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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 20, 2010 7 Intended status: Informational 8 Expires: February 21, 2011 10 Overview and Framework for Internationalized Email 11 draft-ietf-eai-frmwrk-4952bis-04 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 21, 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 -02 and -03 . . . . . . . . . . . . . . . 28 118 1. Introduction 120 Note in Draft and to RFC Editor: The keyword represented in this 121 document by "UTF8SMTPbis" (and in the XML source by &EAISMTPkeyword;) 122 is a placeholder. The actual keyword will be assigned when the 123 standards track SMTP extension in this series [RFC5336bis-SMTP] is 124 approved for publication and should be substituted here. This 125 paragraph should be treated as normative reference to that SMTP 126 extension draft, creating a reference hold until it is approved by 127 the IESG. The paragraph should be removed before RFC publication. 129 In order to use internationalized email addresses, we need to 130 internationalize both the domain part and the local part of email 131 addresses. The domain part of email addresses is already 132 internationalized [RFC5890], while the local part is not. Without 133 the extensions specified in this document, the mailbox name is 134 restricted to a subset of 7-bit ASCII [RFC5321]. Though MIME 135 [RFC2045] enables the transport of non-ASCII data, it does not 136 provide a mechanism for internationalized email addresses. In RFC 137 2047 [RFC2047], MIME defines an encoding mechanism for some specific 138 message header fields to accommodate non-ASCII data. However, it 139 does not permit the use of email addresses that include non-ASCII 140 characters. Without the extensions defined here, or some equivalent 141 set, the only way to incorporate non-ASCII characters in any part of 142 email addresses is to use RFC 2047 coding to embed them in what RFC 143 5322 [RFC5322] calls the "display name" (known as a "name phrase" or 144 by other terms elsewhere) of the relevant header fields. Information 145 coded into the display name is invisible in the message envelope and, 146 for many purposes, is not part of the address at all. 148 This document is an update of RFC 4952 [RFC4952]; it reflects 149 additional issues, shared terminology, and some architectural changes 150 identified since that document was published. 152 The pronouns "he" and "she" are used interchangeably to indicate a 153 human of indeterminate gender. 155 The key words "MUST", "SHALL", "REQUIRED", "SHOULD", "RECOMMENDED", 156 and "MAY" in this document are to be interpreted as described in RFC 157 2119 [RFC2119]. 159 2. Role of This Specification 161 This document presents the overview and framework for an approach to 162 the next stage of email internationalization. This new stage 163 requires not only internationalization of addresses and header 164 fields, but also associated transport and delivery models. A prior 165 version of this specification, RFC 4952 [RFC4952], also provided an 166 introduction to a series of experimental protocols [RFC5335] 167 [RFC5336] [RFC5337] [RFC5504] [RFC5721] [RFC5738] [RFC5825]. This 168 revised form provides overview and conceptual information for the 169 standards-track successors of a subset of those protocols. Details 170 of the documents and the relationships among them appear in Section 5 171 and a discussion of what was learned from the Experimental protocols 172 and their implementations appears in Section 6. 174 Taken together, these specifications provide the details for a way to 175 implement and support internationalized email. The document itself 176 describes how the various elements of email internationalization fit 177 together and the relationships among the primary specifications 178 associated with message transport, header formats, and handling. 180 This document, and others that comprise the collection described 181 above, assume a reasonable familiarity with the basic Internet 182 electronic mail specifications and terminology [RFC5321][RFC5322] and 183 the MIME [RFC2045] and 8BITMIME [RFC1652] ones as well. While not 184 strictly required to implement this specification, a general 185 familiarity with the terminology and functions of IDNA 186 [RFC5890][RFC5891] [RFC5892][RFC5893] [RFC5894] are also assumed. 188 3. Problem Statement 190 Internationalizing Domain Names in Applications (IDNA) [RFC5890] 191 permits internationalized domain names, but deployment has not yet 192 reached most users. One of the reasons for this is that we do not 193 yet have fully internationalized naming schemes. Domain names are 194 just one of the various names and identifiers that are required to be 195 internationalized. In many contexts, until more of those identifiers 196 are internationalized, internationalized domain names alone have 197 little value. 199 Email addresses are prime examples of why it is not good enough to 200 just internationalize the domain name. As most observers have 201 learned from experience, users strongly prefer email addresses that 202 resemble names or initials to those involving seemingly meaningless 203 strings of letters or numbers. Unless the entire email address can 204 use familiar characters and formats, users will perceive email as 205 being culturally unfriendly. If the names and initials used in email 206 addresses can be expressed in the native languages and writing 207 systems of the users, the Internet will be perceived as more natural, 208 especially by those whose native language is not written in a subset 209 of a Roman-derived script. 211 Internationalization of email addresses is not merely a matter of 212 changing the SMTP envelope; or of modifying the From, To, and Cc 213 header fields; or of permitting upgraded Mail User Agents (MUAs) to 214 decode a special coding and respond by displaying local characters. 215 To be perceived as usable, the addresses must be internationalized 216 and handled consistently in all of the contexts in which they occur. 217 This requirement has far-reaching implications: collections of 218 patches and workarounds are not adequate. Even if they were 219 adequate, a workaround-based approach may result in an assortment of 220 implementations with different sets of patches and workarounds having 221 been applied with consequent user confusion about what is actually 222 usable and supported. Instead, we need to build a fully 223 internationalized email environment, focusing on permitting efficient 224 communication among those who share a language or other community. 225 That, in turn, implies changes to the mail header environment to 226 permit the full range of Unicode characters where that makes sense, 227 an SMTP Extension to permit UTF-8 [RFC3629] [RFC5198] mail addressing 228 and delivery of those extended header fields, support for 229 internationalization of delivery and service notifications [RFC3461] 230 [RFC3464], and (finally) a requirement for support of the 8BITMIME 231 SMTP Extension [RFC1652] so that all of these can be transported 232 through the mail system without having to overcome the limitation 233 that header fields do not have content-transfer-encodings. 235 4. Terminology 237 This document assumes a reasonable understanding of the protocols and 238 terminology of the core email standards as documented in [RFC5321] 239 and [RFC5322]. 241 4.1. Mail User and Mail Transfer Agents 243 Much of the description in this document depends on the abstractions 244 of "Mail Transfer Agent" ("MTA") and "Mail User Agent" ("MUA"). 245 However, it is important to understand that those terms and the 246 underlying concepts postdate the design of the Internet's email 247 architecture and the application of the "protocols on the wire" 248 principle to it. That email architecture, as it has evolved, and 249 that "on the wire" principle have prevented any strong and 250 standardized distinctions about how MTAs and MUAs interact on a given 251 origin or destination host (or even whether they are separate). 253 However, the term "final delivery MTA" is used in this document in a 254 fashion equivalent to the term "delivery system" or "final delivery 255 system" of RFC 5321. This is the SMTP server that controls the 256 format of the local parts of addresses and is permitted to inspect 257 and interpret them. It receives messages from the network for 258 delivery to mailboxes or for other local processing, including any 259 forwarding or aliasing that changes envelope addresses, rather than 260 relaying. From the perspective of the network, any local delivery 261 arrangements such as saving to a message store, handoff to specific 262 message delivery programs or agents, and mechanisms for retrieving 263 messages are all "behind" the final delivery MTA and hence are not 264 part of the SMTP transport or delivery process. 266 4.2. Address Character Sets 268 In this document, an address is "all-ASCII", or just an "ASCII 269 address", if every character in the address is in the ASCII character 270 repertoire [ASCII]; an address is "non-ASCII", or an "i18n-address", 271 if any character is not in the ASCII character repertoire. Such 272 addresses may be restricted in other ways, but those restrictions are 273 not relevant to this definition. The term "all-ASCII" is also 274 applied to other protocol elements when the distinction is important, 275 with "non-ASCII" or "internationalized" as its opposite. 277 The umbrella term to describe the email address internationalization 278 specified by this document and its companion documents is 279 "UTF8SMTPbis". 280 [[anchor3: Note in Draft: Keyword to be changed before publication.]] 281 For example, an address permitted by this specification is referred 282 to as a "UTF8SMTPbis (compliant) address". 284 Please note that, according to the definitions given here, the set of 285 all "all-ASCII" addresses and the set of all "non-ASCII" addresses 286 are mutually exclusive. The set of all addresses permitted when 287 UTF8SMTPbis appears is the union of these two sets. 289 4.3. User Types 291 An "ASCII user" (i) exclusively uses email addresses that contain 292 ASCII characters only, and (ii) cannot generate recipient addresses 293 that contain non-ASCII characters. 295 An "i18mail user" has one or more non-ASCII email addresses, or is 296 able to generate recipient addresses that contain non-ASCII 297 characters. Such a user may have ASCII addresses too; if the user 298 has more than one email account and a corresponding address, or more 299 than one alias for the same address, he or she has some method to 300 choose which address to use on outgoing email. Note that under this 301 definition, it is not possible to tell from an ASCII address if the 302 owner of that address is an i18mail user or not. (A non-ASCII 303 address implies a belief that the owner of that address is an i18mail 304 user.) There is no such thing as an "i18mail message"; the term 305 applies only to users and their agents and capabilities. In 306 particular, the use of non-ASCII message content is an integral part 307 of the MIME specifications [RFC2045] and does not require these 308 extensions (although it is compatible with them). 310 4.4. Messages 312 A "message" is sent from one user (sender) using a particular email 313 address to one or more other recipient email addresses (often 314 referred to just as "users" or "recipient users"). 316 4.5. Mailing Lists 318 A "mailing list" is a mechanism whereby a message may be distributed 319 to multiple recipients by sending it to one recipient address. An 320 agent (typically not a human being) at that single address then 321 causes the message to be redistributed to the target recipients. 322 This agent sets the envelope return address of the redistributed 323 message to a different address from that of the original single 324 recipient message. Using a different envelope return address 325 (reverse-path) causes error (and other automatically generated) 326 messages to go to an error handling address. 328 Special provisions for managing mailing lists that might contain non- 329 ASCII addresses are discussed in a document that is specific to that 330 topic [EAI-Mailinglist] [RFCNNNNbis-MailingList]. 332 4.6. Conventional Message and Internationalized Message 334 o A conventional message is one that does not use any extension 335 defined in the SMTP extension document [RFC5336] or in the 336 UTF8header specification [RFC5335], and is strictly conformant to 337 RFC 5322 [RFC5322]. 339 o An internationalized message is a message utilizing one or more of 340 the extensions defined in this set of specifications, so that it 341 is no longer conformant to the traditional specification of an 342 email message or its transport. 344 4.7. Undeliverable Messages, Notification, and Delivery Receipts 346 As specified in RFC 5321, a message that is undeliverable for some 347 reason is expected to result in notification to the sender. This can 348 occur in either of two ways. One, typically called "Rejection", 349 occurs when an SMTP server returns a reply code indicating a fatal 350 error (a "5yz" code) or persistently returns a temporary failure 351 error (a "4yz" code). The other involves accepting the message 352 during SMTP processing and then generating a message to the sender, 353 typically known as a "Non-delivery Notification" or "NDN". Current 354 practice often favors rejection over NDNs because of the reduced 355 likelihood that the generation of NDNs will be used as a spamming 356 technique. The latter, NDN, case is unavoidable if an intermediate 357 MTA accepts a message that is then rejected by the next-hop server. 359 A sender may also explicitly request message receipts [RFC3461] that 360 raise the same issues for these internationalization extensions as 361 NDNs. 363 5. Overview of the Approach and Document Plan 365 This set of specifications changes both SMTP and the character 366 encoding of email message headers to permit non-ASCII characters to 367 be represented directly. Each important component of the work is 368 described in a separate document. The document set, whose members 369 are described below, also contains informational documents whose 370 purpose is to provide implementation suggestions and guidance for the 371 protocols. 373 In addition to this document, the following documents make up this 374 specification and provide advice and context for it. 376 o SMTP extensions. This document [RFC5336bis-SMTP] provides an SMTP 377 extension (as provided for in RFC 5321) for internationalized 378 addresses. 380 o Email message headers in UTF-8. This document [RFC5335bis-Hdrs] 381 essentially updates RFC 5322 to permit some information in email 382 message headers to be expressed directly by Unicode characters 383 encoded in UTF-8 when the SMTP extension described above is used. 384 This document, possibly with one or more supplemental ones, will 385 also need to address the interactions with MIME, including 386 relationships between UTF8SMTPbis and internal MIME headers and 387 content types. 389 o Extensions to delivery status and notification handling to adapt 390 to internationalized addresses [RFC5337bis-DSN]. 392 o Extensions to the IMAP protocol to support internationalized 393 message headers [RFC5738bis-IMAP]. 395 o Parallel extensions to the POP protocol [RFC5721] 396 [RFC5721bis-POP3]. 398 6. Review of Experimental Results 400 The key difference between this set of protocols and the experimental 401 set that preceded them [RFC5335] [RFC5336] [RFC5337] [RFC5504] 402 [RFC5721] [RFC5738] [RFC5825] is that the earlier group provided a 403 mechanism for in-transit downgrading of messages (described in detail 404 in RFC 5504). That mechanism permitted, and essentially required, 405 that each non-ASCII address be accompanied by an all-ASCII 406 equivalent. That, in turn, raised security concerns associated with 407 pairing of addresses that could not be authenticated. It also 408 introduced the first incompatible change to Internet mail addressing 409 in many years, raising concerns about interoperability issues if the 410 new address forms "leaked" into legacy email implementations. The WG 411 concluded that the advantages of in-transit downgrading, were it 412 feasible operationally, would be significant enough to overcome those 413 concerns. 415 Operationally that turned out to not be the case, with 416 interoperability problems among initial implementations. Prior to 417 starting on the work that led to this set of specifications, the WG 418 concluded that the combination of requirements and long-term 419 implications of that earlier model were too complex to be 420 satisfactory and that work should move ahead without it. 422 7. Overview of Protocol Extensions and Changes 424 7.1. SMTP Extension for Internationalized Email Address 426 An SMTP extension, "UTF8SMTPbis" is specified as follows: 428 o Permits the use of UTF-8 strings in email addresses, both local 429 parts and domain names. 431 o Permits the selective use of UTF-8 strings in email message 432 headers (see Section 7.2). 434 o Requires that the server advertise the 8BITMIME extension 435 [RFC1652] and that the client support 8-bit transmission so that 436 header information can be transmitted without using a special 437 content-transfer-encoding. 439 Some general principles affect the development decisions underlying 440 this work. 442 1. Email addresses enter subsystems (such as a user interface) that 443 may perform charset conversions or other encoding changes. When 444 the left hand side of the address includes characters outside the 445 US-ASCII character repertoire, use of ASCII-compatible (ACE) 446 encoding [RFC3492] [RFC5890] on the right hand side is 447 discouraged to promote consistent processing of characters 448 throughout the address. 450 2. An SMTP relay must 452 * Either recognize the format explicitly, agreeing to do so via 453 an ESMTP option, or 455 * Reject the message or, if necessary, return a non-delivery 456 notification message, so that the sender can make another 457 plan. 459 3. If the message cannot be forwarded because the next-hop system 460 cannot accept the extension it MUST be rejected or a non-delivery 461 message generated and sent. 463 4. In the interest of interoperability, charsets other than UTF-8 464 are prohibited in mail addresses and message headers being 465 transmitted over the Internet. There is no practical way to 466 identify multiple charsets properly with an extension similar to 467 this without introducing great complexity. 469 Conformance to the group of standards specified here for email 470 transport and delivery requires implementation of the SMTP Extension 471 specification and the UTF-8 Header specification. If the system 472 implements IMAP or POP, it MUST conform to the i18n IMAP or POP 473 specifications respectively. 475 7.2. Transmission of Email Header Fields in UTF-8 Encoding 477 There are many places in MUAs or in a user presentation in which 478 email addresses or domain names appear. Examples include the 479 conventional From, To, or Cc header fields; Message-ID and 480 In-Reply-To header fields that normally contain domain names (but 481 that may be a special case); and in message bodies. Each of these 482 must be examined from an internationalization perspective. The user 483 will expect to see mailbox and domain names in local characters, and 484 to see them consistently. If non-obvious encodings, such as 485 protocol-specific ASCII-Compatible Encoding (ACE) variants, are used, 486 the user will inevitably, if only occasionally, see them rather than 487 "native" characters and will find that discomfiting or astonishing. 488 Similarly, if different codings are used for mail transport and 489 message bodies, the user is particularly likely to be surprised, if 490 only as a consequence of the long-established "things leak" 491 principle. The only practical way to avoid these sources of 492 discomfort, in both the medium and the longer term, is to have the 493 encodings used in transport be as similar to the encodings used in 494 message headers and message bodies as possible. 496 When email local parts are internationalized, they should be 497 accompanied by arrangements for the message headers to be in the 498 fully internationalized form. That form should presumably use UTF-8 499 rather than ASCII as the base character set for the contents of 500 header fields (protocol elements such as the header field names 501 themselves are unchanged and remain entirely in ASCII). For 502 transition purposes and compatibility with legacy systems, this can 503 done by extending the traditional MIME encoding models for non-ASCII 504 characters in headers [RFC2045] [RFC2231]. However, the target is 505 fully internationalized message headers, as discussed in 506 [RFC5335bis-Hdrs] and not an extended and painful transition. 508 7.3. SMTP Service Extension for DSNs 510 The existing Draft Standard Delivery status notifications (DSNs) 511 specification [RFC3461] is limited to ASCII text in the machine 512 readable portions of the protocol. "International Delivery and 513 Disposition Notifications" [RFC5337bis-DSN] adds a new address type 514 for international email addresses so an original recipient address 515 with non-ASCII characters can be correctly preserved even after 516 downgrading. If an SMTP server advertises both the UTF8SMTPbis and 517 the DSN extension, that server MUST implement internationalized DSNs 518 including support for the ORCPT parameter specified in RFC 3461 519 [RFC3461]. 521 8. Downgrading before and after SMTP Transactions 523 An important issue with these extensions is how to handle 524 interactions between systems that support non-ASCII addresses and 525 legacy systems that expect ASCII. There is, of course, no problem 526 with ASCII-only systems sending to those that can handle 527 internationalized forms because the ASCII forms are just a proper 528 subset. But, when systems that support these extensions send mail, 529 they may include non-ASCII addresses for senders, receivers, or both 530 and might also provide non-ASCII header information other than 531 addresses. If the extension is not supported by the first-hop system 532 (SMTP server accessed by the Submission server acting as an SMTP 533 client), message originating systems should be prepared to either 534 send conventional envelopes and message headers or to return the 535 message to the originating user so the message may be manually 536 downgraded to the traditional form, possibly using encoded words 537 [RFC2047] in the message headers. Of course, such transformations 538 imply that the originating user or system must have ASCII-only 539 addresses available for all senders and recipients. Mechanisms by 540 which such addresses may be found or identified are outside the scope 541 of these specifications as are decisions about the design of 542 originating systems such as whether any required transformations are 543 made by the user, the originating MUA, or the Submission server. 545 A somewhat more complex situation arises when the first-hop system 546 supports these extensions but some subsequent server in the SMTP 547 transmission chain does not. It is important to note that most cases 548 of that situation with forward-pointing addresses will be the result 549 of configuration errors: especially if it hosts non-ASCII addresses, 550 a final delivery MTA that accepts these extensions should not be 551 configured with lower-preference MX hosts that do not. When the only 552 non-ASCII address being transmitted is backward-pointing (e.g., in an 553 SMTP MAIL command), recipient configuration can not help in general. 554 On the other hand, alternate, all-ASCII, addresses for senders are 555 those most likely to be authoritatively known by the submission 556 environment or the sender herself. Consequently, if an intermediate 557 SMTP relay that requires these extensions then discovers that the 558 next system in the chain does not support them, it will have little 559 choice other than to reject or return the message. 561 As discussed above, downgrading to an ASCII-only form may occur 562 before or during the initial message submission. It might also occur 563 after the delivery to the final delivery MTA in order to accommodate 564 messages stores or IMAP or POP servers or clients that have different 565 capabilities than the delivery MTA. These two cases are discussed in 566 the subsections below. 568 8.1. Downgrading before or during Message Submission 570 It is likely that the most common cases in which a message that 571 requires these extensions is sent to a system that does not will 572 involve the combination of ASCII-only forward-pointing addresses with 573 a non-ASCII backward-pointing one. Until the extensions described 574 here have been universally implemented in the Internet email 575 environment, senders who prefer to use non-ASCII addresses (or raw 576 UTF-8 characters in header fields) even when their intended 577 recipients use and expect all-ASCII ones will need to be especially 578 careful about the error conditions that can arise, especially if they 579 are working in an environment in which non-delivery messages (or 580 other indications from submission servers) are routinely dropped or 581 ignored. 583 Perhaps obviously, the most convenient time to find an ASCII address 584 corresponding to an internationalized address is at the originating 585 MUA or closely-associated systems. This can occur either before the 586 message is sent or after the internationalized form of the message is 587 rejected. It is also the most convenient time to convert a message 588 from the internationalized form into conventional ASCII form or to 589 generate a non-delivery message to the sender if either is necessary. 590 At that point, the user has a full range of choices available, 591 including changing backward-pointing addresses, contacting the 592 intended recipient out of band for an alternate address, consulting 593 appropriate directories, arranging for translation of both addresses 594 and message content into a different language, and so on. While it 595 is natural to think of message downgrading as optimally being a 596 fully-automated process, we should not underestimate the capabilities 597 of a user of at least moderate intelligence who wishes to communicate 598 with another such user. 600 In this context, one can easily imagine modifications to message 601 submission servers (as described in RFC 4409 [RFC4409]) so that they 602 would perform downgrading operations or perhaps even upgrading ones. 603 Such operations would permit receiving messages with one or more of 604 the internationalization extensions discussed here and adapting the 605 outgoing message, as needed, to respond to the delivery or next-hop 606 environment the submission server encounters. 608 8.2. Downgrading or Other Processing After Final SMTP Delivery 610 When an email message is received by a final delivery MTA, it is 611 usually stored in some form. Then it is retrieved either by software 612 that reads the stored form directly or by client software via some 613 email retrieval mechanisms such as POP or IMAP. 615 The SMTP extension described in Section 7.1 provides protection only 616 in transport. It does not prevent MUAs and email retrieval 617 mechanisms that have not been upgraded to understand 618 internationalized addresses and UTF-8 message headers from accessing 619 stored internationalized emails. 621 Since the final delivery MTA (or, to be more specific, its 622 corresponding mail storage agent) cannot safely assume that agents 623 accessing email storage will always be capable of handling the 624 extensions proposed here, it MAY downgrade internationalized emails, 625 specially identify messages that utilize these extensions, or both. 626 If this is done, the final delivery MTA SHOULD include a mechanism to 627 preserve or recover the original internationalized forms without 628 information loss to support access by UTF8SMTPbis-aware agents. 630 9. Downgrading in Transit 632 The base SMTP specification (Section 2.3.11 of RFC 5321 [RFC5321]) 633 states that "due to a long history of problems when intermediate 634 hosts have attempted to optimize transport by modifying them, the 635 local-part MUST be interpreted and assigned semantics only by the 636 host specified in the domain part of the address". This is not a new 637 requirement; equivalent statements appeared in specifications in 2001 638 [RFC2821] and even in 1989 [RFC1123]. 640 Adherence to this rule means that a downgrade mechanism that 641 transforms the local-part of an email address cannot be utilized in 642 transit. It can only be applied at the endpoints, specifically by 643 the MUA or submission server or by the final delivery MTA. 645 One of the reasons for this rule has to do with legacy email systems 646 that embed mail routing information in the local-part of the address 647 field. Transforming the email address destroys such routing 648 information. There is no way a server other than the final delivery 649 server can know, for example, whether the local-part of 650 user%foo@example.com is a route ("user" is reached via "foo") or 651 simply a local address. 653 10. User Interface and Configuration Issues 655 Internationalization of addresses and message headers, especially in 656 combination with variations on character coding that are inherent to 657 Unicode, may make careful choices of addresses and careful 658 configuration of servers and DNS records even more important than 659 they are for traditional Internet email. It is likely that, as 660 experience develops with the use of these protocols, it will be 661 desirable to produce one or more additional documents that offer 662 guidance for configuration and interfaces. A document that discusses 663 issues with mail user agents (MUAs), especially with regard to 664 downgrading [EAI-MUA-issues], is expected to be developed in the EAI 665 Working Group. The subsections below address some other issues. 667 10.1. Choices of Mailbox Names and Unicode Normalization 669 It has long been the case that the email syntax permits choices about 670 mailbox names that are unwise in practice if one actually intends the 671 mailboxes to be accessible to a broad range of senders. The most- 672 often-cited examples involve the use of case-sensitivity and tricky 673 quoting of embedded characters in mailbox local parts. While these 674 are permitted by the protocols and servers are expected to support 675 them and there are special cases where they can provide value, taking 676 advantage of those features is almost always bad practice unless the 677 intent is to create some form of security by obscurity. 679 In the absence of these extensions, SMTP clients and servers are 680 constrained to using only those addresses permitted by RFC 5321. The 681 local parts of those addresses MAY be made up of any ASCII characters 682 except the control characters that 5321 prohibits, although some of 683 them MUST be quoted as specified there. It is notable in an 684 internationalization context that there is a long history on some 685 systems of using overstruck ASCII characters (a character, a 686 backspace, and another character) within a quoted string to 687 approximate non-ASCII characters. This form of internationalization 688 was permitted by RFC 821 [RFC0821] but is prohibited by RFC 5321 689 because it requires a backspace character (a prohibited C0 control). 690 The practice SHOULD be phased out as this extension becomes widely 691 deployed but backward-compatibility considerations may require that 692 it continue to be recognized. 694 For the particular case of EAI mailbox names, special attention must 695 be paid to Unicode normalization [Unicode-UAX15], in part because 696 Unicode strings may be normalized by other processes independent of 697 what a mail protocol specifies (this is exactly analogous to what may 698 happen with quoting and dequoting in traditional addresses). 699 Consequently, the following principles are offered as advice to those 700 who are selecting names for mailboxes: 702 o In general, it is wise to support addresses in Normalized form, 703 using either Normalization Form NFC and, except in unusual 704 circumstances, NFKC. 706 o It may be wise to support other forms of the same local-part 707 string, either as aliases or by normalization of strings reaching 708 the delivery server, in the event that the sender does not send 709 the strings in normalized form. 711 o Stated differently and in more specific terms, the rules of the 712 protocol for local-part strings essentially provide that: 714 * Unnormalized strings are valid, but sufficiently bad practice 715 that they may not work reliably on a global basis. 717 * C0 (and presumably C1) controls (see The Unicode Standard 718 [Unicode52]) are prohibited, the first in RFC 5321 and the 719 second by an obvious extension from it [RFC5198]. 721 * Other kinds of punctuation, spaces, etc., are risky practice. 722 Perhaps they will work, and SMTP receiver code is required to 723 handle them, but creating dependencies on them in mailbox names 724 that are chosen is usually a bad practice and may lead to 725 interoperability problems. 727 11. Additional Issues 729 This section identifies issues that are not covered, or not covered 730 comprehensively, as part of this set of specifications, but that will 731 require ongoing review as part of deployment of email address and 732 header internationalization. 734 11.1. Impact on URIs and IRIs 736 The mailto: schema [RFC2368] and discussed in the Internationalized 737 Resource Identifier (IRI) specification [RFC3987] may need to be 738 modified when this work is completed and standardized. 740 11.2. Use of Email Addresses as Identifiers 742 There are a number of places in contemporary Internet usage in which 743 email addresses are used as identifiers for individuals, including as 744 identifiers to Web servers supporting some electronic commerce sites 745 and in some X.509 certificates [RFC5280]. These documents do not 746 address those uses, but it is reasonable to expect that some 747 difficulties will be encountered when internationalized addresses are 748 first used in those contexts, many of which cannot even handle the 749 full range of addresses permitted today. 751 11.3. Encoded Words, Signed Messages, and Downgrading 753 One particular characteristic of the email format is its persistency: 754 MUAs are expected to handle messages that were originally sent 755 decades ago and not just those delivered seconds ago. As such, MUAs 756 and mail filtering software, such as that specified in Sieve 757 [RFC5228], will need to continue to accept and decode header fields 758 that use the "encoded word" mechanism [RFC2047] to accommodate non- 759 ASCII characters in some header fields. While extensions to both 760 POP3 [RFC1939] and IMAP [RFC3501] have been defined that include 761 automatic upgrading of messages that carry non-ASCII information in 762 encoded form -- including RFC 2047 decoding -- of messages by the 763 POP3 [RFC5721bis-POP3] or IMAP [RFC5738bis-IMAP] server, there are 764 message structures and MIME content-types for which that cannot be 765 done or where the change would have unacceptable side effects. 767 For example, message parts that are cryptographically signed, using 768 e.g., S/MIME [RFC3851] or Pretty Good Privacy (PGP) [RFC3156], cannot 769 be upgraded from the RFC 2047 form to normal UTF-8 characters without 770 breaking the signature. Similarly, message parts that are encrypted 771 may contain, when decrypted, header fields that use the RFC 2047 772 encoding; such messages cannot be 'fully' upgraded without access to 773 cryptographic keys. 775 Similar issues may arise if messages are signed and then subsequently 776 downgraded, e.g., as discussed in Section 8.1, and then an attempt is 777 made to upgrade them to the original form and then verify the 778 signatures. Even the very subtle changes that may result from 779 algorithms to downgrade and then upgrade again may be sufficient to 780 invalidate the signatures if they impact either the primary or MIME 781 bodypart headers. When signatures are present, downgrading must be 782 performed with extreme care if at all. 784 11.4. LMTP 786 LMTP [RFC2033] may be used as part of the final delivery agent. In 787 such cases, LMTP may be arranged to deliver the mail to the mail 788 store. The mail store may not have UTF8SMTPbis capability. LMTP may 789 need to be updated to deal with these situations. 791 11.5. Other Uses of Local Parts 793 Local parts are sometimes used to construct domain labels, e.g., the 794 local part "user" in the address user@domain.example could be 795 converted into a vanity host user.domain.example with its Web space 796 at and the catchall addresses 797 any.thing.goes@user.domain.example. 799 Such schemes are obviously limited by, among other things, the SMTP 800 rules for domain names, and will not work without further 801 restrictions for other local parts such as the 802 specified in [RFC5335bis-Hdrs]. Whether those limitations are 803 relevant to these specifications is an open question. It may be 804 simply another case of the considerable flexibility accorded to 805 delivery MTAs in determining the mailbox names they will accept and 806 how they are interpreted. 808 11.6. Non-Standard Encapsulation Formats 810 Some applications use formats similar to the application/mbox format 811 defined in [RFC4155] instead of the message/digest form described in 812 RFC 2046, Section 5.1.5 [RFC2046] to transfer multiple messages as 813 single units. Insofar as such applications assume that all stored 814 messages use the message/rfc822 format described in RFC 2046, Section 815 5.2.1 [RFC2046] with US-ASCII message headers, they are not ready for 816 the extensions specified in this series of documents and special 817 measures may be needed to properly detect and process them. 819 12. IANA Considerations 821 This overview description and framework document does not contemplate 822 any IANA registrations or other actions. Some of the documents in 823 the group have their own IANA considerations sections and 824 requirements. 826 13. Security Considerations 828 Any expansion of permitted characters and encoding forms in email 829 addresses raises some risks. There have been discussions on so 830 called "IDN-spoofing" or "IDN homograph attacks". These attacks 831 allow an attacker (or "phisher") to spoof the domain or URLs of 832 businesses. The same kind of attack is also possible on the local 833 part of internationalized email addresses. It should be noted that 834 the proposed fix involving forcing all displayed elements into 835 normalized lower-case works for domain names in URLs, but not email 836 local parts since those are case sensitive. 838 Since email addresses are often transcribed from business cards and 839 notes on paper, they are subject to problems arising from confusable 840 characters (see [RFC4690]). These problems are somewhat reduced if 841 the domain associated with the mailbox is unambiguous and supports a 842 relatively small number of mailboxes whose names follow local system 843 conventions. They are increased with very large mail systems in 844 which users can freely select their own addresses. 846 The internationalization of email addresses and message headers must 847 not leave the Internet less secure than it is without the required 848 extensions. The requirements and mechanisms documented in this set 849 of specifications do not, in general, raise any new security issues. 851 They do require a review of issues associated with confusable 852 characters -- a topic that is being explored thoroughly elsewhere 853 (see, e.g., RFC 4690 [RFC4690]) -- and, potentially, some issues with 854 UTF-8 normalization, discussed in RFC 3629 [RFC3629], and other 855 transformations. Normalization and other issues associated with 856 transformations and standard forms are also part of the subject of 857 work described elsewhere [RFC5198] [RFC5893] [IAB-idn-encoding]. 859 Some issues specifically related to internationalized addresses and 860 message headers are discussed in more detail in the other documents 861 in this set. However, in particular, caution should be taken that 862 any "downgrading" mechanism, or use of downgraded addresses, does not 863 inappropriately assume authenticated bindings between the 864 internationalized and ASCII addresses. Expecting and most or all 865 such transformations prior to final delivery be done by systems that 866 are presumed to be under the administrative control of the sending 867 user ameliorates the potential problem somewhat as compared to what 868 it would be if the relationships were changed in transit. 870 The new UTF-8 header and message formats might also raise, or 871 aggravate, another known issue. If the model creates new forms of an 872 'invalid' or 'malformed' message, then a new email attack is created: 873 in an effort to be robust, some or most agents will accept such 874 message and interpret them as if they were well-formed. If a filter 875 interprets such a message differently than the MUA used by the 876 recipient, then it may be possible to create a message that appears 877 acceptable under the filter's interpretation but should be rejected 878 under the interpretation given to it by that MUA. Such attacks 879 already exist for existing messages and encoding layers, e.g., 880 invalid MIME syntax, invalid HTML markup, and invalid coding of 881 particular image types. 883 In addition, email addresses are used in many contexts other than 884 sending mail, such as for identifiers under various circumstances 885 (see Section 11.2). Each of those contexts will need to be 886 evaluated, in turn, to determine whether the use of non-ASCII forms 887 is appropriate and what particular issues they raise. 889 This work will clearly affect any systems or mechanisms that are 890 dependent on digital signatures or similar integrity protection for 891 email message headers (see also the discussion in Section 11.3). 892 Many conventional uses of PGP and S/MIME are not affected since they 893 are used to sign body parts but not message headers. On the other 894 hand, the developing work on domain keys identified mail (DKIM) 895 [RFC5863] will eventually need to consider this work and vice versa: 896 while this specification does not address or solve the issues raised 897 by DKIM and other signed header mechanisms, the issues will have to 898 be coordinated and resolved eventually if the two sets of protocols 899 are to co-exist. In addition, to the degree to which email addresses 900 appear in PKI (Public Key Infrastructure) certificates, standards 901 addressing such certificates will need to be upgraded to address 902 these internationalized addresses. Those upgrades will need to 903 address questions of spoofing by look-alikes of the addresses 904 themselves. 906 14. Acknowledgments 908 This document is an update to, and derived from, RFC 4952. This 909 document would have been impossible without the work and 910 contributions acknowledged in it. The present document benefited 911 significantly from discussions in the EAI WG and elsewhere after RFC 912 4952 was published, especially discussions about the experimental 913 versions of other documents in the internationalized email 914 collection, and from RFC errata on RFC 4952 itself. 916 Special thanks are due to Ernie Dainow for careful reviews and 917 suggested text in this version. 919 15. References 921 15.1. Normative References 923 [ASCII] American National Standards Institute 924 (formerly United States of America 925 Standards Institute), "USA Code for 926 Information Interchange", ANSI X3.4-1968, 927 1968. 929 ANSI X3.4-1968 has been replaced by newer 930 versions with slight modifications, but the 931 1968 version remains definitive for the 932 Internet. 934 [RFC1652] Klensin, J., Freed, N., Rose, M., 935 Stefferud, E., and D. Crocker, "SMTP 936 Service Extension for 8bit-MIMEtransport", 937 RFC 1652, July 1994. 939 [RFC2119] Bradner, S., "Key words for use in RFCs to 940 Indicate Requirement Levels", BCP 14, 941 RFC 2119, March 1997. 943 [RFC3629] Yergeau, F., "UTF-8, a transformation 944 format of ISO 10646", STD 63, RFC 3629, 945 November 2003. 947 [RFC5321] Klensin, J., "Simple Mail Transfer 948 Protocol", RFC 5321, October 2008. 950 [RFC5322] Resnick, P., Ed., "Internet Message 951 Format", RFC 5322, October 2008. 953 [RFC5335bis-Hdrs] Yang, A. and S. Steele, "Internationalized 954 Email Headers", July 2010, . 958 [RFC5336bis-SMTP] Yao, J. and W. Mao, "SMTP Extension for 959 Internationalized Email Address", 960 August 2010, . 963 [RFC5337bis-DSN] Not yet posted?, "Internationalized 964 Delivery Status and Disposition 965 Notifications", Unwritten waiting for I-D, 966 2010. 968 [RFC5721bis-POP3] Not yet posted?, "POP3 Support for UTF-8", 969 Unwritten waiting for I-D, 2010. 971 [RFC5738bis-IMAP] Not yet posted?, "IMAP Support for UTF-8", 972 Unwritten waiting for I-D, 2010. 974 [RFC5890] Klensin, J., "Internationalized Domain 975 Names for Applications (IDNA): Definitions 976 and Document Framework", RFC 5890, 977 August 2010. 979 [RFCNNNNbis-MailingList] Not yet posted?, "Mailing Lists and 980 Internationalized Email Addresses", First 981 Version still not in RFC Editor queue https 982 ://datatracker.ietf.org/doc/ 983 draft-ietf-eai-mailinglist/, 984 Unwritten waiting for I-D, 2010. 986 15.2. Informative References 988 [EAI-MUA-issues] EAI WG, "Still-unnamed proposed document on 989 MUA issues", Not assigned or agreed to yet, 990 2011. 992 Note to IESG and RFC Editor: While there is 993 provision for a document on this subject in 994 the WG Charter, there is, as yet, no plan 995 for producing it or even for adding it to 996 the WG's task list with benchmarks. If the 997 present document is approved for 998 publication before the is at least a title 999 and author(s) for an I-D, the citation and 1000 reference should simply be dropped. 1002 [EAI-Mailinglist] Gellens, R., "Mailing Lists and 1003 Internationalized Email Addresses", 1004 June 2010, . 1007 [IAB-idn-encoding] Thaler, D., Klensin, J., and S. Cheshire, 1008 "IAB Thoughts on Encodings for 1009 Internationalized Domain Names", 2010, . 1013 [RFC0821] Postel, J., "Simple Mail Transfer 1014 Protocol", STD 10, RFC 821, August 1982. 1016 [RFC1123] Braden, R., "Requirements for Internet 1017 Hosts - Application and Support", STD 3, 1018 RFC 1123, October 1989. 1020 [RFC1939] Myers, J. and M. Rose, "Post Office 1021 Protocol - Version 3", STD 53, RFC 1939, 1022 May 1996. 1024 [RFC2033] Myers, J., "Local Mail Transfer Protocol", 1025 RFC 2033, October 1996. 1027 [RFC2045] Freed, N. and N. Borenstein, "Multipurpose 1028 Internet Mail Extensions (MIME) Part One: 1029 Format of Internet Message Bodies", 1030 RFC 2045, November 1996. 1032 [RFC2046] Freed, N. and N. Borenstein, "Multipurpose 1033 Internet Mail Extensions (MIME) Part Two: 1034 Media Types", RFC 2046, November 1996. 1036 [RFC2047] Moore, K., "MIME (Multipurpose Internet 1037 Mail Extensions) Part Three: Message Header 1038 Extensions for Non-ASCII Text", RFC 2047, 1039 November 1996. 1041 [RFC2231] Freed, N. and K. Moore, "MIME Parameter 1042 Value and Encoded Word Extensions: Characte 1043 r Sets, Languages, and Continuations", 1044 RFC 2231, November 1997. 1046 [RFC2368] Hoffman, P., Masinter, L., and J. Zawinski, 1047 "The mailto URL scheme", RFC 2368, 1048 July 1998. 1050 [RFC2821] Klensin, J., "Simple Mail Transfer 1051 Protocol", RFC 2821, April 2001. 1053 [RFC3156] Elkins, M., Del Torto, D., Levien, R., and 1054 T. Roessler, "MIME Security with OpenPGP", 1055 RFC 3156, August 2001. 1057 [RFC3461] Moore, K., "Simple Mail Transfer Protocol 1058 (SMTP) Service Extension for Delivery 1059 Status Notifications (DSNs)", RFC 3461, 1060 January 2003. 1062 [RFC3464] Moore, K. and G. Vaudreuil, "An Extensible 1063 Message Format for Delivery Status 1064 Notifications", RFC 3464, January 2003. 1066 [RFC3492] Costello, A., "Punycode: A Bootstring 1067 encoding of Unicode for Internationalized 1068 Domain Names in Applications (IDNA)", 1069 RFC 3492, March 2003. 1071 [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS 1072 PROTOCOL - VERSION 4rev1", RFC 3501, 1073 March 2003. 1075 [RFC3851] Ramsdell, B., "Secure/Multipurpose Internet 1076 Mail Extensions (S/MIME) Version 3.1 1077 Message Specification", RFC 3851, 1078 July 2004. 1080 [RFC3987] Duerst, M. and M. Suignard, 1081 "Internationalized Resource Identifiers 1082 (IRIs)", RFC 3987, January 2005. 1084 [RFC4155] Hall, E., "The application/mbox Media 1085 Type", RFC 4155, September 2005. 1087 [RFC4409] Gellens, R. and J. Klensin, "Message 1088 Submission for Mail", RFC 4409, April 2006. 1090 [RFC4690] Klensin, J., Faltstrom, P., Karp, C., and 1091 IAB, "Review and Recommendations for 1092 Internationalized Domain Names (IDNs)", 1093 RFC 4690, September 2006. 1095 [RFC4952] Klensin, J. and Y. Ko, "Overview and 1096 Framework for Internationalized Email", 1097 RFC 4952, July 2007. 1099 [RFC5198] Klensin, J. and M. Padlipsky, "Unicode 1100 Format for Network Interchange", RFC 5198, 1101 March 2008. 1103 [RFC5228] Guenther, P. and T. Showalter, "Sieve: An 1104 Email Filtering Language", RFC 5228, 1105 January 2008. 1107 [RFC5280] Cooper, D., Santesson, S., Farrell, S., 1108 Boeyen, S., Housley, R., and W. Polk, 1109 "Internet X.509 Public Key Infrastructure 1110 Certificate and Certificate Revocation List 1111 (CRL) Profile", RFC 5280, May 2008. 1113 [RFC5335] Abel, Y., "Internationalized Email 1114 Headers", RFC 5335, September 2008. 1116 [RFC5336] Yao, J. and W. Mao, "SMTP Extension for 1117 Internationalized Email Addresses", 1118 RFC 5336, September 2008. 1120 [RFC5337] Newman, C. and A. Melnikov, 1121 "Internationalized Delivery Status and 1122 Disposition Notifications", RFC 5337, 1123 September 2008. 1125 [RFC5504] Fujiwara, K. and Y. Yoneya, "Downgrading 1126 Mechanism for Email Address 1127 Internationalization", RFC 5504, 1128 March 2009. 1130 [RFC5721] Gellens, R. and C. Newman, "POP3 Support 1131 for UTF-8", RFC 5721, February 2010. 1133 [RFC5738] Resnick, P. and C. Newman, "IMAP Support 1134 for UTF-8", RFC 5738, March 2010. 1136 [RFC5825] Fujiwara, K. and B. Leiba, "Displaying 1137 Downgraded Messages for Email Address 1138 Internationalization", RFC 5825, 1139 April 2010. 1141 [RFC5863] Hansen, T., Siegel, E., Hallam-Baker, P., 1142 and D. Crocker, "DomainKeys Identified Mail 1143 (DKIM) Development, Deployment, and 1144 Operations", RFC 5863, May 2010. 1146 [RFC5891] Klensin, J., "Internationalized Domain 1147 Names in Applications (IDNA): Protocol", 1148 RFC 5891, August 2010. 1150 [RFC5892] Faltstrom, P., "The Unicode Code Points and 1151 Internationalized Domain Names for 1152 Applications (IDNA)", RFC 5892, 1153 August 2010. 1155 [RFC5893] Alvestrand, H. and C. Karp, "Right-to-Left 1156 Scripts for Internationalized Domain Names 1157 for Applications (IDNA)", RFC 5893, 1158 August 2010. 1160 [RFC5894] Klensin, J., "Internationalized Domain 1161 Names for Applications (IDNA): Background, 1162 Explanation, and Rationale", RFC 5894, 1163 August 2010. 1165 [Unicode-UAX15] The Unicode Consortium, "Unicode Standard 1166 Annex #15: Unicode Normalization Forms", 1167 March 2008, 1168 . 1170 [Unicode52] The Unicode Consortium. The Unicode 1171 Standard, Version 5.2.0, defined by:, "The 1172 Unicode Standard, Version 5.2.0", (Mountain 1173 View, CA: The Unicode Consortium, 1174 2009. ISBN 978-1-936213-00-9)., . 1177 Appendix A. Change Log 1179 [[RFC Editor: Please remove this section prior to publication.]] 1181 A.1. Changes between -00 and -01 1183 o Because there has been no feedback on the mailing list, updated 1184 the various questions to refer to this version as well. 1186 o Reflected RFC Editor erratum #1507 by correcting terminology for 1187 headers and header fields and distinguishing between "message 1188 headers" and different sorts of headers (e.g., the MIME ones). 1190 A.2. Changes between -01 and -02 1192 Note that section numbers in the list that follows may refer to -01 1193 and not -02. 1195 o Discussion of RFC 5825 ("downgraded display") has been removed per 1196 the earlier note and on-list discussion. Any needed discussion 1197 about reconstructed messages will need to appear in the IMAP and 1198 POP documents. However, the introductory material has been 1199 reworded to permit keeping 5504 and 5825 on the list there, 1200 without which the back chain would not be complete. For 1201 consistency with this change, 5504 and 5825 have been added to the 1202 "Obsoletes" list (as far as I know, an Informational spec can 1203 obsolete or update Experimental ones, so no downref problem here 1204 --JcK). 1206 o Reference to alternate addresses dropped from (former) Section 1207 7.1. 1209 o Reference to RFC 5504 added to (former) Section 8 for 1210 completeness. 1212 o Ernie's draft comments added (with some minor edits) to replace 1213 the placeholder in (former) Section 9 ("Downgrading in Transit"). 1214 It is intended to capture at least an introduction the earlier 1215 discussions of algorithmic downgrading generally and ACE/Punycode 1216 transformations in particular. Anyone who is unhappy with it 1217 should say so and propose alternate text. RSN. 1219 o In the interest of clarity and consistency with the terminology in 1220 Section 4.1, all uses of "final delivery SMTP server" and "final 1221 delivery server" have been changed to "final delivery MTA". 1223 o Placeholder at the end of Section 2 has been removed and the text 1224 revised to promise less. The "Document Plan" (Section 5) has been 1225 revised accordingly. We need to discuss this at IETF 78 if not 1226 sooner. 1228 o Sections 5 and 6 have been collapsed into one -- there wasn't 1229 enough left in the former Section 5 to justify a separate section. 1231 o Former Section 11.1 has been dropped and the DSN document moved up 1232 into the "Document Plan" as suggested earlier. 1234 o Section 12, "Experimental Targets", has been removed. 1236 o Updated references for the new version EAI documents and added 1237 placeholders for all of the known remaining drafts that will 1238 become part of the core EAI series but that have not been written. 1240 o Inserted an additional clarification about the relationship of 1241 these extensions to non-ASCII messages. 1243 o Changed some normative/informative reference classifications based 1244 on review of the new text. 1246 o Removed references to the pre-EAI documents that were cited for 1247 historical context in 4952. 1249 o Got rid of a remaining pointer to address downgrading in the 1250 discussion of an updated MAILTO URI. 1252 o Minor additional editorial cleanups and tuning. 1254 A.3. Changes between -02 and -03 1256 o Inserted paragraph clarifying the status of the UTF8SMTPbis 1257 keyword as a result of discussion prior to and during IETF 79. 1259 o Adjusted some references including adding an explicit citation of 1260 RFC 821. 1262 o Removed the discussion of the experimental work from an inline 1263 aside to a separate section, Section 6. 1265 o Rewrote the discussion of configuration errors in MX setups to 1266 make it clear that they are an issue with forward-pointing 1267 addresses only and improved the discussion of backward-pointing 1268 addresses. 1270 o Removed some now-obsolete placeholder notes and resolved the 1271 remaining one to a dangling reference. 1273 A.4. Changes between -02 and -03 1275 o Several minor editorial changes. 1277 o Added a discussion of the relationship to the base email, MIME, 1278 and IDNA specifications. 1280 Authors' Addresses 1282 John C KLENSIN 1283 1770 Massachusetts Ave, #322 1284 Cambridge, MA 02140 1285 USA 1287 Phone: +1 617 491 5735 1288 EMail: john-ietf@jck.com 1290 YangWoo KO 1291 ICU 1292 119 Munjiro 1293 Yuseong-gu, Daejeon 305-732 1294 Republic of Korea 1296 EMail: yw@mrko.pe.kr