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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Email Address Internationalization J. Klensin 3 (EAI) 4 Internet-Draft Y. Ko 5 Obsoletes: RFCs 4952, 5504, 5825 ICU 6 (if approved) August 22, 2010 7 Intended status: Informational 8 Expires: February 23, 2011 10 Overview and Framework for Internationalized Email 11 draft-ietf-eai-frmwrk-4952bis-05 13 Abstract 15 Full use of electronic mail throughout the world requires that 16 (subject to other constraints) people be able to use close variations 17 on their own names (written correctly in their own languages and 18 scripts) as mailbox names in email addresses. This document 19 introduces a series of specifications that define mechanisms and 20 protocol extensions needed to fully support internationalized email 21 addresses. These changes include an SMTP extension and extension of 22 email header syntax to accommodate UTF-8 data. The document set also 23 includes discussion of key assumptions and issues in deploying fully 24 internationalized email. This document is an update of RFC 4952; it 25 reflects additional issues identified since that document was 26 published. 28 Status of This Memo 30 This Internet-Draft is submitted in full conformance with the 31 provisions of BCP 78 and BCP 79. 33 Internet-Drafts are working documents of the Internet Engineering 34 Task Force (IETF). Note that other groups may also distribute 35 working documents as Internet-Drafts. The list of current Internet- 36 Drafts is at http://datatracker.ietf.org/drafts/current/. 38 Internet-Drafts are draft documents valid for a maximum of six months 39 and may be updated, replaced, or obsoleted by other documents at any 40 time. It is inappropriate to use Internet-Drafts as reference 41 material or to cite them other than as "work in progress." 43 This Internet-Draft will expire on February 23, 2011. 45 Copyright Notice 47 Copyright (c) 2010 IETF Trust and the persons identified as the 48 document authors. All rights reserved. 50 This document is subject to BCP 78 and the IETF Trust's Legal 51 Provisions Relating to IETF Documents 52 (http://trustee.ietf.org/license-info) in effect on the date of 53 publication of this document. Please review these documents 54 carefully, as they describe your rights and restrictions with respect 55 to this document. Code Components extracted from this document must 56 include Simplified BSD License text as described in Section 4.e of 57 the Trust Legal Provisions and are provided without warranty as 58 described in the Simplified BSD License. 60 This document may contain material from IETF Documents or IETF 61 Contributions published or made publicly available before November 62 10, 2008. The person(s) controlling the copyright in some of this 63 material may not have granted the IETF Trust the right to allow 64 modifications of such material outside the IETF Standards Process. 65 Without obtaining an adequate license from the person(s) controlling 66 the copyright in such materials, this document may not be modified 67 outside the IETF Standards Process, and derivative works of it may 68 not be created outside the IETF Standards Process, except to format 69 it for publication as an RFC or to translate it into languages other 70 than English. 72 Table of Contents 74 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 75 2. Role of This Specification . . . . . . . . . . . . . . . . . . 4 76 3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 5 77 4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6 78 4.1. Mail User and Mail Transfer Agents . . . . . . . . . . . . 6 79 4.2. Address Character Sets . . . . . . . . . . . . . . . . . . 7 80 4.3. User Types . . . . . . . . . . . . . . . . . . . . . . . . 7 81 4.4. Messages . . . . . . . . . . . . . . . . . . . . . . . . . 8 82 4.5. Mailing Lists . . . . . . . . . . . . . . . . . . . . . . 8 83 4.6. Conventional Message and Internationalized Message . . . . 8 84 4.7. Undeliverable Messages, Notification, and Delivery 85 Receipts . . . . . . . . . . . . . . . . . . . . . . . . . 8 86 5. Overview of the Approach and Document Plan . . . . . . . . . . 9 87 6. Review of Experimental Results . . . . . . . . . . . . . . . . 9 88 7. Overview of Protocol Extensions and Changes . . . . . . . . . 10 89 7.1. SMTP Extension for Internationalized Email Address . . . . 10 90 7.2. Transmission of Email Header Fields in UTF-8 Encoding . . 11 91 7.3. SMTP Service Extension for DSNs . . . . . . . . . . . . . 12 92 8. Downgrading before and after SMTP Transactions . . . . . . . . 12 93 8.1. Downgrading before or during Message Submission . . . . . 13 94 8.2. Downgrading or Other Processing After Final SMTP 95 Delivery . . . . . . . . . . . . . . . . . . . . . . . . . 14 96 9. Downgrading in Transit . . . . . . . . . . . . . . . . . . . . 14 97 10. User Interface and Configuration Issues . . . . . . . . . . . 15 98 10.1. Choices of Mailbox Names and Unicode Normalization . . . . 15 99 11. Additional Issues . . . . . . . . . . . . . . . . . . . . . . 16 100 11.1. Impact on URIs and IRIs . . . . . . . . . . . . . . . . . 16 101 11.2. Use of Email Addresses as Identifiers . . . . . . . . . . 16 102 11.3. Encoded Words, Signed Messages, and Downgrading . . . . . 17 103 11.4. LMTP . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 104 11.5. Other Uses of Local Parts . . . . . . . . . . . . . . . . 18 105 11.6. Non-Standard Encapsulation Formats . . . . . . . . . . . . 18 106 12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 107 13. Security Considerations . . . . . . . . . . . . . . . . . . . 18 108 14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20 109 15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20 110 15.1. Normative References . . . . . . . . . . . . . . . . . . . 20 111 15.2. Informative References . . . . . . . . . . . . . . . . . . 22 112 Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 26 113 A.1. Changes between -00 and -01 . . . . . . . . . . . . . . . 26 114 A.2. Changes between -01 and -02 . . . . . . . . . . . . . . . 26 115 A.3. Changes between -02 and -03 . . . . . . . . . . . . . . . 27 116 A.4. Changes between -03 and -04 . . . . . . . . . . . . . . . 28 117 A.5. Changes between -04 and -05 . . . . . . . . . . . . . . . 28 119 1. Introduction 121 Note in Draft and to RFC Editor: The keyword represented in this 122 document by "UTF8SMTPbis" (and in the XML source by &EAISMTPkeyword;) 123 is a placeholder. The actual keyword will be assigned when the 124 standards track SMTP extension in this series [RFC5336bis-SMTP] is 125 approved for publication and should be substituted here. This 126 paragraph should be treated as normative reference to that SMTP 127 extension draft, creating a reference hold until it is approved by 128 the IESG. The paragraph should be removed before RFC publication. 130 In order to use internationalized email addresses, we need to 131 internationalize both the domain part and the local part of email 132 addresses. The domain part of email addresses is already 133 internationalized [RFC5890], while the local part is not. Without 134 the extensions specified in this document, the mailbox name is 135 restricted to a subset of 7-bit ASCII [RFC5321]. Though MIME 136 [RFC2045] enables the transport of non-ASCII data, it does not 137 provide a mechanism for internationalized email addresses. In RFC 138 2047 [RFC2047], MIME defines an encoding mechanism for some specific 139 message header fields to accommodate non-ASCII data. However, it 140 does not permit the use of email addresses that include non-ASCII 141 characters. Without the extensions defined here, or some equivalent 142 set, the only way to incorporate non-ASCII characters in any part of 143 email addresses is to use RFC 2047 coding to embed them in what RFC 144 5322 [RFC5322] calls the "display name" (known as a "name phrase" or 145 by other terms elsewhere) of the relevant header fields. Information 146 coded into the display name is invisible in the message envelope and, 147 for many purposes, is not part of the address at all. 149 This document is an update of RFC 4952 [RFC4952]; it reflects 150 additional issues, shared terminology, and some architectural changes 151 identified since that document was published. 153 The pronouns "he" and "she" are used interchangeably to indicate a 154 human of indeterminate gender. 156 The key words "MUST", "SHALL", "REQUIRED", "SHOULD", "RECOMMENDED", 157 and "MAY" in this document are to be interpreted as described in RFC 158 2119 [RFC2119]. 160 2. Role of This Specification 162 This document presents the overview and framework for an approach to 163 the next stage of email internationalization. This new stage 164 requires not only internationalization of addresses and header 165 fields, but also associated transport and delivery models. A prior 166 version of this specification, RFC 4952 [RFC4952], also provided an 167 introduction to a series of experimental protocols [RFC5335] 168 [RFC5336] [RFC5337] [RFC5504] [RFC5721] [RFC5738] [RFC5825]. This 169 revised form provides overview and conceptual information for the 170 standards-track successors of a subset of those protocols. Details 171 of the documents and the relationships among them appear in Section 5 172 and a discussion of what was learned from the Experimental protocols 173 and their implementations appears in Section 6. 175 Taken together, these specifications provide the details for a way to 176 implement and support internationalized email. The document itself 177 describes how the various elements of email internationalization fit 178 together and the relationships among the primary specifications 179 associated with message transport, header formats, and handling. 181 This document, and others that comprise the collection described 182 above, assume a reasonable familiarity with the basic Internet 183 electronic mail specifications and terminology [RFC5321][RFC5322] and 184 the MIME [RFC2045] and 8BITMIME [RFC1652] ones as well. While not 185 strictly required to implement this specification, a general 186 familiarity with the terminology and functions of IDNA 187 [RFC5890][RFC5891] [RFC5892][RFC5893] [RFC5894] are also assumed. 189 3. Problem Statement 191 Internationalizing Domain Names in Applications (IDNA) [RFC5890] 192 permits internationalized domain names, but deployment has not yet 193 reached most users. One of the reasons for this is that we do not 194 yet have fully internationalized naming schemes. Domain names are 195 just one of the various names and identifiers that are required to be 196 internationalized. In many contexts, until more of those identifiers 197 are internationalized, internationalized domain names alone have 198 little value. 200 Email addresses are prime examples of why it is not good enough to 201 just internationalize the domain name. As most observers have 202 learned from experience, users strongly prefer email addresses that 203 resemble names or initials to those involving seemingly meaningless 204 strings of letters or numbers. Unless the entire email address can 205 use familiar characters and formats, users will perceive email as 206 being culturally unfriendly. If the names and initials used in email 207 addresses can be expressed in the native languages and writing 208 systems of the users, the Internet will be perceived as more natural, 209 especially by those whose native language is not written in a subset 210 of a Roman-derived script. 212 Internationalization of email addresses is not merely a matter of 213 changing the SMTP envelope; or of modifying the From, To, and Cc 214 header fields; or of permitting upgraded Mail User Agents (MUAs) to 215 decode a special coding and respond by displaying local characters. 216 To be perceived as usable, the addresses must be internationalized 217 and handled consistently in all of the contexts in which they occur. 218 This requirement has far-reaching implications: collections of 219 patches and workarounds are not adequate. Even if they were 220 adequate, a workaround-based approach may result in an assortment of 221 implementations with different sets of patches and workarounds having 222 been applied with consequent user confusion about what is actually 223 usable and supported. Instead, we need to build a fully 224 internationalized email environment, focusing on permitting efficient 225 communication among those who share a language or other community. 226 That, in turn, implies changes to the mail header environment to 227 permit the full range of Unicode characters where that makes sense, 228 an SMTP Extension to permit UTF-8 [RFC3629] [RFC5198] mail addressing 229 and delivery of those extended header fields, support for 230 internationalization of delivery and service notifications [RFC3461] 231 [RFC3464], and (finally) a requirement for support of the 8BITMIME 232 SMTP Extension [RFC1652] so that all of these can be transported 233 through the mail system without having to overcome the limitation 234 that header fields do not have content-transfer-encodings. 236 4. Terminology 238 This document assumes a reasonable understanding of the protocols and 239 terminology of the core email standards as documented in [RFC5321] 240 and [RFC5322]. 242 4.1. Mail User and Mail Transfer Agents 244 Much of the description in this document depends on the abstractions 245 of "Mail Transfer Agent" ("MTA") and "Mail User Agent" ("MUA"). 246 However, it is important to understand that those terms and the 247 underlying concepts postdate the design of the Internet's email 248 architecture and the application of the "protocols on the wire" 249 principle to it. That email architecture, as it has evolved, and 250 that "on the wire" principle have prevented any strong and 251 standardized distinctions about how MTAs and MUAs interact on a given 252 origin or destination host (or even whether they are separate). 254 However, the term "final delivery MTA" is used in this document in a 255 fashion equivalent to the term "delivery system" or "final delivery 256 system" of RFC 5321. This is the SMTP server that controls the 257 format of the local parts of addresses and is permitted to inspect 258 and interpret them. It receives messages from the network for 259 delivery to mailboxes or for other local processing, including any 260 forwarding or aliasing that changes envelope addresses, rather than 261 relaying. From the perspective of the network, any local delivery 262 arrangements such as saving to a message store, handoff to specific 263 message delivery programs or agents, and mechanisms for retrieving 264 messages are all "behind" the final delivery MTA and hence are not 265 part of the SMTP transport or delivery process. 267 4.2. Address Character Sets 269 In this document, an address is "all-ASCII", or just an "ASCII 270 address", if every character in the address is in the ASCII character 271 repertoire [ASCII]; an address is "non-ASCII", or an "i18n-address", 272 if any character is not in the ASCII character repertoire. Such 273 addresses may be restricted in other ways, but those restrictions are 274 not relevant to this definition. The term "all-ASCII" is also 275 applied to other protocol elements when the distinction is important, 276 with "non-ASCII" or "internationalized" as its opposite. 278 The umbrella term to describe the email address internationalization 279 specified by this document and its companion documents is 280 "UTF8SMTPbis". 281 [[anchor3: Note in Draft: Keyword to be changed before publication.]] 282 For example, an address permitted by this specification is referred 283 to as a "UTF8SMTPbis (compliant) address". 285 Please note that, according to the definitions given here, the set of 286 all "all-ASCII" addresses and the set of all "non-ASCII" addresses 287 are mutually exclusive. The set of all addresses permitted when 288 UTF8SMTPbis appears is the union of these two sets. 290 4.3. User Types 292 An "ASCII user" (i) exclusively uses email addresses that contain 293 ASCII characters only, and (ii) cannot generate recipient addresses 294 that contain non-ASCII characters. 296 An "i18mail user" has one or more non-ASCII email addresses, or is 297 able to generate recipient addresses that contain non-ASCII 298 characters. Such a user may have ASCII addresses too; if the user 299 has more than one email account and a corresponding address, or more 300 than one alias for the same address, he or she has some method to 301 choose which address to use on outgoing email. Note that under this 302 definition, it is not possible to tell from an ASCII address if the 303 owner of that address is an i18mail user or not. (A non-ASCII 304 address implies a belief that the owner of that address is an i18mail 305 user.) There is no such thing as an "i18mail message"; the term 306 applies only to users and their agents and capabilities. In 307 particular, the use of non-ASCII message content is an integral part 308 of the MIME specifications [RFC2045] and does not require these 309 extensions (although it is compatible with them). 311 4.4. Messages 313 A "message" is sent from one user (sender) using a particular email 314 address to one or more other recipient email addresses (often 315 referred to just as "users" or "recipient users"). 317 4.5. Mailing Lists 319 A "mailing list" is a mechanism whereby a message may be distributed 320 to multiple recipients by sending it to one recipient address. An 321 agent (typically not a human being) at that single address then 322 causes the message to be redistributed to the target recipients. 323 This agent sets the envelope return address of the redistributed 324 message to a different address from that of the original single 325 recipient message. Using a different envelope return address 326 (reverse-path) causes error (and other automatically generated) 327 messages to go to an error handling address. 329 Special provisions for managing mailing lists that might contain non- 330 ASCII addresses are discussed in a document that is specific to that 331 topic [EAI-Mailinglist] [RFCNNNNbis-MailingList]. 333 4.6. Conventional Message and Internationalized Message 335 o A conventional message is one that does not use any extension 336 defined in the SMTP extension document [RFC5336] or in the 337 UTF8header specification [RFC5335], and is strictly conformant to 338 RFC 5322 [RFC5322]. 340 o An internationalized message is a message utilizing one or more of 341 the extensions defined in this set of specifications, so that it 342 is no longer conformant to the traditional specification of an 343 email message or its transport. 345 4.7. Undeliverable Messages, Notification, and Delivery Receipts 347 As specified in RFC 5321, a message that is undeliverable for some 348 reason is expected to result in notification to the sender. This can 349 occur in either of two ways. One, typically called "Rejection", 350 occurs when an SMTP server returns a reply code indicating a fatal 351 error (a "5yz" code) or persistently returns a temporary failure 352 error (a "4yz" code). The other involves accepting the message 353 during SMTP processing and then generating a message to the sender, 354 typically known as a "Non-delivery Notification" or "NDN". Current 355 practice often favors rejection over NDNs because of the reduced 356 likelihood that the generation of NDNs will be used as a spamming 357 technique. The latter, NDN, case is unavoidable if an intermediate 358 MTA accepts a message that is then rejected by the next-hop server. 360 A sender may also explicitly request message receipts [RFC3461] that 361 raise the same issues for these internationalization extensions as 362 NDNs. 364 5. Overview of the Approach and Document Plan 366 This set of specifications changes both SMTP and the character 367 encoding of email message headers to permit non-ASCII characters to 368 be represented directly. Each important component of the work is 369 described in a separate document. The document set, whose members 370 are described below, also contains informational documents whose 371 purpose is to provide implementation suggestions and guidance for the 372 protocols. 374 In addition to this document, the following documents make up this 375 specification and provide advice and context for it. 377 o SMTP extensions. This document [RFC5336bis-SMTP] provides an SMTP 378 extension (as provided for in RFC 5321) for internationalized 379 addresses. 381 o Email message headers in UTF-8. This document [RFC5335bis-Hdrs] 382 essentially updates RFC 5322 to permit some information in email 383 message headers to be expressed directly by Unicode characters 384 encoded in UTF-8 when the SMTP extension described above is used. 385 This document, possibly with one or more supplemental ones, will 386 also need to address the interactions with MIME, including 387 relationships between UTF8SMTPbis and internal MIME headers and 388 content types. 390 o Extensions to delivery status and notification handling to adapt 391 to internationalized addresses [RFC5337bis-DSN]. 393 o Extensions to the IMAP protocol to support internationalized 394 message headers [RFC5738bis-IMAP]. 396 o Parallel extensions to the POP protocol [RFC5721] 397 [RFC5721bis-POP3]. 399 6. Review of Experimental Results 401 The key difference between this set of protocols and the experimental 402 set that preceded them [RFC5335] [RFC5336] [RFC5337] [RFC5504] 403 [RFC5721] [RFC5738] [RFC5825] is that the earlier group provided a 404 mechanism for in-transit downgrading of messages (described in detail 405 in RFC 5504). That mechanism permitted, and essentially required, 406 that each non-ASCII address be accompanied by an all-ASCII 407 equivalent. That, in turn, raised security concerns associated with 408 pairing of addresses that could not be authenticated. It also 409 introduced the first incompatible change to Internet mail addressing 410 in many years, raising concerns about interoperability issues if the 411 new address forms "leaked" into legacy email implementations. The WG 412 concluded that the advantages of in-transit downgrading, were it 413 feasible operationally, would be significant enough to overcome those 414 concerns. 416 Operationally that turned out to not be the case, with 417 interoperability problems among initial implementations. Prior to 418 starting on the work that led to this set of specifications, the WG 419 concluded that the combination of requirements and long-term 420 implications of that earlier model were too complex to be 421 satisfactory and that work should move ahead without it. 423 7. Overview of Protocol Extensions and Changes 425 7.1. SMTP Extension for Internationalized Email Address 427 An SMTP extension, "UTF8SMTPbis" is specified as follows: 429 o Permits the use of UTF-8 strings in email addresses, both local 430 parts and domain names. 432 o Permits the selective use of UTF-8 strings in email message 433 headers (see Section 7.2). 435 o Requires that the server advertise the 8BITMIME extension 436 [RFC1652] and that the client support 8-bit transmission so that 437 header information can be transmitted without using a special 438 content-transfer-encoding. 440 Some general principles affect the development decisions underlying 441 this work. 443 1. Email addresses enter subsystems (such as a user interface) that 444 may perform charset conversions or other encoding changes. When 445 the left hand side of the address includes characters outside the 446 US-ASCII character repertoire, use of ASCII-compatible encoding 447 (ACE) [RFC3492] [RFC5890] on the right hand side is discouraged 448 to promote consistent processing of characters throughout the 449 address. 451 2. An SMTP relay must 453 * Either recognize the format explicitly, agreeing to do so via 454 an ESMTP option, or 456 * Reject the message or, if necessary, return a non-delivery 457 notification message, so that the sender can make another 458 plan. 460 3. If the message cannot be forwarded because the next-hop system 461 cannot accept the extension it MUST be rejected or a non-delivery 462 message MUST be generated and sent. 464 4. In the interest of interoperability, charsets other than UTF-8 465 are prohibited in mail addresses and message headers being 466 transmitted over the Internet. There is no practical way to 467 identify multiple charsets properly with an extension similar to 468 this without introducing great complexity. 470 Conformance to the group of standards specified here for email 471 transport and delivery requires implementation of the SMTP Extension 472 specification and the UTF-8 Header specification. If the system 473 implements IMAP or POP, it MUST conform to the i18n IMAP or POP 474 specifications respectively. 476 7.2. Transmission of Email Header Fields in UTF-8 Encoding 478 There are many places in MUAs or in a user presentation in which 479 email addresses or domain names appear. Examples include the 480 conventional From, To, or Cc header fields; Message-ID and 481 In-Reply-To header fields that normally contain domain names (but 482 that may be a special case); and in message bodies. Each of these 483 must be examined from an internationalization perspective. The user 484 will expect to see mailbox and domain names in local characters, and 485 to see them consistently. If non-obvious encodings, such as 486 protocol-specific ASCII-Compatible Encoding (ACE) variants, are used, 487 the user will inevitably, if only occasionally, see them rather than 488 "native" characters and will find that discomfiting or astonishing. 489 Similarly, if different codings are used for mail transport and 490 message bodies, the user is particularly likely to be surprised, if 491 only as a consequence of the long-established "things leak" 492 principle. The only practical way to avoid these sources of 493 discomfort, in both the medium and the longer term, is to have the 494 encodings used in transport be as similar to the encodings used in 495 message headers and message bodies as possible. 497 When email local parts are internationalized, they should be 498 accompanied by arrangements for the message headers to be in the 499 fully internationalized form. That form should presumably use UTF-8 500 rather than ASCII as the base character set for the contents of 501 header fields (protocol elements such as the header field names 502 themselves are unchanged and remain entirely in ASCII). For 503 transition purposes and compatibility with legacy systems, this can 504 done by extending the traditional MIME encoding models for non-ASCII 505 characters in headers [RFC2045] [RFC2231]. However, the target is 506 fully internationalized message headers, as discussed in 507 [RFC5335bis-Hdrs] and not an extended and painful transition. 509 7.3. SMTP Service Extension for DSNs 511 The existing Draft Standard Delivery status notifications (DSNs) 512 specification [RFC3461] is limited to ASCII text in the machine 513 readable portions of the protocol. "International Delivery and 514 Disposition Notifications" [RFC5337bis-DSN] adds a new address type 515 for international email addresses so an original recipient address 516 with non-ASCII characters can be correctly preserved even after 517 downgrading. If an SMTP server advertises both the UTF8SMTPbis and 518 the DSN extension, that server MUST implement internationalized DSNs 519 including support for the ORCPT parameter specified in RFC 3461 520 [RFC3461]. 522 8. Downgrading before and after SMTP Transactions 524 An important issue with these extensions is how to handle 525 interactions between systems that support non-ASCII addresses and 526 legacy systems that expect ASCII. There is, of course, no problem 527 with ASCII-only systems sending to those that can handle 528 internationalized forms because the ASCII forms are just a proper 529 subset. But, when systems that support these extensions send mail, 530 they may include non-ASCII addresses for senders, receivers, or both 531 and might also provide non-ASCII header information other than 532 addresses. If the extension is not supported by the first-hop system 533 (SMTP server accessed by the Submission server acting as an SMTP 534 client), message originating systems should be prepared to either 535 send conventional envelopes and message headers or to return the 536 message to the originating user so the message may be manually 537 downgraded to the traditional form, possibly using encoded words 538 [RFC2047] in the message headers. Of course, such transformations 539 imply that the originating user or system must have ASCII-only 540 addresses available for all senders and recipients. Mechanisms by 541 which such addresses may be found or identified are outside the scope 542 of these specifications as are decisions about the design of 543 originating systems such as whether any required transformations are 544 made by the user, the originating MUA, or the Submission server. 546 A somewhat more complex situation arises when the first-hop system 547 supports these extensions but some subsequent server in the SMTP 548 transmission chain does not. It is important to note that most cases 549 of that situation with forward-pointing addresses will be the result 550 of configuration errors: especially if it hosts non-ASCII addresses, 551 a final delivery MTA that accepts these extensions should not be 552 configured with lower-preference MX hosts that do not. When the only 553 non-ASCII address being transmitted is backward-pointing (e.g., in an 554 SMTP MAIL command), recipient configuration can not help in general. 555 On the other hand, alternate, all-ASCII, addresses for senders are 556 those most likely to be authoritatively known by the submission 557 environment or the sender herself. Consequently, if an intermediate 558 SMTP relay that requires these extensions then discovers that the 559 next system in the chain does not support them, it will have little 560 choice other than to reject or return the message. 562 As discussed above, downgrading to an ASCII-only form may occur 563 before or during the initial message submission. It might also occur 564 after the delivery to the final delivery MTA in order to accommodate 565 messages stores or IMAP or POP servers or clients that have different 566 capabilities than the delivery MTA. These two cases are discussed in 567 the subsections below. 569 8.1. Downgrading before or during Message Submission 571 It is likely that the most common cases in which a message that 572 requires these extensions is sent to a system that does not will 573 involve the combination of ASCII-only forward-pointing addresses with 574 a non-ASCII backward-pointing one. Until the extensions described 575 here have been universally implemented in the Internet email 576 environment, senders who prefer to use non-ASCII addresses (or raw 577 UTF-8 characters in header fields) even when their intended 578 recipients use and expect all-ASCII ones will need to be especially 579 careful about the error conditions that can arise, especially if they 580 are working in an environment in which non-delivery messages (or 581 other indications from submission servers) are routinely dropped or 582 ignored. 584 Perhaps obviously, the most convenient time to find an ASCII address 585 corresponding to an internationalized address is at the originating 586 MUA or closely-associated systems. This can occur either before the 587 message is sent or after the internationalized form of the message is 588 rejected. It is also the most convenient time to convert a message 589 from the internationalized form into conventional ASCII form or to 590 generate a non-delivery message to the sender if either is necessary. 591 At that point, the user has a full range of choices available, 592 including changing backward-pointing addresses, contacting the 593 intended recipient out of band for an alternate address, consulting 594 appropriate directories, arranging for translation of both addresses 595 and message content into a different language, and so on. While it 596 is natural to think of message downgrading as optimally being a 597 fully-automated process, we should not underestimate the capabilities 598 of a user of at least moderate intelligence who wishes to communicate 599 with another such user. 601 In this context, one can easily imagine modifications to message 602 submission servers (as described in RFC 4409 [RFC4409]) so that they 603 would perform downgrading operations or perhaps even upgrading ones. 604 Such operations would permit receiving messages with one or more of 605 the internationalization extensions discussed here and adapting the 606 outgoing message, as needed, to respond to the delivery or next-hop 607 environment the submission server encounters. 609 8.2. Downgrading or Other Processing After Final SMTP Delivery 611 When an email message is received by a final delivery MTA, it is 612 usually stored in some form. Then it is retrieved either by software 613 that reads the stored form directly or by client software via some 614 email retrieval mechanisms such as POP or IMAP. 616 The SMTP extension described in Section 7.1 provides protection only 617 in transport. It does not prevent MUAs and email retrieval 618 mechanisms that have not been upgraded to understand 619 internationalized addresses and UTF-8 message headers from accessing 620 stored internationalized emails. 622 Since the final delivery MTA (or, to be more specific, its 623 corresponding mail storage agent) cannot safely assume that agents 624 accessing email storage will always be capable of handling the 625 extensions proposed here, it MAY downgrade internationalized emails, 626 specially identify messages that utilize these extensions, or both. 627 If this is done, the final delivery MTA SHOULD include a mechanism to 628 preserve or recover the original internationalized forms without 629 information loss to support access by UTF8SMTPbis-aware agents. 631 9. Downgrading in Transit 633 The base SMTP specification (Section 2.3.11 of RFC 5321 [RFC5321]) 634 states that "due to a long history of problems when intermediate 635 hosts have attempted to optimize transport by modifying them, the 636 local-part MUST be interpreted and assigned semantics only by the 637 host specified in the domain part of the address". This is not a new 638 requirement; equivalent statements appeared in specifications in 2001 639 [RFC2821] and even in 1989 [RFC1123]. 641 Adherence to this rule means that a downgrade mechanism that 642 transforms the local-part of an email address cannot be utilized in 643 transit. It can only be applied at the endpoints, specifically by 644 the MUA or submission server or by the final delivery MTA. 646 One of the reasons for this rule has to do with legacy email systems 647 that embed mail routing information in the local-part of the address 648 field. Transforming the email address destroys such routing 649 information. There is no way a server other than the final delivery 650 server can know, for example, whether the local-part of 651 user%foo@example.com is a route ("user" is reached via "foo") or 652 simply a local address. 654 10. User Interface and Configuration Issues 656 Internationalization of addresses and message headers, especially in 657 combination with variations on character coding that are inherent to 658 Unicode, may make careful choices of addresses and careful 659 configuration of servers and DNS records even more important than 660 they are for traditional Internet email. It is likely that, as 661 experience develops with the use of these protocols, it will be 662 desirable to produce one or more additional documents that offer 663 guidance for configuration and interfaces. A document that discusses 664 issues with mail user agents (MUAs), especially with regard to 665 downgrading [EAI-MUA-issues], is expected to be developed in the EAI 666 Working Group. The subsections below address some other issues. 668 10.1. Choices of Mailbox Names and Unicode Normalization 670 It has long been the case that the email syntax permits choices about 671 mailbox names that are unwise in practice if one actually intends the 672 mailboxes to be accessible to a broad range of senders. The most- 673 often-cited examples involve the use of case-sensitivity and tricky 674 quoting of embedded characters in mailbox local parts. While these 675 are permitted by the protocols and servers are expected to support 676 them and there are special cases where they can provide value, taking 677 advantage of those features is almost always bad practice unless the 678 intent is to create some form of security by obscurity. 680 In the absence of these extensions, SMTP clients and servers are 681 constrained to using only those addresses permitted by RFC 5321. The 682 local parts of those addresses MAY be made up of any ASCII characters 683 except the control characters that 5321 prohibits, although some of 684 them MUST be quoted as specified there. It is notable in an 685 internationalization context that there is a long history on some 686 systems of using overstruck ASCII characters (a character, a 687 backspace, and another character) within a quoted string to 688 approximate non-ASCII characters. This form of internationalization 689 was permitted by RFC 821 [RFC0821] but is prohibited by RFC 5321 690 because it requires a backspace character (a prohibited C0 control). 691 The practice SHOULD be phased out as this extension becomes widely 692 deployed but backward-compatibility considerations may require that 693 it continue to be recognized. 695 For the particular case of EAI mailbox names, special attention must 696 be paid to Unicode normalization [Unicode-UAX15], in part because 697 Unicode strings may be normalized by other processes independent of 698 what a mail protocol specifies (this is exactly analogous to what may 699 happen with quoting and dequoting in traditional addresses). 700 Consequently, the following principles are offered as advice to those 701 who are selecting names for mailboxes: 703 o In general, it is wise to support addresses in Normalized form, 704 using either Normalization Form NFC and, except in unusual 705 circumstances, NFKC. 707 o It may be wise to support other forms of the same local-part 708 string, either as aliases or by normalization of strings reaching 709 the delivery server, in the event that the sender does not send 710 the strings in normalized form. 712 o Stated differently and in more specific terms, the rules of the 713 protocol for local-part strings essentially provide that: 715 * Unnormalized strings are valid, but sufficiently bad practice 716 that they may not work reliably on a global basis. 718 * C0 (and presumably C1) controls (see The Unicode Standard 719 [Unicode52]) are prohibited, the first in RFC 5321 and the 720 second by an obvious extension from it [RFC5198]. 722 * Other kinds of punctuation, spaces, etc., are risky practice. 723 Perhaps they will work, and SMTP receiver code is required to 724 handle them, but creating dependencies on them in mailbox names 725 that are chosen is usually a bad practice and may lead to 726 interoperability problems. 728 11. Additional Issues 730 This section identifies issues that are not covered, or not covered 731 comprehensively, as part of this set of specifications, but that will 732 require ongoing review as part of deployment of email address and 733 header internationalization. 735 11.1. Impact on URIs and IRIs 737 The mailto: schema [RFC2368] and discussed in the Internationalized 738 Resource Identifier (IRI) specification [RFC3987] may need to be 739 modified when this work is completed and standardized. 741 11.2. Use of Email Addresses as Identifiers 743 There are a number of places in contemporary Internet usage in which 744 email addresses are used as identifiers for individuals, including as 745 identifiers to Web servers supporting some electronic commerce sites 746 and in some X.509 certificates [RFC5280]. These documents do not 747 address those uses, but it is reasonable to expect that some 748 difficulties will be encountered when internationalized addresses are 749 first used in those contexts, many of which cannot even handle the 750 full range of addresses permitted today. 752 11.3. Encoded Words, Signed Messages, and Downgrading 754 One particular characteristic of the email format is its persistency: 755 MUAs are expected to handle messages that were originally sent 756 decades ago and not just those delivered seconds ago. As such, MUAs 757 and mail filtering software, such as that specified in Sieve 758 [RFC5228], will need to continue to accept and decode header fields 759 that use the "encoded word" mechanism [RFC2047] to accommodate non- 760 ASCII characters in some header fields. While extensions to both 761 POP3 [RFC1939] and IMAP [RFC3501] have been defined that include 762 automatic upgrading of messages that carry non-ASCII information in 763 encoded form -- including RFC 2047 decoding -- of messages by the 764 POP3 [RFC5721bis-POP3] or IMAP [RFC5738bis-IMAP] server, there are 765 message structures and MIME content-types for which that cannot be 766 done or where the change would have unacceptable side effects. 768 For example, message parts that are cryptographically signed, using 769 e.g., S/MIME [RFC3851] or Pretty Good Privacy (PGP) [RFC3156], cannot 770 be upgraded from the RFC 2047 form to normal UTF-8 characters without 771 breaking the signature. Similarly, message parts that are encrypted 772 may contain, when decrypted, header fields that use the RFC 2047 773 encoding; such messages cannot be 'fully' upgraded without access to 774 cryptographic keys. 776 Similar issues may arise if messages are signed and then subsequently 777 downgraded, e.g., as discussed in Section 8.1, and then an attempt is 778 made to upgrade them to the original form and then verify the 779 signatures. Even the very subtle changes that may result from 780 algorithms to downgrade and then upgrade again may be sufficient to 781 invalidate the signatures if they impact either the primary or MIME 782 bodypart headers. When signatures are present, downgrading must be 783 performed with extreme care if at all. 785 11.4. LMTP 787 LMTP [RFC2033] may be used as part of the final delivery agent. In 788 such cases, LMTP may be arranged to deliver the mail to the mail 789 store. The mail store may not have UTF8SMTPbis capability. LMTP may 790 need to be updated to deal with these situations. 792 11.5. Other Uses of Local Parts 794 Local parts are sometimes used to construct domain labels, e.g., the 795 local part "user" in the address user@domain.example could be 796 converted into a vanity host user.domain.example with its Web space 797 at and the catchall addresses 798 any.thing.goes@user.domain.example. 800 Such schemes are obviously limited by, among other things, the SMTP 801 rules for domain names, and will not work without further 802 restrictions for other local parts such as the 803 specified in [RFC5335bis-Hdrs]. Whether those limitations are 804 relevant to these specifications is an open question. It may be 805 simply another case of the considerable flexibility accorded to 806 delivery MTAs in determining the mailbox names they will accept and 807 how they are interpreted. 809 11.6. Non-Standard Encapsulation Formats 811 Some applications use formats similar to the application/mbox format 812 defined in [RFC4155] instead of the message/digest form described in 813 RFC 2046, Section 5.1.5 [RFC2046] to transfer multiple messages as 814 single units. Insofar as such applications assume that all stored 815 messages use the message/rfc822 format described in RFC 2046, Section 816 5.2.1 [RFC2046] with US-ASCII message headers, they are not ready for 817 the extensions specified in this series of documents and special 818 measures may be needed to properly detect and process them. 820 12. IANA Considerations 822 This overview description and framework document does not contemplate 823 any IANA registrations or other actions. Some of the documents in 824 the group have their own IANA considerations sections and 825 requirements. 827 13. Security Considerations 829 Any expansion of permitted characters and encoding forms in email 830 addresses raises some risks. There have been discussions on so 831 called "IDN-spoofing" or "IDN homograph attacks". These attacks 832 allow an attacker (or "phisher") to spoof the domain or URLs of 833 businesses. The same kind of attack is also possible on the local 834 part of internationalized email addresses. It should be noted that 835 the proposed fix involving forcing all displayed elements into 836 normalized lower-case works for domain names in URLs, but not email 837 local parts since those are case sensitive. 839 Since email addresses are often transcribed from business cards and 840 notes on paper, they are subject to problems arising from confusable 841 characters (see [RFC4690]). These problems are somewhat reduced if 842 the domain associated with the mailbox is unambiguous and supports a 843 relatively small number of mailboxes whose names follow local system 844 conventions. They are increased with very large mail systems in 845 which users can freely select their own addresses. 847 The internationalization of email addresses and message headers must 848 not leave the Internet less secure than it is without the required 849 extensions. The requirements and mechanisms documented in this set 850 of specifications do not, in general, raise any new security issues. 852 They do require a review of issues associated with confusable 853 characters -- a topic that is being explored thoroughly elsewhere 854 (see, e.g., RFC 4690 [RFC4690]) -- and, potentially, some issues with 855 UTF-8 normalization, discussed in RFC 3629 [RFC3629], and other 856 transformations. Normalization and other issues associated with 857 transformations and standard forms are also part of the subject of 858 work described elsewhere [RFC5198] [RFC5893] [IAB-idn-encoding]. 860 Some issues specifically related to internationalized addresses and 861 message headers are discussed in more detail in the other documents 862 in this set. However, in particular, caution should be taken that 863 any "downgrading" mechanism, or use of downgraded addresses, does not 864 inappropriately assume authenticated bindings between the 865 internationalized and ASCII addresses. Expecting and most or all 866 such transformations prior to final delivery be done by systems that 867 are presumed to be under the administrative control of the sending 868 user ameliorates the potential problem somewhat as compared to what 869 it would be if the relationships were changed in transit. 871 The new UTF-8 header and message formats might also raise, or 872 aggravate, another known issue. If the model creates new forms of an 873 'invalid' or 'malformed' message, then a new email attack is created: 874 in an effort to be robust, some or most agents will accept such 875 message and interpret them as if they were well-formed. If a filter 876 interprets such a message differently than the MUA used by the 877 recipient, then it may be possible to create a message that appears 878 acceptable under the filter's interpretation but should be rejected 879 under the interpretation given to it by that MUA. Such attacks 880 already exist for existing messages and encoding layers, e.g., 881 invalid MIME syntax, invalid HTML markup, and invalid coding of 882 particular image types. 884 In addition, email addresses are used in many contexts other than 885 sending mail, such as for identifiers under various circumstances 886 (see Section 11.2). Each of those contexts will need to be 887 evaluated, in turn, to determine whether the use of non-ASCII forms 888 is appropriate and what particular issues they raise. 890 This work will clearly affect any systems or mechanisms that are 891 dependent on digital signatures or similar integrity protection for 892 email message headers (see also the discussion in Section 11.3). 893 Many conventional uses of PGP and S/MIME are not affected since they 894 are used to sign body parts but not message headers. On the other 895 hand, the developing work on domain keys identified mail (DKIM) 896 [RFC5863] will eventually need to consider this work and vice versa: 897 while this specification does not address or solve the issues raised 898 by DKIM and other signed header mechanisms, the issues will have to 899 be coordinated and resolved eventually if the two sets of protocols 900 are to co-exist. In addition, to the degree to which email addresses 901 appear in PKI (Public Key Infrastructure) certificates, standards 902 addressing such certificates will need to be upgraded to address 903 these internationalized addresses. Those upgrades will need to 904 address questions of spoofing by look-alikes of the addresses 905 themselves. 907 14. Acknowledgments 909 This document is an update to, and derived from, RFC 4952. This 910 document would have been impossible without the work and 911 contributions acknowledged in it. The present document benefited 912 significantly from discussions in the EAI WG and elsewhere after RFC 913 4952 was published, especially discussions about the experimental 914 versions of other documents in the internationalized email 915 collection, and from RFC errata on RFC 4952 itself. 917 Special thanks are due to Ernie Dainow for careful reviews and 918 suggested text in this version. 920 15. References 922 15.1. Normative References 924 [ASCII] American National Standards Institute 925 (formerly United States of America 926 Standards Institute), "USA Code for 927 Information Interchange", ANSI X3.4-1968, 928 1968. 930 ANSI X3.4-1968 has been replaced by newer 931 versions with slight modifications, but the 932 1968 version remains definitive for the 933 Internet. 935 [RFC1652] Klensin, J., Freed, N., Rose, M., 936 Stefferud, E., and D. Crocker, "SMTP 937 Service Extension for 8bit-MIMEtransport", 938 RFC 1652, July 1994. 940 [RFC2119] Bradner, S., "Key words for use in RFCs to 941 Indicate Requirement Levels", BCP 14, 942 RFC 2119, March 1997. 944 [RFC3629] Yergeau, F., "UTF-8, a transformation 945 format of ISO 10646", STD 63, RFC 3629, 946 November 2003. 948 [RFC5321] Klensin, J., "Simple Mail Transfer 949 Protocol", RFC 5321, October 2008. 951 [RFC5322] Resnick, P., Ed., "Internet Message 952 Format", RFC 5322, October 2008. 954 [RFC5335bis-Hdrs] Yang, A. and S. Steele, "Internationalized 955 Email Headers", July 2010, . 959 [RFC5336bis-SMTP] Yao, J. and W. Mao, "SMTP Extension for 960 Internationalized Email Address", 961 August 2010, . 964 [RFC5337bis-DSN] Not yet posted?, "Internationalized 965 Delivery Status and Disposition 966 Notifications", Unwritten waiting for I-D, 967 2010. 969 [RFC5721bis-POP3] Not yet posted?, "POP3 Support for UTF-8", 970 Unwritten waiting for I-D, 2010. 972 [RFC5738bis-IMAP] Not yet posted?, "IMAP Support for UTF-8", 973 Unwritten waiting for I-D, 2010. 975 [RFC5890] Klensin, J., "Internationalized Domain 976 Names for Applications (IDNA): Definitions 977 and Document Framework", RFC 5890, 978 August 2010. 980 [RFCNNNNbis-MailingList] Not yet posted?, "Mailing Lists and 981 Internationalized Email Addresses", First 982 Version still not in RFC Editor queue https 983 ://datatracker.ietf.org/doc/ 984 draft-ietf-eai-mailinglist/, 985 Unwritten waiting for I-D, 2010. 987 15.2. Informative References 989 [EAI-MUA-issues] EAI WG, "Still-unnamed proposed document on 990 MUA issues", Not assigned or agreed to yet, 991 2011. 993 Note to IESG and RFC Editor: While there is 994 provision for a document on this subject in 995 the WG Charter, there is, as yet, no plan 996 for producing it or even for adding it to 997 the WG's task list with benchmarks. If the 998 present document is approved for 999 publication before the is at least a title 1000 and author(s) for an I-D, the citation and 1001 reference should simply be dropped. 1003 [EAI-Mailinglist] Gellens, R., "Mailing Lists and 1004 Internationalized Email Addresses", 1005 June 2010, . 1008 [IAB-idn-encoding] Thaler, D., Klensin, J., and S. Cheshire, 1009 "IAB Thoughts on Encodings for 1010 Internationalized Domain Names", 2010, . 1014 [RFC0821] Postel, J., "Simple Mail Transfer 1015 Protocol", STD 10, RFC 821, August 1982. 1017 [RFC1123] Braden, R., "Requirements for Internet 1018 Hosts - Application and Support", STD 3, 1019 RFC 1123, October 1989. 1021 [RFC1939] Myers, J. and M. Rose, "Post Office 1022 Protocol - Version 3", STD 53, RFC 1939, 1023 May 1996. 1025 [RFC2033] Myers, J., "Local Mail Transfer Protocol", 1026 RFC 2033, October 1996. 1028 [RFC2045] Freed, N. and N. Borenstein, "Multipurpose 1029 Internet Mail Extensions (MIME) Part One: 1030 Format of Internet Message Bodies", 1031 RFC 2045, November 1996. 1033 [RFC2046] Freed, N. and N. Borenstein, "Multipurpose 1034 Internet Mail Extensions (MIME) Part Two: 1035 Media Types", RFC 2046, November 1996. 1037 [RFC2047] Moore, K., "MIME (Multipurpose Internet 1038 Mail Extensions) Part Three: Message Header 1039 Extensions for Non-ASCII Text", RFC 2047, 1040 November 1996. 1042 [RFC2231] Freed, N. and K. Moore, "MIME Parameter 1043 Value and Encoded Word Extensions: Characte 1044 r Sets, Languages, and Continuations", 1045 RFC 2231, November 1997. 1047 [RFC2368] Hoffman, P., Masinter, L., and J. Zawinski, 1048 "The mailto URL scheme", RFC 2368, 1049 July 1998. 1051 [RFC2821] Klensin, J., "Simple Mail Transfer 1052 Protocol", RFC 2821, April 2001. 1054 [RFC3156] Elkins, M., Del Torto, D., Levien, R., and 1055 T. Roessler, "MIME Security with OpenPGP", 1056 RFC 3156, August 2001. 1058 [RFC3461] Moore, K., "Simple Mail Transfer Protocol 1059 (SMTP) Service Extension for Delivery 1060 Status Notifications (DSNs)", RFC 3461, 1061 January 2003. 1063 [RFC3464] Moore, K. and G. Vaudreuil, "An Extensible 1064 Message Format for Delivery Status 1065 Notifications", RFC 3464, January 2003. 1067 [RFC3492] Costello, A., "Punycode: A Bootstring 1068 encoding of Unicode for Internationalized 1069 Domain Names in Applications (IDNA)", 1070 RFC 3492, March 2003. 1072 [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS 1073 PROTOCOL - VERSION 4rev1", RFC 3501, 1074 March 2003. 1076 [RFC3851] Ramsdell, B., "Secure/Multipurpose Internet 1077 Mail Extensions (S/MIME) Version 3.1 1078 Message Specification", RFC 3851, 1079 July 2004. 1081 [RFC3987] Duerst, M. and M. Suignard, 1082 "Internationalized Resource Identifiers 1083 (IRIs)", RFC 3987, January 2005. 1085 [RFC4155] Hall, E., "The application/mbox Media 1086 Type", RFC 4155, September 2005. 1088 [RFC4409] Gellens, R. and J. Klensin, "Message 1089 Submission for Mail", RFC 4409, April 2006. 1091 [RFC4690] Klensin, J., Faltstrom, P., Karp, C., and 1092 IAB, "Review and Recommendations for 1093 Internationalized Domain Names (IDNs)", 1094 RFC 4690, September 2006. 1096 [RFC4952] Klensin, J. and Y. Ko, "Overview and 1097 Framework for Internationalized Email", 1098 RFC 4952, July 2007. 1100 [RFC5198] Klensin, J. and M. Padlipsky, "Unicode 1101 Format for Network Interchange", RFC 5198, 1102 March 2008. 1104 [RFC5228] Guenther, P. and T. Showalter, "Sieve: An 1105 Email Filtering Language", RFC 5228, 1106 January 2008. 1108 [RFC5280] Cooper, D., Santesson, S., Farrell, S., 1109 Boeyen, S., Housley, R., and W. Polk, 1110 "Internet X.509 Public Key Infrastructure 1111 Certificate and Certificate Revocation List 1112 (CRL) Profile", RFC 5280, May 2008. 1114 [RFC5335] Abel, Y., "Internationalized Email 1115 Headers", RFC 5335, September 2008. 1117 [RFC5336] Yao, J. and W. Mao, "SMTP Extension for 1118 Internationalized Email Addresses", 1119 RFC 5336, September 2008. 1121 [RFC5337] Newman, C. and A. Melnikov, 1122 "Internationalized Delivery Status and 1123 Disposition Notifications", RFC 5337, 1124 September 2008. 1126 [RFC5504] Fujiwara, K. and Y. Yoneya, "Downgrading 1127 Mechanism for Email Address 1128 Internationalization", RFC 5504, 1129 March 2009. 1131 [RFC5721] Gellens, R. and C. Newman, "POP3 Support 1132 for UTF-8", RFC 5721, February 2010. 1134 [RFC5738] Resnick, P. and C. Newman, "IMAP Support 1135 for UTF-8", RFC 5738, March 2010. 1137 [RFC5825] Fujiwara, K. and B. Leiba, "Displaying 1138 Downgraded Messages for Email Address 1139 Internationalization", RFC 5825, 1140 April 2010. 1142 [RFC5863] Hansen, T., Siegel, E., Hallam-Baker, P., 1143 and D. Crocker, "DomainKeys Identified Mail 1144 (DKIM) Development, Deployment, and 1145 Operations", RFC 5863, May 2010. 1147 [RFC5891] Klensin, J., "Internationalized Domain 1148 Names in Applications (IDNA): Protocol", 1149 RFC 5891, August 2010. 1151 [RFC5892] Faltstrom, P., "The Unicode Code Points and 1152 Internationalized Domain Names for 1153 Applications (IDNA)", RFC 5892, 1154 August 2010. 1156 [RFC5893] Alvestrand, H. and C. Karp, "Right-to-Left 1157 Scripts for Internationalized Domain Names 1158 for Applications (IDNA)", RFC 5893, 1159 August 2010. 1161 [RFC5894] Klensin, J., "Internationalized Domain 1162 Names for Applications (IDNA): Background, 1163 Explanation, and Rationale", RFC 5894, 1164 August 2010. 1166 [Unicode-UAX15] The Unicode Consortium, "Unicode Standard 1167 Annex #15: Unicode Normalization Forms", 1168 March 2008, 1169 . 1171 [Unicode52] The Unicode Consortium. The Unicode 1172 Standard, Version 5.2.0, defined by:, "The 1173 Unicode Standard, Version 5.2.0", (Mountain 1174 View, CA: The Unicode Consortium, 1175 2009. ISBN 978-1-936213-00-9)., . 1178 Appendix A. Change Log 1180 [[RFC Editor: Please remove this section prior to publication.]] 1182 A.1. Changes between -00 and -01 1184 o Because there has been no feedback on the mailing list, updated 1185 the various questions to refer to this version as well. 1187 o Reflected RFC Editor erratum #1507 by correcting terminology for 1188 headers and header fields and distinguishing between "message 1189 headers" and different sorts of headers (e.g., the MIME ones). 1191 A.2. Changes between -01 and -02 1193 Note that section numbers in the list that follows may refer to -01 1194 and not -02. 1196 o Discussion of RFC 5825 ("downgraded display") has been removed per 1197 the earlier note and on-list discussion. Any needed discussion 1198 about reconstructed messages will need to appear in the IMAP and 1199 POP documents. However, the introductory material has been 1200 reworded to permit keeping 5504 and 5825 on the list there, 1201 without which the back chain would not be complete. For 1202 consistency with this change, 5504 and 5825 have been added to the 1203 "Obsoletes" list (as far as I know, an Informational spec can 1204 obsolete or update Experimental ones, so no downref problem here 1205 --JcK). 1207 o Reference to alternate addresses dropped from (former) Section 1208 7.1. 1210 o Reference to RFC 5504 added to (former) Section 8 for 1211 completeness. 1213 o Ernie's draft comments added (with some minor edits) to replace 1214 the placeholder in (former) Section 9 ("Downgrading in Transit"). 1215 It is intended to capture at least an introduction the earlier 1216 discussions of algorithmic downgrading generally and ACE/Punycode 1217 transformations in particular. Anyone who is unhappy with it 1218 should say so and propose alternate text. RSN. 1220 o In the interest of clarity and consistency with the terminology in 1221 Section 4.1, all uses of "final delivery SMTP server" and "final 1222 delivery server" have been changed to "final delivery MTA". 1224 o Placeholder at the end of Section 2 has been removed and the text 1225 revised to promise less. The "Document Plan" (Section 5) has been 1226 revised accordingly. We need to discuss this at IETF 78 if not 1227 sooner. 1229 o Sections 5 and 6 have been collapsed into one -- there wasn't 1230 enough left in the former Section 5 to justify a separate section. 1232 o Former Section 11.1 has been dropped and the DSN document moved up 1233 into the "Document Plan" as suggested earlier. 1235 o Section 12, "Experimental Targets", has been removed. 1237 o Updated references for the new version EAI documents and added 1238 placeholders for all of the known remaining drafts that will 1239 become part of the core EAI series but that have not been written. 1241 o Inserted an additional clarification about the relationship of 1242 these extensions to non-ASCII messages. 1244 o Changed some normative/informative reference classifications based 1245 on review of the new text. 1247 o Removed references to the pre-EAI documents that were cited for 1248 historical context in 4952. 1250 o Got rid of a remaining pointer to address downgrading in the 1251 discussion of an updated MAILTO URI. 1253 o Minor additional editorial cleanups and tuning. 1255 A.3. Changes between -02 and -03 1257 o Inserted paragraph clarifying the status of the UTF8SMTPbis 1258 keyword as a result of discussion prior to and during IETF 79. 1260 o Adjusted some references including adding an explicit citation of 1261 RFC 821. 1263 o Removed the discussion of the experimental work from an inline 1264 aside to a separate section, Section 6. 1266 o Rewrote the discussion of configuration errors in MX setups to 1267 make it clear that they are an issue with forward-pointing 1268 addresses only and improved the discussion of backward-pointing 1269 addresses. 1271 o Removed some now-obsolete placeholder notes and resolved the 1272 remaining one to a dangling reference. 1274 A.4. Changes between -03 and -04 1276 o Several minor editorial changes. 1278 o Added a discussion of the relationship to the base email, MIME, 1279 and IDNA specifications. 1281 A.5. Changes between -04 and -05 1283 o Several more minor editorial changes. 1285 Authors' Addresses 1287 John C KLENSIN 1288 1770 Massachusetts Ave, #322 1289 Cambridge, MA 02140 1290 USA 1292 Phone: +1 617 491 5735 1293 EMail: john-ietf@jck.com 1295 YangWoo KO 1296 ICU 1297 119 Munjiro 1298 Yuseong-gu, Daejeon 305-732 1299 Republic of Korea 1301 EMail: yw@mrko.pe.kr