idnits 2.17.1 draft-ietf-eai-frmwrk-4952bis-06.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 'Obsoletes: ' line in the draft header should list only the _numbers_ of the RFCs which will be obsoleted by this document (if approved); it should not include the word 'RFC' in the list. -- 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 RFC5825, 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 (August 23, 2010) is 4989 days in the past. Is this intentional? Checking references for intended status: Informational ---------------------------------------------------------------------------- ** Obsolete normative reference: RFC 1652 (Obsoleted by RFC 6152) ** Obsolete normative reference: RFC 5336 (Obsoleted by RFC 6531) ** Obsolete normative reference: RFC 5337 (Obsoleted by RFC 6533) ** Obsolete normative reference: RFC 5721 (Obsoleted by RFC 6856) ** Obsolete normative reference: RFC 5738 (Obsoleted by RFC 6855) -- Obsolete informational reference (is this intentional?): RFC 821 (Obsoleted by RFC 2821) -- Obsolete informational reference (is this intentional?): RFC 2368 (Obsoleted by RFC 6068) -- 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 3851 (Obsoleted by RFC 5751) -- 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) -- Duplicate reference: RFC5337, mentioned in 'RFC5337', was also mentioned in 'RFC5337bis-DSN'. -- Obsolete informational reference (is this intentional?): RFC 5337 (Obsoleted by RFC 6533) -- Obsolete informational reference (is this intentional?): RFC 5504 (Obsoleted by RFC 6530) -- Duplicate reference: RFC5721, mentioned in 'RFC5721', was also mentioned in 'RFC5721bis-POP3'. -- Obsolete informational reference (is this intentional?): RFC 5721 (Obsoleted by RFC 6856) -- Duplicate reference: RFC5738, mentioned in 'RFC5738', was also mentioned in 'RFC5738bis-IMAP'. -- Obsolete informational reference (is this intentional?): RFC 5738 (Obsoleted by RFC 6855) -- Obsolete informational reference (is this intentional?): RFC 5825 (Obsoleted by RFC 6530) Summary: 5 errors (**), 0 flaws (~~), 3 warnings (==), 22 comments (--). 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 23, 2010 7 Intended status: Informational 8 Expires: February 24, 2011 10 Overview and Framework for Internationalized Email 11 draft-ietf-eai-frmwrk-4952bis-06 13 Abstract 15 Full use of electronic mail throughout the world requires that 16 (subject to other constraints) people be able to use close variations 17 on their own names (written correctly in their own languages and 18 scripts) as mailbox names in email addresses. This document 19 introduces a series of specifications that define mechanisms and 20 protocol extensions needed to fully support internationalized email 21 addresses. These changes include an SMTP extension and extension of 22 email header syntax to accommodate UTF-8 data. The document set also 23 includes discussion of key assumptions and issues in deploying fully 24 internationalized email. This document is an update of RFC 4952; it 25 reflects additional issues identified since that document was 26 published. 28 Status of This Memo 30 This Internet-Draft is submitted in full conformance with the 31 provisions of BCP 78 and BCP 79. 33 Internet-Drafts are working documents of the Internet Engineering 34 Task Force (IETF). Note that other groups may also distribute 35 working documents as Internet-Drafts. The list of current Internet- 36 Drafts is at http://datatracker.ietf.org/drafts/current/. 38 Internet-Drafts are draft documents valid for a maximum of six months 39 and may be updated, replaced, or obsoleted by other documents at any 40 time. It is inappropriate to use Internet-Drafts as reference 41 material or to cite them other than as "work in progress." 43 This Internet-Draft will expire on February 24, 2011. 45 Copyright Notice 47 Copyright (c) 2010 IETF Trust and the persons identified as the 48 document authors. All rights reserved. 50 This document is subject to BCP 78 and the IETF Trust's Legal 51 Provisions Relating to IETF Documents 52 (http://trustee.ietf.org/license-info) in effect on the date of 53 publication of this document. Please review these documents 54 carefully, as they describe your rights and restrictions with respect 55 to this document. Code Components extracted from this document must 56 include Simplified BSD License text as described in Section 4.e of 57 the Trust Legal Provisions and are provided without warranty as 58 described in the Simplified BSD License. 60 This document may contain material from IETF Documents or IETF 61 Contributions published or made publicly available before November 62 10, 2008. The person(s) controlling the copyright in some of this 63 material may not have granted the IETF Trust the right to allow 64 modifications of such material outside the IETF Standards Process. 65 Without obtaining an adequate license from the person(s) controlling 66 the copyright in such materials, this document may not be modified 67 outside the IETF Standards Process, and derivative works of it may 68 not be created outside the IETF Standards Process, except to format 69 it for publication as an RFC or to translate it into languages other 70 than English. 72 Table of Contents 74 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 75 2. Role of This Specification . . . . . . . . . . . . . . . . . . 4 76 3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 5 77 4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6 78 4.1. Mail User and Mail Transfer Agents . . . . . . . . . . . . 6 79 4.2. Address Character Sets . . . . . . . . . . . . . . . . . . 7 80 4.3. User Types . . . . . . . . . . . . . . . . . . . . . . . . 7 81 4.4. Messages . . . . . . . . . . . . . . . . . . . . . . . . . 8 82 4.5. Mailing Lists . . . . . . . . . . . . . . . . . . . . . . 8 83 4.6. Conventional Message and Internationalized Message . . . . 8 84 4.7. Undeliverable Messages, Notification, and Delivery 85 Receipts . . . . . . . . . . . . . . . . . . . . . . . . . 8 86 5. Overview of the Approach and Document Plan . . . . . . . . . . 9 87 6. Review of Experimental Results . . . . . . . . . . . . . . . . 9 88 7. Overview of Protocol Extensions and Changes . . . . . . . . . 10 89 7.1. SMTP Extension for Internationalized Email Address . . . . 10 90 7.2. Transmission of Email Header Fields in UTF-8 Encoding . . 11 91 7.3. SMTP Service Extension for DSNs . . . . . . . . . . . . . 12 92 8. Downgrading before and after SMTP Transactions . . . . . . . . 12 93 8.1. Downgrading before or during Message Submission . . . . . 13 94 8.2. Downgrading or Other Processing After Final SMTP 95 Delivery . . . . . . . . . . . . . . . . . . . . . . . . . 14 96 9. Downgrading in Transit . . . . . . . . . . . . . . . . . . . . 14 97 10. User Interface and Configuration Issues . . . . . . . . . . . 15 98 10.1. Choices of Mailbox Names and Unicode Normalization . . . . 15 99 11. Additional Issues . . . . . . . . . . . . . . . . . . . . . . 16 100 11.1. Impact on URIs and IRIs . . . . . . . . . . . . . . . . . 16 101 11.2. Use of Email Addresses as Identifiers . . . . . . . . . . 17 102 11.3. Encoded Words, Signed Messages, and Downgrading . . . . . 17 103 11.4. LMTP . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 104 11.5. Other Uses of Local Parts . . . . . . . . . . . . . . . . 18 105 11.6. Non-Standard Encapsulation Formats . . . . . . . . . . . . 18 106 12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 107 13. Security Considerations . . . . . . . . . . . . . . . . . . . 18 108 14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20 109 15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20 110 15.1. Normative References . . . . . . . . . . . . . . . . . . . 20 111 15.2. Informative References . . . . . . . . . . . . . . . . . . 22 112 Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 26 113 A.1. Changes between -00 and -01 . . . . . . . . . . . . . . . 26 114 A.2. Changes between -01 and -02 . . . . . . . . . . . . . . . 26 115 A.3. Changes between -02 and -03 . . . . . . . . . . . . . . . 27 116 A.4. Changes between -03 and -04 . . . . . . . . . . . . . . . 28 117 A.5. Changes between -04 and -05 . . . . . . . . . . . . . . . 28 118 A.6. Changes between -05 and -06 . . . . . . . . . . . . . . . 28 120 1. Introduction 122 Note in Draft and to RFC Editor: The keyword represented in this 123 document by "UTF8SMTPbis" (and in the XML source by &EAISMTPkeyword;) 124 is a placeholder. The actual keyword will be assigned when the 125 standards track SMTP extension in this series [RFC5336bis-SMTP] is 126 approved for publication and should be substituted here. This 127 paragraph should be treated as normative reference to that SMTP 128 extension draft, creating a reference hold until it is approved by 129 the IESG. The paragraph should be removed before RFC publication. 131 In order to use internationalized email addresses, we need to 132 internationalize both the domain part and the local part of email 133 addresses. The domain part of email addresses is already 134 internationalized [RFC5890], while the local part is not. Without 135 the extensions specified in this document, the mailbox name is 136 restricted to a subset of 7-bit ASCII [RFC5321]. Though MIME 137 [RFC2045] enables the transport of non-ASCII data, it does not 138 provide a mechanism for internationalized email addresses. In RFC 139 2047 [RFC2047], MIME defines an encoding mechanism for some specific 140 message header fields to accommodate non-ASCII data. However, it 141 does not permit the use of email addresses that include non-ASCII 142 characters. Without the extensions defined here, or some equivalent 143 set, the only way to incorporate non-ASCII characters in any part of 144 email addresses is to use RFC 2047 coding to embed them in what RFC 145 5322 [RFC5322] calls the "display name" (known as a "name phrase" or 146 by other terms elsewhere) of the relevant header fields. Information 147 coded into the display name is invisible in the message envelope and, 148 for many purposes, is not part of the address at all. 150 This document is an update of RFC 4952 [RFC4952]; it reflects 151 additional issues, shared terminology, and some architectural changes 152 identified since that document was published. 154 The pronouns "he" and "she" are used interchangeably to indicate a 155 human of indeterminate gender. 157 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 158 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 159 document are to be interpreted as described in BCP 14, RFC 2119 160 [RFC2119]. Although this document is Informational, those 161 requirements are consistent with requirements specified in the 162 Standards Track documents in this set as described in Section 5. 164 2. Role of This Specification 166 This document presents the overview and framework for an approach to 167 the next stage of email internationalization. This new stage 168 requires not only internationalization of addresses and header 169 fields, but also associated transport and delivery models. A prior 170 version of this specification, RFC 4952 [RFC4952], also provided an 171 introduction to a series of experimental protocols [RFC5335] 172 [RFC5336] [RFC5337] [RFC5504] [RFC5721] [RFC5738] [RFC5825]. This 173 revised form provides overview and conceptual information for the 174 standards-track successors of a subset of those protocols. Details 175 of the documents and the relationships among them appear in Section 5 176 and a discussion of what was learned from the Experimental protocols 177 and their implementations appears in Section 6. 179 Taken together, these specifications provide the details for a way to 180 implement and support internationalized email. The document itself 181 describes how the various elements of email internationalization fit 182 together and the relationships among the primary specifications 183 associated with message transport, header formats, and handling. 185 This document, and others that comprise the collection described 186 above, assume a reasonable familiarity with the basic Internet 187 electronic mail specifications and terminology [RFC5321][RFC5322] and 188 the MIME [RFC2045] and 8BITMIME [RFC1652] ones as well. While not 189 strictly required to implement this specification, a general 190 familiarity with the terminology and functions of IDNA 191 [RFC5890][RFC5891] [RFC5892][RFC5893] [RFC5894] are also assumed. 193 3. Problem Statement 195 Internationalizing Domain Names in Applications (IDNA) [RFC5890] 196 permits internationalized domain names, but deployment has not yet 197 reached most users. One of the reasons for this is that we do not 198 yet have fully internationalized naming schemes. Domain names are 199 just one of the various names and identifiers that are required to be 200 internationalized. In many contexts, until more of those identifiers 201 are internationalized, internationalized domain names alone have 202 little value. 204 Email addresses are prime examples of why it is not good enough to 205 just internationalize the domain name. As most observers have 206 learned from experience, users strongly prefer email addresses that 207 resemble names or initials to those involving seemingly meaningless 208 strings of letters or numbers. Unless the entire email address can 209 use familiar characters and formats, users will perceive email as 210 being culturally unfriendly. If the names and initials used in email 211 addresses can be expressed in the native languages and writing 212 systems of the users, the Internet will be perceived as more natural, 213 especially by those whose native language is not written in a subset 214 of a Roman-derived script. 216 Internationalization of email addresses is not merely a matter of 217 changing the SMTP envelope; or of modifying the From, To, and Cc 218 header fields; or of permitting upgraded Mail User Agents (MUAs) to 219 decode a special coding and respond by displaying local characters. 220 To be perceived as usable, the addresses must be internationalized 221 and handled consistently in all of the contexts in which they occur. 222 This requirement has far-reaching implications: collections of 223 patches and workarounds are not adequate. Even if they were 224 adequate, a workaround-based approach may result in an assortment of 225 implementations with different sets of patches and workarounds having 226 been applied with consequent user confusion about what is actually 227 usable and supported. Instead, we need to build a fully 228 internationalized email environment, focusing on permitting efficient 229 communication among those who share a language or other community. 230 That, in turn, implies changes to the mail header environment to 231 permit the full range of Unicode characters where that makes sense, 232 an SMTP Extension to permit UTF-8 [RFC3629] [RFC5198] mail addressing 233 and delivery of those extended header fields, support for 234 internationalization of delivery and service notifications [RFC3461] 235 [RFC3464], and (finally) a requirement for support of the 8BITMIME 236 SMTP Extension [RFC1652] so that all of these can be transported 237 through the mail system without having to overcome the limitation 238 that header fields do not have content-transfer-encodings. 240 4. Terminology 242 This document assumes a reasonable understanding of the protocols and 243 terminology of the core email standards as documented in [RFC5321] 244 and [RFC5322]. 246 4.1. Mail User and Mail Transfer Agents 248 Much of the description in this document depends on the abstractions 249 of "Mail Transfer Agent" ("MTA") and "Mail User Agent" ("MUA"). 250 However, it is important to understand that those terms and the 251 underlying concepts postdate the design of the Internet's email 252 architecture and the application of the "protocols on the wire" 253 principle to it. That email architecture, as it has evolved, and 254 that "on the wire" principle have prevented any strong and 255 standardized distinctions about how MTAs and MUAs interact on a given 256 origin or destination host (or even whether they are separate). 258 However, the term "final delivery MTA" is used in this document in a 259 fashion equivalent to the term "delivery system" or "final delivery 260 system" of RFC 5321. This is the SMTP server that controls the 261 format of the local parts of addresses and is permitted to inspect 262 and interpret them. It receives messages from the network for 263 delivery to mailboxes or for other local processing, including any 264 forwarding or aliasing that changes envelope addresses, rather than 265 relaying. From the perspective of the network, any local delivery 266 arrangements such as saving to a message store, handoff to specific 267 message delivery programs or agents, and mechanisms for retrieving 268 messages are all "behind" the final delivery MTA and hence are not 269 part of the SMTP transport or delivery process. 271 4.2. Address Character Sets 273 In this document, an address is "all-ASCII", or just an "ASCII 274 address", if every character in the address is in the ASCII character 275 repertoire [ASCII]; an address is "non-ASCII", or an "i18n-address", 276 if any character is not in the ASCII character repertoire. Such 277 addresses MAY be restricted in other ways, but those restrictions are 278 not relevant to this definition. The term "all-ASCII" is also 279 applied to other protocol elements when the distinction is important, 280 with "non-ASCII" or "internationalized" as its opposite. 282 The umbrella term to describe the email address internationalization 283 specified by this document and its companion documents is 284 "UTF8SMTPbis". 285 [[anchor3: Note in Draft: Keyword to be changed before publication.]] 286 For example, an address permitted by this specification is referred 287 to as a "UTF8SMTPbis (compliant) address". 289 Please note that, according to the definitions given here, the set of 290 all "all-ASCII" addresses and the set of all "non-ASCII" addresses 291 are mutually exclusive. The set of all addresses permitted when 292 UTF8SMTPbis appears is the union of these two sets. 294 4.3. User Types 296 An "ASCII user" (i) exclusively uses email addresses that contain 297 ASCII characters only, and (ii) cannot generate recipient addresses 298 that contain non-ASCII characters. 300 An "i18mail user" has one or more non-ASCII email addresses, or is 301 able to generate recipient addresses that contain non-ASCII 302 characters. Such a user may have ASCII addresses too; if the user 303 has more than one email account and a corresponding address, or more 304 than one alias for the same address, he or she has some method to 305 choose which address to use on outgoing email. Note that under this 306 definition, it is not possible to tell from an ASCII address if the 307 owner of that address is an i18mail user or not. (A non-ASCII 308 address implies a belief that the owner of that address is an i18mail 309 user.) There is no such thing as an "i18mail message"; the term 310 applies only to users and their agents and capabilities. In 311 particular, the use of non-ASCII message content is an integral part 312 of the MIME specifications [RFC2045] and does not require these 313 extensions (although it is compatible with them). 315 4.4. Messages 317 A "message" is sent from one user (sender) using a particular email 318 address to one or more other recipient email addresses (often 319 referred to just as "users" or "recipient users"). 321 4.5. Mailing Lists 323 A "mailing list" is a mechanism whereby a message may be distributed 324 to multiple recipients by sending it to one recipient address. An 325 agent (typically not a human being) at that single address then 326 causes the message to be redistributed to the target recipients. 327 This agent sets the envelope return address of the redistributed 328 message to a different address from that of the original single 329 recipient message. Using a different envelope return address 330 (reverse-path) causes error (and other automatically generated) 331 messages to go to an error handling address. 333 Special provisions for managing mailing lists that might contain non- 334 ASCII addresses are discussed in a document that is specific to that 335 topic [EAI-Mailinglist] [RFCNNNNbis-MailingList]. 337 4.6. Conventional Message and Internationalized Message 339 o A conventional message is one that does not use any extension 340 defined in the SMTP extension document [RFC5336] or in the 341 UTF8header specification [RFC5335], and is strictly conformant to 342 RFC 5322 [RFC5322]. 344 o An internationalized message is a message utilizing one or more of 345 the extensions defined in this set of specifications, so that it 346 is no longer conformant to the traditional specification of an 347 email message or its transport. 349 4.7. Undeliverable Messages, Notification, and Delivery Receipts 351 As specified in RFC 5321, a message that is undeliverable for some 352 reason is expected to result in notification to the sender. This can 353 occur in either of two ways. One, typically called "Rejection", 354 occurs when an SMTP server returns a reply code indicating a fatal 355 error (a "5yz" code) or persistently returns a temporary failure 356 error (a "4yz" code). The other involves accepting the message 357 during SMTP processing and then generating a message to the sender, 358 typically known as a "Non-delivery Notification" or "NDN". Current 359 practice often favors rejection over NDNs because of the reduced 360 likelihood that the generation of NDNs will be used as a spamming 361 technique. The latter, NDN, case is unavoidable if an intermediate 362 MTA accepts a message that is then rejected by the next-hop server. 364 A sender MAY also explicitly request message receipts [RFC3461] that 365 raise the same issues for these internationalization extensions as 366 NDNs. 368 5. Overview of the Approach and Document Plan 370 This set of specifications changes both SMTP and the character 371 encoding of email message headers to permit non-ASCII characters to 372 be represented directly. Each important component of the work is 373 described in a separate document. The document set, whose members 374 are described below, also contains informational documents whose 375 purpose is to provide implementation suggestions and guidance for the 376 protocols. 378 In addition to this document, the following documents make up this 379 specification and provide advice and context for it. 381 o SMTP extensions. This document [RFC5336bis-SMTP] provides an SMTP 382 extension (as provided for in RFC 5321) for internationalized 383 addresses. 385 o Email message headers in UTF-8. This document [RFC5335bis-Hdrs] 386 essentially updates RFC 5322 to permit some information in email 387 message headers to be expressed directly by Unicode characters 388 encoded in UTF-8 when the SMTP extension described above is used. 389 This document, possibly with one or more supplemental ones, will 390 also need to address the interactions with MIME, including 391 relationships between UTF8SMTPbis and internal MIME headers and 392 content types. 394 o Extensions to delivery status and notification handling to adapt 395 to internationalized addresses [RFC5337bis-DSN]. 397 o Extensions to the IMAP protocol to support internationalized 398 message headers [RFC5738bis-IMAP]. 400 o Parallel extensions to the POP protocol [RFC5721] 401 [RFC5721bis-POP3]. 403 6. Review of Experimental Results 405 The key difference between this set of protocols and the experimental 406 set that preceded them [RFC5335] [RFC5336] [RFC5337] [RFC5504] 407 [RFC5721] [RFC5738] [RFC5825] is that the earlier group provided a 408 mechanism for in-transit downgrading of messages (described in detail 409 in RFC 5504). That mechanism permitted, and essentially required, 410 that each non-ASCII address be accompanied by an all-ASCII 411 equivalent. That, in turn, raised security concerns associated with 412 pairing of addresses that could not be authenticated. It also 413 introduced the first incompatible change to Internet mail addressing 414 in many years, raising concerns about interoperability issues if the 415 new address forms "leaked" into legacy email implementations. The WG 416 concluded that the advantages of in-transit downgrading, were it 417 feasible operationally, would be significant enough to overcome those 418 concerns. 420 Operationally that turned out to not be the case, with 421 interoperability problems among initial implementations. Prior to 422 starting on the work that led to this set of specifications, the WG 423 concluded that the combination of requirements and long-term 424 implications of that earlier model were too complex to be 425 satisfactory and that work should move ahead without it. 427 7. Overview of Protocol Extensions and Changes 429 7.1. SMTP Extension for Internationalized Email Address 431 An SMTP extension, "UTF8SMTPbis" is specified as follows: 433 o Permits the use of UTF-8 strings in email addresses, both local 434 parts and domain names. 436 o Permits the selective use of UTF-8 strings in email message 437 headers (see Section 7.2). 439 o Requires that the server advertise the 8BITMIME extension 440 [RFC1652] and that the client support 8-bit transmission so that 441 header information can be transmitted without using a special 442 content-transfer-encoding. 444 Some general principles affect the development decisions underlying 445 this work. 447 1. Email addresses enter subsystems (such as a user interface) that 448 may perform charset conversions or other encoding changes. When 449 the left hand side of the address includes characters outside the 450 US-ASCII character repertoire, use of ASCII-compatible encoding 451 (ACE) [RFC3492] [RFC5890] on the right hand side is discouraged 452 to promote consistent processing of characters throughout the 453 address. 455 2. An SMTP relay MUST 457 * Either recognize the format explicitly, agreeing to do so via 458 an ESMTP option, or 460 * Reject the message or, if necessary, return a non-delivery 461 notification message, so that the sender can make another 462 plan. 464 3. If the message cannot be forwarded because the next-hop system 465 cannot accept the extension it MUST be rejected or a non-delivery 466 message MUST be generated and sent. 468 4. In the interest of interoperability, charsets other than UTF-8 469 are prohibited in mail addresses and message headers being 470 transmitted over the Internet. There is no practical way to 471 identify multiple charsets properly with an extension similar to 472 this without introducing great complexity. 474 Conformance to the group of standards specified here for email 475 transport and delivery requires implementation of the SMTP Extension 476 specification and the UTF-8 Header specification. If the system 477 implements IMAP or POP, it MUST conform to the i18n IMAP or POP 478 specifications respectively. 480 7.2. Transmission of Email Header Fields in UTF-8 Encoding 482 There are many places in MUAs or in a user presentation in which 483 email addresses or domain names appear. Examples include the 484 conventional From, To, or Cc header fields; Message-ID and 485 In-Reply-To header fields that normally contain domain names (but 486 that may be a special case); and in message bodies. Each of these 487 must be examined from an internationalization perspective. The user 488 will expect to see mailbox and domain names in local characters, and 489 to see them consistently. If non-obvious encodings, such as 490 protocol-specific ASCII-Compatible Encoding (ACE) variants, are used, 491 the user will inevitably, if only occasionally, see them rather than 492 "native" characters and will find that discomfiting or astonishing. 493 Similarly, if different codings are used for mail transport and 494 message bodies, the user is particularly likely to be surprised, if 495 only as a consequence of the long-established "things leak" 496 principle. The only practical way to avoid these sources of 497 discomfort, in both the medium and the longer term, is to have the 498 encodings used in transport be as similar to the encodings used in 499 message headers and message bodies as possible. 501 When email local parts are internationalized, they SHOULD be 502 accompanied by arrangements for the message headers to be in the 503 fully internationalized form. That form SHOULD presumably use UTF-8 504 rather than ASCII as the base character set for the contents of 505 header fields (protocol elements such as the header field names 506 themselves are unchanged and remain entirely in ASCII). For 507 transition purposes and compatibility with legacy systems, this can 508 done by extending the traditional MIME encoding models for non-ASCII 509 characters in headers [RFC2045] [RFC2231]. However, the target is 510 fully internationalized message headers, as discussed in 511 [RFC5335bis-Hdrs] and not an extended and painful transition. 513 7.3. SMTP Service Extension for DSNs 515 The existing Draft Standard Delivery status notifications (DSNs) 516 specification [RFC3461] is limited to ASCII text in the machine 517 readable portions of the protocol. "International Delivery and 518 Disposition Notifications" [RFC5337bis-DSN] adds a new address type 519 for international email addresses so an original recipient address 520 with non-ASCII characters can be correctly preserved even after 521 downgrading. If an SMTP server advertises both the UTF8SMTPbis and 522 the DSN extension, that server MUST implement internationalized DSNs 523 including support for the ORCPT parameter specified in RFC 3461 524 [RFC3461]. 526 8. Downgrading before and after SMTP Transactions 528 An important issue with these extensions is how to handle 529 interactions between systems that support non-ASCII addresses and 530 legacy systems that expect ASCII. There is, of course, no problem 531 with ASCII-only systems sending to those that can handle 532 internationalized forms because the ASCII forms are just a proper 533 subset. But, when systems that support these extensions send mail, 534 they MAY include non-ASCII addresses for senders, receivers, or both 535 and might also provide non-ASCII header information other than 536 addresses. If the extension is not supported by the first-hop system 537 (SMTP server accessed by the Submission server acting as an SMTP 538 client), message originating systems SHOULD be prepared to either 539 send conventional envelopes and message headers or to return the 540 message to the originating user so the message may be manually 541 downgraded to the traditional form, possibly using encoded words 542 [RFC2047] in the message headers. Of course, such transformations 543 imply that the originating user or system must have ASCII-only 544 addresses available for all senders and recipients. Mechanisms by 545 which such addresses may be found or identified are outside the scope 546 of these specifications as are decisions about the design of 547 originating systems such as whether any required transformations are 548 made by the user, the originating MUA, or the Submission server. 550 A somewhat more complex situation arises when the first-hop system 551 supports these extensions but some subsequent server in the SMTP 552 transmission chain does not. It is important to note that most cases 553 of that situation with forward-pointing addresses will be the result 554 of configuration errors: especially if it hosts non-ASCII addresses, 555 a final delivery MTA that accepts these extensions SHOULD NOT be 556 configured with lower-preference MX hosts that do not. When the only 557 non-ASCII address being transmitted is backward-pointing (e.g., in an 558 SMTP MAIL command), recipient configuration can not help in general. 559 On the other hand, alternate, all-ASCII, addresses for senders are 560 those most likely to be authoritatively known by the submission 561 environment or the sender herself. Consequently, if an intermediate 562 SMTP relay that requires these extensions then discovers that the 563 next system in the chain does not support them, it will have little 564 choice other than to reject or return the message. 566 As discussed above, downgrading to an ASCII-only form may occur 567 before or during the initial message submission. It might also occur 568 after the delivery to the final delivery MTA in order to accommodate 569 messages stores or IMAP or POP servers or clients that have different 570 capabilities than the delivery MTA. These two cases are discussed in 571 the subsections below. 573 8.1. Downgrading before or during Message Submission 575 It is likely that the most common cases in which a message that 576 requires these extensions is sent to a system that does not will 577 involve the combination of ASCII-only forward-pointing addresses with 578 a non-ASCII backward-pointing one. Until the extensions described 579 here have been universally implemented in the Internet email 580 environment, senders who prefer to use non-ASCII addresses (or raw 581 UTF-8 characters in header fields) even when their intended 582 recipients use and expect all-ASCII ones will need to be especially 583 careful about the error conditions that can arise, especially if they 584 are working in an environment in which non-delivery messages (or 585 other indications from submission servers) are routinely dropped or 586 ignored. 588 Perhaps obviously, the most convenient time to find an ASCII address 589 corresponding to an internationalized address is at the originating 590 MUA or closely-associated systems. This can occur either before the 591 message is sent or after the internationalized form of the message is 592 rejected. It is also the most convenient time to convert a message 593 from the internationalized form into conventional ASCII form or to 594 generate a non-delivery message to the sender if either is necessary. 595 At that point, the user has a full range of choices available, 596 including changing backward-pointing addresses, contacting the 597 intended recipient out of band for an alternate address, consulting 598 appropriate directories, arranging for translation of both addresses 599 and message content into a different language, and so on. While it 600 is natural to think of message downgrading as optimally being a 601 fully-automated process, we should not underestimate the capabilities 602 of a user of at least moderate intelligence who wishes to communicate 603 with another such user. 605 In this context, one can easily imagine modifications to message 606 submission servers (as described in RFC 4409 [RFC4409]) so that they 607 would perform downgrading operations or perhaps even upgrading ones. 608 Such operations would permit receiving messages with one or more of 609 the internationalization extensions discussed here and adapting the 610 outgoing message, as needed, to respond to the delivery or next-hop 611 environment the submission server encounters. 613 8.2. Downgrading or Other Processing After Final SMTP Delivery 615 When an email message is received by a final delivery MTA, it is 616 usually stored in some form. Then it is retrieved either by software 617 that reads the stored form directly or by client software via some 618 email retrieval mechanisms such as POP or IMAP. 620 The SMTP extension described in Section 7.1 provides protection only 621 in transport. It does not prevent MUAs and email retrieval 622 mechanisms that have not been upgraded to understand 623 internationalized addresses and UTF-8 message headers from accessing 624 stored internationalized emails. 626 Since the final delivery MTA (or, to be more specific, its 627 corresponding mail storage agent) cannot safely assume that agents 628 accessing email storage will always be capable of handling the 629 extensions proposed here, it MAY downgrade internationalized emails, 630 specially identify messages that utilize these extensions, or both. 631 If this is done, the final delivery MTA SHOULD include a mechanism to 632 preserve or recover the original internationalized forms without 633 information loss to support access by UTF8SMTPbis-aware agents. 635 9. Downgrading in Transit 637 The base SMTP specification (Section 2.3.11 of RFC 5321 [RFC5321]) 638 states that "due to a long history of problems when intermediate 639 hosts have attempted to optimize transport by modifying them, the 640 local-part MUST be interpreted and assigned semantics only by the 641 host specified in the domain part of the address". This is not a new 642 requirement; equivalent statements appeared in specifications in 2001 643 [RFC2821] and even in 1989 [RFC1123]. 645 Adherence to this rule means that a downgrade mechanism that 646 transforms the local-part of an email address cannot be utilized in 647 transit. It can only be applied at the endpoints, specifically by 648 the MUA or submission server or by the final delivery MTA. 650 One of the reasons for this rule has to do with legacy email systems 651 that embed mail routing information in the local-part of the address 652 field. Transforming the email address destroys such routing 653 information. There is no way a server other than the final delivery 654 server can know, for example, whether the local-part of 655 user%foo@example.com is a route ("user" is reached via "foo") or 656 simply a local address. 658 10. User Interface and Configuration Issues 660 Internationalization of addresses and message headers, especially in 661 combination with variations on character coding that are inherent to 662 Unicode, may make careful choices of addresses and careful 663 configuration of servers and DNS records even more important than 664 they are for traditional Internet email. It is likely that, as 665 experience develops with the use of these protocols, it will be 666 desirable to produce one or more additional documents that offer 667 guidance for configuration and interfaces. A document that discusses 668 issues with mail user agents (MUAs), especially with regard to 669 downgrading [EAI-MUA-issues], is expected to be developed in the EAI 670 Working Group. The subsections below address some other issues. 672 10.1. Choices of Mailbox Names and Unicode Normalization 674 It has long been the case that the email syntax permits choices about 675 mailbox names that are unwise in practice if one actually intends the 676 mailboxes to be accessible to a broad range of senders. The most- 677 often-cited examples involve the use of case-sensitivity and tricky 678 quoting of embedded characters in mailbox local parts. While these 679 are permitted by the protocols and servers are expected to support 680 them and there are special cases where they can provide value, taking 681 advantage of those features is almost always bad practice unless the 682 intent is to create some form of security by obscurity. 684 In the absence of these extensions, SMTP clients and servers are 685 constrained to using only those addresses permitted by RFC 5321. The 686 local parts of those addresses MAY be made up of any ASCII characters 687 except the control characters that 5321 prohibits, although some of 688 them MUST be quoted as specified there. It is notable in an 689 internationalization context that there is a long history on some 690 systems of using overstruck ASCII characters (a character, a 691 backspace, and another character) within a quoted string to 692 approximate non-ASCII characters. This form of internationalization 693 was permitted by RFC 821 [RFC0821] but is prohibited by RFC 5321 694 because it requires a backspace character (a prohibited C0 control). 696 The practice SHOULD be phased out as this extension becomes widely 697 deployed but backward-compatibility considerations may require that 698 it continue to be recognized. 700 For the particular case of EAI mailbox names, special attention MUST 701 be paid to Unicode normalization [Unicode-UAX15], in part because 702 Unicode strings may be normalized by other processes independent of 703 what a mail protocol specifies (this is exactly analogous to what may 704 happen with quoting and dequoting in traditional addresses). 705 Consequently, the following principles are offered as advice to those 706 who are selecting names for mailboxes: 708 o In general, it is wise to support addresses in Normalized form, 709 using either Normalization Form NFC and, except in unusual 710 circumstances, NFKC. 712 o It may be wise to support other forms of the same local-part 713 string, either as aliases or by normalization of strings reaching 714 the delivery server, in the event that the sender does not send 715 the strings in normalized form. 717 o Stated differently and in more specific terms, the rules of the 718 protocol for local-part strings essentially provide that: 720 * Unnormalized strings are valid, but sufficiently bad practice 721 that they may not work reliably on a global basis. 723 * C0 (and presumably C1) controls (see The Unicode Standard 724 [Unicode52]) are prohibited, the first in RFC 5321 and the 725 second by an obvious extension from it [RFC5198]. 727 * Other kinds of punctuation, spaces, etc., are risky practice. 728 Perhaps they will work, and SMTP receiver code is required to 729 handle them, but creating dependencies on them in mailbox names 730 that are chosen is usually a bad practice and may lead to 731 interoperability problems. 733 11. Additional Issues 735 This section identifies issues that are not covered, or not covered 736 comprehensively, as part of this set of specifications, but that will 737 require ongoing review as part of deployment of email address and 738 header internationalization. 740 11.1. Impact on URIs and IRIs 742 The mailto: schema [RFC2368] and discussed in the Internationalized 743 Resource Identifier (IRI) specification [RFC3987] may need to be 744 modified when this work is completed and standardized. 746 11.2. Use of Email Addresses as Identifiers 748 There are a number of places in contemporary Internet usage in which 749 email addresses are used as identifiers for individuals, including as 750 identifiers to Web servers supporting some electronic commerce sites 751 and in some X.509 certificates [RFC5280]. These documents do not 752 address those uses, but it is reasonable to expect that some 753 difficulties will be encountered when internationalized addresses are 754 first used in those contexts, many of which cannot even handle the 755 full range of addresses permitted today. 757 11.3. Encoded Words, Signed Messages, and Downgrading 759 One particular characteristic of the email format is its persistency: 760 MUAs are expected to handle messages that were originally sent 761 decades ago and not just those delivered seconds ago. As such, MUAs 762 and mail filtering software, such as that specified in Sieve 763 [RFC5228], will need to continue to accept and decode header fields 764 that use the "encoded word" mechanism [RFC2047] to accommodate non- 765 ASCII characters in some header fields. While extensions to both 766 POP3 [RFC1939] and IMAP [RFC3501] have been defined that include 767 automatic upgrading of messages that carry non-ASCII information in 768 encoded form -- including RFC 2047 decoding -- of messages by the 769 POP3 [RFC5721bis-POP3] or IMAP [RFC5738bis-IMAP] server, there are 770 message structures and MIME content-types for which that cannot be 771 done or where the change would have unacceptable side effects. 773 For example, message parts that are cryptographically signed, using 774 e.g., S/MIME [RFC3851] or Pretty Good Privacy (PGP) [RFC3156], cannot 775 be upgraded from the RFC 2047 form to normal UTF-8 characters without 776 breaking the signature. Similarly, message parts that are encrypted 777 may contain, when decrypted, header fields that use the RFC 2047 778 encoding; such messages cannot be 'fully' upgraded without access to 779 cryptographic keys. 781 Similar issues may arise if messages are signed and then subsequently 782 downgraded, e.g., as discussed in Section 8.1, and then an attempt is 783 made to upgrade them to the original form and then verify the 784 signatures. Even the very subtle changes that may result from 785 algorithms to downgrade and then upgrade again may be sufficient to 786 invalidate the signatures if they impact either the primary or MIME 787 bodypart headers. When signatures are present, downgrading MUST be 788 performed with extreme care if at all. 790 11.4. LMTP 792 LMTP [RFC2033] may be used as part of the final delivery agent. In 793 such cases, LMTP may be arranged to deliver the mail to the mail 794 store. The mail store may not have UTF8SMTPbis capability. LMTP may 795 need to be updated to deal with these situations. 797 11.5. Other Uses of Local Parts 799 Local parts are sometimes used to construct domain labels, e.g., the 800 local part "user" in the address user@domain.example could be 801 converted into a vanity host user.domain.example with its Web space 802 at and the catchall addresses 803 any.thing.goes@user.domain.example. 805 Such schemes are obviously limited by, among other things, the SMTP 806 rules for domain names, and will not work without further 807 restrictions for other local parts such as the 808 specified in [RFC5335bis-Hdrs]. Whether those limitations are 809 relevant to these specifications is an open question. It may be 810 simply another case of the considerable flexibility accorded to 811 delivery MTAs in determining the mailbox names they will accept and 812 how they are interpreted. 814 11.6. Non-Standard Encapsulation Formats 816 Some applications use formats similar to the application/mbox format 817 defined in [RFC4155] instead of the message/digest form described in 818 RFC 2046, Section 5.1.5 [RFC2046] to transfer multiple messages as 819 single units. Insofar as such applications assume that all stored 820 messages use the message/rfc822 format described in RFC 2046, Section 821 5.2.1 [RFC2046] with US-ASCII message headers, they are not ready for 822 the extensions specified in this series of documents and special 823 measures may be needed to properly detect and process them. 825 12. IANA Considerations 827 This overview description and framework document does not contemplate 828 any IANA registrations or other actions. Some of the documents in 829 the group have their own IANA considerations sections and 830 requirements. 832 13. Security Considerations 834 Any expansion of permitted characters and encoding forms in email 835 addresses raises some risks. There have been discussions on so 836 called "IDN-spoofing" or "IDN homograph attacks". These attacks 837 allow an attacker (or "phisher") to spoof the domain or URLs of 838 businesses. The same kind of attack is also possible on the local 839 part of internationalized email addresses. It should be noted that 840 the proposed fix involving forcing all displayed elements into 841 normalized lower-case works for domain names in URLs, but not email 842 local parts since those are case sensitive. 844 Since email addresses are often transcribed from business cards and 845 notes on paper, they are subject to problems arising from confusable 846 characters (see [RFC4690]). These problems are somewhat reduced if 847 the domain associated with the mailbox is unambiguous and supports a 848 relatively small number of mailboxes whose names follow local system 849 conventions. They are increased with very large mail systems in 850 which users can freely select their own addresses. 852 The internationalization of email addresses and message headers must 853 not leave the Internet less secure than it is without the required 854 extensions. The requirements and mechanisms documented in this set 855 of specifications do not, in general, raise any new security issues. 857 They do require a review of issues associated with confusable 858 characters -- a topic that is being explored thoroughly elsewhere 859 (see, e.g., RFC 4690 [RFC4690]) -- and, potentially, some issues with 860 UTF-8 normalization, discussed in RFC 3629 [RFC3629], and other 861 transformations. Normalization and other issues associated with 862 transformations and standard forms are also part of the subject of 863 work described elsewhere [RFC5198] [RFC5893] [IAB-idn-encoding]. 865 Some issues specifically related to internationalized addresses and 866 message headers are discussed in more detail in the other documents 867 in this set. However, in particular, caution should be taken that 868 any "downgrading" mechanism, or use of downgraded addresses, does not 869 inappropriately assume authenticated bindings between the 870 internationalized and ASCII addresses. Expecting and most or all 871 such transformations prior to final delivery be done by systems that 872 are presumed to be under the administrative control of the sending 873 user ameliorates the potential problem somewhat as compared to what 874 it would be if the relationships were changed in transit. 876 The new UTF-8 header and message formats might also raise, or 877 aggravate, another known issue. If the model creates new forms of an 878 'invalid' or 'malformed' message, then a new email attack is created: 879 in an effort to be robust, some or most agents will accept such 880 message and interpret them as if they were well-formed. If a filter 881 interprets such a message differently than the MUA used by the 882 recipient, then it may be possible to create a message that appears 883 acceptable under the filter's interpretation but that should be 884 rejected under the interpretation given to it by that MUA. Such 885 attacks already exist for existing messages and encoding layers, 886 e.g., invalid MIME syntax, invalid HTML markup, and invalid coding of 887 particular image types. 889 In addition, email addresses are used in many contexts other than 890 sending mail, such as for identifiers under various circumstances 891 (see Section 11.2). Each of those contexts will need to be 892 evaluated, in turn, to determine whether the use of non-ASCII forms 893 is appropriate and what particular issues they raise. 895 This work will clearly affect any systems or mechanisms that are 896 dependent on digital signatures or similar integrity protection for 897 email message headers (see also the discussion in Section 11.3). 898 Many conventional uses of PGP and S/MIME are not affected since they 899 are used to sign body parts but not message headers. On the other 900 hand, the developing work on domain keys identified mail (DKIM) 901 [RFC5863] will eventually need to consider this work and vice versa: 902 while this specification does not address or solve the issues raised 903 by DKIM and other signed header mechanisms, the issues will have to 904 be coordinated and resolved eventually if the two sets of protocols 905 are to co-exist. In addition, to the degree to which email addresses 906 appear in PKI (Public Key Infrastructure) certificates, standards 907 addressing such certificates will need to be upgraded to address 908 these internationalized addresses. Those upgrades will need to 909 address questions of spoofing by look-alikes of the addresses 910 themselves. 912 14. Acknowledgments 914 This document is an update to, and derived from, RFC 4952. This 915 document would have been impossible without the work and 916 contributions acknowledged in it. The present document benefited 917 significantly from discussions in the EAI WG and elsewhere after RFC 918 4952 was published, especially discussions about the experimental 919 versions of other documents in the internationalized email 920 collection, and from RFC errata on RFC 4952 itself. 922 Special thanks are due to Ernie Dainow for careful reviews and 923 suggested text in this version. 925 15. References 927 15.1. Normative References 929 [ASCII] American National Standards Institute 930 (formerly United States of America 931 Standards Institute), "USA Code for 932 Information Interchange", ANSI X3.4-1968, 933 1968. 935 ANSI X3.4-1968 has been replaced by newer 936 versions with slight modifications, but the 937 1968 version remains definitive for the 938 Internet. 940 [RFC1652] Klensin, J., Freed, N., Rose, M., 941 Stefferud, E., and D. Crocker, "SMTP 942 Service Extension for 8bit-MIMEtransport", 943 RFC 1652, July 1994. 945 [RFC2119] Bradner, S., "Key words for use in RFCs to 946 Indicate Requirement Levels", BCP 14, 947 RFC 2119, March 1997. 949 [RFC3629] Yergeau, F., "UTF-8, a transformation 950 format of ISO 10646", STD 63, RFC 3629, 951 November 2003. 953 [RFC5321] Klensin, J., "Simple Mail Transfer 954 Protocol", RFC 5321, October 2008. 956 [RFC5322] Resnick, P., Ed., "Internet Message 957 Format", RFC 5322, October 2008. 959 [RFC5335bis-Hdrs] Yang, A. and S. Steele, "Internationalized 960 Email Headers", July 2010, . 964 [RFC5336bis-SMTP] Yao, J. and W. Mao, "SMTP Extension for 965 Internationalized Email Address", 966 August 2010, . 969 [RFC5337bis-DSN] Not yet posted?, "Internationalized 970 Delivery Status and Disposition 971 Notifications", Unwritten waiting for I-D, 972 2010. 974 [RFC5721bis-POP3] Not yet posted?, "POP3 Support for UTF-8", 975 Unwritten waiting for I-D, 2010. 977 [RFC5738bis-IMAP] Not yet posted?, "IMAP Support for UTF-8", 978 Unwritten waiting for I-D, 2010. 980 [RFC5890] Klensin, J., "Internationalized Domain 981 Names for Applications (IDNA): Definitions 982 and Document Framework", RFC 5890, 983 August 2010. 985 [RFCNNNNbis-MailingList] Not yet posted?, "Mailing Lists and 986 Internationalized Email Addresses", First 987 Version still not in RFC Editor queue https 988 ://datatracker.ietf.org/doc/ 989 draft-ietf-eai-mailinglist/, 990 Unwritten waiting for I-D, 2010. 992 15.2. Informative References 994 [EAI-MUA-issues] EAI WG, "Still-unnamed proposed document on 995 MUA issues", Not assigned or agreed to yet, 996 2011. 998 Note to IESG and RFC Editor: While there is 999 provision for a document on this subject in 1000 the WG Charter, there is, as yet, no plan 1001 for producing it or even for adding it to 1002 the WG's task list with benchmarks. If the 1003 present document is approved for 1004 publication before the is at least a title 1005 and author(s) for an I-D, the citation and 1006 reference should simply be dropped. 1008 [EAI-Mailinglist] Gellens, R., "Mailing Lists and 1009 Internationalized Email Addresses", 1010 June 2010, . 1013 [IAB-idn-encoding] Thaler, D., Klensin, J., and S. Cheshire, 1014 "IAB Thoughts on Encodings for 1015 Internationalized Domain Names", 2010, . 1019 [RFC0821] Postel, J., "Simple Mail Transfer 1020 Protocol", STD 10, RFC 821, August 1982. 1022 [RFC1123] Braden, R., "Requirements for Internet 1023 Hosts - Application and Support", STD 3, 1024 RFC 1123, October 1989. 1026 [RFC1939] Myers, J. and M. Rose, "Post Office 1027 Protocol - Version 3", STD 53, RFC 1939, 1028 May 1996. 1030 [RFC2033] Myers, J., "Local Mail Transfer Protocol", 1031 RFC 2033, October 1996. 1033 [RFC2045] Freed, N. and N. Borenstein, "Multipurpose 1034 Internet Mail Extensions (MIME) Part One: 1035 Format of Internet Message Bodies", 1036 RFC 2045, November 1996. 1038 [RFC2046] Freed, N. and N. Borenstein, "Multipurpose 1039 Internet Mail Extensions (MIME) Part Two: 1040 Media Types", RFC 2046, November 1996. 1042 [RFC2047] Moore, K., "MIME (Multipurpose Internet 1043 Mail Extensions) Part Three: Message Header 1044 Extensions for Non-ASCII Text", RFC 2047, 1045 November 1996. 1047 [RFC2231] Freed, N. and K. Moore, "MIME Parameter 1048 Value and Encoded Word Extensions: Characte 1049 r Sets, Languages, and Continuations", 1050 RFC 2231, November 1997. 1052 [RFC2368] Hoffman, P., Masinter, L., and J. Zawinski, 1053 "The mailto URL scheme", RFC 2368, 1054 July 1998. 1056 [RFC2821] Klensin, J., "Simple Mail Transfer 1057 Protocol", RFC 2821, April 2001. 1059 [RFC3156] Elkins, M., Del Torto, D., Levien, R., and 1060 T. Roessler, "MIME Security with OpenPGP", 1061 RFC 3156, August 2001. 1063 [RFC3461] Moore, K., "Simple Mail Transfer Protocol 1064 (SMTP) Service Extension for Delivery 1065 Status Notifications (DSNs)", RFC 3461, 1066 January 2003. 1068 [RFC3464] Moore, K. and G. Vaudreuil, "An Extensible 1069 Message Format for Delivery Status 1070 Notifications", RFC 3464, January 2003. 1072 [RFC3492] Costello, A., "Punycode: A Bootstring 1073 encoding of Unicode for Internationalized 1074 Domain Names in Applications (IDNA)", 1075 RFC 3492, March 2003. 1077 [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS 1078 PROTOCOL - VERSION 4rev1", RFC 3501, 1079 March 2003. 1081 [RFC3851] Ramsdell, B., "Secure/Multipurpose Internet 1082 Mail Extensions (S/MIME) Version 3.1 1083 Message Specification", RFC 3851, 1084 July 2004. 1086 [RFC3987] Duerst, M. and M. Suignard, 1087 "Internationalized Resource Identifiers 1088 (IRIs)", RFC 3987, January 2005. 1090 [RFC4155] Hall, E., "The application/mbox Media 1091 Type", RFC 4155, September 2005. 1093 [RFC4409] Gellens, R. and J. Klensin, "Message 1094 Submission for Mail", RFC 4409, April 2006. 1096 [RFC4690] Klensin, J., Faltstrom, P., Karp, C., and 1097 IAB, "Review and Recommendations for 1098 Internationalized Domain Names (IDNs)", 1099 RFC 4690, September 2006. 1101 [RFC4952] Klensin, J. and Y. Ko, "Overview and 1102 Framework for Internationalized Email", 1103 RFC 4952, July 2007. 1105 [RFC5198] Klensin, J. and M. Padlipsky, "Unicode 1106 Format for Network Interchange", RFC 5198, 1107 March 2008. 1109 [RFC5228] Guenther, P. and T. Showalter, "Sieve: An 1110 Email Filtering Language", RFC 5228, 1111 January 2008. 1113 [RFC5280] Cooper, D., Santesson, S., Farrell, S., 1114 Boeyen, S., Housley, R., and W. Polk, 1115 "Internet X.509 Public Key Infrastructure 1116 Certificate and Certificate Revocation List 1117 (CRL) Profile", RFC 5280, May 2008. 1119 [RFC5335] Abel, Y., "Internationalized Email 1120 Headers", RFC 5335, September 2008. 1122 [RFC5336] Yao, J. and W. Mao, "SMTP Extension for 1123 Internationalized Email Addresses", 1124 RFC 5336, September 2008. 1126 [RFC5337] Newman, C. and A. Melnikov, 1127 "Internationalized Delivery Status and 1128 Disposition Notifications", RFC 5337, 1129 September 2008. 1131 [RFC5504] Fujiwara, K. and Y. Yoneya, "Downgrading 1132 Mechanism for Email Address 1133 Internationalization", RFC 5504, 1134 March 2009. 1136 [RFC5721] Gellens, R. and C. Newman, "POP3 Support 1137 for UTF-8", RFC 5721, February 2010. 1139 [RFC5738] Resnick, P. and C. Newman, "IMAP Support 1140 for UTF-8", RFC 5738, March 2010. 1142 [RFC5825] Fujiwara, K. and B. Leiba, "Displaying 1143 Downgraded Messages for Email Address 1144 Internationalization", RFC 5825, 1145 April 2010. 1147 [RFC5863] Hansen, T., Siegel, E., Hallam-Baker, P., 1148 and D. Crocker, "DomainKeys Identified Mail 1149 (DKIM) Development, Deployment, and 1150 Operations", RFC 5863, May 2010. 1152 [RFC5891] Klensin, J., "Internationalized Domain 1153 Names in Applications (IDNA): Protocol", 1154 RFC 5891, August 2010. 1156 [RFC5892] Faltstrom, P., "The Unicode Code Points and 1157 Internationalized Domain Names for 1158 Applications (IDNA)", RFC 5892, 1159 August 2010. 1161 [RFC5893] Alvestrand, H. and C. Karp, "Right-to-Left 1162 Scripts for Internationalized Domain Names 1163 for Applications (IDNA)", RFC 5893, 1164 August 2010. 1166 [RFC5894] Klensin, J., "Internationalized Domain 1167 Names for Applications (IDNA): Background, 1168 Explanation, and Rationale", RFC 5894, 1169 August 2010. 1171 [Unicode-UAX15] The Unicode Consortium, "Unicode Standard 1172 Annex #15: Unicode Normalization Forms", 1173 March 2008, 1174 . 1176 [Unicode52] The Unicode Consortium. The Unicode 1177 Standard, Version 5.2.0, defined by:, "The 1178 Unicode Standard, Version 5.2.0", (Mountain 1179 View, CA: The Unicode Consortium, 1180 2009. ISBN 978-1-936213-00-9)., . 1183 Appendix A. Change Log 1185 [[RFC Editor: Please remove this section prior to publication.]] 1187 A.1. Changes between -00 and -01 1189 o Because there has been no feedback on the mailing list, updated 1190 the various questions to refer to this version as well. 1192 o Reflected RFC Editor erratum #1507 by correcting terminology for 1193 headers and header fields and distinguishing between "message 1194 headers" and different sorts of headers (e.g., the MIME ones). 1196 A.2. Changes between -01 and -02 1198 Note that section numbers in the list that follows may refer to -01 1199 and not -02. 1201 o Discussion of RFC 5825 ("downgraded display") has been removed per 1202 the earlier note and on-list discussion. Any needed discussion 1203 about reconstructed messages will need to appear in the IMAP and 1204 POP documents. However, the introductory material has been 1205 reworded to permit keeping 5504 and 5825 on the list there, 1206 without which the back chain would not be complete. For 1207 consistency with this change, 5504 and 5825 have been added to the 1208 "Obsoletes" list (as far as I know, an Informational spec can 1209 obsolete or update Experimental ones, so no downref problem here 1210 --JcK). 1212 o Reference to alternate addresses dropped from (former) Section 1213 7.1. 1215 o Reference to RFC 5504 added to (former) Section 8 for 1216 completeness. 1218 o Ernie's draft comments added (with some minor edits) to replace 1219 the placeholder in (former) Section 9 ("Downgrading in Transit"). 1220 It is intended to capture at least an introduction the earlier 1221 discussions of algorithmic downgrading generally and ACE/Punycode 1222 transformations in particular. Anyone who is unhappy with it 1223 should say so and propose alternate text. RSN. 1225 o In the interest of clarity and consistency with the terminology in 1226 Section 4.1, all uses of "final delivery SMTP server" and "final 1227 delivery server" have been changed to "final delivery MTA". 1229 o Placeholder at the end of Section 2 has been removed and the text 1230 revised to promise less. The "Document Plan" (Section 5) has been 1231 revised accordingly. We need to discuss this at IETF 78 if not 1232 sooner. 1234 o Sections 5 and 6 have been collapsed into one -- there wasn't 1235 enough left in the former Section 5 to justify a separate section. 1237 o Former Section 11.1 has been dropped and the DSN document moved up 1238 into the "Document Plan" as suggested earlier. 1240 o Section 12, "Experimental Targets", has been removed. 1242 o Updated references for the new version EAI documents and added 1243 placeholders for all of the known remaining drafts that will 1244 become part of the core EAI series but that have not been written. 1246 o Inserted an additional clarification about the relationship of 1247 these extensions to non-ASCII messages. 1249 o Changed some normative/informative reference classifications based 1250 on review of the new text. 1252 o Removed references to the pre-EAI documents that were cited for 1253 historical context in 4952. 1255 o Got rid of a remaining pointer to address downgrading in the 1256 discussion of an updated MAILTO URI. 1258 o Minor additional editorial cleanups and tuning. 1260 A.3. Changes between -02 and -03 1262 o Inserted paragraph clarifying the status of the UTF8SMTPbis 1263 keyword as a result of discussion prior to and during IETF 79. 1265 o Adjusted some references including adding an explicit citation of 1266 RFC 821. 1268 o Removed the discussion of the experimental work from an inline 1269 aside to a separate section, Section 6. 1271 o Rewrote the discussion of configuration errors in MX setups to 1272 make it clear that they are an issue with forward-pointing 1273 addresses only and improved the discussion of backward-pointing 1274 addresses. 1276 o Removed some now-obsolete placeholder notes and resolved the 1277 remaining one to a dangling reference. 1279 A.4. Changes between -03 and -04 1281 o Several minor editorial changes. 1283 o Added a discussion of the relationship to the base email, MIME, 1284 and IDNA specifications. 1286 A.5. Changes between -04 and -05 1288 o Several more minor editorial changes. 1290 A.6. Changes between -05 and -06 1292 o Corrections to more precisely reflect RFC 2119 language 1293 requirements and closely-related issues.. 1295 Authors' Addresses 1297 John C KLENSIN 1298 1770 Massachusetts Ave, #322 1299 Cambridge, MA 02140 1300 USA 1302 Phone: +1 617 491 5735 1303 EMail: john-ietf@jck.com 1305 YangWoo KO 1306 ICU 1307 119 Munjiro 1308 Yuseong-gu, Daejeon 305-732 1309 Republic of Korea 1311 EMail: yw@mrko.pe.kr