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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group R. Gellens 3 Internet-Draft QUALCOMM Incorporated 4 Updates: 1939 (if approved) C. Newman 5 Intended status: Experimental Sun Microsystems 6 Expires: April 9, 2010 October 6, 2009 8 POP3 Support for UTF-8 9 draft-ietf-eai-pop-07.txt 11 Status of this Memo 13 This Internet-Draft is submitted to IETF in full conformance with the 14 provisions of BCP 78 and BCP 79. This document may contain material 15 from IETF Documents or IETF Contributions published or made publicly 16 available before November 10, 2008. The person(s) controlling the 17 copyright in some of this material may not have granted the IETF 18 Trust the right to allow modifications of such material outside the 19 IETF Standards Process. Without obtaining an adequate license from 20 the person(s) controlling the copyright in such materials, this 21 document may not be modified outside the IETF Standards Process, and 22 derivative works of it may not be created outside the IETF Standards 23 Process, except to format it for publication as an RFC or to 24 translate it into languages other than English. 26 Internet-Drafts are working documents of the Internet Engineering 27 Task Force (IETF), its areas, and its working groups. Note that 28 other groups may also distribute working documents as Internet- 29 Drafts. 31 Internet-Drafts are draft documents valid for a maximum of six months 32 and may be updated, replaced, or obsoleted by other documents at any 33 time. It is inappropriate to use Internet-Drafts as reference 34 material or to cite them other than as "work in progress." 36 The list of current Internet-Drafts can be accessed at 37 http://www.ietf.org/ietf/1id-abstracts.txt. 39 The list of Internet-Draft Shadow Directories can be accessed at 40 http://www.ietf.org/shadow.html. 42 This Internet-Draft will expire on April 9, 2010. 44 Copyright Notice 46 Copyright (c) 2009 IETF Trust and the persons identified as the 47 document authors. All rights reserved. 49 This document is subject to BCP 78 and the IETF Trust's Legal 50 Provisions Relating to IETF Documents in effect on the date of 51 publication of this document (http://trustee.ietf.org/license-info). 52 Please review these documents carefully, as they describe your rights 53 and restrictions with respect to this document. 55 Abstract 57 This specification extends the Post Office Protocol version 3 (POP3) 58 to support un-encoded international characters in user names, 59 passwords, mail addresses, message headers, and protocol-level 60 textual error strings. 62 Table of Contents 64 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 65 1.1. Conventions Used in this Document . . . . . . . . . . . . 3 66 1.2. Change History . . . . . . . . . . . . . . . . . . . . . . 3 67 1.2.1. Changes from -06 to -07 . . . . . . . . . . . . . . . 3 68 1.2.2. Changes from -05 to -06 . . . . . . . . . . . . . . . 4 69 1.2.3. Changes from -04 to -05 . . . . . . . . . . . . . . . 4 70 1.2.4. Changes from -03 to -04 . . . . . . . . . . . . . . . 5 71 1.2.5. Changes from -02 to -03 . . . . . . . . . . . . . . . 5 72 1.2.6. Changes from -01 to -02 . . . . . . . . . . . . . . . 5 73 1.2.7. Changes from -00 to -01 . . . . . . . . . . . . . . . 6 74 1.2.8. Changes from draft-newman-ima-pop . . . . . . . . . . 6 75 1.3. Open Issues . . . . . . . . . . . . . . . . . . . . . . . 6 76 2. LANG Capability . . . . . . . . . . . . . . . . . . . . . . . 6 77 3. UTF8 Capability . . . . . . . . . . . . . . . . . . . . . . . 9 78 3.1. The UTF8 Command . . . . . . . . . . . . . . . . . . . . . 9 79 3.2. USER Argument to UTF8 Capability . . . . . . . . . . . . . 11 80 4. Issues with UTF-8 Header maildrop . . . . . . . . . . . . . . 11 81 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 82 6. Security Considerations . . . . . . . . . . . . . . . . . . . 12 83 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12 84 7.1. Normative References . . . . . . . . . . . . . . . . . . . 12 85 7.2. Informative References . . . . . . . . . . . . . . . . . . 13 86 Appendix A. Design Rationale . . . . . . . . . . . . . . . . . . 13 87 Appendix B. Acknowledgments . . . . . . . . . . . . . . . . . . . 14 88 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14 90 1. Introduction 92 This specification extends POP3 [RFC1939] using the POP3 Extension 93 Mechanism [RFC2449] to permit un-encoded UTF-8 [RFC3629] in headers 94 as described in Internationalized Email Headers [RFC5335]. It also 95 adds a mechanism to support login names outside the ASCII character 96 set, and a mechanism to support UTF-8 protocol-level error strings in 97 a language appropriate for the user. 99 Within this specification, the term down-conversion refers to the 100 process of modifying a message containing UTF8 headers [RFC5335] or 101 body parts with 8bit content-transfer-encoding as defined in MIME 102 section 2.8 [RFC2045] into conforming 7-bit Internet Message Format 103 [RFC5322] with Message Header Extensions for Non-ASCII Text [RFC2047] 104 and other 7-bit encodings. Down-conversion is specified by 105 Downgrading mechanism for Email Address Internationalization 106 [RFC5504]. 108 1.1. Conventions Used in this Document 110 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 111 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 112 document are to be interpreted as described in "Key words for use in 113 RFCs to Indicate Requirement Levels" [RFC2119]. 115 The formal syntax uses the Augmented Backus-Naur Form (ABNF) 116 [RFC5234] notation including the core rules defined in Appendix B of 117 RFC 5234. 119 In examples, "C:" and "S:" indicate lines sent by the client and 120 server respectively. If a single "C:" or "S:" label applies to 121 multiple lines, then the line breaks between those lines are for 122 editorial clarity only and are not part of the actual protocol 123 exchange. 125 1.2. Change History 127 This section describes the change history of this Internet draft and 128 will be removed when/if this is published as an RFC. 130 1.2.1. Changes from -06 to -07 132 o Added discussion about accuracy of size. 134 o Added mention of potential buffer overflow problems because of 135 inaccurate sizes to the Security Considerations. 137 o Added informative reference to SASL for POP3 (RFC 5034). 139 o Removed text making changes to AUTH, as this is handled by POP3 140 SASL. 142 o Fixed typo ("depricated" instead of "deprecated"). 144 o Reworded Design Rationale appendix. 146 1.2.2. Changes from -05 to -06 148 o Removed LIST and TOP as possible arguments to the UTF8 tag in the 149 CAPA response. 151 o Clarified that the UTF8 command has no parameters. 153 o Changed "arguments" to "arguments with CAPA tag" to clarify that 154 these are possible arguments to the tag in the CAPA response and 155 not command parameters. 157 o Clarified use of "argument" to refer to CAPA tag and "parameter" 158 to refer to commands. 160 o Clarified that free-form text is non-standard. 162 o Removed open issue (downgrading). 164 o Added discussion of downgrading to Appendix A. 166 o Updated downgrade reference to RFC 5504. 168 o Tweaked RFC 2119 text to satisfy I-D nit checker. 170 1.2.3. Changes from -04 to -05 172 o Downgrading is back to an informative, not normative reference, 173 and is suggested as a good idea but explicitly not required. 175 o Language listing now specifies that the human-readable description 176 of a language is in the language itself. 178 o Updated 2822 reference to 5322, made text "Internet Message 179 Format". 181 o Updated reference to utf8headers draft to RFC5335. 183 o Updated reference to RFC4234 to RFC5234. 185 1.2.4. Changes from -03 to -04 187 o Specified that it is an error to issue STLS after UTF8. 189 o Removed prior open issues. 191 o Downgrading added as open issue. 193 1.2.5. Changes from -02 to -03 195 o Updated references. 197 o Replaced US-ASCII with ASCII. 199 o Added comment to language listing failure example. 201 o Replaced RET8, LST8, and TOP8 commands with a single mode-switch 202 UTF8 command issued before authentication. This simplifies the 203 protocol, and allows servers to optionally down-convert a cache of 204 the maildrop prior to issuing the +OK response entering 205 TRANSACTION state. 207 o Removed most up-conversion material. 209 o Removed definition of up-conversion. 211 o Removed IMAP4 reference. 213 o Added AUTH command to those affected by UTF8 capability. 215 o Removed LST8 and TOP8 capability parameters and commands. 217 o Removed NO-RETR capability. POP servers are now unconditionally 218 required to support down-conversion of UTF8-native maildrops. 220 o Added sentence about modifying authentication code to Security 221 Considerations. 223 o eai-downgrade draft is now normative and required. 225 o Deleted references to RFCs 1341, 1847, 2049, 2183, 3501, 3516, and 226 3490. 228 1.2.6. Changes from -01 to -02 230 o Minor grammatical tweaks. 232 o Add passwords to Abstract. 234 o Removed new editor's name from Acknowledgments. 236 1.2.7. Changes from -00 to -01 238 o Update references 240 1.2.8. Changes from draft-newman-ima-pop 242 o Change title to make this a WG document. 244 o Add LANG command and extension. 246 o Rename RET8 capability to UTF8 and add sub-sections for arguments. 248 o Add TOP8 command. 250 o Add definition of up-conversion and down-conversion. 252 o Some grammar fix-ups and section re-ordering based on RFC editor 253 style. 255 1.3. Open Issues 257 1. none 259 2. LANG Capability 261 CAPA tag: 262 LANG 264 Arguments with CAPA tag: 265 none 267 Added Commands: 268 LANG 270 Standard commands affected: 271 All 273 Announced states / possible differences: 274 both / no 276 Commands valid in states: 277 AUTHENTICATION, TRANSACTION 279 Specification reference: 280 this document 282 Discussion: 284 POP3 allows most +OK and -ERR server responses to include human- 285 readable text that in some cases needs to be presented to the user. 286 But that text is limited to ASCII by the POP3 specification 287 [RFC1939]. The LANG capability and command permit a POP3 client to 288 negotiate which language the server should use when sending human- 289 readable text. 291 A server that advertises the LANG extension MUST use the language 292 "i-default" as described in [RFC2277] as its default language until 293 another supported language is negotiated by the client. A server 294 MUST include "i-default" as one of its supported languages. 296 The LANG command requests that human-readable text included in all 297 subsequent +OK and -ERR responses be localized to a language matching 298 the language range argument as described by [RFC4647]. If the 299 command succeeds, the server returns a +OK response followed by a 300 single space, the exact language tag selected, another space, and the 301 rest of the line is human-readable text in the appropriate language. 302 This and subsequent protocol-level human readable text is encoded in 303 the UTF-8 charset. 305 If the command fails, the server returns an -ERR response and 306 subsequent human-readable response text continues to use the language 307 that was previously active (typically i-default). 309 The special "*" language range argument indicates a request to use a 310 language designated as preferred by the server administrator. The 311 preferred language MAY vary based on the currently active user. 313 If no argument is given and the POP3 server issues a positive 314 response, then the response given is multi-line. After the initial 315 +OK, for each language tag the server supports, the POP3 server 316 responds with a line for that language. This line is called a 317 "language listing". 319 In order to simplify parsing, all POP3 servers are required to use a 320 certain format for language listings. A language listing consists of 321 the language tag [RFC4646] of the message, optionally followed by a 322 single space and a human readable description of the language in the 323 language itself, using the UTF-8 charset. 325 < The server defaults to using English i-default responses until 326 the client explicitly changes the language. > 328 C: USER karen 329 S: +OK Hello, karen 330 C: PASS password 331 S: +OK karen's maildrop contains 2 messages (320 octets) 333 < Client requests deprecated MUL language. Server replies 334 with -ERR response > 336 C: LANG MUL 337 S: -ERR invalid language MUL 339 < A LANG command with no parameters is a request for 340 a language listing. > 342 C: LANG 343 S: +OK Language listing follows: 344 S: en English 345 S: en-boont English Boontling dialect 346 S: de Deutsch 347 S: it Italiano 348 S: i-default Default language 349 S: . 351 < A request for a language listing might fail > 353 C: LANG 354 S: -ERR Server is unable to list languages 356 < Once the client changes the language, all responses will be in 357 that language starting with the response to the LANG command. 358 Note: the example does not include the correct character accents 359 due to limitations of this document format. > 361 C: LANG fr 362 S: +OK fr La Language commande a ete execute avec success 364 < If a server does not support the requested primary language, 365 responses will continue to be returned in the current language 366 the server is using. > 368 C: LANG uga 369 S: -ERR Ce Language n'est pas supporte 371 C: LANG fr-ca 372 S: +OK fr La Language commande a ete execute avec success 373 C: LANG * 374 S: +OK fr La Language commande a ete execute avec success 376 Examples 378 3. UTF8 Capability 380 CAPA tag: 381 UTF8 383 Arguments with CAPA tag: 384 USER 386 Added Commands: 387 UTF8 389 Standard commands affected: 390 USER, PASS, APOP, LIST, TOP, RETR 392 Announced states / possible differences: 393 both / no 395 Commands valid in states: 396 AUTHORIZATION 398 Specification reference: 399 this document 401 Discussion: 403 This capability adds the "UTF8" command to POP3. The UTF8 command 404 switches the session from ASCII to UTF8 mode. 406 3.1. The UTF8 Command 408 The UTF8 command enables UTF8 mode. The UTF8 command has no 409 parameters. 411 Maildrops can natively store UTF8 or be limited to ASCII. UTF8 mode 412 has no effect on messages in an ACII-only maildrop. Messages in 413 native-UTF8 maildrops can be ASCII or UTF8 using internationalized 414 headers [RFC5335] and/or 8bit content-transfer-encoding as defined in 415 MIME section 2.8 [RFC2045]. In UTF8 mode, both UTF8 and ASCII 416 messages are sent to the client as-is (without conversion). When not 417 in UTF8 mode, UTF8 messages in a native UTF8 maildrop MUST be down- 418 converted (downgraded) to comply with unextended POP and Internet 419 Mail Format. POP servers (unlike SMTP and Submit servers) are not 420 required to use Downgrading mechanism for Email Address 421 Internationalization [RFC5504]. 423 Discussion: The main argument against a single required mechanism for 424 downgrade by a POP server is that the only clients that have any use 425 for a standardized downgraded message (because they wish to interpret 426 downgrade headers, for example) are ones that can support UTF8 and 427 hence will issue the UTF8 command in the first place. The counter 428 argument to this is that non-UTF8 clients might be upgraded in the 429 future; it's desirable for an upgraded client to be capable of 430 interpreting prior downgraded messages in the local mail store, which 431 is most likely if the messages were downgraded using one standardized 432 procedure. 434 Therefore, while POP servers are not required to use the Downgrading 435 mechanism for Email Address Internationalization [RFC5504], there are 436 advantages to them doing so. 438 Note that even in UTF8 mode, MIME binary content-transfer-encoding is 439 still not permitted. 441 The octet count (size) of a message reported in a response to the 442 LIST command SHOULD match the actual number of octets sent in a RETR 443 response. Sizes reported elsewhere, such as in STAT responses and 444 non-standardized free-form text in positive status indicators 445 (following "+OK") need not be accurate, but it is preferable if they 446 are. 448 Discussion: Mail stores are either ASCII or native UTF-8, and clients 449 either issue the UTF8 command or not. The message needs converting 450 only when it is native UTF8 and the client has not issued the UTF8 451 command, in which case the server must downconvert it. The 452 downconverted message may be larger. The server may choose various 453 strategies regarding downconversion, which include when to 454 downconvert, whether to cache or store the downconverted form of a 455 message (and if so, for how long), and whether to calculate or retain 456 the size of a downconverted message independently of the 457 downconverted content. If the server does not have immediate access 458 to the accurate downconverted size, it may be faster to estimate 459 rather than calculate it. Servers are expected to normally follow 460 the RFC 1939 [RFC1939] text on using the "exact size" in a scan 461 listing, but there may be situations with maildrops containing very 462 large numbers of messages in which this might be a problem. If the 463 server does estimate, reporting a scan listing size smaller than what 464 it turns out to be could be a problem for some clients. In summary, 465 it is better for servers to report accurate sizes, but if not, high 466 guesses are better than small ones. Some POP servers include the 467 message size in the non-standardized text response following "+OK" 468 (the 'text' production of RFC 2449 [RFC2449]), in a RETR or TOP 469 response (possibly because some examples in POP3 [RFC1939] do so). 470 There has been at least one known case of a client relying on this to 471 know when it had received all of the message rather than following 472 the POP3 [RFC1939] rule of looking for a line consisting of a 473 termination octet (".") and a CRLF pair. While any such client is 474 non-compliant, if a server does include the size in such text, it is 475 better if it is accurate. 477 Clients MUST NOT issue the STLS command [RFC2595] after issuing UTF8; 478 servers MAY (but are not required to) enforce this by rejecting with 479 an "-ERR" response an STLS command issued subsequent to a successful 480 UTF8 command. (Because this is a protocol error as opposed to a 481 failure based on conditions, an extended response code [RFC2449] is 482 not specified.) 484 3.2. USER Argument to UTF8 Capability 486 If the USER argument is included with this capability, it indicates 487 that the server accepts UTF-8 user names and passwords and applies 488 SASLprep [RFC4013] to the arguments of the USER, PASS and APOP 489 commands. A client that supports APOP and permits UTF-8 in user 490 names or passwords MUST also implement SASLprep [RFC4013] on the user 491 name and password used to compute the APOP digest. 493 The client does not need to issue the UTF8 command prior to using 494 UTF8 in authentication. However, clients MUST NOT use UTF8 in USER, 495 PASS, or APOP commands unless the USER argument is included with the 496 UTF8 capability. 498 Use of UTF8 in the AUTH command is governed by the POP3 SASL 499 [RFC5034] mechanism. 501 4. Issues with UTF-8 Header maildrop 503 When a POP3 server uses a UTF8-native maildrop, it is the 504 responsibility of the server to comply with the POP3 base 505 specification [RFC1939] and Internet Message Format [RFC5322] when 506 not in UTF8 mode. Mechanisms for 7-bit downgrading to help comply 507 with the standards are described in Downgrading mechanism for Email 508 Address Internationalization [RFC5504]. 510 5. IANA Considerations 512 This adds two new capabilities ("UTF8" and "LANG") to the POP3 513 capability registry [RFC2449]. 515 6. Security Considerations 517 The security considerations of UTF-8 [RFC3629] and SASLprep [RFC4013] 518 apply to this specification, particularly with respect to use of 519 UTF-8 in user names and passwords. 521 The "LANG *" command can reveal the existence and preferred language 522 of a user to an active attacker probing the system if the active 523 language changes in response to the USER, PASS, or APOP commands 524 prior to validating the user's credentials. Servers MUST implement a 525 configuration to prevent this exposure. 527 It is possible for a man-in-the-middle attacker to insert a LANG 528 command in the command stream thus making protocol-level diagnostic 529 responses unintelligible to the user. A mechanism to integrity 530 protect the session, such as TLS [RFC2595] can be used to defeat such 531 attacks. 533 Modifying server authentication code (in this case, to support UTF8) 534 needs to be done with care to avoid introducing vulnerabilities (for 535 example, in string parsing). 537 The UTF8 Command (Section 3.1) description contains a discussion on 538 reporting inaccurate sizes. An additional risk to doing so is that, 539 if a client allocates buffers based on the reported size, it may 540 overrun the buffer, crash, or have other problems if the message data 541 is larger than reported. 543 7. References 545 7.1. Normative References 547 [RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3", 548 STD 53, RFC 1939, May 1996. 550 [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail 551 Extensions (MIME) Part One: Format of Internet Message 552 Bodies", RFC 2045, November 1996. 554 [RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions) 555 Part Three: Message Header Extensions for Non-ASCII Text", 556 RFC 2047, November 1996. 558 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 559 Requirement Levels", BCP 14, RFC 2119, March 1997. 561 [RFC2277] Alvestrand, H., "IETF Policy on Character Sets and 562 Languages", BCP 18, RFC 2277, January 1998. 564 [RFC2449] Gellens, R., Newman, C., and L. Lundblade, "POP3 Extension 565 Mechanism", RFC 2449, November 1998. 567 [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, 568 October 2008. 570 [RFC4646] Phillips, A. and M. Davis, "Tags for Identifying 571 Languages", BCP 47, RFC 4646, September 2006. 573 [RFC4647] Phillips, A. and M. Davis, "Matching of Language Tags", 574 BCP 47, RFC 4647, September 2006. 576 [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 577 10646", STD 63, RFC 3629, November 2003. 579 [RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User Names 580 and Passwords", RFC 4013, February 2005. 582 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 583 Specifications: ABNF", STD 68, RFC 5234, January 2008. 585 [RFC5335] Abel, Y., "Internationalized Email Headers", RFC 5335, 586 September 2008. 588 7.2. Informative References 590 [RFC2595] Newman, C., "Using TLS with IMAP, POP3 and ACAP", 591 RFC 2595, June 1999. 593 [RFC5034] Siemborski, R. and A. Menon-Sen, "The Post Office Protocol 594 (POP3) Simple Authentication and Security Layer (SASL) 595 Authentication Mechanism", RFC 5034, July 2007. 597 [RFC5504] Fujiwara, K. and Y. Yoneya, "Downgrading Mechanism for 598 Email Address Internationalization", RFC 5504, March 2009. 600 Appendix A. Design Rationale 602 This non-normative section discusses the reasons behind some of the 603 design choices in the above specification. 605 Having servers perform up-conversion so that, at a minimum, RFC2047- 606 encoded words are decoded into UTF8 is tempting, since this is an 607 area that clients often fail to correctly implement. However, after 608 much discussion the group felt that the benefits did not justify the 609 burden. 611 Due to interoperability problems with RFC 2047 and limited deployment 612 of RFC 2231, it is hoped these 7-bit encoding mechanisms can be 613 deprecated in the future when UTF-8 header support becomes prevalent. 615 USER is optional because the implementation burden of SASLprep 616 [RFC4013] is not well understood and mandating such support in all 617 cases could negatively impact deployment. 619 While it is possible to provide useful examples for language 620 negotiation without support for non-ASCII characters, it is difficult 621 to provide useful examples for commands specifically designed to use 622 the UTF-8 charset un-encoded when the document format is limited to 623 ASCII. As a result, there are no plans to provide examples for that 624 part of the specification as long as this remains an experimental 625 proposal. However, implementers of this specification are encouraged 626 to provide examples to the document author for a future revision. 628 While down-conversion of native-UTF8 messages is mandatory in the 629 absence of the UTF8 command, servers are not required to do so as 630 specified in Downgrading Mechanism [RFC5504]. As clients are 631 upgraded with UTF8 support and the ability to intelligently handle 632 (e.g., display and reply to) UTF8 messages that were downgraded in 633 transit, it is better if they are also able to handle messages in the 634 local mail store that were downgraded by the POP server. This is 635 more likely if the POP server downgrades messages using the same 636 mechanism as an SMTP server. 638 Appendix B. Acknowledgments 640 Thanks to John Klensin, Tony Hansen and other EAI working group 641 participants who provided helpful suggestions and interesting debate 642 that improved this specification. 644 Authors' Addresses 646 Randall Gellens 647 QUALCOMM Incorporated 648 5775 Morehouse Drive 649 San Diego, CA 92651 650 US 652 Email: rg+ietf@qualcomm.com 653 Chris Newman 654 Sun Microsystems 655 800 Royal Oaks 656 Monrovia, CA 91016-6347 657 US 659 Email: chris.newman@sun.com