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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 16, 2010 7 Intended status: Informational 8 Expires: February 17, 2011 10 Overview and Framework for Internationalized Email 11 draft-ietf-eai-frmwrk-4952bis-03 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 that 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 17, 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 . . . . . . . . . . . . . . . . . . 6 80 4.3. User Types . . . . . . . . . . . . . . . . . . . . . . . . 7 81 4.4. Messages . . . . . . . . . . . . . . . . . . . . . . . . . 7 82 4.5. Mailing Lists . . . . . . . . . . . . . . . . . . . . . . 8 83 4.6. Conventional Message and Internationalized Message . . . . 8 84 4.7. Undeliverable Messages and Notification . . . . . . . . . 8 85 5. Overview of the Approach and Document Plan . . . . . . . . . . 8 86 6. Review of Experimental Results . . . . . . . . . . . . . . . . 9 87 7. Overview of Protocol Extensions and Changes . . . . . . . . . 10 88 7.1. SMTP Extension for Internationalized Email Address . . . . 10 89 7.2. Transmission of Email Header Fields in UTF-8 Encoding . . 11 90 7.3. SMTP Service Extension for DSNs . . . . . . . . . . . . . 12 91 8. Downgrading before and after SMTP Transactions . . . . . . . . 12 92 8.1. Downgrading before or during Message Submission . . . . . 13 93 8.2. Downgrading or Other Processing After Final SMTP 94 Delivery . . . . . . . . . . . . . . . . . . . . . . . . . 14 95 9. Downgrading in Transit . . . . . . . . . . . . . . . . . . . . 14 96 10. User Interface and Configuration Issues . . . . . . . . . . . 15 97 10.1. Choices of Mailbox Names and Unicode Normalization . . . . 15 98 11. Additional Issues . . . . . . . . . . . . . . . . . . . . . . 16 99 11.1. Impact on URIs and IRIs . . . . . . . . . . . . . . . . . 16 100 11.2. Use of Email Addresses as Identifiers . . . . . . . . . . 16 101 11.3. Encoded Words, Signed Messages, and Downgrading . . . . . 17 102 11.4. LMTP . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 103 11.5. Other Uses of Local Parts . . . . . . . . . . . . . . . . 17 104 11.6. Non-Standard Encapsulation Formats . . . . . . . . . . . . 18 105 12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 106 13. Security Considerations . . . . . . . . . . . . . . . . . . . 18 107 14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20 108 15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20 109 15.1. Normative References . . . . . . . . . . . . . . . . . . . 20 110 15.2. Informative References . . . . . . . . . . . . . . . . . . 22 111 Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 25 112 A.1. Changes between -00 and -01 . . . . . . . . . . . . . . . 25 113 A.2. Changes between -01 and -02 . . . . . . . . . . . . . . . 26 114 A.3. Changes between -02 and -03 . . . . . . . . . . . . . . . 27 116 1. Introduction 118 Note in Draft and to RFC Editor: The keyword represented in this 119 document by "UTF8SMTPbis" (and in the XML source by &EAISMTPkeyword;) 120 is a placeholder. The actual keyword will be assigned when the 121 standards track SMTP extension in this series [RFC5336bis-SMTP] is 122 approved for publication and should be substituted here. This 123 paragraph should be treated as normative reference to that SMTP 124 extension draft, creating a reference hold until it is approved by 125 the IESG. The paragraph should be removed before RFC publication. 127 In order to use internationalized email addresses, we need to 128 internationalize both the domain part and the local part of email 129 addresses. The domain part of email addresses is already 130 internationalized [RFC5890], while the local part is not. Without 131 the extensions specified in this document, the mailbox name is 132 restricted to a subset of 7-bit ASCII [RFC5321]. Though MIME 133 [RFC2045] enables the transport of non-ASCII data, it does not 134 provide a mechanism for internationalized email addresses. In RFC 135 2047 [RFC2047], MIME defines an encoding mechanism for some specific 136 message header fields to accommodate non-ASCII data. However, it 137 does not permit the use of email addresses that include non-ASCII 138 characters. Without the extensions defined here, or some equivalent 139 set, the only way to incorporate non-ASCII characters in any part of 140 email addresses is to use RFC 2047 coding to embed them in what RFC 141 5322 [RFC5322] calls the "display name" (known as a "name phrase" or 142 by other terms elsewhere) of the relevant header fields. Information 143 coded into the display name is invisible in the message envelope and, 144 for many purposes, is not part of the address at all. 146 This document is an update of RFC 4952 [RFC4952] that reflects 147 additional issues, shared terminology, and some architectural changes 148 identified since that document was published. 150 The pronouns "he" and "she" are used interchangeably to indicate a 151 human of indeterminate gender. 153 The key words "MUST", "SHALL", "REQUIRED", "SHOULD", "RECOMMENDED", 154 and "MAY" in this document are to be interpreted as described in RFC 155 2119 [RFC2119]. 157 2. Role of This Specification 159 This document presents the overview and framework for an approach to 160 the next stage of email internationalization. This new stage 161 requires not only internationalization of addresses and header 162 fields, but also associated transport and delivery models. A prior 163 version of this specification, RFC 4952 [RFC4952], also provided an 164 introduction to a series of experimental protocols [RFC5335] 165 [RFC5336] [RFC5337] [RFC5504] [RFC5721] [RFC5738] [RFC5825]. This 166 revised form provides overview and conceptual information for the 167 standards-track successors of a subset of those protocols. Details 168 of the documents and the relationships among them appear in Section 5 169 and a discussion of what was learned from the Experimental protocols 170 and their implementations appears in Section 6. 172 Taken together, these specifications provide the details for a way to 173 implement and support internationalized email. The document itself 174 describes how the various elements of email internationalization fit 175 together and the relationships among the primary specifications 176 associated with message transport, header formats, and handling. 178 3. Problem Statement 180 Internationalizing Domain Names in Applications (IDNA) [RFC5890] 181 permits internationalized domain names, but deployment has not yet 182 reached most users. One of the reasons for this is that we do not 183 yet have fully internationalized naming schemes. Domain names are 184 just one of the various names and identifiers that are required to be 185 internationalized. In many contexts, until more of those identifiers 186 are internationalized, internationalized domain names alone have 187 little value. 189 Email addresses are prime examples of why it is not good enough to 190 just internationalize the domain name. As most observers have 191 learned from experience, users strongly prefer email addresses that 192 resemble names or initials to those involving seemingly meaningless 193 strings of letters or numbers. Unless the entire email address can 194 use familiar characters and formats, users will perceive email as 195 being culturally unfriendly. If the names and initials used in email 196 addresses can be expressed in the native languages and writing 197 systems of the users, the Internet will be perceived as more natural, 198 especially by those whose native language is not written in a subset 199 of a Roman-derived script. 201 Internationalization of email addresses is not merely a matter of 202 changing the SMTP envelope; or of modifying the From, To, and Cc 203 header fields; or of permitting upgraded Mail User Agents (MUAs) to 204 decode a special coding and respond by displaying local characters. 205 To be perceived as usable, the addresses must be internationalized 206 and handled consistently in all of the contexts in which they occur. 207 This requirement has far-reaching implications: collections of 208 patches and workarounds are not adequate. Even if they were 209 adequate, a workaround-based approach may result in an assortment of 210 implementations with different sets of patches and workarounds having 211 been applied with consequent user confusion about what is actually 212 usable and supported. Instead, we need to build a fully 213 internationalized email environment, focusing on permitting efficient 214 communication among those who share a language or other community. 215 That, in turn, implies changes to the mail header environment to 216 permit the full range of Unicode characters where that makes sense, 217 an SMTP Extension to permit UTF-8 [RFC3629] [RFC5198] mail addressing 218 and delivery of those extended header fields, support for 219 internationalized delivery and service notifications [RFC3461] 220 [RFC3464], and (finally) a requirement for support of the 8BITMIME 221 SMTP Extension [RFC1652] so that all of these can be transported 222 through the mail system without having to overcome the limitation 223 that header fields do not have content-transfer-encodings. 225 4. Terminology 227 This document assumes a reasonable understanding of the protocols and 228 terminology of the core email standards as documented in [RFC5321] 229 and [RFC5322]. 231 4.1. Mail User and Mail Transfer Agents 233 Much of the description in this document depends on the abstractions 234 of "Mail Transfer Agent" ("MTA") and "Mail User Agent" ("MUA"). 235 However, it is important to understand that those terms and the 236 underlying concepts postdate the design of the Internet's email 237 architecture and the application of the "protocols on the wire" 238 principle to it. That email architecture, as it has evolved, and 239 that "on the wire" principle have prevented any strong and 240 standardized distinctions about how MTAs and MUAs interact on a given 241 origin or destination host (or even whether they are separate). 243 However, the term "final delivery MTA" is used in this document in a 244 fashion equivalent to the term "delivery system" or "final delivery 245 system" of RFC 5321. This is the SMTP server that controls the 246 format of the local parts of addresses and is permitted to inspect 247 and interpret them. It receives messages from the network for 248 delivery to mailboxes or for other local processing, including any 249 forwarding or aliasing that changes envelope addresses, rather than 250 relaying. From the perspective of the network, any local delivery 251 arrangements such as saving to a message store, handoff to specific 252 message delivery programs or agents, and mechanisms for retrieving 253 messages are all "behind" the final delivery MTA and hence are not 254 part of the SMTP transport or delivery process. 256 4.2. Address Character Sets 258 In this document, an address is "all-ASCII", or just an "ASCII 259 address", if every character in the address is in the ASCII character 260 repertoire [ASCII]; an address is "non-ASCII", or an "i18n-address", 261 if any character is not in the ASCII character repertoire. Such 262 addresses may be restricted in other ways, but those restrictions are 263 not relevant to this definition. The term "all-ASCII" is also 264 applied to other protocol elements when the distinction is important, 265 with "non-ASCII" or "internationalized" as its opposite. 267 The umbrella term to describe the email address internationalization 268 specified by this document and its companion documents is 269 "UTF8SMTPbis". 270 [[anchor3: Note in Draft: Keyword to be changed before publication.]] 271 For example, an address permitted by this specification is referred 272 to as a "UTF8SMTPbis (compliant) address". 274 Please note that, according to the definitions given here, the set of 275 all "all-ASCII" addresses and the set of all "non-ASCII" addresses 276 are mutually exclusive. The set of all addresses permitted when 277 UTF8SMTPbis appears is the union of these two sets. 279 4.3. User Types 281 An "ASCII user" (i) exclusively uses email addresses that contain 282 ASCII characters only, and (ii) cannot generate recipient addresses 283 that contain non-ASCII characters. 285 An "i18mail user" has one or more non-ASCII email addresses. Such a 286 user may have ASCII addresses too; if the user has more than one 287 email account and a corresponding address, or more than one alias for 288 the same address, he or she has some method to choose which address 289 to use on outgoing email. Note that under this definition, it is not 290 possible to tell from an ASCII address if the owner of that address 291 is an i18mail user or not. (A non-ASCII address implies a belief 292 that the owner of that address is an i18mail user.) There is no such 293 thing as an "i18mail message"; the term applies only to users and 294 their agents and capabilities. In particular, the use of non-ASCII 295 message content is an integral part of the MIME specifications 296 [RFC2045] and does not require these extensions (although it is 297 compatible with them). 299 4.4. Messages 301 A "message" is sent from one user (sender) using a particular email 302 address to one or more other recipient email addresses (often 303 referred to just as "users" or "recipient users"). 305 4.5. Mailing Lists 307 A "mailing list" is a mechanism whereby a message may be distributed 308 to multiple recipients by sending it to one recipient address. An 309 agent (typically not a human being) at that single address then 310 causes the message to be redistributed to the target recipients. 311 This agent sets the envelope return address of the redistributed 312 message to a different address from that of the original single 313 recipient message. Using a different envelope return address 314 (reverse-path) causes error (and other automatically generated) 315 messages to go to an error handling address. 317 Special provisions for managing mailing lists that might contain non- 318 ASCII addresses are discussed in a document that is specific to that 319 topic [EAI-Mailinglist] [RFCNNNNbis-MailingList]. 321 4.6. Conventional Message and Internationalized Message 323 o A conventional message is one that does not use any extension 324 defined in the SMTP extension document [RFC5336] or in the 325 UTF8header specification [RFC5335], and is strictly conformant to 326 RFC 5322 [RFC5322]. 328 o An internationalized message is a message utilizing one or more of 329 the extensions defined in this set of specifications, so that it 330 is no longer conformant to the traditional specification of an 331 email message or its transport. 333 4.7. Undeliverable Messages and Notification 335 As specified in RFC 5321, a message that is undeliverable for some 336 reason is expected to result in notification to the sender. This can 337 occur in either of two ways. One, typically called "Rejection", 338 occurs when an SMTP server returns a reply code indicating a fatal 339 error (a "5yz" code) or persistently returns a temporary failure 340 error (a "4yz" code). The other involves accepting the message 341 during SMTP processing and then generating a message to the sender, 342 typically known as a "Non-delivery Notification" or "NDN". Current 343 practice often favors rejection over NDNs because of the reduced 344 likelihood that the generation of NDNs will be used as a spamming 345 technique. The latter, NDN, case is unavoidable if an intermediate 346 MTA accepts a message that is then rejected by the next-hop server. 348 5. Overview of the Approach and Document Plan 350 This set of specifications changes both SMTP and the character 351 encoding of email message headers to permit non-ASCII characters to 352 be represented directly. Each important component of the work is 353 described in a separate document. The document set, whose members 354 are described below, also contains informational documents whose 355 purpose is to provide implementation suggestions and guidance for the 356 protocols. 358 In addition to this document, the following documents make up this 359 specification and provide advice and context for it. 361 o SMTP extensions. This document [RFC5336bis-SMTP] provides an SMTP 362 extension (as provided for in RFC 5321) for internationalized 363 addresses. 365 o Email message headers in UTF-8. This document [RFC5335bis-Hdrs] 366 essentially updates RFC 5322 to permit some information in email 367 message headers to be expressed directly by Unicode characters 368 encoded in UTF-8 when the SMTP extension described above is used. 369 This document, possibly with one or more supplemental ones, will 370 also need to address the interactions with MIME, including 371 relationships between UTF8SMTPbis and internal MIME headers and 372 content types. 374 o Extensions to delivery status and notification handling to adapt 375 to internationalized addresses [RFC5337bis-DSN]. 377 o Extensions to the IMAP protocol to support internationalized 378 message headers [RFC5738bis-IMAP]. 380 o Parallel extensions to the POP protocol [RFC5721] 381 [RFC5721bis-POP3]. 383 6. Review of Experimental Results 385 The key difference between this set of protocols and the experimental 386 set that preceded them [RFC5335] [RFC5336] [RFC5337] [RFC5504] 387 [RFC5721] [RFC5738] [RFC5825] is that the earlier group provided a 388 mechanism for in-transit downgrading of messages (described in detail 389 in RFC 5504). That mechanism permitted, and essentially required, 390 that each non-ASCII address be accompanied by an all-ASCII 391 equivalent. That, in turn, raised security concerns associated with 392 pairing of addresses that could not be authenticated. It also 393 introduced the first incompatible change to Internet mail addressing 394 in many years, raising concerns about interoperability issues if the 395 new address forms "leaked" into legacy email implementations. The WG 396 concluded that the advantages of in-transit downgrading, were it 397 feasible operationally, would be significant enough to overcome those 398 concerns. 400 Operationally that turned out to not be the case, with 401 interoperability problems among initial implementations. Prior to 402 starting on the work that led to this set of specifications, the WG 403 concluded that the combination of requirements and long-term 404 implications of that earlier model were too complex to be 405 satisfactory and that work should move ahead without it. 407 7. Overview of Protocol Extensions and Changes 409 7.1. SMTP Extension for Internationalized Email Address 411 An SMTP extension, "UTF8SMTPbis" is specified as follows: 413 o Permits the use of UTF-8 strings in email addresses, both local 414 parts and domain names. 416 o Permits the selective use of UTF-8 strings in email message 417 headers (see Section 7.2). 419 o Requires that the server advertise the 8BITMIME extension 420 [RFC1652] and that the client support 8-bit transmission so that 421 header information can be transmitted without using a special 422 content-transfer-encoding. 424 Some general principles affect the development decisions underlying 425 this work. 427 1. Email addresses enter subsystems (such as a user interface) that 428 may perform charset conversions or other encoding changes. When 429 the left hand side of the address includes characters outside the 430 US-ASCII character repertoire, use of ASCII-compatible (ACE) 431 encoding [RFC3492] [RFC5890] on the right hand side is 432 discouraged to promote consistent processing of characters 433 throughout the address. 435 2. An SMTP relay must 437 * Either recognize the format explicitly, agreeing to do so via 438 an ESMTP option, or 440 * Reject the message or, if necessary, return a non-delivery 441 notification message, so that the sender can make another 442 plan. 444 3. If the message cannot be forwarded because the next-hop system 445 cannot accept the extension it MUST be rejected or a non-delivery 446 message generated and sent. 448 4. In the interest of interoperability, charsets other than UTF-8 449 are prohibited in mail addresses and message headers being 450 transmitted over the Internet. There is no practical way to 451 identify multiple charsets properly with an extension similar to 452 this without introducing great complexity. 454 Conformance to the group of standards specified here for email 455 transport and delivery requires implementation of the SMTP Extension 456 specification and the UTF-8 Header specification. If the system 457 implements IMAP or POP, it MUST conform to the i18n IMAP or POP 458 specifications respectively. 460 7.2. Transmission of Email Header Fields in UTF-8 Encoding 462 There are many places in MUAs or in a user presentation in which 463 email addresses or domain names appear. Examples include the 464 conventional From, To, or Cc header fields; Message-ID and 465 In-Reply-To header fields that normally contain domain names (but 466 that may be a special case); and in message bodies. Each of these 467 must be examined from an internationalization perspective. The user 468 will expect to see mailbox and domain names in local characters, and 469 to see them consistently. If non-obvious encodings, such as 470 protocol-specific ASCII-Compatible Encoding (ACE) variants, are used, 471 the user will inevitably, if only occasionally, see them rather than 472 "native" characters and will find that discomfiting or astonishing. 473 Similarly, if different codings are used for mail transport and 474 message bodies, the user is particularly likely to be surprised, if 475 only as a consequence of the long-established "things leak" 476 principle. The only practical way to avoid these sources of 477 discomfort, in both the medium and the longer term, is to have the 478 encodings used in transport be as similar to the encodings used in 479 message headers and message bodies as possible. 481 When email local parts are internationalized, it seems clear that 482 they should be accompanied by arrangements for the message headers to 483 be in the fully internationalized form. That form should presumably 484 use UTF-8 rather than ASCII as the base character set for the 485 contents of header fields (protocol elements such as the header field 486 names themselves are unchanged and remain entirely in ASCII). For 487 transition purposes and compatibility with legacy systems, this can 488 done by extending the traditional MIME encoding models for non-ASCII 489 characters in headers [RFC2045] [RFC2231]. However, the target is 490 fully internationalized message headers, as discussed in 491 [RFC5335bis-Hdrs] and not an extended and painful transition. 493 7.3. SMTP Service Extension for DSNs 495 The existing Draft Standard Delivery status notifications (DSNs) 496 specification [RFC3461] is limited to ASCII text in the machine 497 readable portions of the protocol. "International Delivery and 498 Disposition Notifications" [RFC5337bis-DSN] adds a new address type 499 for international email addresses so an original recipient address 500 with non-ASCII characters can be correctly preserved even after 501 downgrading. If an SMTP server advertises both the UTF8SMTPbis and 502 the DSN extension, that server MUST implement internationalized DSNs 503 including support for the ORCPT parameter specified in RFC 3461 504 [RFC3461]. 506 8. Downgrading before and after SMTP Transactions 508 An important issue with these extensions is how to handle 509 interactions between systems that support non-ASCII addresses and 510 legacy systems that expect ASCII. There is, of course, no problem 511 with ASCII-only systems sending to those that can handle 512 internationalized forms because the ASCII forms are just a proper 513 subset. But, when systems that support these extensions send mail, 514 they may include non-ASCII addresses for senders, receivers, or both 515 and might also provide non-ASCII header information other than 516 addresses. If the extension is not supported by the first-hop system 517 (SMTP server accessed by the Submission server acting as an SMTP 518 client), message originating systems should be prepared to either 519 send conventional envelopes and message headers or to return the 520 message to the originating user so the message may be manually 521 downgraded to the traditional form, possibly using encoded words 522 [RFC2047] in the message headers. Of course, such transformations 523 imply that the originating user or system must have ASCII-only 524 addresses available for all senders and recipients. Mechanisms by 525 which such addresses may be found or identified are outside the scope 526 of these specifications as are decisions about the design of 527 originating systems such as whether any required transformations are 528 made by the user, the originating MUA, or the Submission server. 530 A somewhat more complex situation arises when the first-hop system 531 supports these extensions but some subsequent server in the SMTP 532 transmission chain does not. It is important to note that most cases 533 of that situation with forward-pointing addresses will be the result 534 of configuration errors: especially if it hosts non-ASCII addresses, 535 a final delivery MTA that accepts these extensions should not be 536 configured with lower-preference MX hosts that do not. When the only 537 non-ASCII address being transmitted is backward-pointing (e.g., in an 538 SMTP MAIL command), recipient configuration can not help in general. 539 On the other hand, alternate, all-ASCII, addresses for senders are 540 those most likely to be authoritatively known by the submission 541 environment or the sender herself. Consequently, if an intermediate 542 SMTP relay that is transmitting a message that requires these 543 extensions and discovers that the next system in the chain does not 544 support them, it will have little choice other than to reject or 545 return the message. 547 As discussed above, downgrading to an ASCII-only form may occur 548 before or during the initial message submission. It might also occur 549 after the delivery to the final delivery MTA in order to accommodate 550 messages stores or IMAP or POP servers or clients that have different 551 capabilities than the delivery MTA. These two cases are discussed in 552 the subsections below. 554 8.1. Downgrading before or during Message Submission 556 It is likely that the most common cases in which a message that 557 requires these extensions is sent to a system that does not will 558 involve the combination of ASCII-only forward-pointing addresses with 559 a non-ASCII backward-pointing one. Until the extensions described 560 here have been universally implemented in the Internet email 561 environment, senders who prefer to use non-ASCII addresses (or raw 562 UTF-8 characters in header fields) even when their intended 563 recipients use and expect all-ASCII ones will need to be especially 564 careful about the error conditions that can arise, especially if they 565 are working in an environment in which non-delivery messages (or 566 other indications from submission servers) are routinely dropped or 567 ignored. 569 Perhaps obviously, the most convenient time to find an ASCII address 570 corresponding to an internationalized address is at the originating 571 MUA or closely-associated systems. This can occur either before the 572 message is sent or after the internationalized form of the message is 573 rejected. It is also the most convenient time to convert a message 574 from the internationalized form into conventional ASCII form or to 575 generate a non-delivery message to the sender if either is necessary. 576 At that point, the user has a full range of choices available, 577 including changing backward-pointing addresses, contacting the 578 intended recipient out of band for an alternate address, consulting 579 appropriate directories, arranging for translation of both addresses 580 and message content into a different language, and so on. While it 581 is natural to think of message downgrading as optimally being a 582 fully-automated process, we should not underestimate the capabilities 583 of a user of at least moderate intelligence who wishes to communicate 584 with another such user. 586 In this context, one can easily imagine modifications to message 587 submission servers (as described in [RFC4409]) so that they would 588 perform downgrading, or perhaps even upgrading, operations, receiving 589 messages with one or more of the internationalization extensions 590 discussed here and adapting the outgoing message, as needed, to 591 respond to the delivery or next-hop environment it encounters. 593 8.2. Downgrading or Other Processing After Final SMTP Delivery 595 When an email message is received by a final delivery MTA, it is 596 usually stored in some form. Then it is retrieved either by software 597 that reads the stored form directly or by client software via some 598 email retrieval mechanisms such as POP or IMAP. 600 The SMTP extension described in Section 7.1 provides protection only 601 in transport. It does not prevent MUAs and email retrieval 602 mechanisms that have not been upgraded to understand 603 internationalized addresses and UTF-8 message headers from accessing 604 stored internationalized emails. 606 Since the final delivery MTA (or, to be more specific, its 607 corresponding mail storage agent) cannot safely assume that agents 608 accessing email storage will always be capable of handling the 609 extensions proposed here, it MAY either downgrade internationalized 610 emails or specially identify messages that utilize these extensions, 611 or both. If this is done, the final delivery MTA SHOULD include a 612 mechanism to preserve or recover the original internationalized forms 613 without information loss to support access by UTF8SMTPbis-aware 614 agents. 616 9. Downgrading in Transit 618 The base SMTP specification (Section 2.3.11 of RFC 5321 [RFC5321]) 619 states that "due to a long history of problems when intermediate 620 hosts have attempted to optimize transport by modifying them, the 621 local-part MUST be interpreted and assigned semantics only by the 622 host specified in the domain part of the address". This is not a new 623 requirement; equivalent statements appeared in specifications in 2001 624 [RFC2821] and even in 1989 [RFC1123]. 626 Adherence to this rule means that a downgrade mechanism that 627 transforms the local-part of an email address cannot be utilized in 628 transit. It can only be applied at the endpoints, specifically by 629 the MUA or submission server or by the final delivery MTA. 631 One of the reasons for this rule has to do with legacy email systems 632 that embed mail routing information in the local-part of the address 633 field. Transforming the email address destroys such routing 634 information. There is no way a server other than the final delivery 635 server can know, for example, whether the local-part of 636 user%foo@example.com is a route ("user" is reached via "foo") or 637 simply a local address. 639 10. User Interface and Configuration Issues 641 Internationalization of addresses and message headers, especially in 642 combination with variations on character coding that are inherent to 643 Unicode, may make careful choices of addresses and careful 644 configuration of servers and DNS records even more important than 645 they are for traditional Internet email. It is likely that, as 646 experience develops with the use of these protocols, it will be 647 desirable to produce one or more additional documents that offer 648 guidance for configuration and interfaces. A document that discusses 649 issues with mail user agents (MUAs), especially with regard to 650 downgrading [EAI-MUA-issues], is expected to be developed in the EAI 651 Working Group. The subsections below address some other issues. 653 10.1. Choices of Mailbox Names and Unicode Normalization 655 It has long been the case that the email syntax permits choices about 656 mailbox names that are unwise in practice if one actually intends the 657 mailboxes to be accessible to a broad range of senders. The most- 658 often-cited examples involve the use of case-sensitivity and tricky 659 quoting of embedded characters in mailbox local parts. While these 660 are permitted by the protocols and servers are expected to support 661 them and there are special cases where they can provide value, taking 662 advantage of those features is almost always bad practice unless the 663 intent is to create some form of security by obscurity. 665 In the absence of these extensions, SMTP clients and servers are 666 constrained to using only those addresses permitted by RFC 5321. The 667 local parts of those addresses MAY be made up of any ASCII characters 668 except the control characters that 5321 prohibits, although some of 669 them MUST be quoted as specified there. It is notable in an 670 internationalization context that there is a long history on some 671 systems of using overstruck ASCII characters (a character, a 672 backspace, and another character) within a quoted string to 673 approximate non-ASCII characters. This form of internationalization 674 was permitted by RFC 821 [RFC0821] but is prohibited by RFC 5321 675 because it requires a backspace character (a prohibited C0 control). 676 The practice SHOULD be phased out as this extension becomes widely 677 deployed but backward-compatibility considerations may require that 678 it continue to be recognized. 680 For the particular case of EAI mailbox names, special attention must 681 be paid to Unicode normalization [Unicode-UAX15], in part because 682 Unicode strings may be normalized by other processes independent of 683 what a mail protocol specifies (this is exactly analogous to what may 684 happen with quoting and dequoting in traditional addresses). 686 Consequently, the following principles are offered as advice to those 687 who are selecting names for mailboxes: 689 o In general, it is wise to support addresses in Normalized form, 690 using either Normalization Form NFC and, except in unusual 691 circumstances, NFKC. 693 o It may be wise to support other forms of the same local-part 694 string, either as aliases or by normalization of strings reaching 695 the delivery server, in the event that the sender does not send 696 the strings in normalized form. 698 o Stated differently and in more specific terms, the rules of the 699 protocol for local-part strings essentially provide that: 701 * Unnormalized strings are valid, but sufficiently bad practice 702 that they may not work reliably on a global basis. 704 * C0 (and presumably C1) controls (see The Unicode Standard 705 [Unicode52]) are prohibited, the first in RFC 5321 and the 706 second by an obvious extension from it [RFC5198]. 708 * Other kinds of punctuation, spaces, etc., are risky practice. 709 Perhaps they will work, and SMTP receiver code is required to 710 handle them, but creating dependencies on them in mailbox names 711 that are chosen is usually a bad practice and may lead to 712 interoperability problems. 714 11. Additional Issues 716 This section identifies issues that are not covered, or not covered 717 comprehensively, as part of this set of specifications, but that will 718 require ongoing review as part of deployment of email address and 719 header internationalization. 721 11.1. Impact on URIs and IRIs 723 The mailto: schema [RFC2368] and discussed in the Internationalized 724 Resource Identifier (IRI) specification [RFC3987] may need to be 725 modified when this work is completed and standardized. 727 11.2. Use of Email Addresses as Identifiers 729 There are a number of places in contemporary Internet usage in which 730 email addresses are used as identifiers for individuals, including as 731 identifiers to Web servers supporting some electronic commerce sites 732 and in some X.509 certificates [RFC5280]. These documents do not 733 address those uses, but it is reasonable to expect that some 734 difficulties will be encountered when internationalized addresses are 735 first used in those contexts, many of which cannot even handle the 736 full range of addresses permitted today. 738 11.3. Encoded Words, Signed Messages, and Downgrading 740 One particular characteristic of the email format is its persistency: 741 MUAs are expected to handle messages that were originally sent 742 decades ago and not just those delivered seconds ago. As such, MUAs 743 and mail filtering software, such as that specified in Sieve 744 [RFC5228], will need to continue to accept and decode header fields 745 that use the "encoded word" mechanism [RFC2047] to accommodate non- 746 ASCII characters in some header fields. While extensions to both 747 POP3 [RFC1939] and IMAP [RFC3501] have been defined that include 748 automatic upgrading of messages that carry non-ASCII information in 749 encoded form -- including RFC 2047 decoding -- of messages by the 750 POP3 [RFC5721bis-POP3] or IMAP [RFC5738bis-IMAP] server, there are 751 message structures and MIME content-types for which that cannot be 752 done or where the change would have unacceptable side effects. 754 For example, message parts that are cryptographically signed, using 755 e.g., S/MIME [RFC3851] or Pretty Good Privacy (PGP) [RFC3156], cannot 756 be upgraded from the RFC 2047 form to normal UTF-8 characters without 757 breaking the signature. Similarly, message parts that are encrypted 758 may contain, when decrypted, header fields that use the RFC 2047 759 encoding; such messages cannot be 'fully' upgraded without access to 760 cryptographic keys. 762 Similar issues may arise if messages are signed and then subsequently 763 downgraded, e.g., as discussed in Section 8.1, and then an attempt is 764 made to upgrade them to the original form and then verify the 765 signatures. Even the very subtle changes that may result from 766 algorithms to downgrade and then upgrade again may be sufficient to 767 invalidate the signatures if they impact either the primary or MIME 768 bodypart headers. When signatures are present, downgrading must be 769 performed with extreme care if at all. 771 11.4. LMTP 773 LMTP [RFC2033] may be used as part of the final delivery agent. In 774 such cases, LMTP may be arranged to deliver the mail to the mail 775 store. The mail store may not have UTF8SMTPbis capability. LMTP may 776 need to be updated to deal with these situations. 778 11.5. Other Uses of Local Parts 780 Local parts are sometimes used to construct domain labels, e.g., the 781 local part "user" in the address user@domain.example could be 782 converted into a vanity host user.domain.example with its Web space 783 at and the catchall addresses 784 any.thing.goes@user.domain.example. 786 Such schemes are obviously limited by, among other things, the SMTP 787 rules for domain names, and will not work without further 788 restrictions for other local parts such as the 789 specified in [RFC5335bis-Hdrs]. Whether those limitations are 790 relevant to these specifications is an open question. It may be 791 simply another case of the considerable flexibility accorded to 792 delivery MTAs in determining the mailbox names they will accept and 793 how they are interpreted. 795 11.6. Non-Standard Encapsulation Formats 797 Some applications use formats similar to the application/mbox format 798 defined in [RFC4155] instead of the message/digest form described in 799 RFC 2046, Section 5.1.5 [RFC2046] to transfer multiple messages as 800 single units. Insofar as such applications assume that all stored 801 messages use the message/rfc822 format described in RFC 2046, Section 802 5.2.1 [RFC2046] with US-ASCII message headers, they are not ready for 803 the extensions specified in this series of documents and special 804 measures may be needed to properly detect and process them. 806 12. IANA Considerations 808 This overview description and framework document does not contemplate 809 any IANA registrations or other actions. Some of the documents in 810 the group have their own IANA considerations sections and 811 requirements. 813 13. Security Considerations 815 Any expansion of permitted characters and encoding forms in email 816 addresses raises some risks. There have been discussions on so 817 called "IDN-spoofing" or "IDN homograph attacks". These attacks 818 allow an attacker (or "phisher") to spoof the domain or URLs of 819 businesses. The same kind of attack is also possible on the local 820 part of internationalized email addresses. It should be noted that 821 the proposed fix involving forcing all displayed elements into 822 normalized lower-case works for domain names in URLs, but not email 823 local parts since those are case sensitive. 825 Since email addresses are often transcribed from business cards and 826 notes on paper, they are subject to problems arising from confusable 827 characters (see [RFC4690]). These problems are somewhat reduced if 828 the domain associated with the mailbox is unambiguous and supports a 829 relatively small number of mailboxes whose names follow local system 830 conventions. They are increased with very large mail systems in 831 which users can freely select their own addresses. 833 The internationalization of email addresses and message headers must 834 not leave the Internet less secure than it is without the required 835 extensions. The requirements and mechanisms documented in this set 836 of specifications do not, in general, raise any new security issues. 838 They do require a review of issues associated with confusable 839 characters -- a topic that is being explored thoroughly elsewhere 840 (see, e.g., RFC 4690 [RFC4690]) -- and, potentially, some issues with 841 UTF-8 normalization, discussed in RFC 3629 [RFC3629], and other 842 transformations. Normalization and other issues associated with 843 transformations and standard forms are also part of the subject of 844 work described elsewhere [RFC5198] [RFC5893] [IAB-idn-encoding]. 846 Some issues specifically related to internationalized addresses and 847 message headers are discussed in more detail in the other documents 848 in this set. However, in particular, caution should be taken that 849 any "downgrading" mechanism, or use of downgraded addresses, does not 850 inappropriately assume authenticated bindings between the 851 internationalized and ASCII addresses. Expecting and most or all 852 such transformations prior to final delivery be done by systems that 853 are presumed to be under the administrative control of the sending 854 user ameliorates the potential problem somewhat as compared to what 855 it would be if the relationships were changed in transit. 857 The new UTF-8 header and message formats might also raise, or 858 aggravate, another known issue. If the model creates new forms of an 859 'invalid' or 'malformed' message, then a new email attack is created: 860 in an effort to be robust, some or most agents will accept such 861 message and interpret them as if they were well-formed. If a filter 862 interprets such a message differently than the MUA used by the 863 recipient, then it may be possible to create a message that appears 864 acceptable under the filter's interpretation but should be rejected 865 under the interpretation given to it by that MUA. Such attacks 866 already exist for existing messages and encoding layers, e.g., 867 invalid MIME syntax, invalid HTML markup, and invalid coding of 868 particular image types. 870 In addition, email addresses are used in many contexts other than 871 sending mail, such as for identifiers under various circumstances 872 (see Section 11.2). Each of those contexts will need to be 873 evaluated, in turn, to determine whether the use of non-ASCII forms 874 is appropriate and what particular issues they raise. 876 This work will clearly affect any systems or mechanisms that are 877 dependent on digital signatures or similar integrity protection for 878 email message headers (see also the discussion in Section 11.3). 879 Many conventional uses of PGP and S/MIME are not affected since they 880 are used to sign body parts but not message headers. On the other 881 hand, the developing work on domain keys identified mail (DKIM) 882 [RFC5863] will eventually need to consider this work and vice versa: 883 while this specification does not address or solve the issues raised 884 by DKIM and other signed header mechanisms, the issues will have to 885 be coordinated and resolved eventually if the two sets of protocols 886 are to co-exist. In addition, to the degree to which email addresses 887 appear in PKI (Public Key Infrastructure) certificates, standards 888 addressing such certificates will need to be upgraded to address 889 these internationalized addresses. Those upgrades will need to 890 address questions of spoofing by look-alikes of the addresses 891 themselves. 893 14. Acknowledgments 895 This document is an update to, and derived from, RFC 4952. This 896 document would have been impossible without the work and 897 contributions acknowledged in it. The present document benefited 898 significantly from discussions in the EAI WG and elsewhere after RFC 899 4952 was published, especially discussions about the experimental 900 versions of other documents in the internationalized email 901 collection, and from RFC errata on RFC 4952 itself. 903 Special thanks are due to Ernie Dainow for careful reviews and 904 suggested text in this version. 906 15. References 908 15.1. Normative References 910 [ASCII] American National Standards Institute 911 (formerly United States of America 912 Standards Institute), "USA Code for 913 Information Interchange", ANSI X3.4-1968, 914 1968. 916 ANSI X3.4-1968 has been replaced by newer 917 versions with slight modifications, but the 918 1968 version remains definitive for the 919 Internet. 921 [RFC1652] Klensin, J., Freed, N., Rose, M., 922 Stefferud, E., and D. Crocker, "SMTP 923 Service Extension for 8bit-MIMEtransport", 924 RFC 1652, July 1994. 926 [RFC2119] Bradner, S., "Key words for use in RFCs to 927 Indicate Requirement Levels'", RFC 2119, 928 BCP 14, March 1997. 930 [RFC3629] Yergeau, F., "UTF-8, a transformation 931 format of ISO 10646", STD 63, RFC 3629, 932 November 2003. 934 [RFC5321] Klensin, J., "Simple Mail Transfer 935 Protocol", RFC 5321, October 2008. 937 [RFC5322] Resnick, P., Ed., "Internet Message 938 Format", RFC 5322, October 2008. 940 [RFC5335bis-Hdrs] Yang, A. and S. Steele, "Internationalized 941 Email Headers", July 2010, . 945 [RFC5336bis-SMTP] Yao, J. and W. Mao, "SMTP Extension for 946 Internationalized Email Address", 947 August 2010, . 950 [RFC5337bis-DSN] Not yet posted?, "Internationalized 951 Delivery Status and Disposition 952 Notifications", Unwritten waiting for I-D, 953 2010. 955 [RFC5721bis-POP3] Not yet posted?, "POP3 Support for UTF-8", 956 Unwritten waiting for I-D, 2010. 958 [RFC5738bis-IMAP] Not yet posted?, "IMAP Support for UTF-8", 959 Unwritten waiting for I-D, 2010. 961 [RFC5890] Klensin, J., "Internationalized Domain 962 Names for Applications (IDNA): Definitions 963 and Document Framework", RFC 5890, 964 June 2010. 966 [RFCNNNNbis-MailingList] Not yet posted?, "Mailing Lists and 967 Internationalized Email Addresses", First 968 Version still not in RFC Editor queue https 969 ://datatracker.ietf.org/doc/ 970 draft-ietf-eai-mailinglist/, 971 Unwritten waiting for I-D, 2010. 973 15.2. Informative References 975 [EAI-MUA-issues] EAI WG, "Still-unnamed proposed document on 976 MUA issues", Not assigned or agreed to yet, 977 2011. 979 Note to IESG and RFC Editor: While there is 980 provision for a document on this subject in 981 the WG Charter, there is, as yet, no plan 982 for producing it or even for adding it to 983 the WG's task list with benchmarks. If the 984 present document is approved for 985 publication before the is at least a title 986 and author(s) for an I-D, the citation and 987 reference should simply be dropped. 989 [EAI-Mailinglist] Gellens, R., "Mailing Lists and 990 Internationalized Email Addresses", 991 June 2010, . 994 [IAB-idn-encoding] Thaler, D., Klensin, J., and S. Cheshire, 995 "IAB Thoughts on Encodings for 996 Internationalized Domain Names", 2010, . 1000 [RFC0821] Postel, J., "Simple Mail Transfer 1001 Protocol", STD 10, RFC 821, August 1982. 1003 [RFC1123] Braden, R., "Requirements for Internet 1004 Hosts - Application and Support", STD 3, 1005 RFC 1123, October 1989. 1007 [RFC1939] Myers, J. and M. Rose, "Post Office 1008 Protocol - Version 3", STD 53, RFC 1939, 1009 May 1996. 1011 [RFC2033] Myers, J., "Local Mail Transfer Protocol", 1012 RFC 2033, October 1996. 1014 [RFC2045] Freed, N. and N. Borenstein, "Multipurpose 1015 Internet Mail Extensions (MIME) Part One: 1016 Format of Internet Message Bodies", 1017 RFC 2045, November 1996. 1019 [RFC2046] Freed, N. and N. Borenstein, "Multipurpose 1020 Internet Mail Extensions (MIME) Part Two: 1022 Media Types", RFC 2046, November 1996. 1024 [RFC2047] Moore, K., "MIME (Multipurpose Internet 1025 Mail Extensions) Part Three: Message Header 1026 Extensions for Non-ASCII Text", RFC 2047, 1027 November 1996. 1029 [RFC2231] Freed, N. and K. Moore, "MIME Parameter 1030 Value and Encoded Word Extensions: Characte 1031 r Sets, Languages, and Continuations", 1032 RFC 2231, November 1997. 1034 [RFC2368] Hoffman, P., Masinter, L., and J. Zawinski, 1035 "The mailto URL scheme", RFC 2368, 1036 July 1998. 1038 [RFC2821] Klensin, J., "Simple Mail Transfer 1039 Protocol", RFC 2821, April 2001. 1041 [RFC3156] Elkins, M., Del Torto, D., Levien, R., and 1042 T. Roessler, "MIME Security with OpenPGP", 1043 RFC 3156, August 2001. 1045 [RFC3461] Moore, K., "Simple Mail Transfer Protocol 1046 (SMTP) Service Extension for Delivery 1047 Status Notifications (DSNs)", RFC 3461, 1048 January 2003. 1050 [RFC3464] Moore, K. and G. Vaudreuil, "An Extensible 1051 Message Format for Delivery Status 1052 Notifications", RFC 3464, January 2003. 1054 [RFC3492] Costello, A., "Punycode: A Bootstring 1055 encoding of Unicode for Internationalized 1056 Domain Names in Applications (IDNA)", 1057 RFC 3492, March 2003. 1059 [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS 1060 PROTOCOL - VERSION 4rev1", RFC 3501, 1061 March 2003. 1063 [RFC3851] Ramsdell, B., "Secure/Multipurpose Internet 1064 Mail Extensions (S/MIME) Version 3.1 1065 Message Specification", RFC 3851, 1066 July 2004. 1068 [RFC3987] Duerst, M. and M. Suignard, 1069 "Internationalized Resource Identifiers 1070 (IRIs)", RFC 3987, January 2005. 1072 [RFC4155] Hall, E., "The application/mbox Media 1073 Type", RFC 4155, September 2005. 1075 [RFC4409] Gellens, R. and J. Klensin, "Message 1076 Submission for Mail", RFC 4409, April 2006. 1078 [RFC4690] Klensin, J., Faltstrom, P., Karp, C., and 1079 IAB, "Review and Recommendations for 1080 Internationalized Domain Names (IDNs)", 1081 RFC 4690, September 2006. 1083 [RFC4952] Klensin, J. and Y. Ko, "Overview and 1084 Framework for Internationalized Email", 1085 RFC 4952, July 2007. 1087 [RFC5198] Klensin, J. and M. Padlipsky, "Unicode 1088 Format for Network Interchange", RFC 5198, 1089 March 2008. 1091 [RFC5228] Guenther, P. and T. Showalter, "Sieve: An 1092 Email Filtering Language", RFC 5228, 1093 January 2008. 1095 [RFC5280] Cooper, D., Santesson, S., Farrell, S., 1096 Boeyen, S., Housley, R., and W. Polk, 1097 "Internet X.509 Public Key Infrastructure 1098 Certificate and Certificate Revocation List 1099 (CRL) Profile", RFC 5280, May 2008. 1101 [RFC5335] Abel, Y., "Internationalized Email 1102 Headers", RFC 5335, September 2008. 1104 [RFC5336] Yao, J. and W. Mao, "SMTP Extension for 1105 Internationalized Email Addresses", 1106 RFC 5336, September 2008. 1108 [RFC5337] Newman, C. and A. Melnikov, 1109 "Internationalized Delivery Status and 1110 Disposition Notifications", RFC 5337, 1111 September 2008. 1113 [RFC5504] Fujiwara, K. and Y. Yoneya, "Downgrading 1114 Mechanism for Email Address 1115 Internationalization", RFC 5504, 1116 March 2009. 1118 [RFC5721] Gellens, R. and C. Newman, "POP3 Support 1119 for UTF-8", RFC 5721, February 2010. 1121 [RFC5738] Resnick, P. and C. Newman, "IMAP Support 1122 for UTF-8", RFC 5738, March 2010. 1124 [RFC5825] Fujiwara, K. and B. Leiba, "Displaying 1125 Downgraded Messages for Email Address 1126 Internationalization", RFC 5825, 1127 April 2010. 1129 [RFC5863] Hansen, T., Siegel, E., Hallam-Baker, P., 1130 and D. Crocker, "DomainKeys Identified Mail 1131 (DKIM) Development, Deployment, and 1132 Operations", RFC 5863, May 2010. 1134 [RFC5893] Alvestrand, H. and C. Karp, "Right-to-Left 1135 Scripts for Internationalized Domain Names 1136 for Applications (IDNA)", RFC 5893, 1137 June 2010. 1139 [Unicode-UAX15] The Unicode Consortium, "Unicode Standard 1140 Annex #15: Unicode Normalization Forms", 1141 March 2008, 1142 . 1144 [Unicode52] The Unicode Consortium. The Unicode 1145 Standard, Version 5.2.0, defined by:, "The 1146 Unicode Standard, Version 5.2.0", (Mountain 1147 View, CA: The Unicode Consortium, 1148 2009. ISBN 978-1-936213-00-9)., . 1151 Appendix A. Change Log 1153 [[RFC Editor: Please remove this section prior to publication.]] 1155 A.1. Changes between -00 and -01 1157 o Because there has been no feedback on the mailing list, updated 1158 the various questions to refer to this version as well. 1160 o Reflected RFC Editor erratum #1507 by correcting terminology for 1161 headers and header fields and distinguishing between "message 1162 headers" and different sorts of headers (e.g., the MIME ones). 1164 A.2. Changes between -01 and -02 1166 Note that section numbers in the list that follows may refer to -01 1167 and not -02. 1169 o Discussion of RFC 5825 ("downgraded display") has been removed per 1170 the earlier note and on-list discussion. Any needed discussion 1171 about reconstructed messages will need to appear in the IMAP and 1172 POP documents. However, the introductory material has been 1173 reworded to permit keeping 5504 and 5825 on the list there, 1174 without which the back chain would not be complete. For 1175 consistency with this change, 5504 and 5825 have been added to the 1176 "Obsoletes" list (as far as I know, an Informational spec can 1177 obsolete or update Experimental ones, so no downref problem here 1178 --JcK). 1180 o Reference to alternate addresses dropped from (former) Section 1181 7.1. 1183 o Reference to RFC 5504 added to (former) Section 8 for 1184 completeness. 1186 o Ernie's draft comments added (with some minor edits) to replace 1187 the placeholder in (former) Section 9 ("Downgrading in Transit"). 1188 It is intended to capture at least an introduction the earlier 1189 discussions of algorithmic downgrading generally and ACE/Punycode 1190 transformations in particular. Anyone who is unhappy with it 1191 should say so and propose alternate text. RSN. 1193 o In the interest of clarity and consistency with the terminology in 1194 Section 4.1, all uses of "final delivery SMTP server" and "final 1195 delivery server" have been changed to "final delivery MTA". 1197 o Placeholder at the end of Section 2 has been removed and the text 1198 revised to promise less. The "Document Plan" (Section 5) has been 1199 revised accordingly. We need to discuss this at IETF 78 if not 1200 sooner. 1202 o Sections 5 and 6 have been collapsed into one -- there wasn't 1203 enough left in the former Section 5 to justify a separate section. 1205 o Former Section 11.1 has been dropped and the DSN document moved up 1206 into the "Document Plan" as suggested earlier. 1208 o Section 12, "Experimental Targets", has been removed. 1210 o Updated references for the new version EAI documents and added 1211 placeholders for all of the known remaining drafts that will 1212 become part of the core EAI series but that have not been written. 1214 o Inserted an additional clarification about the relationship of 1215 these extensions to non-ASCII messages. 1217 o Changed some normative/informative reference classifications based 1218 on review of the new text. 1220 o Removed references to the pre-EAI documents that were cited for 1221 historical context in 4952. 1223 o Got rid of a remaining pointer to address downgrading in the 1224 discussion of an updated MAILTO URI. 1226 o Minor additional editorial cleanups and tuning. 1228 A.3. Changes between -02 and -03 1230 o Inserted paragraph clarifying the status of the UTF8SMTPbis 1231 keyword as a result of discussion prior to and during IETF 79. 1233 o Adjusted some references including adding an explicit citation of 1234 RFC 821. 1236 o Removed the discussion of the experimental work from an inline 1237 aside to a separate section, Section 6. 1239 o Rewrote the discussion of configuration errors in MX setups to 1240 make it clear that they are an issue with forward-pointing 1241 addresses only and improved the discussion of backward-pointing 1242 addresses. 1244 o Removed some now-obsolete placeholder notes and resolved the 1245 remaining one to a dangling reference. 1247 Authors' Addresses 1249 John C KLENSIN 1250 1770 Massachusetts Ave, #322 1251 Cambridge, MA 02140 1252 USA 1254 Phone: +1 617 491 5735 1255 EMail: john-ietf@jck.com 1256 YangWoo KO 1257 ICU 1258 119 Munjiro 1259 Yuseong-gu, Daejeon 305-732 1260 Republic of Korea 1262 EMail: yw@mrko.pe.kr