7Network Working Group P. Resnick
8Request for Comments: 5738 Qualcomm Incorporated
10Category: Experimental Sun Microsystems
14 IMAP Support for UTF-8
18 This specification extends the Internet Message Access Protocol
19 version 4rev1 (IMAP4rev1) to support UTF-8 encoded international
20 characters in user names, mail addresses, and message headers.
24 This document is not an Internet Standards Track specification; it is
25 published for examination, experimental implementation, and
28 This document defines an Experimental Protocol for the Internet
29 community. This document is a product of the Internet Engineering
30 Task Force (IETF). It represents the consensus of the IETF
31 community. It has received public review and has been approved for
32 publication by the Internet Engineering Steering Group (IESG). Not
33 all documents approved by the IESG are a candidate for any level of
34 Internet Standard; see Section 2 of RFC 5741.
36 Information about the current status of this document, any errata,
37 and how to provide feedback on it may be obtained at
38 http://www.rfc-editor.org/info/rfc5738.
42 Copyright (c) 2010 IETF Trust and the persons identified as the
43 document authors. All rights reserved.
45 This document is subject to BCP 78 and the IETF Trust's Legal
46 Provisions Relating to IETF Documents
47 (http://trustee.ietf.org/license-info) in effect on the date of
48 publication of this document. Please review these documents
49 carefully, as they describe your rights and restrictions with respect
50 to this document. Code Components extracted from this document must
51 include Simplified BSD License text as described in Section 4.e of
52 the Trust Legal Provisions and are provided without warranty as
53 described in the Simplified BSD License.
58Resnick & Newman Experimental [Page 1]
60RFC 5738 IMAP Support for UTF-8 March 2010
63 This document may contain material from IETF Documents or IETF
64 Contributions published or made publicly available before November
65 10, 2008. The person(s) controlling the copyright in some of this
66 material may not have granted the IETF Trust the right to allow
67 modifications of such material outside the IETF Standards Process.
68 Without obtaining an adequate license from the person(s) controlling
69 the copyright in such materials, this document may not be modified
70 outside the IETF Standards Process, and derivative works of it may
71 not be created outside the IETF Standards Process, except to format
72 it for publication as an RFC or to translate it into languages other
77 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
78 2. Conventions Used in This Document . . . . . . . . . . . . . . 3
79 3. UTF8=ACCEPT IMAP Capability . . . . . . . . . . . . . . . . . 3
80 3.1. IMAP UTF-8 Quoted Strings . . . . . . . . . . . . . . . . 3
81 3.2. UTF8 Parameter to SELECT and EXAMINE . . . . . . . . . . . 5
82 3.3. UTF-8 LIST and LSUB Responses . . . . . . . . . . . . . . 5
83 3.4. UTF-8 Interaction with IMAP4 LIST Command Extensions . . . 6
84 3.4.1. UTF8 and UTF8ONLY LIST Selection Options . . . . . . . 6
85 3.4.2. UTF8 LIST Return Option . . . . . . . . . . . . . . . 6
86 4. UTF8=APPEND Capability . . . . . . . . . . . . . . . . . . . . 7
87 5. UTF8=USER Capability . . . . . . . . . . . . . . . . . . . . . 7
88 6. UTF8=ALL Capability . . . . . . . . . . . . . . . . . . . . . 7
89 7. UTF8=ONLY Capability . . . . . . . . . . . . . . . . . . . . . 8
90 8. Up-Conversion Server Requirements . . . . . . . . . . . . . . 8
91 9. Issues with UTF-8 Header Mailstore . . . . . . . . . . . . . . 9
92 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
93 11. Security Considerations . . . . . . . . . . . . . . . . . . . 11
94 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
95 12.1. Normative References . . . . . . . . . . . . . . . . . . . 11
96 12.2. Informative References . . . . . . . . . . . . . . . . . . 13
97 Appendix A. Design Rationale . . . . . . . . . . . . . . . . . . 14
98 Appendix B. Examples Demonstrating Relationships between
99 UTF8= Capabilities . . . . . . . . . . . . . . . . . 15
100 Appendix C. Acknowledgments . . . . . . . . . . . . . . . . . . . 15
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121 This specification extends IMAP4rev1 [RFC3501] to permit UTF-8
122 [RFC3629] in headers as described in "Internationalized Email
123 Headers" [RFC5335]. It also adds a mechanism to support mailbox
124 names, login names, and passwords using the UTF-8 charset. This
125 specification creates five new IMAP capabilities to allow servers to
126 advertise these new extensions, along with two new IMAP LIST
127 selection options and a new IMAP LIST return option.
1292. Conventions Used in This Document
131 The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
132 in this document are to be interpreted as defined in "Key words for
133 use in RFCs to Indicate Requirement Levels" [RFC2119].
135 The formal syntax uses the Augmented Backus-Naur Form (ABNF)
136 [RFC5234] notation including the core rules defined in Appendix B of
137 [RFC5234]. In addition, rules from IMAP4rev1 [RFC3501], UTF-8
138 [RFC3629], "Collected Extensions to IMAP4 ABNF" [RFC4466], and IMAP4
139 LIST Command Extensions [RFC5258] are also referenced.
141 In examples, "C:" and "S:" indicate lines sent by the client and
142 server, respectively. If a single "C:" or "S:" label applies to
143 multiple lines, then the line breaks between those lines are for
144 editorial clarity only and are not part of the actual protocol
1473. UTF8=ACCEPT IMAP Capability
149 The "UTF8=ACCEPT" capability indicates that the server supports UTF-8
150 quoted strings, the "UTF8" parameter to SELECT and EXAMINE, and UTF-8
151 responses from the LIST and LSUB commands.
153 A client MUST use the "ENABLE UTF8=ACCEPT" command (defined in
154 [RFC5161]) to indicate to the server that the client accepts UTF-8
155 quoted-strings. The "ENABLE UTF8=ACCEPT" command MUST only be used
156 in the authenticated state. (Note that the "UTF8=ONLY" capability
157 described in Section 7 and the "UTF8=ALL" capability described in
158 Section 6 imply the "UTF8=ACCEPT" capability. See additional
159 information in these sections.)
1613.1. IMAP UTF-8 Quoted Strings
163 The IMAP4rev1 [RFC3501] base specification forbids the use of 8-bit
164 characters in atoms or quoted strings. Thus, a UTF-8 string can only
165 be sent as a literal. This can be inconvenient from a coding
166 standpoint, and unless the server offers IMAP4 non-synchronizing
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172RFC 5738 IMAP Support for UTF-8 March 2010
175 literals [RFC2088], this requires an extra round trip for each UTF-8
176 string sent by the client. When the IMAP server advertises the
177 "UTF8=ACCEPT" capability, it informs the client that it supports
178 native UTF-8 quoted-strings with the following syntax:
180 string =/ utf8-quoted
182 utf8-quoted = "*" DQUOTE *UQUOTED-CHAR DQUOTE
184 UQUOTED-CHAR = QUOTED-CHAR / UTF8-2 / UTF8-3 / UTF8-4
185 ; UTF8-2, UTF8-3, and UTF8-4 are as defined in RFC 3629
187 When this quoting mechanism is used by the client (specifically an
188 octet sequence beginning with *" and ending with "), then the server
189 MUST reject octet sequences with the high bit set that fail to comply
190 with the formal syntax in [RFC3629] with a BAD response.
192 The IMAP server MUST NOT send utf8-quoted syntax to the client unless
193 the client has indicated support for that syntax by using the "ENABLE
194 UTF8=ACCEPT" command.
196 If the server advertises the "UTF8=ACCEPT" capability, the client MAY
197 use utf8-quoted syntax with any IMAP argument that permits a string
198 (including astring and nstring). However, if characters outside the
199 US-ASCII repertoire are used in an inappropriate place, the results
200 would be the same as if other syntactically valid but semantically
201 invalid characters were used. For example, if the client includes
202 UTF-8 characters in the user or password arguments (and the server
203 has not advertised "UTF8=USER"), the LOGIN command will fail as it
204 would with any other invalid user name or password. Specific cases
205 where UTF-8 characters are permitted or not permitted are described
206 in the following paragraphs.
208 All IMAP servers that advertise the "UTF8=ACCEPT" capability SHOULD
209 accept UTF-8 in mailbox names, and those that also support the
210 "Mailbox International Naming Convention" described in RFC 3501,
211 Section 5.1.3 MUST accept utf8-quoted mailbox names and convert them
212 to the appropriate internal format. Mailbox names MUST comply with
213 the Net-Unicode Definition (Section 2 of [RFC5198]) with the specific
214 exception that they MUST NOT contain control characters (0000-001F,
215 0080-009F), delete (007F), line separator (2028), or paragraph
218 An IMAP client MUST NOT issue a SEARCH command that uses a mixture of
219 utf8-quoted syntax and a SEARCH CHARSET other than UTF-8. If an IMAP
220 server receives such a SEARCH command, it SHOULD reject the command
221 with a BAD response (due to the conflicting charset labels).
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228RFC 5738 IMAP Support for UTF-8 March 2010
2313.2. UTF8 Parameter to SELECT and EXAMINE
233 The "UTF8=ACCEPT" capability also indicates that the server supports
234 the "UTF8" parameter to SELECT and EXAMINE. When a mailbox is
235 selected with the "UTF8" parameter, it alters the behavior of all
236 IMAP commands related to message sizes, message headers, and MIME
237 body headers so they refer to the message with UTF-8 headers. If the
238 mailstore is not UTF-8 header native and the SELECT or EXAMINE
239 command with UTF-8 header modifier succeeds, then the server MUST
240 return results as if the mailstore were UTF-8 header native with
241 upconversion requirements as described in Section 8. The server MAY
242 reject the SELECT or EXAMINE command with the [NOT-UTF-8] response
243 code, unless the "UTF8=ALL" or "UTF8=ONLY" capability is advertised.
245 Servers MAY include mailboxes that can only be selected or examined
246 if the "UTF8" parameter is provided. However, such mailboxes MUST
247 NOT be included in the output of an unextended LIST, LSUB, or
248 equivalent command. If a client attempts to SELECT or EXAMINE such
249 mailboxes without the "UTF8" parameter, the server MUST reject the
250 command with a [UTF-8-ONLY] response code. As a result, such
251 mailboxes will not be accessible by IMAP clients written prior to
252 this specification and are discouraged unless the server advertises
253 "UTF8=ONLY" or the server implements IMAP4 LIST Command Extensions
256 utf8-select-param = "UTF8"
257 ;; Conforms to <select-param> from RFC 4466
259 C: a SELECT newmailbox (UTF8)
261 S: a OK SELECT completed
262 C: b FETCH 1 (SIZE ENVELOPE BODY)
263 S: ... < UTF-8 header native results >
264 S: b OK FETCH completed
266 C: c EXAMINE legacymailbox (UTF8)
267 S: c NO [NOT-UTF-8] Mailbox does not support UTF-8 access
269 C: d SELECT funky-new-mailbox
270 S: d NO [UTF-8-ONLY] Mailbox requires UTF-8 client
2723.3. UTF-8 LIST and LSUB Responses
274 After an IMAP client successfully issues an "ENABLE UTF8=ACCEPT"
275 command, the server MUST NOT return in LIST results any mailbox names
276 to the client following the IMAP4 Mailbox International Naming
277 Convention. Instead, the server MUST return any mailbox names with
278 characters outside the US-ASCII repertoire using utf8-quoted syntax.
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284RFC 5738 IMAP Support for UTF-8 March 2010
287 (The IMAP4 Mailbox International Naming Convention has proved
288 problematic in the past, so the desire is to make this syntax
289 obsolete as quickly as possible.)
2913.4. UTF-8 Interaction with IMAP4 LIST Command Extensions
293 When an IMAP server advertises both the "UTF8=ACCEPT" capability and
294 the "LIST-EXTENDED" [RFC5258] capability, the server MUST support the
295 LIST extensions described in this section.
2973.4.1. UTF8 and UTF8ONLY LIST Selection Options
299 The "UTF8" LIST selection option tells the server to include
300 mailboxes that only support UTF-8 headers in the output of the list
301 command. The "UTF8ONLY" LIST selection option tells the server to
302 include all mailboxes that support UTF-8 headers and to exclude
303 mailboxes that don't support UTF-8 headers. Note that "UTF8ONLY"
304 implies "UTF8", so it is not necessary for the client to request
305 both. Use of either selection option will also result in UTF-8
306 mailbox names in the result as described in Section 3.3 and implies
307 the "UTF8" List return option described in Section 3.4.2.
3093.4.2. UTF8 LIST Return Option
311 If the client supplies the "UTF8" LIST return option, then the server
312 MUST include either the "\NoUTF8" or the "\UTF8Only" mailbox
313 attribute as appropriate. The "\NoUTF8" mailbox attribute indicates
314 that an attempt to SELECT or EXAMINE that mailbox with the "UTF8"
315 parameter will fail with a [NOT-UTF-8] response code. The
316 "\UTF8Only" mailbox attribute indicates that an attempt to SELECT or
317 EXAMINE that mailbox without the "UTF8" parameter will fail with a
318 [UTF-8-ONLY] response code. Note that computing this information may
319 be expensive on some server implementations, so this return option
320 should not be used unless necessary.
322 The ABNF [RFC5234] for these LIST extensions follows:
324 list-select-independent-opt =/ "UTF8"
326 list-select-base-opt =/ "UTF8ONLY"
328 mbx-list-oflag =/ "\NoUTF8" / "\UTF8Only"
330 return-option =/ "UTF8"
332 resp-text-code =/ "NOT-UTF-8" / "UTF-8-ONLY"
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3434. UTF8=APPEND Capability
345 If the "UTF8=APPEND" capability is advertised, then the server
346 accepts UTF-8 headers in the APPEND command message argument. A
347 client that sends a message with UTF-8 headers to the server MUST
348 send them using the "UTF8" APPEND data extension. If the server also
349 advertises the CATENATE capability (as specified in [RFC4469]), the
350 client can use the same data extension to include such a message in a
351 CATENATE message part. The ABNF for the APPEND data extension and
352 CATENATE extension follows:
354 utf8-literal = "UTF8" SP "(" literal8 ")"
356 append-data =/ utf8-literal
358 cat-part =/ utf8-literal
360 A server that advertises "UTF8=APPEND" has to comply with the
361 requirements of the IMAP base specification and [RFC5322] for message
362 fetching. Mechanisms for 7-bit downgrading to help comply with the
363 standards are discussed in Downgrading mechanism for
364 Internationalized eMail Address (IMA) [RFC5504].
366 IMAP servers that do not advertise the "UTF8=APPEND" or "UTF8=ONLY"
367 capability SHOULD reject an APPEND command that includes any 8-bit in
368 the message headers with a "NO" response.
370 Note that the "UTF8=ONLY" capability described in Section 7 implies
371 the "UTF8=APPEND" capability. See additional information in that
3745. UTF8=USER Capability
376 If the "UTF8=USER" capability is advertised, that indicates the
377 server accepts UTF-8 user names and passwords and applies SASLprep
378 [RFC4013] to both arguments of the LOGIN command. The server MUST
379 reject UTF-8 that fails to comply with the formal syntax in RFC 3629
380 [RFC3629] or if it encounters Unicode characters listed in Section
381 2.3 of SASLprep RFC 4013 [RFC4013].
3836. UTF8=ALL Capability
385 The "UTF8=ALL" capability indicates all server mailboxes support
386 UTF-8 headers. Specifically, SELECT and EXAMINE with the "UTF8"
387 parameter will never fail with a [NOT-UTF-8] response code.
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399 Note that the "UTF8=ONLY" capability described in Section 7 implies
400 the "UTF8=ALL" capability. See additional information in that
403 Note that the "UTF8=ALL" capability implies the "UTF8=ACCEPT"
4067. UTF8=ONLY Capability
408 The "UTF8=ONLY" capability permits an IMAP server to advertise that
409 it does not support the international mailbox name convention
410 (modified UTF-7), and does not permit selection or examination of any
411 mailbox unless the "UTF8" parameter is provided. As this is an
412 incompatible change to IMAP, a clear warning is necessary. IMAP
413 clients that find implementation of the "UTF8=ONLY" capability
414 problematic are encouraged to at least detect the "UTF8=ONLY"
415 capability and provide an informative error message to the end-user.
417 When an IMAP mailbox internally uses UTF-8 header native storage, the
418 down-conversion step is necessary to permit selection or examination
419 of the mailbox in a backwards compatible fashion will become more
420 difficult to support. Although it is hoped that deployed IMAP
421 servers will not advertise "UTF8=ONLY" for some years, this
422 capability is intended to minimize the disruption when legacy support
425 The "UTF8=ONLY" capability implies the "UTF8=ACCEPT" capability, the
426 "UTF8=ALL" capability, and the "UTF8=APPEND" capability. A server
427 that advertises "UTF8=ONLY" need not advertise the three implicit
4308. Up-Conversion Server Requirements
432 When an IMAP4 server uses a traditional mailbox format that includes
433 7-bit headers and it chooses to permit access to that mailbox with
434 the "UTF8" parameter, it MUST support minimal up-conversion as
435 described in this section.
437 The server MUST support up-conversion of the following address
438 header-fields in the message header: From, Sender, To, CC, Bcc,
439 Resent-From, Resent-Sender, Resent-To, Resent-CC, Resent-Bcc, and
440 Reply-To. This up-conversion MUST include address local-parts in
441 fields downgraded according to [RFC5504], address domains encoded
442 according to Internationalizing Domain Names in Applications (IDNA)
443 [RFC3490], and MIME header encoding [RFC2047] of display-names and
444 any [RFC5322] comments.
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452RFC 5738 IMAP Support for UTF-8 March 2010
455 The following charsets MUST be supported for up-conversion of MIME
456 header encoding [RFC2047]: UTF-8, US-ASCII, ISO-8859-1, ISO-8859-2,
457 ISO-8859-3, ISO-8859-4, ISO-8859-5, ISO-8859-6, ISO-8859-7,
458 ISO-8859-8, ISO-8859-9, ISO-8859-10, ISO-8859-14, and ISO-8859-15.
459 If the server supports other charsets in IMAP SEARCH or IMAP CONVERT
460 [RFC5259], it SHOULD also support those charsets in this conversion.
462 Up-conversion of MIME header encoding of the following headers MUST
463 also be implemented: Subject, Date ([RFC5322] comments only),
464 Comments, Keywords, and Content-Description.
466 Server implementations also SHOULD up-convert all MIME body headers
467 [RFC2045], SHOULD up-convert or remove the deprecated (and misused)
468 "name" parameter [RFC1341] on Content-Type, and MUST up-convert the
469 Content-Disposition [RFC2183] "filename" parameter, except when any
470 of these are contained within a multipart/signed MIME body part (see
471 below). These parameters can be encoded using the standard MIME
472 parameter encoding [RFC2231] mechanism, or via non-standard use of
473 MIME header encoding [RFC2047] in quoted strings.
475 The IMAP server MUST NOT perform up-conversion of headers and content
476 of multipart/signed, as well as Original-Recipient and Return-Path.
4789. Issues with UTF-8 Header Mailstore
480 When an IMAP server uses a mailbox format that supports UTF-8 headers
481 and it permits selection or examination of that mailbox without the
482 "UTF8" parameter, it is the responsibility of the server to comply
483 with the IMAP4rev1 base specification [RFC3501] and [RFC5322] with
484 respect to all header information transmitted over the wire.
485 Mechanisms for 7-bit downgrading to help comply with the standards
486 are discussed in "Downgrading Mechanism for Email Address
487 Internationalization" [RFC5504].
489 An IMAP server with a mailbox that supports UTF-8 headers MUST comply
490 with the protocol requirements implicit from Section 8. However, the
491 code necessary for such compliance need not be part of the IMAP
492 server itself in this case. For example, the minimal required up-
493 conversion could be performed when a message is inserted into the
494 IMAP-accessible mailbox.
49610. IANA Considerations
498 This adds five new capabilities ("UTF8=ACCEPT", "UTF8=USER",
499 "UTF8=APPEND", "UTF8=ALL", and "UTF8=ONLY") to the IMAP4rev1
500 Capabilities registry [RFC3501].
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511 This adds two new IMAP4 list selection options and one new IMAP4 list
514 1. LIST-EXTENDED option name: UTF8
516 LIST-EXTENDED option type: SELECTION
518 Implied return options(s): UTF8
520 LIST-EXTENDED option description: Causes the LIST response to
521 include mailboxes that mandate the UTF8 SELECT/EXAMINE parameter.
523 Published specification: RFC 5738, Section 3.4.1
525 Security considerations: RFC 5738, Section 11
527 Intended usage: COMMON
529 Person and email address to contact for further information: see
530 the Authors' Addresses at the end of this specification
532 Owner/Change controller: iesg@ietf.org
534 2. LIST-EXTENDED option name: UTF8ONLY
536 LIST-EXTENDED option type: SELECTION
538 Implied return options(s): UTF8
540 LIST-EXTENDED option description: Causes the LIST response to
541 include mailboxes that mandate the UTF8 SELECT/EXAMINE parameter
542 and exclude mailboxes that do not support the UTF8 SELECT/EXAMINE
545 Published specification: RFC 5738, Section 3.4.1
547 Security considerations: RFC 5738, Section 11
549 Intended usage: COMMON
551 Person and email address to contact for further information: see
552 the Authors' Addresses at the end of this specification
554 Owner/Change controller: iesg@ietf.org
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564RFC 5738 IMAP Support for UTF-8 March 2010
567 3. LIST-EXTENDED option name: UTF8
569 LIST-EXTENDED option type: RETURN
571 Implied return options(s): none
573 LIST-EXTENDED option description: Causes the LIST response to
574 include \NoUTF8 and \UTF8Only mailbox attributes.
576 Published specification: RFC 5738, Section 3.4.1
578 Security considerations: RFC 5738, Section 11
580 Intended usage: COMMON
582 Person and email address to contact for further information: see
583 the Authors' Addresses at the end of this specification
585 Owner/Change controller: iesg@ietf.org
58711. Security Considerations
589 The security considerations of UTF-8 [RFC3629] and SASLprep [RFC4013]
590 apply to this specification, particularly with respect to use of
591 UTF-8 in user names and passwords. Otherwise, this is not believed
592 to alter the security considerations of IMAP4rev1.
59612.1. Normative References
598 [RFC1341] Borenstein, N. and N. Freed, "MIME (Multipurpose Internet
599 Mail Extensions): Mechanisms for Specifying and Describing
600 the Format of Internet Message Bodies", RFC 1341,
603 [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
604 Extensions (MIME) Part One: Format of Internet Message
605 Bodies", RFC 2045, November 1996.
607 [RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
608 Part Three: Message Header Extensions for Non-ASCII Text",
609 RFC 2047, November 1996.
611 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
612 Requirement Levels", BCP 14, RFC 2119, March 1997.
618Resnick & Newman Experimental [Page 11]
620RFC 5738 IMAP Support for UTF-8 March 2010
623 [RFC2183] Troost, R., Dorner, S., and K. Moore, "Communicating
624 Presentation Information in Internet Messages: The
625 Content-Disposition Header Field", RFC 2183, August 1997.
627 [RFC2231] Freed, N. and K. Moore, "MIME Parameter Value and Encoded
629 Character Sets, Languages, and Continuations", RFC 2231,
632 [RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
633 "Internationalizing Domain Names in Applications (IDNA)",
634 RFC 3490, March 2003.
636 [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
637 4rev1", RFC 3501, March 2003.
639 [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
640 10646", STD 63, RFC 3629, November 2003.
642 [RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User Names
643 and Passwords", RFC 4013, February 2005.
645 [RFC4466] Melnikov, A. and C. Daboo, "Collected Extensions to IMAP4
646 ABNF", RFC 4466, April 2006.
648 [RFC4469] Resnick, P., "Internet Message Access Protocol (IMAP)
649 CATENATE Extension", RFC 4469, April 2006.
651 [RFC5161] Gulbrandsen, A. and A. Melnikov, "The IMAP ENABLE
652 Extension", RFC 5161, March 2008.
654 [RFC5198] Klensin, J. and M. Padlipsky, "Unicode Format for Network
655 Interchange", RFC 5198, March 2008.
657 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
658 Specifications: ABNF", STD 68, RFC 5234, January 2008.
660 [RFC5258] Leiba, B. and A. Melnikov, "Internet Message Access
661 Protocol version 4 - LIST Command Extensions", RFC 5258,
664 [RFC5259] Melnikov, A. and P. Coates, "Internet Message Access
665 Protocol - CONVERT Extension", RFC 5259, July 2008.
667 [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
674Resnick & Newman Experimental [Page 12]
676RFC 5738 IMAP Support for UTF-8 March 2010
679 [RFC5335] Abel, Y., "Internationalized Email Headers", RFC 5335,
682 [RFC5504] Fujiwara, K. and Y. Yoneya, "Downgrading Mechanism for
683 Email Address Internationalization", RFC 5504, March 2009.
68512.2. Informative References
687 [RFC2049] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
688 Extensions (MIME) Part Five: Conformance Criteria and
689 Examples", RFC 2049, November 1996.
691 [RFC2088] Myers, J., "IMAP4 non-synchronizing literals", RFC 2088,
694 [RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
695 Languages", BCP 18, RFC 2277, January 1998.
697 [RFC5721] Gellens, R. and C. Newman, "POP3 Support for UTF-8",
698 RFC 5721, February 2010.
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732RFC 5738 IMAP Support for UTF-8 March 2010
735Appendix A. Design Rationale
737 This non-normative section discusses the reasons behind some of the
738 design choices in the above specification.
740 The basic approach of advertising the ability to access a mailbox in
741 UTF-8 mode is intended to permit graceful upgrade, including servers
742 that support multiple mailbox formats. In particular, it would be
743 undesirable to force conversion of an entire server mailstore to
744 UTF-8 headers, so being able to phase-in support for new mailboxes
745 and gradually migrate old mailboxes is permitted by this design.
747 "UTF8=USER" is optional because many identity systems are US-ASCII
748 only, so it's helpful to inform the client up front that UTF-8 won't
751 "UTF8=APPEND" is optional because it effectively requires IMAP server
752 support for down-conversion, which is a much more complex operation
755 The "UTF8=ONLY" mechanism simplifies diagnosis of interoperability
756 problems when legacy support goes away. In the situation where
757 backwards compatibility is broken anyway, just-send-UTF-8 IMAP has
758 the advantage that it might work with some legacy clients. However,
759 the difficulty of diagnosing interoperability problems caused by a
760 just-send-UTF-8 IMAP mechanism is the reason the "UTF8=ONLY"
761 capability mechanism was chosen.
763 The up-conversion requirements are designed to balance the desire to
764 deprecate and eventually eliminate complicated encodings (like MIME
765 header encodings) without creating a significant deployment burden
766 for servers. As IMAP4 servers already require a MIME parser, this
767 includes additional server up-conversion requirements not present in
768 POP3 Support for UTF-8 [RFC5721].
770 The set of mandatory charsets comes from two sources: MIME
771 requirements [RFC2049] and IETF Policy on Character Sets [RFC2277].
772 Including a requirement to up-convert widely deployed encoded
773 ideographic charsets to UTF-8 would be reasonable for most scenarios,
774 but may require unacceptable table sizes for some embedded devices.
775 The open-ended recommendation to support widely deployed charsets
776 avoids the political ramifications of attempting to list such
777 charsets. The authors believe market forces, existing open-source
778 software, and public conversion tables are sufficient to deploy the
779 appropriate charsets.
786Resnick & Newman Experimental [Page 14]
788RFC 5738 IMAP Support for UTF-8 March 2010
791Appendix B. Examples Demonstrating Relationships between UTF8=
795 UTF8=ACCEPT UTF8=USER UTF8=APPEND
797 UTF8=ALL ; Note, same as above
798 UTF8=ACCEPT UTF8=USER UTF8=APPEND UTF8=ALL UTF8=ONLY
799 UTF8=USER UTF8=ONLY ; Note, same as above
801Appendix C. Acknowledgments
803 The authors wish to thank the participants of the EAI working group
804 for their contributions to this document with particular thanks to
805 Harald Alvestrand, David Black, Randall Gellens, Arnt Gulbrandsen,
806 Kari Hurtta, John Klensin, Xiaodong Lee, Charles Lindsey, Alexey
807 Melnikov, Subramanian Moonesamy, Shawn Steele, Daniel Taharlev, and
808 Joseph Yee for their specific contributions to the discussion.
813 Qualcomm Incorporated
815 San Diego, CA 92121-1714
818 Phone: +1 858 651 4478
819 EMail: presnick@qualcomm.com
820 URI: http://www.qualcomm.com/~presnick/
828 EMail: chris.newman@sun.com
842Resnick & Newman Experimental [Page 15]