idnits 2.17.1 draft-ietf-eai-frmwrk-4952bis-11.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- -- The draft header indicates that this document obsoletes RFC5504, but the abstract doesn't seem to mention this, which it should. -- The draft header indicates that this document obsoletes RFC4952, but the abstract doesn't seem to directly say this. It does mention RFC4952 though, so this could be OK. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year == The document seems to contain a disclaimer for pre-RFC5378 work, but was first submitted on or after 10 November 2008. The disclaimer is usually necessary only for documents that revise or obsolete older RFCs, and that take significant amounts of text from those RFCs. If you can contact all authors of the source material and they are willing to grant the BCP78 rights to the IETF Trust, you can and should remove the disclaimer. Otherwise, the disclaimer is needed and you can ignore this comment. (See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (October 25, 2011) is 4567 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) -- Possible downref: Non-RFC (?) normative reference: ref. 'ASCII' ** Obsolete normative reference: RFC 5336 (Obsoleted by RFC 6531) -- Obsolete informational reference (is this intentional?): RFC 821 (Obsoleted by RFC 2821) -- Obsolete informational reference (is this intentional?): RFC 2821 (Obsoleted by RFC 5321) -- Obsolete informational reference (is this intentional?): RFC 3501 (Obsoleted by RFC 9051) -- Obsolete informational reference (is this intentional?): RFC 4409 (Obsoleted by RFC 6409) -- Obsolete informational reference (is this intentional?): RFC 4952 (Obsoleted by RFC 6530) -- Obsolete informational reference (is this intentional?): RFC 5335 (Obsoleted by RFC 6532) -- Duplicate reference: RFC5336, mentioned in 'RFC5336', was also mentioned in 'RFC5336bis-SMTP'. -- Obsolete informational reference (is this intentional?): RFC 5336 (Obsoleted by RFC 6531) -- Obsolete informational reference (is this intentional?): RFC 5337 (Obsoleted by RFC 6533) -- Obsolete informational reference (is this intentional?): RFC 5504 (Obsoleted by RFC 6530) -- Obsolete informational reference (is this intentional?): RFC 5721 (Obsoleted by RFC 6856) -- Duplicate reference: RFC5721, mentioned in 'RFC5721bis-POP3', was also mentioned in 'RFC5721'. -- Obsolete informational reference (is this intentional?): RFC 5721 (Obsoleted by RFC 6856) -- Obsolete informational reference (is this intentional?): RFC 5738 (Obsoleted by RFC 6855) -- Duplicate reference: RFC5738, mentioned in 'RFC5738bis-IMAP', was also mentioned in 'RFC5738'. -- Obsolete informational reference (is this intentional?): RFC 5738 (Obsoleted by RFC 6855) -- Obsolete informational reference (is this intentional?): RFC 5751 (Obsoleted by RFC 8551) -- Obsolete informational reference (is this intentional?): RFC 5825 (Obsoleted by RFC 6530) -- Obsolete informational reference (is this intentional?): RFC 5983 (Obsoleted by RFC 6783) -- Duplicate reference: RFC5983, mentioned in 'RFC5983bis-MailingList', was also mentioned in 'RFC5983'. -- Obsolete informational reference (is this intentional?): RFC 5983 (Obsoleted by RFC 6783) Summary: 1 error (**), 0 flaws (~~), 2 warnings (==), 25 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Email Address Internationalization J. Klensin 3 (EAI) Y. Ko 4 Internet-Draft October 25, 2011 5 Obsoletes: 4952, 5504, 5825 6 (if approved) 7 Intended status: Standards Track 8 Expires: April 27, 2012 10 Overview and Framework for Internationalized Email 11 draft-ietf-eai-frmwrk-4952bis-11 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 April 27, 2012. 45 Copyright Notice 47 Copyright (c) 2011 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 . . . . . . . . . . . . . . . . . . 5 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 . . . . . . . . . . . . . . . . 10 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 . . . . . . . . . . . . . . . . . . . . 15 97 10. User Interface and Configuration Issues . . . . . . . . . . . 15 98 10.1. Choices of Mailbox Names and Unicode Normalization . . . . 16 99 11. Additional Issues . . . . . . . . . . . . . . . . . . . . . . 17 100 11.1. Impact on URIs and IRIs . . . . . . . . . . . . . . . . . 17 101 11.2. Use of Email Addresses as Identifiers . . . . . . . . . . 17 102 11.3. Encoded Words, Signed Messages, and Downgrading . . . . . 18 103 11.4. Other Uses of Local Parts . . . . . . . . . . . . . . . . 18 104 11.5. Non-Standard Encapsulation Formats . . . . . . . . . . . . 19 105 12. Key Changes From the Experimental Protocols and Framework . . 19 106 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19 107 14. Security Considerations . . . . . . . . . . . . . . . . . . . 19 108 15. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21 109 16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21 110 16.1. Normative References . . . . . . . . . . . . . . . . . . . 21 111 16.2. 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 . . . . . . . . . . . . . . . 27 115 A.3. Changes between -02 and -03 . . . . . . . . . . . . . . . 28 116 A.4. Changes between -03 and -04 . . . . . . . . . . . . . . . 28 117 A.5. Changes between -04 and -05 . . . . . . . . . . . . . . . 29 118 A.6. Changes between -05 and -06 . . . . . . . . . . . . . . . 29 119 A.7. Changes between -06 and -07 . . . . . . . . . . . . . . . 29 120 A.8. Changes between -07 and -08 (after IETF Last Call) . . . . 29 121 A.9. Changes between -08 and -09 . . . . . . . . . . . . . . . 29 122 A.10. Changes between -09 and -10 . . . . . . . . . . . . . . . 29 123 A.11. Changes between -10 and -11 . . . . . . . . . . . . . . . 29 125 1. Introduction 127 Note in Draft and to RFC Editor: The keyword represented in this 128 document by "UTF8SMTPbis" (and in the XML source by &EAISMTPkeyword;) 129 is a placeholder. The actual keyword will be assigned when the 130 standards track SMTP extension in this series [RFC5336bis-SMTP] is 131 approved for publication and should be substituted here. This 132 paragraph should be treated as normative reference to that SMTP 133 extension draft, creating a reference hold until it is approved by 134 the IESG. The paragraph should be removed before RFC publication. 136 In order to use internationalized email addresses, we need to 137 internationalize both the domain part and the local part of email 138 addresses. The domain part of email addresses is already 139 internationalized [RFC5890], while the local part is not. Without 140 the extensions specified in this document, the mailbox name is 141 restricted to a subset of 7-bit ASCII [RFC5321]. Though MIME 142 [RFC2045] enables the transport of non-ASCII data, it does not 143 provide a mechanism for internationalized email addresses. In RFC 144 2047 [RFC2047], MIME defines an encoding mechanism for some specific 145 message header fields to accommodate non-ASCII data. However, it 146 does not permit the use of email addresses that include non-ASCII 147 characters. Without the extensions defined here, or some equivalent 148 set, the only way to incorporate non-ASCII characters in any part of 149 email addresses is to use RFC 2047 coding to embed them in what RFC 150 5322 [RFC5322] calls the "display name" (known as a "name phrase" or 151 by other terms elsewhere) of the relevant header fields. Information 152 coded into the display name is invisible in the message envelope and, 153 for many purposes, is not part of the address at all. 155 This document is a replacement for RFC 4952 [RFC4952]; it reflects 156 additional issues, shared terminology, and some architectural changes 157 identified since that document was published. It obsoletes that 158 document and RFCs 5504 [RFC5504] and 5825 [RFC5825], which are no 159 longer needed due to the changes discussed in Section 12. The RFC 160 Editor is requested to move all three of those documents to Historic. 162 The pronouns "he" and "she" are used interchangeably to indicate a 163 human of indeterminate gender. 165 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 166 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 167 document are to be interpreted as described in BCP 14, RFC 2119 168 [RFC2119]. Although this document is Informational, those 169 requirements are consistent with requirements specified in the 170 Standards Track documents in this set as described in Section 5. 172 2. Role of This Specification 174 This document presents the overview and framework for an approach to 175 the next stage of email internationalization. This new stage 176 requires not only internationalization of addresses and header 177 fields, but also associated transport and delivery models. A prior 178 version of this specification, RFC 4952 [RFC4952], also provided an 179 introduction to a series of experimental protocols [RFC5335] 180 [RFC5336] [RFC5337] [RFC5504] [RFC5721] [RFC5738] [RFC5825]. This 181 revised form provides overview and conceptual information for the 182 standards-track successors of a subset of those protocols. Details 183 of the documents and the relationships among them appear in Section 5 184 and a discussion of what was learned from the Experimental protocols 185 and their implementations appears in Section 6. 187 Taken together, these specifications provide the details for a way to 188 implement and support internationalized email. The document itself 189 describes how the various elements of email internationalization fit 190 together and the relationships among the primary specifications 191 associated with message transport, header formats, and handling. 193 This document, and others that comprise the collection described 194 above, assume a reasonable familiarity with the basic Internet 195 electronic mail specifications and terminology [RFC5321][RFC5322] and 196 the MIME [RFC2045] and 8BITMIME [RFC6152] ones as well. While not 197 strictly required to implement this specification, a general 198 familiarity with the terminology and functions of IDNA 199 [RFC5890][RFC5891] [RFC5892][RFC5893] [RFC5894] are also assumed. 201 3. Problem Statement 203 Internationalizing Domain Names in Applications (IDNA) [RFC5890] 204 permits internationalized domain names, but deployment has not yet 205 reached most users. One of the reasons for this is that we do not 206 yet have fully internationalized naming schemes. Domain names are 207 just one of the various names and identifiers that are required to be 208 internationalized. In many contexts, until more of those identifiers 209 are internationalized, internationalized domain names alone have 210 little value. 212 Email addresses are prime examples of why it is not good enough to 213 just internationalize the domain name. As most observers have 214 learned from experience, users strongly prefer email addresses that 215 resemble names or initials to those involving seemingly meaningless 216 strings of letters or numbers. Unless the entire email address can 217 use familiar characters and formats, users will perceive email as 218 being culturally unfriendly. If the names and initials used in email 219 addresses can be expressed in the native languages and writing 220 systems of the users, the Internet will be perceived as more natural, 221 especially by those whose native language is not written in a subset 222 of a Roman-derived script. 224 Internationalization of email addresses is not merely a matter of 225 changing the SMTP envelope; or of modifying the From, To, and Cc 226 header fields; or of permitting upgraded Mail User Agents (MUAs) to 227 decode a special coding and respond by displaying local characters. 228 To be perceived as usable, the addresses must be internationalized 229 and handled consistently in all of the contexts in which they occur. 230 This requirement has far-reaching implications: collections of 231 patches and workarounds are not adequate. Even if they were 232 adequate, a workaround-based approach may result in an assortment of 233 implementations with different sets of patches and workarounds having 234 been applied with consequent user confusion about what is actually 235 usable and supported. Instead, we need to build a fully 236 internationalized email environment, focusing on permitting efficient 237 communication among those who share a language and writing system. 238 That, in turn, implies changes to the mail header environment to 239 permit those header fields that are appropriately internationalized 240 to utilize the full range of Unicode characters, an SMTP Extension to 241 permit UTF-8 [RFC3629] [RFC5198] mail addressing and delivery of 242 those extended header fields, support for internationalization of 243 delivery and service notifications [RFC3461] [RFC3464], and (finally) 244 a requirement for support of the 8BITMIME SMTP Extension [RFC6152] so 245 that all of these can be transported through the mail system without 246 having to overcome the limitation that header fields do not have 247 content-transfer-encodings. 249 4. Terminology 251 This document assumes a reasonable understanding of the protocols and 252 terminology of the core email standards as documented in [RFC5321] 253 and [RFC5322]. 255 4.1. Mail User and Mail Transfer Agents 257 Much of the description in this document depends on the abstractions 258 of "Mail Transfer Agent" ("MTA") and "Mail User Agent" ("MUA"). 259 However, it is important to understand that those terms and the 260 underlying concepts postdate the design of the Internet's email 261 architecture and the application of the "protocols on the wire" 262 principle to it. That email architecture, as it has evolved, and 263 that "on the wire" principle have prevented any strong and 264 standardized distinctions about how MTAs and MUAs interact on a given 265 origin or destination host (or even whether they are separate). 267 However, the term "final delivery MTA" is used in this document in a 268 fashion equivalent to the term "delivery system" or "final delivery 269 system" of RFC 5321. This is the SMTP server that controls the 270 format of the local parts of addresses and is permitted to inspect 271 and interpret them. It receives messages from the network for 272 delivery to mailboxes or for other local processing, including any 273 forwarding or aliasing that changes envelope addresses, rather than 274 relaying. From the perspective of the network, any local delivery 275 arrangements such as saving to a message store, handoff to specific 276 message delivery programs or agents, and mechanisms for retrieving 277 messages are all "behind" the final delivery MTA and hence are not 278 part of the SMTP transport or delivery process. 280 4.2. Address Character Sets 282 In this document, an address is "all-ASCII", or just an "ASCII 283 address", if every character in the address is in the ASCII character 284 repertoire [ASCII]; an address is "non-ASCII", or an "i18n-address", 285 if any character is not in the ASCII character repertoire. Such 286 addresses MAY be restricted in other ways, but those restrictions are 287 not relevant to this definition. The term "all-ASCII" is also 288 applied to other protocol elements when the distinction is important, 289 with "non-ASCII" or "internationalized" as its opposite. 291 The umbrella term to describe the email address internationalization 292 specified by this document and its companion documents is 293 "UTF8SMTPbis". 294 [[anchor3: Note in Draft: Keyword to be changed before publication.]] 295 For example, an address permitted by this specification is referred 296 to as a "UTF8SMTPbis (compliant) address". 298 Please note that, according to the definitions given here, the set of 299 all "all-ASCII" addresses and the set of all "non-ASCII" addresses 300 are mutually exclusive. The set of all addresses permitted when 301 UTF8SMTPbis appears is the union of these two sets. 303 4.3. User Types 305 An "ASCII user" (i) exclusively uses email addresses that contain 306 ASCII characters only, and (ii) cannot generate recipient addresses 307 that contain non-ASCII characters. 309 An "i18mail user" has one or more non-ASCII email addresses, or is 310 able to generate recipient addresses that contain non-ASCII 311 characters. Such a user may have ASCII addresses too; if the user 312 has more than one email account and a corresponding address, or more 313 than one alias for the same address, he or she has some method to 314 choose which address to use on outgoing email. Note that under this 315 definition, it is not possible to tell from an ASCII address if the 316 owner of that address is an i18mail user or not. (A non-ASCII 317 address implies a belief that the owner of that address is an i18mail 318 user.) There is no such thing as an "i18mail message"; the term 319 applies only to users and their agents and capabilities. In 320 particular, the use of non-ASCII message content is an integral part 321 of the MIME specifications [RFC2045] and does not require these 322 extensions (although it is compatible with them). 324 4.4. Messages 326 A "message" is sent from one user (sender) using a particular email 327 address to one or more other recipient email addresses (often 328 referred to just as "users" or "recipient users"). 330 4.5. Mailing Lists 332 A "mailing list" is a mechanism whereby a message may be distributed 333 to multiple recipients by sending it to one recipient address. An 334 agent (typically not a human being) at that single address then 335 causes the message to be redistributed to the target recipients. 336 This agent sets the envelope return address of the redistributed 337 message to a different address from that of the original single 338 recipient message. Using a different envelope return address 339 (reverse-path) causes error (and other automatically generated) 340 messages to go to an error handling address. 342 Special provisions for managing mailing lists that might contain non- 343 ASCII addresses are discussed in a document that is specific to that 344 topic [RFC5983] [RFC5983bis-MailingList]. 346 4.6. Conventional Message and Internationalized Message 348 o A conventional message is one that does not use any extension 349 defined in the SMTP extension document [RFC5336] or in the 350 UTF8header specification [RFC5335], and is strictly conformant to 351 RFC 5322 [RFC5322]. 353 o An internationalized message is a message utilizing one or more of 354 the extensions defined in this set of specifications, so that it 355 is no longer conformant to the traditional specification of an 356 email message or its transport. 358 4.7. Undeliverable Messages, Notification, and Delivery Receipts 360 As specified in RFC 5321, a message that is undeliverable for some 361 reason is expected to result in notification to the sender. This can 362 occur in either of two ways. One, typically called "Rejection", 363 occurs when an SMTP server returns a reply code indicating a fatal 364 error (a "5yz" code) or persistently returns a temporary failure 365 error (a "4yz" code). The other involves accepting the message 366 during SMTP processing and then generating a message to the sender, 367 typically known as a "Non-delivery Notification" or "NDN". Current 368 practice often favors rejection over NDNs because of the reduced 369 likelihood that the generation of NDNs will be used as a spamming 370 technique. The latter, NDN, case is unavoidable if an intermediate 371 MTA accepts a message that is then rejected by the next-hop server. 373 A sender MAY also explicitly request message receipts [RFC3461] that 374 raise the same issues for these internationalization extensions as 375 NDNs. 377 5. Overview of the Approach and Document Plan 379 This set of specifications changes both SMTP and the character 380 encoding of email message headers to permit non-ASCII characters to 381 be represented directly. Each important component of the work is 382 described in a separate document. The document set, whose members 383 are described below, also contains informational documents whose 384 purpose is to provide implementation suggestions and guidance for the 385 protocols. 387 In addition to this document, the following documents make up this 388 specification and provide advice and context for it. 390 o SMTP extension. The SMTP extension document [RFC5336bis-SMTP] 391 provides an SMTP extension (as provided for in RFC 5321) for 392 internationalized addresses. 394 o Email message headers in UTF-8. The email message header document 395 [RFC5335bis-Hdrs] essentially updates RFC 5322 to permit some 396 information in email message headers to be expressed directly by 397 Unicode characters encoded in UTF-8 when the SMTP extension 398 described above is used. This document, possibly with one or more 399 supplemental ones, will also need to address the interactions with 400 MIME, including relationships between UTF8SMTPbis and internal 401 MIME headers and content types. 403 o Extensions to delivery status and notification handling to adapt 404 to internationalized addresses [RFC5337bis-DSN]. 406 o Forthcoming documents will specify extensions to the IMAP protocol 407 [RFC3501] to support internationalized message headers 408 [RFC5738bis-IMAP], Parallel extensions to the POP protocol 409 [RFC5721] [RFC5721bis-POP3], and some common properties of the two 410 [POPIMAP-downgrade]. 412 6. Review of Experimental Results 414 The key difference between this set of protocols and the experimental 415 set that preceded them [RFC5335] [RFC5336] [RFC5337] [RFC5504] 416 [RFC5721] [RFC5738] [RFC5825] is that the earlier group provided a 417 mechanism for in-transit downgrading of messages (described in detail 418 in RFC 5504). That mechanism permitted, and essentially required, 419 that each non-ASCII address be accompanied by an all-ASCII 420 equivalent. That, in turn, raised security concerns associated with 421 pairing of addresses that could not be authenticated. It also 422 introduced the first incompatible change to Internet mail addressing 423 in many years, raising concerns about interoperability issues if the 424 new address forms "leaked" into legacy email implementations. The WG 425 concluded that the advantages of in-transit downgrading, were it 426 feasible operationally, would be significant enough to overcome those 427 concerns. 429 That turned out not to be the case, with interoperability problems 430 among initial implementations. Prior to starting on the work that 431 led to this set of specifications, the WG concluded that the 432 combination of requirements and long-term implications of that 433 earlier model were too complex to be satisfactory and that work 434 should move ahead without it. 436 The other significant change to the protocols themselves is that the 437 UTF8SMTPbis keyword is now required as an SMTP client announcement if 438 the extension is needed; in the experimental version, only the server 439 announcement that an extended envelope and/or content were permitted 440 was necessary. 442 7. Overview of Protocol Extensions and Changes 444 7.1. SMTP Extension for Internationalized Email Address 446 An SMTP extension, "UTF8SMTPbis" is specified as follows: 448 o Permits the use of UTF-8 strings in email addresses, both local 449 parts and domain names. 451 o Permits the selective use of UTF-8 strings in email message 452 headers (see Section 7.2). 454 o Requires that the server advertise the 8BITMIME extension 455 [RFC6152] and that the client support 8-bit transmission so that 456 header information can be transmitted without using a special 457 content-transfer-encoding. 459 Some general principles affect the development decisions underlying 460 this work. 462 1. Email addresses enter subsystems (such as a user interface) that 463 may perform charset conversions or other encoding changes. When 464 the local part of the address includes characters outside the 465 ASCII character repertoire, use of ASCII-compatible encoding 466 (ACE) [RFC3492] [RFC5890] in the domain part is discouraged to 467 promote consistent processing of characters throughout the 468 address. 470 2. An SMTP relay MUST 472 * Either recognize the format explicitly, agreeing to do so via 473 an ESMTP option, or 475 * Reject the message or, if necessary, return a non-delivery 476 notification message, so that the sender can make another 477 plan. 479 3. If the message cannot be forwarded because the next-hop system 480 cannot accept the extension, it MUST be rejected or a non- 481 delivery message MUST be generated and sent. 483 4. In the interest of interoperability, charsets other than UTF-8 484 are prohibited in mail addresses and message headers being 485 transmitted over the Internet. There is no practical way to 486 identify multiple charsets properly with an extension similar to 487 this without introducing great complexity. 489 Conformance to the group of standards specified here for email 490 transport and delivery requires implementation of the SMTP Extension 491 specification and the UTF-8 Header specification. If the system 492 implements IMAP or POP, it MUST conform to the i18n IMAP 493 [RFC5738bis-IMAP] or POP [RFC5721bis-POP3] specifications 494 respectively. 496 7.2. Transmission of Email Header Fields in UTF-8 Encoding 498 There are many places in MUAs or in a user presentation in which 499 email addresses or domain names appear. Examples include the 500 conventional From, To, or Cc header fields; Message-ID and 501 In-Reply-To header fields that normally contain domain names (but 502 that may be a special case); and in message bodies. Each of these 503 must be examined from an internationalization perspective. The user 504 will expect to see mailbox and domain names in local characters, and 505 to see them consistently. If non-obvious encodings, such as 506 protocol-specific ASCII-Compatible Encoding (ACE) variants, are used, 507 the user will inevitably, if only occasionally, see them rather than 508 "native" characters and will find that discomfiting or astonishing. 509 Similarly, if different codings are used for mail transport and 510 message bodies, the user is particularly likely to be surprised, if 511 only as a consequence of the long-established "things leak" 512 principle. The only practical way to avoid these sources of 513 discomfort, in both the medium and the longer term, is to have the 514 encodings used in transport be as similar to the encodings used in 515 message headers and message bodies as possible. 517 When email local parts are internationalized, they SHOULD be 518 accompanied by arrangements for the message headers to be in the 519 fully internationalized form. That form SHOULD use UTF-8 rather than 520 ASCII as the base character set for the contents of header fields 521 (protocol elements such as the header field names themselves are 522 unchanged and remain entirely in ASCII). For transition purposes and 523 compatibility with legacy systems, this can done by extending the 524 traditional MIME encoding models for non-ASCII characters in headers 525 [RFC2045] [RFC2231], but even these should be based on UTF-8, rather 526 than other encodings, if at all possible [RFC6055]. However, the 527 target is fully internationalized message headers, as discussed in 528 [RFC5335bis-Hdrs] and not an extended and painful transition. 530 7.3. SMTP Service Extension for DSNs 532 The existing Draft Standard Delivery status notifications (DSNs) 533 specification [RFC3461] is limited to ASCII text in the machine 534 readable portions of the protocol. "International Delivery and 535 Disposition Notifications" [RFC5337bis-DSN] adds a new address type 536 for international email addresses so an original recipient address 537 with non-ASCII characters can be correctly preserved even after 538 downgrading. If an SMTP server advertises both the UTF8SMTPbis and 539 the DSN extension, that server MUST implement internationalized DSNs 540 including support for the ORCPT parameter specified in RFC 3461 541 [RFC3461]. 543 8. Downgrading before and after SMTP Transactions 545 An important issue with these extensions is how to handle 546 interactions between systems that support non-ASCII addresses and 547 legacy systems that expect ASCII. There is, of course, no problem 548 with ASCII-only systems sending to those that can handle 549 internationalized forms because the ASCII forms are just a proper 550 subset. But, when systems that support these extensions send mail, 551 they MAY include non-ASCII addresses for senders, receivers, or both 552 and might also provide non-ASCII header information other than 553 addresses. If the extension is not supported by the first-hop system 554 (SMTP server accessed by the Submission server acting as an SMTP 555 client), message originating systems SHOULD be prepared to either 556 send conventional envelopes and message headers or to return the 557 message to the originating user so the message may be manually 558 downgraded to the traditional form, possibly using encoded words 559 [RFC2047] in the message headers. Of course, such transformations 560 imply that the originating user or system must have ASCII-only 561 addresses available for all senders and recipients. Mechanisms by 562 which such addresses may be found or identified are outside the scope 563 of these specifications as are decisions about the design of 564 originating systems such as whether any required transformations are 565 made by the user, the originating MUA, or the Submission server. 567 A somewhat more complex situation arises when the first-hop system 568 supports these extensions but some subsequent server in the SMTP 569 transmission chain does not. It is important to note that most cases 570 of that situation with forward-pointing addresses will be the result 571 of configuration errors: especially if it hosts non-ASCII addresses, 572 a final delivery MTA that accepts these extensions SHOULD NOT be 573 configured with lower-preference MX hosts that do not. When the only 574 non-ASCII address being transmitted is backward-pointing (e.g., in an 575 SMTP MAIL command), recipient configuration can not help in general. 576 On the other hand, alternate, all-ASCII, addresses for senders are 577 those most likely to be authoritatively known by the submission 578 environment or the sender herself. Consequently, if an intermediate 579 SMTP relay that requires these extensions then discovers that the 580 next system in the chain does not support them, it will have little 581 choice other than to reject or return the message. 583 As discussed above, downgrading to an ASCII-only form may occur 584 before or during the initial message submission. It might also occur 585 after the delivery to the final delivery MTA in order to accommodate 586 messages stores or IMAP or POP servers or clients that have different 587 capabilities than the delivery MTA. These two cases are discussed in 588 the subsections below. 590 8.1. Downgrading before or during Message Submission 592 The IETF has traditionally avoided specifying the precise behavior of 593 MUAs to provide maximum flexibility in the associated user 594 interfaces. The SMTP standard [RFC5321], Section 6.4, gives wide 595 latitude to MUAs and Submission servers as to what might be supplied 596 by the user as long as the result conforms with "on the wire" 597 standards once it is injected into the public Internet. In that 598 tradition, the discussion in the remainder of Section 8 is provided 599 as general guidance rather than normative requirements. 601 Messages that require these extensions will sometimes be transferred 602 to a system that does not support these extensions; it is likely that 603 the most common cases will involve the combination of ASCII-only 604 forward-pointing addresses with a non-ASCII backward-pointing one. 605 Until the extensions described here have been universally implemented 606 in the Internet email environment, senders who prefer to use non- 607 ASCII addresses (or raw UTF-8 characters in header fields) even when 608 their intended recipients use and expect all-ASCII ones will need to 609 be especially careful about the error conditions that can arise, 610 especially if they are working in an environment in which non- 611 delivery messages (or other indications from submission servers) are 612 routinely dropped or ignored. 614 Perhaps obviously, the most convenient time to find an ASCII address 615 corresponding to an internationalized address is at the originating 616 MUA or closely-associated systems. This can occur either before the 617 message is sent or after the internationalized form of the message is 618 rejected. It is also the most convenient time to convert a message 619 from the internationalized form into conventional ASCII form or to 620 generate a non-delivery message to the sender if either is necessary. 621 At that point, the user has a full range of choices available, 622 including changing backward-pointing addresses, contacting the 623 intended recipient out of band for an alternate address, consulting 624 appropriate directories, arranging for translation of both addresses 625 and message content into a different language, and so on. While it 626 is natural to think of message downgrading as optimally being a 627 fully-automated process, we should not underestimate the capabilities 628 of a user of at least moderate intelligence who wishes to communicate 629 with another such user. 631 In this context, one can easily imagine modifications to message 632 submission servers (as described in RFC 4409 [RFC4409]) so that they 633 would perform downgrading operations or perhaps even upgrading ones. 634 Such operations would permit receiving messages with one or more of 635 the internationalization extensions discussed here and adapting the 636 outgoing message, as needed, to respond to the delivery or next-hop 637 environment the submission server encounters. 639 8.2. Downgrading or Other Processing After Final SMTP Delivery 641 When an email message is received by a final delivery MTA, it is 642 usually stored in some form. Then it is retrieved either by software 643 that reads the stored form directly or by client software via some 644 email retrieval mechanisms such as POP or IMAP. 646 The SMTP extension described in Section 7.1 provides protection only 647 in transport. It does not prevent MUAs and email retrieval 648 mechanisms that have not been upgraded to understand 649 internationalized addresses and UTF-8 message headers from accessing 650 stored internationalized emails. 652 Since the final delivery MTA (or, to be more specific, its 653 corresponding mail storage agent) cannot safely assume that agents 654 accessing email storage will always be capable of handling the 655 extensions proposed here, it MAY downgrade internationalized emails, 656 specially identify messages that utilize these extensions, or both. 657 If this is done, the final delivery MTA SHOULD include a mechanism to 658 preserve or recover the original internationalized forms without 659 information loss to support access by UTF8SMTPbis-aware agents. 661 9. Downgrading in Transit 663 The base SMTP specification (Section 2.3.11 of RFC 5321 [RFC5321]) 664 states that "due to a long history of problems when intermediate 665 hosts have attempted to optimize transport by modifying them, the 666 local-part MUST be interpreted and assigned semantics only by the 667 host specified in the domain part of the address". This is not a new 668 requirement; equivalent statements appeared in specifications in 2001 669 [RFC2821] and even in 1989 [RFC1123]. 671 Adherence to this rule means that a downgrade mechanism that 672 transforms the local-part of an email address cannot be utilized in 673 transit. It can only be applied at the endpoints, specifically by 674 the MUA or submission server or by the final delivery MTA. 676 One of the reasons for this rule has to do with legacy email systems 677 that embed mail routing information in the local-part of the address 678 field. Transforming the email address destroys such routing 679 information. There is no way a server other than the final delivery 680 server can know, for example, whether the local-part of 681 user%foo@example.com is a route ("user" is reached via "foo") or 682 simply a local address. 684 10. User Interface and Configuration Issues 686 Internationalization of addresses and message headers, especially in 687 combination with variations on character coding that are inherent to 688 Unicode, may make careful choices of addresses and careful 689 configuration of servers and DNS records even more important than 690 they are for traditional Internet email. It is likely that, as 691 experience develops with the use of these protocols, it will be 692 desirable to produce one or more additional documents that offer 693 guidance for configuration and interfaces. A document that discusses 694 issues with mail user agents (MUAs), especially with regard to 695 downgrading, is expected to be developed in the EAI Working Group. 696 The subsections below address some other issues. 698 10.1. Choices of Mailbox Names and Unicode Normalization 700 It has long been the case that the email syntax permits choices about 701 mailbox names that are unwise in practice if one actually intends the 702 mailboxes to be accessible to a broad range of senders. The most- 703 often-cited examples involve the use of case-sensitivity and tricky 704 quoting of embedded characters in mailbox local parts. These 705 deliberately-unusual constructions are permitted by the protocols and 706 servers are expected to support them. Although they can provide 707 value in special cases, taking advantage of them is almost always bad 708 practice unless the intent is to create some form of security by 709 obscurity. 711 In the absence of these extensions, SMTP clients and servers are 712 constrained to using only those addresses permitted by RFC 5321. The 713 local parts of those addresses MAY be made up of any ASCII characters 714 except the control characters that 5321 prohibits, although some of 715 them MUST be quoted as specified there. It is notable in an 716 internationalization context that there is a long history on some 717 systems of using overstruck ASCII characters (a character, a 718 backspace, and another character) within a quoted string to 719 approximate non-ASCII characters. This form of internationalization 720 was permitted by RFC 821 [RFC0821] but is prohibited by RFC 5321 721 because it requires a backspace character (a prohibited C0 control). 722 Because RFC 5321 (and its predecessor, RFC 2821) prohibit the use of 723 this character in ASCII mailbox names and it is even more problematic 724 (for canonicalization and normalization reasons) in non-ASCII 725 strings, backspace MUST NOT appear in UTF8SMTPbis mailbox names. 727 For the particular case of mailbox names that contain non-ASCII 728 characters in the local part, domain part, or both, special attention 729 MUST be paid to Unicode normalization [Unicode-UAX15], in part 730 because Unicode strings may be normalized by other processes 731 independent of what a mail protocol specifies (this is exactly 732 analogous to what may happen with quoting and dequoting in 733 traditional addresses). Consequently, the following principles are 734 offered as advice to those who are selecting names for mailboxes: 736 o In general, it is wise to support addresses in Normalized form, 737 using at least Normalization Form NFC. Except in circumstances in 738 which NFKC would map characters together that the parties 739 responsible for the destination mail server would prefer to be 740 kept distinguishable, supporting the NFKC-conformant form would 741 yield even more predictable behavior for the typical user. 743 o It will usually be wise to support other forms of the same local- 744 part string, either as aliases or by normalization of strings 745 reaching the delivery server: the sender should not be depended 746 upon to send the strings in normalized form. 748 o Stated differently and in more specific terms, the rules of the 749 protocol for local-part strings essentially provide that: 751 * Unnormalized strings are valid, but sufficiently bad practice 752 that they may not work reliably on a global basis. Servers 753 should not depend on clients to send normalized forms but 754 should be aware that procedures on client machines outside the 755 control of the MUA may cause normalized strings to be sent 756 regardless of user intent. 758 * C0 (and presumably C1) controls (see The Unicode Standard 759 [Unicode]) are prohibited, the first in RFC 5321 and the second 760 by an obvious extension from it [RFC5198]. 762 * Other kinds of punctuation, spaces, etc., are risky practice. 763 Perhaps they will work, and SMTP receiver code is required to 764 handle them without severe errors (even if such strings are not 765 accepted in addresses to be delivered on that server), but 766 creating dependencies on them in mailbox names that are chosen 767 is usually a bad practice and may lead to interoperability 768 problems. 770 11. Additional Issues 772 This section identifies issues that are not covered, or not covered 773 comprehensively, as part of this set of specifications, but that will 774 require ongoing review as part of deployment of email address and 775 header internationalization. 777 11.1. Impact on URIs and IRIs 779 The mailto: schema [RFC6068], and the discussion of it in the 780 Internationalized Resource Identifier (IRI) specification [RFC3987] 781 may need to be modified when this work is completed and standardized. 783 11.2. Use of Email Addresses as Identifiers 785 There are a number of places in contemporary Internet usage in which 786 email addresses are used as identifiers for individuals, including as 787 identifiers to Web servers supporting some electronic commerce sites 788 and in some X.509 certificates [RFC5280]. These documents do not 789 address those uses, but it is reasonable to expect that some 790 difficulties will be encountered when internationalized addresses are 791 first used in those contexts, many of which cannot even handle the 792 full range of addresses permitted today. 794 11.3. Encoded Words, Signed Messages, and Downgrading 796 One particular characteristic of the email format is its persistency: 797 MUAs are expected to handle messages that were originally sent 798 decades ago and not just those delivered seconds ago. As such, MUAs 799 and mail filtering software, such as that specified in Sieve 800 [RFC5228], will need to continue to accept and decode header fields 801 that use the "encoded word" mechanism [RFC2047] to accommodate non- 802 ASCII characters in some header fields. While extensions to both 803 POP3 [RFC1939] and IMAP [RFC3501] have been defined that include 804 automatic upgrading of messages that carry non-ASCII information in 805 encoded form -- including RFC 2047 decoding -- of messages by the 806 POP3 [RFC5721bis-POP3] or IMAP [RFC5738bis-IMAP] server, there are 807 message structures and MIME content-types for which that cannot be 808 done or where the change would have unacceptable side effects. 810 For example, message parts that are cryptographically signed, using 811 e.g., S/MIME [RFC5751] or Pretty Good Privacy (PGP) [RFC3156], cannot 812 be upgraded from the RFC 2047 form to normal UTF-8 characters without 813 breaking the signature. Similarly, message parts that are encrypted 814 may contain, when decrypted, header fields that use the RFC 2047 815 encoding; such messages cannot be 'fully' upgraded without access to 816 cryptographic keys. 818 Similar issues may arise if messages are signed and then subsequently 819 downgraded, e.g., as discussed in Section 8.1, and then an attempt is 820 made to upgrade them to the original form and then verify the 821 signatures. Even the very subtle changes that may result from 822 algorithms to downgrade and then upgrade again may be sufficient to 823 invalidate the signatures if they impact either the primary or MIME 824 bodypart headers. When signatures are present, downgrading MUST be 825 performed with extreme care if at all. 827 11.4. Other Uses of Local Parts 829 Local parts are sometimes used to construct domain labels, e.g., the 830 local part "user" in the address user@domain.example could be 831 converted into a vanity host user.domain.example with its Web space 832 at and the catchall addresses 833 any.thing.goes@user.domain.example. 835 Such schemes are obviously limited by, among other things, the SMTP 836 rules for domain names, and will not work without further 837 restrictions for other local parts such as the 838 specified in [RFC5335bis-Hdrs]. Whether those limitations are 839 relevant to these specifications is an open question. It may be 840 simply another case of the considerable flexibility accorded to 841 delivery MTAs in determining the mailbox names they will accept and 842 how they are interpreted. 844 11.5. Non-Standard Encapsulation Formats 846 Some applications use formats similar to the application/mbox format 847 defined in [RFC4155] instead of the message/digest form described in 848 RFC 2046, Section 5.1.5 [RFC2046] to transfer multiple messages as 849 single units. Insofar as such applications assume that all stored 850 messages use the message/rfc822 format described in RFC 2046, Section 851 5.2.1 [RFC2046] with ASCII message headers, they are not ready for 852 the extensions specified in this series of documents and special 853 measures may be needed to properly detect and process them. 855 12. Key Changes From the Experimental Protocols and Framework 857 The original framework for internationalized email addresses and 858 headers was described in RFC 4952 and a subsequent set of 859 experimental protocol documents. Those relationships are described 860 in Section 3. The key architectural difference between the 861 experimental specifications and this newer set is that the earlier 862 specifications supported in-transit downgrading. Those mechanisms 863 included the definition of syntax and functions to support passing 864 alternate, all-ASCII, addresses with the non-ASCII ones as well as 865 special headers to indicate the downgraded status of messages. Those 866 features were eliminated after experimentation indicated that they 867 were more complex and less necessary than had been assumed earlier. 868 Those issues are described in more detail in Section 6 and Section 9. 870 13. IANA Considerations 872 This overview description and framework document does not contemplate 873 any IANA registrations or other actions. Some of the documents in 874 the group have their own IANA considerations sections and 875 requirements. 877 14. Security Considerations 879 Any expansion of permitted characters and encoding forms in email 880 addresses raises some risks. There have been discussions on so 881 called "IDN-spoofing" or "IDN homograph attacks". These attacks 882 allow an attacker (or "phisher") to spoof the domain or URLs of 883 businesses. The same kind of attack is also possible on the local 884 part of internationalized email addresses. It should be noted that 885 the proposed fix involving forcing all displayed elements into 886 normalized lower-case works for domain names in URLs, but not for 887 email local parts since those are case sensitive. 889 Since email addresses are often transcribed from business cards and 890 notes on paper, they are subject to problems arising from confusable 891 characters (see [RFC4690]). These problems are somewhat reduced if 892 the domain associated with the mailbox is unambiguous and supports a 893 relatively small number of mailboxes whose names follow local system 894 conventions. They are increased with very large mail systems in 895 which users can freely select their own addresses. 897 The internationalization of email addresses and message headers must 898 not leave the Internet less secure than it is without the required 899 extensions. The requirements and mechanisms documented in this set 900 of specifications do not, in general, raise any new security issues. 902 They do require a review of issues associated with confusable 903 characters -- a topic that is being explored thoroughly elsewhere 904 (see, e.g., RFC 4690 [RFC4690]) -- and, potentially, some issues with 905 UTF-8 normalization, discussed in RFC 3629 [RFC3629], and other 906 transformations. Normalization and other issues associated with 907 transformations and standard forms are also part of the subject of 908 work described elsewhere [RFC5198] [RFC5893] [RFC6055]. 910 Some issues specifically related to internationalized addresses and 911 message headers are discussed in more detail in the other documents 912 in this set. However, in particular, caution should be taken that 913 any "downgrading" mechanism, or use of downgraded addresses, does not 914 inappropriately assume authenticated bindings between the 915 internationalized and ASCII addresses. This potential problem can be 916 mitigated somewhat by enforcing the expectation that most or all such 917 transformations will be performed prior to final delivery by systems 918 that are presumed to be under the administrative control of the 919 sending user (as opposed to being performed in transit by entities 920 that are not under the administrative control of the sending user). 922 The new UTF-8 header and message formats might also raise, or 923 aggravate, another known issue. If the model creates new forms of an 924 'invalid' or 'malformed' message, then a new email attack is created: 925 in an effort to be robust, some or most agents will accept such 926 message and interpret them as if they were well-formed. If a filter 927 interprets such a message differently than the MUA used by the 928 recipient, then it may be possible to create a message that appears 929 acceptable under the filter's interpretation but that should be 930 rejected under the interpretation given to it by that MUA. Such 931 attacks already exist for existing messages and encoding layers, 932 e.g., invalid MIME syntax, invalid HTML markup, and invalid coding of 933 particular image types. 935 In addition, email addresses are used in many contexts other than 936 sending mail, such as for identifiers under various circumstances 937 (see Section 11.2). Each of those contexts will need to be 938 evaluated, in turn, to determine whether the use of non-ASCII forms 939 is appropriate and what particular issues they raise. 941 This work will clearly affect any systems or mechanisms that are 942 dependent on digital signatures or similar integrity protection for 943 email message headers (see also the discussion in Section 11.3). 944 Many conventional uses of PGP and S/MIME are not affected since they 945 are used to sign body parts but not message headers. On the other 946 hand, the developing work on domain keys identified mail (DKIM) 947 [RFC5863] will eventually need to consider this work and vice versa: 948 while this specification does not address or solve the issues raised 949 by DKIM and other signed header mechanisms, the issues will have to 950 be coordinated and resolved eventually if the two sets of protocols 951 are to co-exist. In addition, to the degree to which email addresses 952 appear in PKI (Public Key Infrastructure) certificates [RFC5280], 953 standards addressing such certificates will need to be upgraded to 954 address these internationalized addresses. Those upgrades will need 955 to address questions of spoofing by look-alikes of the addresses 956 themselves. 958 15. Acknowledgments 960 This document is an update to, and derived from, RFC 4952. This 961 document would have been impossible without the work and 962 contributions acknowledged in it. The present document benefited 963 significantly from discussions in the EAI WG and elsewhere after RFC 964 4952 was published, especially discussions about the experimental 965 versions of other documents in the internationalized email 966 collection, and from RFC errata on RFC 4952 itself. 968 Special thanks are due to Ernie Dainow for careful reviews and 969 suggested text in this version and to several IESG members for a 970 careful review and specific suggestions. 972 16. References 974 16.1. Normative References 976 [ASCII] American National Standards Institute 977 (formerly United States of America 978 Standards Institute), "USA Code for 979 Information Interchange", ANSI X3.4-1968, 980 1968. 982 ANSI X3.4-1968 has been replaced by newer 983 versions with slight modifications, but the 984 1968 version remains definitive for the 985 Internet. 987 [RFC2119] Bradner, S., "Key words for use in RFCs to 988 Indicate Requirement Levels", BCP 14, 989 RFC 2119, March 1997. 991 [RFC3629] Yergeau, F., "UTF-8, a transformation 992 format of ISO 10646", STD 63, RFC 3629, 993 November 2003. 995 [RFC5321] Klensin, J., "Simple Mail Transfer 996 Protocol", RFC 5321, October 2008. 998 [RFC5322] Resnick, P., Ed., "Internet Message 999 Format", RFC 5322, October 2008. 1001 [RFC5335bis-Hdrs] Yang, A., Steele, S., and N. Freed, 1002 "Internationalized Email Headers", 1003 September 2011, . 1007 [RFC5336bis-SMTP] Yao, J. and W. Mao, "SMTP Extension for 1008 Internationalized Email Address", 1009 August 2011, . 1012 [RFC5337bis-DSN] Hansen, T., Newman, C., and A. Melnikov, 1013 "Internationalized Delivery Status and 1014 Disposition Notifications", October 2011, < 1015 https://datatracker.ietf.org/doc/ 1016 draft-ietf-eai-rfc5337bis-dsn/>. 1018 [RFC5890] Klensin, J., "Internationalized Domain 1019 Names for Applications (IDNA): Definitions 1020 and Document Framework", RFC 5890, 1021 August 2010. 1023 [RFC6152] Klensin, J., Freed, N., Rose, M., and D. 1024 Crocker, "SMTP Service Extension for 8-bit 1025 MIME Transport", STD 71, RFC 6152, 1026 March 2011. 1028 16.2. Informative References 1030 [POPIMAP-downgrade] Fujiwara, K., "Post-delivery Message 1031 Downgrading for Internationalized Email 1032 Messages", Work in Progress, July 2011, . 1036 [RFC0821] Postel, J., "Simple Mail Transfer 1037 Protocol", STD 10, RFC 821, August 1982. 1039 [RFC1123] Braden, R., "Requirements for Internet 1040 Hosts - Application and Support", STD 3, 1041 RFC 1123, October 1989. 1043 [RFC1939] Myers, J. and M. Rose, "Post Office 1044 Protocol - Version 3", STD 53, RFC 1939, 1045 May 1996. 1047 [RFC2045] Freed, N. and N. Borenstein, "Multipurpose 1048 Internet Mail Extensions (MIME) Part One: 1049 Format of Internet Message Bodies", 1050 RFC 2045, November 1996. 1052 [RFC2046] Freed, N. and N. Borenstein, "Multipurpose 1053 Internet Mail Extensions (MIME) Part Two: 1054 Media Types", RFC 2046, November 1996. 1056 [RFC2047] Moore, K., "MIME (Multipurpose Internet 1057 Mail Extensions) Part Three: Message Header 1058 Extensions for Non-ASCII Text", RFC 2047, 1059 November 1996. 1061 [RFC2231] Freed, N. and K. Moore, "MIME Parameter 1062 Value and Encoded Word Extensions: Characte 1063 r Sets, Languages, and Continuations", 1064 RFC 2231, November 1997. 1066 [RFC2821] Klensin, J., "Simple Mail Transfer 1067 Protocol", RFC 2821, April 2001. 1069 [RFC3156] Elkins, M., Del Torto, D., Levien, R., and 1070 T. Roessler, "MIME Security with OpenPGP", 1071 RFC 3156, August 2001. 1073 [RFC3461] Moore, K., "Simple Mail Transfer Protocol 1074 (SMTP) Service Extension for Delivery 1075 Status Notifications (DSNs)", RFC 3461, 1076 January 2003. 1078 [RFC3464] Moore, K. and G. Vaudreuil, "An Extensible 1079 Message Format for Delivery Status 1080 Notifications", RFC 3464, January 2003. 1082 [RFC3492] Costello, A., "Punycode: A Bootstring 1083 encoding of Unicode for Internationalized 1084 Domain Names in Applications (IDNA)", 1085 RFC 3492, March 2003. 1087 [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS 1088 PROTOCOL - VERSION 4rev1", RFC 3501, 1089 March 2003. 1091 [RFC3987] Duerst, M. and M. Suignard, 1092 "Internationalized Resource Identifiers 1093 (IRIs)", RFC 3987, January 2005. 1095 [RFC4155] Hall, E., "The application/mbox Media 1096 Type", RFC 4155, September 2005. 1098 [RFC4409] Gellens, R. and J. Klensin, "Message 1099 Submission for Mail", RFC 4409, April 2006. 1101 [RFC4690] Klensin, J., Faltstrom, P., Karp, C., and 1102 IAB, "Review and Recommendations for 1103 Internationalized Domain Names (IDNs)", 1104 RFC 4690, September 2006. 1106 [RFC4952] Klensin, J. and Y. Ko, "Overview and 1107 Framework for Internationalized Email", 1108 RFC 4952, July 2007. 1110 [RFC5198] Klensin, J. and M. Padlipsky, "Unicode 1111 Format for Network Interchange", RFC 5198, 1112 March 2008. 1114 [RFC5228] Guenther, P. and T. Showalter, "Sieve: An 1115 Email Filtering Language", RFC 5228, 1116 January 2008. 1118 [RFC5280] Cooper, D., Santesson, S., Farrell, S., 1119 Boeyen, S., Housley, R., and W. Polk, 1120 "Internet X.509 Public Key Infrastructure 1121 Certificate and Certificate Revocation List 1122 (CRL) Profile", RFC 5280, May 2008. 1124 [RFC5335] Abel, Y., "Internationalized Email 1125 Headers", RFC 5335, September 2008. 1127 [RFC5336] Yao, J. and W. Mao, "SMTP Extension for 1128 Internationalized Email Addresses", 1129 RFC 5336, September 2008. 1131 [RFC5337] Newman, C. and A. Melnikov, 1132 "Internationalized Delivery Status and 1133 Disposition Notifications", RFC 5337, 1134 September 2008. 1136 [RFC5504] Fujiwara, K. and Y. Yoneya, "Downgrading 1137 Mechanism for Email Address 1138 Internationalization", RFC 5504, 1139 March 2009. 1141 [RFC5721] Gellens, R. and C. Newman, "POP3 Support 1142 for UTF-8", RFC 5721, February 2010. 1144 [RFC5721bis-POP3] Gellens, R., Yao, J., and K. Fujiwara, 1145 "POP3 Support for UTF-8", Work in Progress, 1146 July 2011, . 1149 [RFC5738] Resnick, P. and C. Newman, "IMAP Support 1150 for UTF-8", RFC 5738, March 2010. 1152 [RFC5738bis-IMAP] Resnick, P., Newman, C., and S. Shen, "IMAP 1153 Support for UTF-8", Work in Progress, 1154 July 2011, . 1157 [RFC5751] Ramsdell, B. and S. Turner, "Secure/ 1158 Multipurpose Internet Mail Extensions 1159 (S/MIME) Version 3.2 Message 1160 Specification", RFC 5751, January 2010. 1162 [RFC5825] Fujiwara, K. and B. Leiba, "Displaying 1163 Downgraded Messages for Email Address 1164 Internationalization", RFC 5825, 1165 April 2010. 1167 [RFC5863] Hansen, T., Siegel, E., Hallam-Baker, P., 1168 and D. Crocker, "DomainKeys Identified Mail 1169 (DKIM) Development, Deployment, and 1170 Operations", RFC 5863, May 2010. 1172 [RFC5891] Klensin, J., "Internationalized Domain 1173 Names in Applications (IDNA): Protocol", 1174 RFC 5891, August 2010. 1176 [RFC5892] Faltstrom, P., "The Unicode Code Points and 1177 Internationalized Domain Names for 1178 Applications (IDNA)", RFC 5892, 1179 August 2010. 1181 [RFC5893] Alvestrand, H. and C. Karp, "Right-to-Left 1182 Scripts for Internationalized Domain Names 1183 for Applications (IDNA)", RFC 5893, 1184 August 2010. 1186 [RFC5894] Klensin, J., "Internationalized Domain 1187 Names for Applications (IDNA): Background, 1188 Explanation, and Rationale", RFC 5894, 1189 August 2010. 1191 [RFC5983] Gellens, R., "Mailing Lists and 1192 Internationalized Email Addresses", 1193 RFC 5983, October 2010. 1195 [RFC5983bis-MailingList] "Mailing Lists and Internationalized Email 1196 Addresses", Unwritten waiting for I-D, 1197 2011. 1199 [RFC6055] Thaler, D., Klensin, J., and S. Cheshire, 1200 "IAB Thoughts on Encodings for 1201 Internationalized Domain Names", RFC 6055, 1202 February 2011. 1204 [RFC6068] Duerst, M., Masinter, L., and J. Zawinski, 1205 "The 'mailto' URI Scheme", RFC 6068, 1206 October 2010. 1208 [Unicode] The Unicode Consortium. The Unicode 1209 Standard, Version 5.2.0, defined by:, "The 1210 Unicode Standard, Version 5.2.0", (Mountain 1211 View, CA: The Unicode Consortium, 1212 2009. ISBN 978-1-936213-00-9)., . 1215 [Unicode-UAX15] The Unicode Consortium, "Unicode Standard 1216 Annex #15: Unicode Normalization Forms", 1217 March 2008, 1218 . 1220 Appendix A. Change Log 1222 [[RFC Editor: Please remove this section prior to publication.]] 1224 A.1. Changes between -00 and -01 1226 o Because there has been no feedback on the mailing list, updated 1227 the various questions to refer to this version as well. 1229 o Reflected RFC Editor erratum #1507 by correcting terminology for 1230 headers and header fields and distinguishing between "message 1231 headers" and different sorts of headers (e.g., the MIME ones). 1233 A.2. Changes between -01 and -02 1235 Note that section numbers in the list that follows may refer to -01 1236 and not -02. 1238 o Discussion of RFC 5825 ("downgraded display") has been removed per 1239 the earlier note and on-list discussion. Any needed discussion 1240 about reconstructed messages will need to appear in the IMAP and 1241 POP documents. However, the introductory material has been 1242 reworded to permit keeping 5504 and 5825 on the list there, 1243 without which the back chain would not be complete. For 1244 consistency with this change, 5504 and 5825 have been added to the 1245 "Obsoletes" list (as far as I know, an Informational spec can 1246 obsolete or update Experimental ones, so no downref problem here 1247 --JcK). 1249 o Reference to alternate addresses dropped from (former) Section 1250 7.1. 1252 o Reference to RFC 5504 added to (former) Section 8 for 1253 completeness. 1255 o Ernie's draft comments added (with some minor edits) to replace 1256 the placeholder in (former) Section 9 ("Downgrading in Transit"). 1257 It is intended to capture at least an introduction the earlier 1258 discussions of algorithmic downgrading generally and ACE/Punycode 1259 transformations in particular. Anyone who is unhappy with it 1260 should say so and propose alternate text. RSN. 1262 o In the interest of clarity and consistency with the terminology in 1263 Section 4.1, all uses of "final delivery SMTP server" and "final 1264 delivery server" have been changed to "final delivery MTA". 1266 o Placeholder at the end of Section 2 has been removed and the text 1267 revised to promise less. The "Document Plan" (Section 5) has been 1268 revised accordingly. We need to discuss this at IETF 78 if not 1269 sooner. 1271 o Sections 5 and 6 have been collapsed into one -- there wasn't 1272 enough left in the former Section 5 to justify a separate section. 1274 o Former Section 11.1 has been dropped and the DSN document moved up 1275 into the "Document Plan" as suggested earlier. 1277 o Section 12, "Experimental Targets", has been removed. 1279 o Updated references for the new version EAI documents and added 1280 placeholders for all of the known remaining drafts that will 1281 become part of the core EAI series but that have not been written. 1283 o Inserted an additional clarification about the relationship of 1284 these extensions to non-ASCII messages. 1286 o Changed some normative/informative reference classifications based 1287 on review of the new text. 1289 o Removed references to the pre-EAI documents that were cited for 1290 historical context in 4952. 1292 o Got rid of a remaining pointer to address downgrading in the 1293 discussion of an updated MAILTO URI. 1295 o Minor additional editorial cleanups and tuning. 1297 A.3. Changes between -02 and -03 1299 o Inserted paragraph clarifying the status of the UTF8SMTPbis 1300 keyword as a result of discussion prior to and during IETF 79. 1302 o Adjusted some references including adding an explicit citation of 1303 RFC 821. 1305 o Removed the discussion of the experimental work from an inline 1306 aside to a separate section, Section 6. 1308 o Rewrote the discussion of configuration errors in MX setups to 1309 make it clear that they are an issue with forward-pointing 1310 addresses only and improved the discussion of backward-pointing 1311 addresses. 1313 o Removed some now-obsolete placeholder notes and resolved the 1314 remaining one to a dangling reference. 1316 A.4. Changes between -03 and -04 1318 o Several minor editorial changes. 1320 o Added a discussion of the relationship to the base email, MIME, 1321 and IDNA specifications. 1323 A.5. Changes between -04 and -05 1325 o Several more minor editorial changes. 1327 A.6. Changes between -05 and -06 1329 o Corrections to more precisely reflect RFC 2119 language 1330 requirements and closely-related issues.. 1332 A.7. Changes between -06 and -07 1334 o Added a new section (now Section 12) to explicitly discuss the 1335 changes from the previous version. 1337 o Removed the discussion of LMTP from Section 11; it is more 1338 appropriately placed in the SMTP Extension document (5336bis). 1340 A.8. Changes between -07 and -08 (after IETF Last Call) 1342 o Modified Section 7.2 to make the last paragraph less tentative and 1343 more clear. 1345 o Modified Section 8.1 to add an introductory paragraph that 1346 clarifies what the IETF does and does not specify about email 1347 protocols. 1349 A.9. Changes between -08 and -09 1351 This version incorporates responses to a last set of public comments 1352 and changes made in response to IESG discussion and comments as part 1353 of the balloting process. 1355 o Many small editorial changes made at IESG request. 1357 o Several other small editorial corrections, removal of uncited 1358 reference to LMTP, added a few citations for clarity. 1360 A.10. Changes between -09 and -10 1362 This version contains additional small editorial changes resulting 1363 from IESG comments and review of -09 changes. Some more significant 1364 clarifications appear in Section 10.1 1366 A.11. Changes between -10 and -11 1368 While -10 was approved for publication by the IESG (after IETF Last 1369 Call) in September 2010, the document then went into a reference hold 1370 in the RFC Editor queue. Issued identified during and after Last 1371 Call for the other three core EAI documents (5335bis, 5336bis, and 1372 5337bis) required reopening this document and making some minor 1373 additional changes. 1375 o Reworded the descriptions of the POP, IMAP, and mailing list 1376 documents and moved them to Informative. Notes in the XML of 1377 earier versions of this draft indicate that they were listed as 1378 Normative merely as a temporary convenience. Examination and 1379 reclassification of them apparently slipped through the cracks. 1381 o Reclassified the document to standards track to eliminate 1382 normative reference problems from other EAI documents. 1384 o References, other than the two Unicode ones, have been updated for 1385 the convenience of reviewers and the RFC Editor. A note has been 1386 inserted into the XML requesting that the RFC Editor update the 1387 Unicode references to be current at the time of publication. 1389 o Explicitly notes status of documents obsoleted by this one and 1390 moves them to Historic. 1392 o Updated author contact information. 1394 Authors' Addresses 1396 John C KLENSIN 1397 1770 Massachusetts Ave, #322 1398 Cambridge, MA 02140 1399 USA 1401 Phone: +1 617 491 5735 1402 EMail: john-ietf@jck.com 1404 YangWoo KO 1405 112-202 Malgeunachim APT. Nae-dong 1406 Seo-gu, Daejeon 302-981 1407 Republic of Korea 1409 EMail: yangwooko@gmail.com