words because the 281 results after replacing an encoded-word with its decoded equivalent 282 in UTF-8 may be syntactically invalid. Processors that elect to 283 decode encoded-words MUST NOT generate syntactically invalid fields. 285 3.7. The Message/global Media Type 287 Internationalized messages in this format MUST only be transmitted as 288 authorized by [I-D.ietf-eai-rfc5336bis] or within a non-SMTP 289 environment that supports these messages. A message is a "message/ 290 global message" if: 292 o it contains 8-bit UTF-8 header values as specified in this 293 document, or 295 o it contains 8-bit UTF-8 values in the header fields of body parts. 297 The content of a message/global part is otherwise identical to that 298 of a message/rfc822 part. 300 If an object of this type is sent to a 7-bit-only system, it MUST 301 have an appropriate content-transfer-encoding applied. (Note that a 302 system compliant with MIME that doesn't recognize message/global is 303 supposed to treat it as "application/octet-stream" as described in 304 Section 5.2.4 of [RFC2046].) 306 Type name: message 308 Subtype name: global 310 Required parameters: none 312 Optional parameters: none 314 Encoding considerations: Any content-transfer-encoding is permitted. 315 The 8-bit or binary content-transfer-encodings are recommended 316 where permitted. 318 Security considerations: See Section 4. 320 Interoperability considerations: This media type provides 321 functionality similar to the message/rfc822 content type for email 322 messages with internationalized email headers. When there is a 323 need to embed or return such content in another message, there is 324 generally an option to use this media type and leave the content 325 unchanged or down-convert the content to message/rfc822. Both of 326 these choices will interoperate with the installed base, but with 327 different properties. Systems unaware of internationalized 328 headers will typically treat a message/global body part as an 329 unknown attachment, while they will understand the structure of a 330 message/rfc822. However, systems that understand message/global 331 will provide functionality superior to the result of a down- 332 conversion to message/rfc822. The most interoperable choice 333 depends on the deployed software. 335 Published specification: RFC XXXX 337 Applications that use this media type: SMTP servers and email 338 clients that support multipart/report generation or parsing. 339 Email clients that forward messages with internationalized headers 340 as attachments. 342 Additional information: 344 Magic number(s): none 346 File extension(s): The extension ".u8msg" is suggested. 348 Macintosh file type code(s): A uniform type identifier (UTI) of 349 "public.utf8-email-message" is suggested. This conforms to 350 "public.message" and "public.composite-content", but does not 351 necessarily conform to "public.utf8-plain-text". 353 Person & email address to contact for further information: See the 354 Author's Address section of this document. 356 Intended usage: COMMON 358 Restrictions on usage: This is a structured media type that embeds 359 other MIME media types. An 8-bit or binary content-transfer- 360 encoding SHOULD be used unless this media type is sent over a 361 7-bit-only transport. 363 Author: See the Author's Address section of this document. 365 Change controller: IETF Standards Process 367 4. Security Considerations 369 Because UTF-8 often requires several octets to encode a single 370 character, internationalization may cause header field values in 371 general and mail addresses in particular to become longer. As 372 specified in [RFC5322], each line of characters MUST be no more than 373 998 octets, excluding the CRLF. On the other hand, MDA (Mail 374 Delivery Agent) processes that parse, store, or handle email 375 addresses or local parts must take extra care not to overflow 376 buffers, truncate addresses, or exceed storage allotments. Also, 377 they must take care, when comparing, to use the entire lengths of the 378 addresses. 380 There are lots of ways to use UTF-8 to represent something equivalent 381 or similar to a particular displayed character or group of 382 characters; see the security considerations in [RFC3629] for details 383 on the problems this can cause. The normalization process described 384 in Section 3.1 is recommended to minimize these issues. 386 The security impact of UTF-8 headers on email signature systems such 387 as Domain Keys Identified Mail (DKIM), S/MIME, and OpenPGP is 388 discussed in [I-D.ietf-eai-frmwrk-4952bis], Section 14. 390 If a user has a non-ASCII mailbox address and an ASCII mailbox 391 address, a digital certificate that identifies that user might have 392 both addresses in the identity. Having multiple email addresses as 393 identities in a single certificate is already supported in PKIX 394 (Public Key Infrastructure for X.509 Certificates) [RFC5280] and 395 OpenPGP [RFC3156], but there may be user interface issues associated 396 with the introduction of UTF-8 into addresses in this context. 398 5. IANA Considerations 400 IANA is requested to update the registration of the message/global 401 MIME type using the registration form contained in Section 3.7. 403 6. Acknowledgements 405 This document incorporates many ideas first described in Internet- 406 Draft form by Paul Hoffman, although many details have changed from 407 that earlier work. 409 The author especially thanks Jeff Yeh for his efforts and 410 contributions on editing previous versions. 412 Most of the content of this document was provided by John C Klensin 413 and Dave Crocker. Significant comments and suggestions were received 414 from Martin Duerst, Julien Elie, Arnt Gulbrandsen, Kristin Hubner, 415 Kari Hurtta, Yangwoo Ko, Charles H. Lindsey, Alexey Melnikov, Chris 416 Newman, Pete Resnick, Yoshiro Yoneya, and additional members of the 417 JET team (Joint Engineering Team) and were incorporated into the 418 document. The editors wish to sincerely thank them all for their 419 contributions. 421 7. Edit history 423 [[RFC Editor: Please remove this section before publishing.]] 425 7.1. draft-ietf-eai-rfc5335bis-00 426 1. Applied Errata suggested by Alfred Hoenes. 428 2. Adjust [RFC2821] and [RFC2822] to [RFC5321] and [RFC5322]. 430 3. Abrogate in ABNF of . 432 4. Revoke [RFC5504] from this document. 434 5. Upgrade some references from I-Ds to RFC. 436 7.2. draft-ietf-eai-rfc5335bis-01 438 1. Author name revised. 440 7.3. draft-ietf-eai-rfc5335bis-02 442 1. ABNF revised. 444 7.4. draft-ietf-eai-rfc5335bis-03 446 1. Fix typos 448 2. ABNF revised 450 3. Improve sentence 452 7.5. draft-ietf-eai-rfc5335bis-04 454 1. improve sentences and ABNF revised based on AD and Co-chairs 456 7.6. draft-ietf-eai-rfc5335bis-05 458 1. ABNF revised based on AD comments 460 7.7. draft-ietf-eai-rfc5335bis-06 462 1. ABNF revised 464 2. improve Section 5 466 7.8. draft-ietf-eai-rfc5335bis-07 468 1. Minor ABNF revised in Section 3.2 470 2. improve Section 5 472 7.9. draft-ietf-eai-rfc5335bis-09 474 Version -08 was posted in error and withdrawn. Version 09 is is 475 identical to version 07 except for a date change, addition of this 476 note, and some vertical spacing compression on this page. 478 7.10. draft-ietf-eai-rfc5335bis-10 480 1. Add appendix and overview of changes 482 2. Replace polls result in Abstract and Section 1 484 3. Minor Sentence modification 486 7.11. draft-ietf-eai-rfc5335bis-11 488 1. Major rewrite of entire document to incorporate Dave Crocker's 489 simplified ABNF. 491 2. The document has intentionally been refocused on implementors 492 wishing to adapt their software to support EAI, so much of the 493 explanatory and historical text has been removed. (Some of it 494 may be reintroduced later as an appendix. 496 7.12. draft-ietf-eai-rfc5335bis-12 498 1. Added a section on the handling of MIME encoded-words. 500 2. Updated the security considerations to refer to the more complete 501 discussion in RFC 3629. 503 3. Added a section on the effects on line length limits. 505 4. Removed the syntax restriction on the use of 8-bit UTF-8 in 506 message-ids. 508 5. Added text recommending that 8-bit UTF-8 be avoided in message- 509 ids. 511 7.13. draft-ietf-eai-rfc5335bis-13 513 1. Updated and alphabetized the contributor list. 515 2. Corrected various typos, reworded some sections to make them 516 clearer. 518 3. Replaced the reference to RFC 5598 with a reference to RFC 5322. 520 4. Removed the Updates: RFC 5322. RFC 5322 is extended by this 521 document, not updated. 523 5. Added some text to the Introduction referring to the framework 524 document for information about changes between this specification 525 and RFC 5335. 527 6. Added text to the Abstract to say that this document replaces RFC 528 5335 and that RFC 2045 is updated. 530 8. References 532 8.1. Normative References 534 [ASCII] "Coded Character Set -- 7-bit American 535 Standard Code for Information 536 Interchange", ANSI X3.4, 1986. 538 [I-D.ietf-eai-frmwrk-4952bis] Klensin, J. and Y. Ko, "Overview and 539 Framework for Internationalized 540 Email", 541 draft-ietf-eai-frmwrk-4952bis-10 (work 542 in progress), September 2010. 544 [I-D.ietf-eai-rfc5336bis] Yao, J. and W. MAO, "SMTP Extension 545 for Internationalized Email Address", 546 draft-ietf-eai-rfc5336bis-07 (work in 547 progress), December 2010. 549 [RFC2119] Bradner, S., "Key words for use in 550 RFCs to Indicate Requirement Levels", 551 BCP 14, RFC 2119, March 1997. 553 [RFC3629] Yergeau, F., "UTF-8, a transformation 554 format of ISO 10646", STD 63, 555 RFC 3629, November 2003. 557 [RFC5198] Klensin, J. and M. Padlipsky, "Unicode 558 Format for Network Interchange", 559 RFC 5198, March 2008. 561 [RFC5234] Crocker, D. and P. Overell, "Augmented 562 BNF for Syntax Specifications: ABNF", 563 STD 68, RFC 5234, January 2008. 565 [RFC5321] Klensin, J., "Simple Mail Transfer 566 Protocol", RFC 5321, October 2008. 568 [RFC5322] Resnick, P., Ed., "Internet Message 569 Format", RFC 5322, October 2008. 571 [UNF] Davis, M. and K. Whistler, "Unicode 572 Standard Annex #15: Unicode 573 Normalization Forms", September 2010, 574 . 577 8.2. Informative References 579 [RFC2045] Freed, N. and N. Borenstein, 580 "Multipurpose Internet Mail Extensions 581 (MIME) Part One: Format of Internet 582 Message Bodies", RFC 2045, 583 November 1996. 585 [RFC2046] Freed, N. and N. Borenstein, 586 "Multipurpose Internet Mail Extensions 587 (MIME) Part Two: Media Types", 588 RFC 2046, November 1996. 590 [RFC2047] Moore, K., "MIME (Multipurpose 591 Internet Mail Extensions) Part Three: 592 Message Header Extensions for Non- 593 ASCII Text", RFC 2047, November 1996. 595 [RFC3156] Elkins, M., Del Torto, D., Levien, R., 596 and T. Roessler, "MIME Security with 597 OpenPGP", RFC 3156, August 2001. 599 [RFC5280] Cooper, D., Santesson, S., Farrell, 600 S., Boeyen, S., Housley, R., and W. 601 Polk, "Internet X.509 Public Key 602 Infrastructure Certificate and 603 Certificate Revocation List (CRL) 604 Profile", RFC 5280, May 2008. 606 [RFC5335] Abel, Y., "Internationalized Email 607 Headers", RFC 5335, September 2008. 609 [RFC6152] Klensin, J., Freed, N., Rose, M., and 610 D. Crocker, "SMTP Service Extension 611 for 8-bit MIME Transport", STD 71, 612 RFC 6152, March 2011. 614 Authors' Addresses 616 Abel Yang 617 TWNIC 618 4F-2, No. 9, Sec 2, Roosevelt Rd. 619 Taipei, 100 620 Taiwan 622 Phone: +886 2 23411313 ext 505 623 EMail: abelyang@twnic.net.tw 625 Shawn Steele 626 Microsoft 628 EMail: Shawn.Steele@microsoft.com 630 Ned Freed 631 Oracle 632 800 Royal Oaks 633 Monrovia, CA 91016-6347 634 USA 636 EMail: ned+ietf@mrochek.com