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Yao 7 CNNIC 8 K. Fujiwara 9 JPRS 10 April 10, 2012 12 POP3 Support for UTF-8 13 draft-ietf-eai-rfc5721bis-04.txt 15 Abstract 17 This specification extends the Post Office Protocol version 3 (POP3) 18 to support un-encoded international characters in user names, 19 passwords, mail addresses, message headers, and protocol-level 20 textual strings. 22 Status of This Memo 24 This Internet-Draft is submitted in full conformance with the 25 provisions of BCP 78 and BCP 79. 27 Internet-Drafts are working documents of the Internet Engineering 28 Task Force (IETF). Note that other groups may also distribute 29 working documents as Internet-Drafts. The list of current Internet- 30 Drafts is at http://datatracker.ietf.org/drafts/current/. 32 Internet-Drafts are draft documents valid for a maximum of six months 33 and may be updated, replaced, or obsoleted by other documents at any 34 time. It is inappropriate to use Internet-Drafts as reference 35 material or to cite them other than as "work in progress." 37 This Internet-Draft will expire on October 12, 2012. 39 Copyright Notice 41 Copyright (c) 2012 IETF Trust and the persons identified as the 42 document authors. All rights reserved. 44 This document is subject to BCP 78 and the IETF Trust's Legal 45 Provisions Relating to IETF Documents 46 (http://trustee.ietf.org/license-info) in effect on the date of 47 publication of this document. Please review these documents 48 carefully, as they describe your rights and restrictions with respect 49 to this document. Code Components extracted from this document must 50 include Simplified BSD License text as described in Section 4.e of 51 the Trust Legal Provisions and are provided without warranty as 52 described in the Simplified BSD License. 54 This document may contain material from IETF Documents or IETF 55 Contributions published or made publicly available before November 56 10, 2008. The person(s) controlling the copyright in some of this 57 material may not have granted the IETF Trust the right to allow 58 modifications of such material outside the IETF Standards Process. 59 Without obtaining an adequate license from the person(s) controlling 60 the copyright in such materials, this document may not be modified 61 outside the IETF Standards Process, and derivative works of it may 62 not be created outside the IETF Standards Process, except to format 63 it for publication as an RFC or to translate it into languages other 64 than English. 66 Table of Contents 68 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 69 1.1. Conventions Used in This Document . . . . . . . . . . . . 3 70 2. LANG Capability . . . . . . . . . . . . . . . . . . . . . . . 4 71 3. UTF8 Capability . . . . . . . . . . . . . . . . . . . . . . . 6 72 3.1. The UTF8 Command . . . . . . . . . . . . . . . . . . . . . 7 73 3.2. USER Argument to UTF8 Capability . . . . . . . . . . . . . 9 74 4. Native UTF-8 Maildrops . . . . . . . . . . . . . . . . . . . . 9 75 5. UTF-8 Response Code . . . . . . . . . . . . . . . . . . . . . 9 76 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 77 7. Security Considerations . . . . . . . . . . . . . . . . . . . 10 78 8. Change History . . . . . . . . . . . . . . . . . . . . . . . . 11 79 8.1. draft-ietf-eai-rfc5721bis: Version 00 . . . . . . . . . . 11 80 8.2. draft-ietf-eai-rfc5721bis: Version 01 . . . . . . . . . . 11 81 8.3. draft-ietf-eai-rfc5721bis: Version 02 . . . . . . . . . . 11 82 8.4. draft-ietf-eai-rfc5721bis: Version 03 . . . . . . . . . . 11 83 8.5. draft-ietf-eai-rfc5721bis: Version 04 . . . . . . . . . . 11 84 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11 85 9.1. Normative References . . . . . . . . . . . . . . . . . . . 11 86 9.2. Informative References . . . . . . . . . . . . . . . . . . 13 87 Appendix A. Design Rationale . . . . . . . . . . . . . . . . . . 13 88 Appendix B. Acknowledgments . . . . . . . . . . . . . . . . . . . 13 90 1. Introduction 92 This document forms part of the Email Address Internationalization 93 (EAI) protocols described in the EAI Framework document [RFC6530]. 94 As part of the overall EAI work, email messages may be transmitted 95 and delivered containing un-encoded UTF-8 characters in the header 96 and/or body, and mail drops that are accessed using POP3 [RFC1939] 97 might natively store UTF-8. 99 This specification extends POP3 [RFC1939] using the POP3 extension 100 mechanism [RFC2449] to permit un-encoded UTF-8 [RFC3629] in headers, 101 and bodies (e.g., transferred using 8-bit Content Transfer Encoding) 102 as described in "Internationalized Email Headers" [RFC6532]. It also 103 adds a mechanism to support login names and passwords containing 104 UTF-8 characters, and a mechanism to support UTF-8 characters in 105 protocol level response strings as well as the ability to negotiate a 106 language for such response strings. 108 This specification also adds a new response code to indicate that a 109 message could not be returned because it requires UTF-8 mode and the 110 server is unwilling to down-convert. 112 Within this specification, the term "down-convert" refers to the 113 process of modifying a message containing UTF-8 headers [RFC6532] or 114 body parts with 8bit content-transfer-encoding, as defined in MIME 115 Section 2.8 [RFC2045], into conforming 7-bit Internet Message Format 116 [RFC5322] with message header extensions for non-ASCII text [RFC2047] 117 and other 7-bit encodings. The method of down-convert is specified 118 by "Post-delivery Message Downgrading for Internationalized Email 119 Messages" [popimap-downgrade] and EAI: Simplified POP/IMAP 120 downgrading [I-D.ietf-eai-simpledowngrade]. 122 This specification replaces an earlier, experimental, approach to the 123 same problem RFC 5721 [RFC5721]. Section 6 of [RFC6530] describes 124 the changes in approach between RFC 5721 [RFC5721] and this 125 specification. 127 1.1. Conventions Used in This Document 129 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 130 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 131 document are to be interpreted as described in "Key words for use in 132 RFCs to Indicate Requirement Levels" [RFC2119]. 134 In examples, "C:" and "S:" indicate lines sent by the client and 135 server, respectively. If a single "C:" or "S:" label applies to 136 multiple lines, then the line breaks between those lines are for 137 editorial clarity only and are not part of the actual protocol 138 exchange. 140 Note that examples always use 7-bit ASCII characters due to 141 limitations of this document format; in particular, some examples for 142 the "LANG" command may appear silly as a result. 144 2. LANG Capability 146 Per "POP3 Extension Mechanism" [RFC2449], this document adds a new 147 capability response tag to indicate support for a new command: LANG. 148 The capability tag and new command are described below. 150 CAPA tag: 151 LANG 153 Arguments with CAPA tag: 154 none 156 Added Commands: 157 LANG 159 Standard commands affected: 160 All 162 Announced states / possible differences: 163 both / no 165 Commands valid in states: 166 AUTHORIZATION, TRANSACTION 168 Specification reference: 169 this document 171 Discussion: 173 POP3 allows most +OK and -ERR server responses to include human- 174 readable text that, in some cases, might be presented to the user. 175 But that text is limited to ASCII by the POP3 specification 176 [RFC1939]. The LANG capability and command permit a POP3 client to 177 negotiate which language the server uses when sending human-readable 178 text. 180 A server that advertises the LANG extension MUST use the language 181 "i-default" as described in [RFC2277] as its default language until 182 another supported language is negotiated by the client. A server 183 MUST include "i-default" as one of its supported languages. 185 The LANG command requests that human-readable text included in all 186 subsequent +OK and -ERR responses be localized to a language matching 187 the language range argument (the "Basic Language Range" as described 188 by [RFC4647]). If the command succeeds, the server returns a +OK 189 response followed by a single space, the exact language tag selected, 190 another space, and the rest of the line is human-readable text in the 191 appropriate language. This and subsequent protocol-level human- 192 readable text is encoded in the UTF-8 charset. 194 If the command fails, the server returns an -ERR response and 195 subsequent human-readable response text continues to use the language 196 that was previously active (typically i-default). 198 The special "*" language range argument indicates a request to use a 199 language designated as preferred by the server administrator. The 200 preferred language MAY vary based on the currently active user. 202 If no argument is given and the POP3 server issues a positive 203 response, then the response given is multi-line. After the initial 204 +OK, for each language tag the server supports, the POP3 server 205 responds with a line for that language. This line is called a 206 "language listing". 208 In order to simplify parsing, all POP3 servers are required to use a 209 certain format for language listings. A language listing consists of 210 the language tag [RFC5646] of the message, optionally followed by a 211 single space and a human-readable description of the language in the 212 language itself, using the UTF-8 charset. 214 Examples: 216 < Note that some examples do not include the correct character 217 accents due to limitations of this document format. > 219 < The server defaults to using English i-default responses until 220 the client explicitly changes the language. > 222 C: USER karen 223 S: +OK Hello, karen 224 C: PASS password 225 S: +OK karen's maildrop contains 2 messages (320 octets) 227 < Client requests deprecated MUL language. Server replies 228 with -ERR response. > 230 C: LANG MUL 231 S: -ERR invalid language MUL 233 < A LANG command with no parameters is a request for 234 a language listing. > 236 C: LANG 237 S: +OK Language listing follows: 238 S: en English 239 S: en-boont English Boontling dialect 240 S: de Deutsch 241 S: it Italiano 242 S: es Espanol 243 S: sv Svenska 244 S: i-default Default language 245 S: . 247 < A request for a language listing might fail. > 249 C: LANG 250 S: -ERR Server is unable to list languages 252 < Once the client changes the language, all responses will be in 253 that language, starting with the response to the LANG command. > 255 C: LANG es 256 S: +OK es Idioma cambiado 258 < If a server does not support the requested primary language, 259 responses will continue to be returned in the current language 260 the server is using. > 262 C: LANG uga 263 S: -ERR es Idioma <> no es conocido 265 C: LANG sv 266 S: +OK sv Kommandot "LANG" lyckades 268 C: LANG * 269 S: +OK es Idioma cambiado 271 3. UTF8 Capability 273 Per "POP3 Extension Mechanism" [RFC2449], this document adds a new 274 capability response tag to indicate support for new server 275 functionality, including a new command: UTF8. The capability tag and 276 new command and functionality are described below. 278 CAPA tag: 279 UTF8 281 Arguments with CAPA tag: 282 USER 284 Added Commands: 285 UTF8 287 Standard commands affected: 288 USER, PASS, APOP, LIST, TOP, RETR 290 Announced states / possible differences: 291 both / no 293 Commands valid in states: 294 AUTHORIZATION 296 Specification reference: 297 this document 299 Discussion: 301 This capability adds the "UTF8" command to POP3. The UTF8 command 302 switches the session from ASCII to UTF-8 mode. In UTF-8 mode, both 303 servers and clients can send and accept UTF-8 characters. 305 3.1. The UTF8 Command 307 The UTF8 command enables UTF-8 mode. The UTF8 command has no 308 parameters. 310 Maildrops can natively store UTF-8 or be limited to ASCII. UTF-8 311 mode has no effect on messages in an ASCII-only maildrop. Messages 312 in native UTF-8 maildrops can be ASCII or UTF-8 using 313 internationalized headers [RFC6532] and/or 8bit content-transfer- 314 encoding, as defined in MIME Section 2.8 [RFC2045]. In UTF-8 mode, 315 both UTF-8 and ASCII messages are sent to the client as-is (without 316 conversion). When not in UTF-8 mode, UTF-8 messages in a native 317 UTF-8 maildrop MUST NOT be sent to the client as-is. If a client 318 requests a UTF-8 message when not in UTF-8 mode, the server MUST 319 either down-convert (downgrade) the message content it sends to the 320 client to comply with unextended POP and Internet Mail Format without 321 UTF-8 mode support, or fail the request with a -ERR response 322 containing the UTF-8 response code (see section 5). The UTF8 command 323 MAY fail. 325 Note that even in UTF-8 mode, MIME binary content-transfer-encoding 326 is still not permitted. 328 The octet count (size) of a message reported in a response to the 329 LIST command SHOULD match the actual number of octets sent in a RETR 330 response (not counting byte-stuffing). Sizes reported elsewhere, 331 such as in STAT responses and non-standardized, free-form text in 332 positive status indicators (following "+OK") need not be accurate, 333 but it is preferable if they were. 335 Mail stores are either ASCII or native UTF-8, and clients either 336 issue the UTF8 command or not. The message needs converting only 337 when it is native UTF-8 and the client has not issued the UTF8 338 command, in which case the server MAY choose to down-convert it or 339 reject the command which requested the message with the new UTF-8 340 response code (see Section 5). The down-converted message may be 341 larger. The server may choose various strategies regarding down- 342 convert, which include when to down-convert, whether to cache or 343 store the down-converted form of a message (and if so, for how long), 344 and whether to calculate or retain the size of a down-converted 345 message independently of the down-converted content. If the server 346 does not have immediate access to the accurate down-converted size, 347 it may be faster to estimate rather than calculate it. Servers are 348 expected to normally follow the RFC 1939 [RFC1939] text on using the 349 "exact size" in a scan listing, but there may be situations with 350 maildrops containing very large numbers of messages in which this 351 might be a problem. If the server does estimate, reporting a scan 352 listing size smaller than what it turns out to be could be a problem 353 for some clients. In summary, it is better for servers to report 354 accurate sizes, but if this is not possible, high guesses are better 355 than small ones. Some POP servers include the message size in the 356 non-standardized text response following "+OK" (the 'text' production 357 of RFC 2449 [RFC2449]), in a RETR or TOP response (possibly because 358 some examples in POP3 [RFC1939] do so). There has been at least one 359 known case of a client relying on this to know when it had received 360 all of the message rather than following the POP3 [RFC1939] rule of 361 looking for a line consisting of a termination octet (".") and a CRLF 362 pair. While any such client is non-compliant, if a server does 363 include the size in such text, it is better if it is accurate. 365 Clients MUST NOT issue the STLS command [RFC2595] after issuing UTF8; 366 servers MAY (but are not required to) enforce this by rejecting with 367 an "-ERR" response an STLS command issued subsequent to a successful 368 UTF8 command. (Because this is a protocol error as opposed to a 369 failure based on conditions, an extended response code [RFC2449] is 370 not specified.) 372 3.2. USER Argument to UTF8 Capability 374 If the USER argument is included with this capability, it indicates 375 that the server accepts UTF-8 user names and passwords. 377 Servers that include the USER argument in the UTF8 capability 378 response SHOULD apply SASLprep [RFC4013] to the arguments of the USER 379 and PASS commands. 381 A client or server that supports APOP and permits UTF-8 in user names 382 or passwords MUST apply SASLprep [RFC4013] to the user name and 383 password used to compute the APOP digest. 385 When applying SASLprep [RFC4013], servers MUST reject UTF-8 user 386 names or passwords that contain a Unicode character listed in Section 387 2.3 of SASLprep [RFC4013]. When applying SASLprep to the USER 388 argument, the PASS argument, or the APOP username argument, a 389 compliant server or client MUST treat them as a query string 390 [RFC3454](i.e., unassigned Unicode code points are allowed). When 391 applying SASLprep to the APOP password argument, a compliant server 392 or client MUST treat them as a stored string [RFC3454] (i.e., 393 unassigned Unicode code points are prohibited). 395 The client does not need to issue the UTF8 command prior to using 396 UTF-8 in authentication. However, clients MUST NOT use UTF-8 397 characters in USER, PASS, or APOP commands unless the USER argument 398 is included in the UTF8 capability response. 400 The server MUST reject UTF-8 user names or passwords that fail to 401 comply with the formal syntax in UTF-8 [RFC3629]. 403 Use of UTF-8 characters in the AUTH command is governed by the POP3 404 SASL [RFC5034] mechanism. 406 4. Native UTF-8 Maildrops 408 When a POP3 server uses a native UTF-8 maildrop, it is the 409 responsibility of the server to comply with the POP3 base 410 specification [RFC1939] and Internet Message Format [RFC5322] when 411 not in UTF-8 mode. Mechanisms for 7-bit downgrading to help comply 412 with the standards are described in [popimap-downgrade] and 413 [I-D.ietf-eai-simpledowngrade]. The implementors MAY choose one of 414 them to implement downgrade. 416 5. UTF-8 Response Code 418 Per "POP3 Extension Mechanism" [RFC2449], this document adds a new 419 response code: UTF-8, described below. 421 Complete response code: 422 UTF8 424 Valid for responses: 425 -ERR 427 Valid for commands: 428 LIST, TOP, RETR 430 Response code meaning and expected client behavior: 432 The UTF-8 response code indicates that a failure is due to a request 433 when not in UTF-8 mode for message content containing UTF-8 434 characters. 436 The client MAY reissue the command after entering UTF-8 mode (or wait 437 for the server to be in a better mood and willing to downconvert). 439 6. IANA Considerations 441 The section 2 and 3 of this specification update two capabilities 442 ("UTF8" and "LANG") to the POP3 capability registry [RFC2449]. 444 The section 5 of this specification also adds one new response code 445 ("UTF-8") to the POP3 response codes registry [RFC2449]. 447 7. Security Considerations 449 The security considerations of UTF-8 [RFC3629] and SASLprep [RFC4013] 450 apply to this specification, particularly with respect to use of 451 UTF-8 in user names and passwords. 453 The "LANG *" command might reveal the existence and preferred 454 language of a user to an active attacker probing the system if the 455 active language changes in response to the USER, PASS, or APOP 456 commands prior to validating the user's credentials. Servers MUST 457 implement a configuration to prevent this exposure. 459 It is possible for a man-in-the-middle attacker to insert a LANG 460 command in the command stream, thus making protocol-level diagnostic 461 responses unintelligible to the user. A mechanism to integrity- 462 protect the session, such as Transport Layer Security (TLS) [RFC2595] 463 can be used to defeat such attacks. 465 Modifying server authentication code (in this case, to support UTF8 466 command) needs to be done with care to avoid introducing 467 vulnerabilities (for example, in string parsing). 469 The UTF8 command description (Section 3.1) contains a discussion on 470 reporting inaccurate sizes. An additional risk to doing so is that, 471 if a client allocates buffers based on the reported size, it may 472 overrun the buffer, crash, or have other problems if the message data 473 is larger than reported. 475 8. Change History 477 8.1. draft-ietf-eai-rfc5721bis: Version 00 479 following the new charter 481 8.2. draft-ietf-eai-rfc5721bis: Version 01 483 refine the texts 485 8.3. draft-ietf-eai-rfc5721bis: Version 02 487 update the texts based on Joseph's comments 489 8.4. draft-ietf-eai-rfc5721bis: Version 03 491 improve the texts 493 text instructing servers to either downconvert or reject 495 new UTF-8 response code for use 497 8.5. draft-ietf-eai-rfc5721bis: Version 04 499 improve the texts 501 9. References 503 9.1. Normative References 505 [I-D.ietf-eai-simpledowngrade] Gulbrandsen, A., "EAI: Simplified 506 POP/IMAP downgrading", 507 draft-ietf-eai-simpledowngrade-03 508 (work in progress), March 2012. 510 [RFC1939] Myers, J. and M. Rose, "Post Office 511 Protocol - Version 3", STD 53, 512 RFC 1939, May 1996. 514 [RFC2045] Freed, N. and N. Borenstein, 515 "Multipurpose Internet Mail 516 Extensions (MIME) Part One: Format of 517 Internet Message Bodies", RFC 2045, 518 November 1996. 520 [RFC2047] Moore, K., "MIME (Multipurpose 521 Internet Mail Extensions) Part Three: 522 Message Header Extensions for Non- 523 ASCII Text", RFC 2047, November 1996. 525 [RFC2119] Bradner, S., "Key words for use in 526 RFCs to Indicate Requirement Levels", 527 BCP 14, RFC 2119, March 1997. 529 [RFC2277] Alvestrand, H., "IETF Policy on 530 Character Sets and Languages", 531 BCP 18, RFC 2277, January 1998. 533 [RFC2449] Gellens, R., Newman, C., and L. 534 Lundblade, "POP3 Extension 535 Mechanism", RFC 2449, November 1998. 537 [RFC3454] Hoffman, P. and M. Blanchet, 538 "Preparation of Internationalized 539 Strings ("stringprep")", RFC 3454, 540 December 2002. 542 [RFC3629] Yergeau, F., "UTF-8, a transformation 543 format of ISO 10646", STD 63, 544 RFC 3629, November 2003. 546 [RFC4013] Zeilenga, K., "SASLprep: Stringprep 547 Profile for User Names and 548 Passwords", RFC 4013, February 2005. 550 [RFC4647] Phillips, A. and M. Davis, "Matching 551 of Language Tags", BCP 47, RFC 4647, 552 September 2006. 554 [RFC5322] Resnick, P., Ed., "Internet Message 555 Format", RFC 5322, October 2008. 557 [RFC5646] Phillips, A. and M. Davis, "Tags for 558 Identifying Languages", BCP 47, 559 RFC 5646, September 2009. 561 [RFC6530] Klensin, J. and Y. Ko, "Overview and 562 Framework for Internationalized 563 Email", RFC 6530, February 2012. 565 [RFC6532] Yang, A., Steele, S., and N. Freed, 566 "Internationalized Email Headers", 567 RFC 6532, February 2012. 569 [popimap-downgrade] Fujiwara, K., "Post-delivery Message 570 Downgrading for Internationalized 571 Email Messages", 572 draft-ietf-eai-popimap-downgrade-04 573 (work in progress), October 2011. 575 9.2. Informative References 577 [RFC2595] Newman, C., "Using TLS with IMAP, 578 POP3 and ACAP", RFC 2595, June 1999. 580 [RFC5034] Siemborski, R. and A. Menon-Sen, "The 581 Post Office Protocol (POP3) Simple 582 Authentication and Security Layer 583 (SASL) Authentication Mechanism", 584 RFC 5034, July 2007. 586 [RFC5721] Gellens, R. and C. Newman, "POP3 587 Support for UTF-8", RFC 5721, 588 February 2010. 590 Appendix A. Design Rationale 592 This non-normative section discusses the reasons behind some of the 593 design choices in the above specification. 595 Due to interoperability problems with RFC 2047 and limited deployment 596 of RFC 2231, it is hoped these 7-bit encoding mechanisms can be 597 deprecated in the future when UTF-8 header support becomes prevalent. 599 USER is optional because the implementation burden of SASLprep 600 [RFC4013] is not well understood, and mandating such support in all 601 cases could negatively impact deployment. 603 Appendix B. Acknowledgments 605 Thanks to John Klensin, Joseph Yee, Tony Hansen, Alexey Melnikov and 606 other EAI working group participants who provided helpful suggestions 607 and interesting debate that improved this specification. 609 Authors' Addresses 611 Randall Gellens 612 QUALCOMM Incorporated 613 5775 Morehouse Drive 614 San Diego, CA 92651 615 US 617 EMail: rg+ietf@qualcomm.com 619 Chris Newman 620 Oracle 621 800 Royal Oaks 622 Monrovia, CA 91016-6347 623 US 625 EMail: chris.newman@oracle.com 627 Jiankang YAO 628 CNNIC 629 No.4 South 4th Street, Zhongguancun 630 Beijing 632 Phone: +86 10 58813007 633 EMail: yaojk@cnnic.cn 635 Kazunori Fujiwara 636 Japan Registry Services Co., Ltd. 637 Chiyoda First Bldg. East 13F, 3-8-1 Nishi-Kanda 638 Tokyo 640 Phone: +81 3 5215 8451 641 EMail: fujiwara@jprs.co.jp