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diff --git a/doc/rfc/rfc5738.txt b/doc/rfc/rfc5738.txt new file mode 100644 index 0000000..2b5daaa --- /dev/null +++ b/doc/rfc/rfc5738.txt @@ -0,0 +1,843 @@ + + + + + + +Network Working Group P. Resnick +Request for Comments: 5738 Qualcomm Incorporated +Updates: 3501 C. Newman +Category: Experimental Sun Microsystems + March 2010 + + + IMAP Support for UTF-8 + +Abstract + + This specification extends the Internet Message Access Protocol + version 4rev1 (IMAP4rev1) to support UTF-8 encoded international + characters in user names, mail addresses, and message headers. + +Status of This Memo + + This document is not an Internet Standards Track specification; it is + published for examination, experimental implementation, and + evaluation. + + This document defines an Experimental Protocol for the Internet + community. This document is a product of the Internet Engineering + Task Force (IETF). It represents the consensus of the IETF + community. It has received public review and has been approved for + publication by the Internet Engineering Steering Group (IESG). Not + all documents approved by the IESG are a candidate for any level of + Internet Standard; see Section 2 of RFC 5741. + + Information about the current status of this document, any errata, + and how to provide feedback on it may be obtained at + http://www.rfc-editor.org/info/rfc5738. + +Copyright Notice + + Copyright (c) 2010 IETF Trust and the persons identified as the + document authors. All rights reserved. + + This document is subject to BCP 78 and the IETF Trust's Legal + Provisions Relating to IETF Documents + (http://trustee.ietf.org/license-info) in effect on the date of + publication of this document. Please review these documents + carefully, as they describe your rights and restrictions with respect + to this document. Code Components extracted from this document must + include Simplified BSD License text as described in Section 4.e of + the Trust Legal Provisions and are provided without warranty as + described in the Simplified BSD License. + + + + +Resnick & Newman Experimental [Page 1] + +RFC 5738 IMAP Support for UTF-8 March 2010 + + + This document may contain material from IETF Documents or IETF + Contributions published or made publicly available before November + 10, 2008. The person(s) controlling the copyright in some of this + material may not have granted the IETF Trust the right to allow + modifications of such material outside the IETF Standards Process. + Without obtaining an adequate license from the person(s) controlling + the copyright in such materials, this document may not be modified + outside the IETF Standards Process, and derivative works of it may + not be created outside the IETF Standards Process, except to format + it for publication as an RFC or to translate it into languages other + than English. + +Table of Contents + + 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 + 2. Conventions Used in This Document . . . . . . . . . . . . . . 3 + 3. UTF8=ACCEPT IMAP Capability . . . . . . . . . . . . . . . . . 3 + 3.1. IMAP UTF-8 Quoted Strings . . . . . . . . . . . . . . . . 3 + 3.2. UTF8 Parameter to SELECT and EXAMINE . . . . . . . . . . . 5 + 3.3. UTF-8 LIST and LSUB Responses . . . . . . . . . . . . . . 5 + 3.4. UTF-8 Interaction with IMAP4 LIST Command Extensions . . . 6 + 3.4.1. UTF8 and UTF8ONLY LIST Selection Options . . . . . . . 6 + 3.4.2. UTF8 LIST Return Option . . . . . . . . . . . . . . . 6 + 4. UTF8=APPEND Capability . . . . . . . . . . . . . . . . . . . . 7 + 5. UTF8=USER Capability . . . . . . . . . . . . . . . . . . . . . 7 + 6. UTF8=ALL Capability . . . . . . . . . . . . . . . . . . . . . 7 + 7. UTF8=ONLY Capability . . . . . . . . . . . . . . . . . . . . . 8 + 8. Up-Conversion Server Requirements . . . . . . . . . . . . . . 8 + 9. Issues with UTF-8 Header Mailstore . . . . . . . . . . . . . . 9 + 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 + 11. Security Considerations . . . . . . . . . . . . . . . . . . . 11 + 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11 + 12.1. Normative References . . . . . . . . . . . . . . . . . . . 11 + 12.2. Informative References . . . . . . . . . . . . . . . . . . 13 + Appendix A. Design Rationale . . . . . . . . . . . . . . . . . . 14 + Appendix B. Examples Demonstrating Relationships between + UTF8= Capabilities . . . . . . . . . . . . . . . . . 15 + Appendix C. Acknowledgments . . . . . . . . . . . . . . . . . . . 15 + + + + + + + + + + + + + +Resnick & Newman Experimental [Page 2] + +RFC 5738 IMAP Support for UTF-8 March 2010 + + +1. Introduction + + This specification extends IMAP4rev1 [RFC3501] to permit UTF-8 + [RFC3629] in headers as described in "Internationalized Email + Headers" [RFC5335]. It also adds a mechanism to support mailbox + names, login names, and passwords using the UTF-8 charset. This + specification creates five new IMAP capabilities to allow servers to + advertise these new extensions, along with two new IMAP LIST + selection options and a new IMAP LIST return option. + +2. Conventions Used in This Document + + The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY" + in this document are to be interpreted as defined in "Key words for + use in RFCs to Indicate Requirement Levels" [RFC2119]. + + The formal syntax uses the Augmented Backus-Naur Form (ABNF) + [RFC5234] notation including the core rules defined in Appendix B of + [RFC5234]. In addition, rules from IMAP4rev1 [RFC3501], UTF-8 + [RFC3629], "Collected Extensions to IMAP4 ABNF" [RFC4466], and IMAP4 + LIST Command Extensions [RFC5258] are also referenced. + + In examples, "C:" and "S:" indicate lines sent by the client and + server, respectively. If a single "C:" or "S:" label applies to + multiple lines, then the line breaks between those lines are for + editorial clarity only and are not part of the actual protocol + exchange. + +3. UTF8=ACCEPT IMAP Capability + + The "UTF8=ACCEPT" capability indicates that the server supports UTF-8 + quoted strings, the "UTF8" parameter to SELECT and EXAMINE, and UTF-8 + responses from the LIST and LSUB commands. + + A client MUST use the "ENABLE UTF8=ACCEPT" command (defined in + [RFC5161]) to indicate to the server that the client accepts UTF-8 + quoted-strings. The "ENABLE UTF8=ACCEPT" command MUST only be used + in the authenticated state. (Note that the "UTF8=ONLY" capability + described in Section 7 and the "UTF8=ALL" capability described in + Section 6 imply the "UTF8=ACCEPT" capability. See additional + information in these sections.) + +3.1. IMAP UTF-8 Quoted Strings + + The IMAP4rev1 [RFC3501] base specification forbids the use of 8-bit + characters in atoms or quoted strings. Thus, a UTF-8 string can only + be sent as a literal. This can be inconvenient from a coding + standpoint, and unless the server offers IMAP4 non-synchronizing + + + +Resnick & Newman Experimental [Page 3] + +RFC 5738 IMAP Support for UTF-8 March 2010 + + + literals [RFC2088], this requires an extra round trip for each UTF-8 + string sent by the client. When the IMAP server advertises the + "UTF8=ACCEPT" capability, it informs the client that it supports + native UTF-8 quoted-strings with the following syntax: + + string =/ utf8-quoted + + utf8-quoted = "*" DQUOTE *UQUOTED-CHAR DQUOTE + + UQUOTED-CHAR = QUOTED-CHAR / UTF8-2 / UTF8-3 / UTF8-4 + ; UTF8-2, UTF8-3, and UTF8-4 are as defined in RFC 3629 + + When this quoting mechanism is used by the client (specifically an + octet sequence beginning with *" and ending with "), then the server + MUST reject octet sequences with the high bit set that fail to comply + with the formal syntax in [RFC3629] with a BAD response. + + The IMAP server MUST NOT send utf8-quoted syntax to the client unless + the client has indicated support for that syntax by using the "ENABLE + UTF8=ACCEPT" command. + + If the server advertises the "UTF8=ACCEPT" capability, the client MAY + use utf8-quoted syntax with any IMAP argument that permits a string + (including astring and nstring). However, if characters outside the + US-ASCII repertoire are used in an inappropriate place, the results + would be the same as if other syntactically valid but semantically + invalid characters were used. For example, if the client includes + UTF-8 characters in the user or password arguments (and the server + has not advertised "UTF8=USER"), the LOGIN command will fail as it + would with any other invalid user name or password. Specific cases + where UTF-8 characters are permitted or not permitted are described + in the following paragraphs. + + All IMAP servers that advertise the "UTF8=ACCEPT" capability SHOULD + accept UTF-8 in mailbox names, and those that also support the + "Mailbox International Naming Convention" described in RFC 3501, + Section 5.1.3 MUST accept utf8-quoted mailbox names and convert them + to the appropriate internal format. Mailbox names MUST comply with + the Net-Unicode Definition (Section 2 of [RFC5198]) with the specific + exception that they MUST NOT contain control characters (0000-001F, + 0080-009F), delete (007F), line separator (2028), or paragraph + separator (2029). + + An IMAP client MUST NOT issue a SEARCH command that uses a mixture of + utf8-quoted syntax and a SEARCH CHARSET other than UTF-8. If an IMAP + server receives such a SEARCH command, it SHOULD reject the command + with a BAD response (due to the conflicting charset labels). + + + + +Resnick & Newman Experimental [Page 4] + +RFC 5738 IMAP Support for UTF-8 March 2010 + + +3.2. UTF8 Parameter to SELECT and EXAMINE + + The "UTF8=ACCEPT" capability also indicates that the server supports + the "UTF8" parameter to SELECT and EXAMINE. When a mailbox is + selected with the "UTF8" parameter, it alters the behavior of all + IMAP commands related to message sizes, message headers, and MIME + body headers so they refer to the message with UTF-8 headers. If the + mailstore is not UTF-8 header native and the SELECT or EXAMINE + command with UTF-8 header modifier succeeds, then the server MUST + return results as if the mailstore were UTF-8 header native with + upconversion requirements as described in Section 8. The server MAY + reject the SELECT or EXAMINE command with the [NOT-UTF-8] response + code, unless the "UTF8=ALL" or "UTF8=ONLY" capability is advertised. + + Servers MAY include mailboxes that can only be selected or examined + if the "UTF8" parameter is provided. However, such mailboxes MUST + NOT be included in the output of an unextended LIST, LSUB, or + equivalent command. If a client attempts to SELECT or EXAMINE such + mailboxes without the "UTF8" parameter, the server MUST reject the + command with a [UTF-8-ONLY] response code. As a result, such + mailboxes will not be accessible by IMAP clients written prior to + this specification and are discouraged unless the server advertises + "UTF8=ONLY" or the server implements IMAP4 LIST Command Extensions + [RFC5258]. + + utf8-select-param = "UTF8" + ;; Conforms to <select-param> from RFC 4466 + + C: a SELECT newmailbox (UTF8) + S: ... + S: a OK SELECT completed + C: b FETCH 1 (SIZE ENVELOPE BODY) + S: ... < UTF-8 header native results > + S: b OK FETCH completed + + C: c EXAMINE legacymailbox (UTF8) + S: c NO [NOT-UTF-8] Mailbox does not support UTF-8 access + + C: d SELECT funky-new-mailbox + S: d NO [UTF-8-ONLY] Mailbox requires UTF-8 client + +3.3. UTF-8 LIST and LSUB Responses + + After an IMAP client successfully issues an "ENABLE UTF8=ACCEPT" + command, the server MUST NOT return in LIST results any mailbox names + to the client following the IMAP4 Mailbox International Naming + Convention. Instead, the server MUST return any mailbox names with + characters outside the US-ASCII repertoire using utf8-quoted syntax. + + + +Resnick & Newman Experimental [Page 5] + +RFC 5738 IMAP Support for UTF-8 March 2010 + + + (The IMAP4 Mailbox International Naming Convention has proved + problematic in the past, so the desire is to make this syntax + obsolete as quickly as possible.) + +3.4. UTF-8 Interaction with IMAP4 LIST Command Extensions + + When an IMAP server advertises both the "UTF8=ACCEPT" capability and + the "LIST-EXTENDED" [RFC5258] capability, the server MUST support the + LIST extensions described in this section. + +3.4.1. UTF8 and UTF8ONLY LIST Selection Options + + The "UTF8" LIST selection option tells the server to include + mailboxes that only support UTF-8 headers in the output of the list + command. The "UTF8ONLY" LIST selection option tells the server to + include all mailboxes that support UTF-8 headers and to exclude + mailboxes that don't support UTF-8 headers. Note that "UTF8ONLY" + implies "UTF8", so it is not necessary for the client to request + both. Use of either selection option will also result in UTF-8 + mailbox names in the result as described in Section 3.3 and implies + the "UTF8" List return option described in Section 3.4.2. + +3.4.2. UTF8 LIST Return Option + + If the client supplies the "UTF8" LIST return option, then the server + MUST include either the "\NoUTF8" or the "\UTF8Only" mailbox + attribute as appropriate. The "\NoUTF8" mailbox attribute indicates + that an attempt to SELECT or EXAMINE that mailbox with the "UTF8" + parameter will fail with a [NOT-UTF-8] response code. The + "\UTF8Only" mailbox attribute indicates that an attempt to SELECT or + EXAMINE that mailbox without the "UTF8" parameter will fail with a + [UTF-8-ONLY] response code. Note that computing this information may + be expensive on some server implementations, so this return option + should not be used unless necessary. + + The ABNF [RFC5234] for these LIST extensions follows: + + list-select-independent-opt =/ "UTF8" + + list-select-base-opt =/ "UTF8ONLY" + + mbx-list-oflag =/ "\NoUTF8" / "\UTF8Only" + + return-option =/ "UTF8" + + resp-text-code =/ "NOT-UTF-8" / "UTF-8-ONLY" + + + + + +Resnick & Newman Experimental [Page 6] + +RFC 5738 IMAP Support for UTF-8 March 2010 + + +4. UTF8=APPEND Capability + + If the "UTF8=APPEND" capability is advertised, then the server + accepts UTF-8 headers in the APPEND command message argument. A + client that sends a message with UTF-8 headers to the server MUST + send them using the "UTF8" APPEND data extension. If the server also + advertises the CATENATE capability (as specified in [RFC4469]), the + client can use the same data extension to include such a message in a + CATENATE message part. The ABNF for the APPEND data extension and + CATENATE extension follows: + + utf8-literal = "UTF8" SP "(" literal8 ")" + + append-data =/ utf8-literal + + cat-part =/ utf8-literal + + A server that advertises "UTF8=APPEND" has to comply with the + requirements of the IMAP base specification and [RFC5322] for message + fetching. Mechanisms for 7-bit downgrading to help comply with the + standards are discussed in Downgrading mechanism for + Internationalized eMail Address (IMA) [RFC5504]. + + IMAP servers that do not advertise the "UTF8=APPEND" or "UTF8=ONLY" + capability SHOULD reject an APPEND command that includes any 8-bit in + the message headers with a "NO" response. + + Note that the "UTF8=ONLY" capability described in Section 7 implies + the "UTF8=APPEND" capability. See additional information in that + section. + +5. UTF8=USER Capability + + If the "UTF8=USER" capability is advertised, that indicates the + server accepts UTF-8 user names and passwords and applies SASLprep + [RFC4013] to both arguments of the LOGIN command. The server MUST + reject UTF-8 that fails to comply with the formal syntax in RFC 3629 + [RFC3629] or if it encounters Unicode characters listed in Section + 2.3 of SASLprep RFC 4013 [RFC4013]. + +6. UTF8=ALL Capability + + The "UTF8=ALL" capability indicates all server mailboxes support + UTF-8 headers. Specifically, SELECT and EXAMINE with the "UTF8" + parameter will never fail with a [NOT-UTF-8] response code. + + + + + + +Resnick & Newman Experimental [Page 7] + +RFC 5738 IMAP Support for UTF-8 March 2010 + + + Note that the "UTF8=ONLY" capability described in Section 7 implies + the "UTF8=ALL" capability. See additional information in that + section. + + Note that the "UTF8=ALL" capability implies the "UTF8=ACCEPT" + capability. + +7. UTF8=ONLY Capability + + The "UTF8=ONLY" capability permits an IMAP server to advertise that + it does not support the international mailbox name convention + (modified UTF-7), and does not permit selection or examination of any + mailbox unless the "UTF8" parameter is provided. As this is an + incompatible change to IMAP, a clear warning is necessary. IMAP + clients that find implementation of the "UTF8=ONLY" capability + problematic are encouraged to at least detect the "UTF8=ONLY" + capability and provide an informative error message to the end-user. + + When an IMAP mailbox internally uses UTF-8 header native storage, the + down-conversion step is necessary to permit selection or examination + of the mailbox in a backwards compatible fashion will become more + difficult to support. Although it is hoped that deployed IMAP + servers will not advertise "UTF8=ONLY" for some years, this + capability is intended to minimize the disruption when legacy support + finally goes away. + + The "UTF8=ONLY" capability implies the "UTF8=ACCEPT" capability, the + "UTF8=ALL" capability, and the "UTF8=APPEND" capability. A server + that advertises "UTF8=ONLY" need not advertise the three implicit + capabilities. + +8. Up-Conversion Server Requirements + + When an IMAP4 server uses a traditional mailbox format that includes + 7-bit headers and it chooses to permit access to that mailbox with + the "UTF8" parameter, it MUST support minimal up-conversion as + described in this section. + + The server MUST support up-conversion of the following address + header-fields in the message header: From, Sender, To, CC, Bcc, + Resent-From, Resent-Sender, Resent-To, Resent-CC, Resent-Bcc, and + Reply-To. This up-conversion MUST include address local-parts in + fields downgraded according to [RFC5504], address domains encoded + according to Internationalizing Domain Names in Applications (IDNA) + [RFC3490], and MIME header encoding [RFC2047] of display-names and + any [RFC5322] comments. + + + + + +Resnick & Newman Experimental [Page 8] + +RFC 5738 IMAP Support for UTF-8 March 2010 + + + The following charsets MUST be supported for up-conversion of MIME + header encoding [RFC2047]: UTF-8, US-ASCII, ISO-8859-1, ISO-8859-2, + ISO-8859-3, ISO-8859-4, ISO-8859-5, ISO-8859-6, ISO-8859-7, + ISO-8859-8, ISO-8859-9, ISO-8859-10, ISO-8859-14, and ISO-8859-15. + If the server supports other charsets in IMAP SEARCH or IMAP CONVERT + [RFC5259], it SHOULD also support those charsets in this conversion. + + Up-conversion of MIME header encoding of the following headers MUST + also be implemented: Subject, Date ([RFC5322] comments only), + Comments, Keywords, and Content-Description. + + Server implementations also SHOULD up-convert all MIME body headers + [RFC2045], SHOULD up-convert or remove the deprecated (and misused) + "name" parameter [RFC1341] on Content-Type, and MUST up-convert the + Content-Disposition [RFC2183] "filename" parameter, except when any + of these are contained within a multipart/signed MIME body part (see + below). These parameters can be encoded using the standard MIME + parameter encoding [RFC2231] mechanism, or via non-standard use of + MIME header encoding [RFC2047] in quoted strings. + + The IMAP server MUST NOT perform up-conversion of headers and content + of multipart/signed, as well as Original-Recipient and Return-Path. + +9. Issues with UTF-8 Header Mailstore + + When an IMAP server uses a mailbox format that supports UTF-8 headers + and it permits selection or examination of that mailbox without the + "UTF8" parameter, it is the responsibility of the server to comply + with the IMAP4rev1 base specification [RFC3501] and [RFC5322] with + respect to all header information transmitted over the wire. + Mechanisms for 7-bit downgrading to help comply with the standards + are discussed in "Downgrading Mechanism for Email Address + Internationalization" [RFC5504]. + + An IMAP server with a mailbox that supports UTF-8 headers MUST comply + with the protocol requirements implicit from Section 8. However, the + code necessary for such compliance need not be part of the IMAP + server itself in this case. For example, the minimal required up- + conversion could be performed when a message is inserted into the + IMAP-accessible mailbox. + +10. IANA Considerations + + This adds five new capabilities ("UTF8=ACCEPT", "UTF8=USER", + "UTF8=APPEND", "UTF8=ALL", and "UTF8=ONLY") to the IMAP4rev1 + Capabilities registry [RFC3501]. + + + + + +Resnick & Newman Experimental [Page 9] + +RFC 5738 IMAP Support for UTF-8 March 2010 + + + This adds two new IMAP4 list selection options and one new IMAP4 list + return option. + + 1. LIST-EXTENDED option name: UTF8 + + LIST-EXTENDED option type: SELECTION + + Implied return options(s): UTF8 + + LIST-EXTENDED option description: Causes the LIST response to + include mailboxes that mandate the UTF8 SELECT/EXAMINE parameter. + + Published specification: RFC 5738, Section 3.4.1 + + Security considerations: RFC 5738, Section 11 + + Intended usage: COMMON + + Person and email address to contact for further information: see + the Authors' Addresses at the end of this specification + + Owner/Change controller: iesg@ietf.org + + 2. LIST-EXTENDED option name: UTF8ONLY + + LIST-EXTENDED option type: SELECTION + + Implied return options(s): UTF8 + + LIST-EXTENDED option description: Causes the LIST response to + include mailboxes that mandate the UTF8 SELECT/EXAMINE parameter + and exclude mailboxes that do not support the UTF8 SELECT/EXAMINE + parameter. + + Published specification: RFC 5738, Section 3.4.1 + + Security considerations: RFC 5738, Section 11 + + Intended usage: COMMON + + Person and email address to contact for further information: see + the Authors' Addresses at the end of this specification + + Owner/Change controller: iesg@ietf.org + + + + + + + +Resnick & Newman Experimental [Page 10] + +RFC 5738 IMAP Support for UTF-8 March 2010 + + + 3. LIST-EXTENDED option name: UTF8 + + LIST-EXTENDED option type: RETURN + + Implied return options(s): none + + LIST-EXTENDED option description: Causes the LIST response to + include \NoUTF8 and \UTF8Only mailbox attributes. + + Published specification: RFC 5738, Section 3.4.1 + + Security considerations: RFC 5738, Section 11 + + Intended usage: COMMON + + Person and email address to contact for further information: see + the Authors' Addresses at the end of this specification + + Owner/Change controller: iesg@ietf.org + +11. Security Considerations + + The security considerations of UTF-8 [RFC3629] and SASLprep [RFC4013] + apply to this specification, particularly with respect to use of + UTF-8 in user names and passwords. Otherwise, this is not believed + to alter the security considerations of IMAP4rev1. + +12. References + +12.1. Normative References + + [RFC1341] Borenstein, N. and N. Freed, "MIME (Multipurpose Internet + Mail Extensions): Mechanisms for Specifying and Describing + the Format of Internet Message Bodies", RFC 1341, + June 1992. + + [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail + Extensions (MIME) Part One: Format of Internet Message + Bodies", RFC 2045, November 1996. + + [RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions) + Part Three: Message Header Extensions for Non-ASCII Text", + RFC 2047, November 1996. + + [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate + Requirement Levels", BCP 14, RFC 2119, March 1997. + + + + + +Resnick & Newman Experimental [Page 11] + +RFC 5738 IMAP Support for UTF-8 March 2010 + + + [RFC2183] Troost, R., Dorner, S., and K. Moore, "Communicating + Presentation Information in Internet Messages: The + Content-Disposition Header Field", RFC 2183, August 1997. + + [RFC2231] Freed, N. and K. Moore, "MIME Parameter Value and Encoded + Word Extensions: + Character Sets, Languages, and Continuations", RFC 2231, + November 1997. + + [RFC3490] Faltstrom, P., Hoffman, P., and A. Costello, + "Internationalizing Domain Names in Applications (IDNA)", + RFC 3490, March 2003. + + [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION + 4rev1", RFC 3501, March 2003. + + [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO + 10646", STD 63, RFC 3629, November 2003. + + [RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User Names + and Passwords", RFC 4013, February 2005. + + [RFC4466] Melnikov, A. and C. Daboo, "Collected Extensions to IMAP4 + ABNF", RFC 4466, April 2006. + + [RFC4469] Resnick, P., "Internet Message Access Protocol (IMAP) + CATENATE Extension", RFC 4469, April 2006. + + [RFC5161] Gulbrandsen, A. and A. Melnikov, "The IMAP ENABLE + Extension", RFC 5161, March 2008. + + [RFC5198] Klensin, J. and M. Padlipsky, "Unicode Format for Network + Interchange", RFC 5198, March 2008. + + [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax + Specifications: ABNF", STD 68, RFC 5234, January 2008. + + [RFC5258] Leiba, B. and A. Melnikov, "Internet Message Access + Protocol version 4 - LIST Command Extensions", RFC 5258, + June 2008. + + [RFC5259] Melnikov, A. and P. Coates, "Internet Message Access + Protocol - CONVERT Extension", RFC 5259, July 2008. + + [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, + October 2008. + + + + + +Resnick & Newman Experimental [Page 12] + +RFC 5738 IMAP Support for UTF-8 March 2010 + + + [RFC5335] Abel, Y., "Internationalized Email Headers", RFC 5335, + September 2008. + + [RFC5504] Fujiwara, K. and Y. Yoneya, "Downgrading Mechanism for + Email Address Internationalization", RFC 5504, March 2009. + +12.2. Informative References + + [RFC2049] Freed, N. and N. Borenstein, "Multipurpose Internet Mail + Extensions (MIME) Part Five: Conformance Criteria and + Examples", RFC 2049, November 1996. + + [RFC2088] Myers, J., "IMAP4 non-synchronizing literals", RFC 2088, + January 1997. + + [RFC2277] Alvestrand, H., "IETF Policy on Character Sets and + Languages", BCP 18, RFC 2277, January 1998. + + [RFC5721] Gellens, R. and C. Newman, "POP3 Support for UTF-8", + RFC 5721, February 2010. + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +Resnick & Newman Experimental [Page 13] + +RFC 5738 IMAP Support for UTF-8 March 2010 + + +Appendix A. Design Rationale + + This non-normative section discusses the reasons behind some of the + design choices in the above specification. + + The basic approach of advertising the ability to access a mailbox in + UTF-8 mode is intended to permit graceful upgrade, including servers + that support multiple mailbox formats. In particular, it would be + undesirable to force conversion of an entire server mailstore to + UTF-8 headers, so being able to phase-in support for new mailboxes + and gradually migrate old mailboxes is permitted by this design. + + "UTF8=USER" is optional because many identity systems are US-ASCII + only, so it's helpful to inform the client up front that UTF-8 won't + work. + + "UTF8=APPEND" is optional because it effectively requires IMAP server + support for down-conversion, which is a much more complex operation + than up-conversion. + + The "UTF8=ONLY" mechanism simplifies diagnosis of interoperability + problems when legacy support goes away. In the situation where + backwards compatibility is broken anyway, just-send-UTF-8 IMAP has + the advantage that it might work with some legacy clients. However, + the difficulty of diagnosing interoperability problems caused by a + just-send-UTF-8 IMAP mechanism is the reason the "UTF8=ONLY" + capability mechanism was chosen. + + The up-conversion requirements are designed to balance the desire to + deprecate and eventually eliminate complicated encodings (like MIME + header encodings) without creating a significant deployment burden + for servers. As IMAP4 servers already require a MIME parser, this + includes additional server up-conversion requirements not present in + POP3 Support for UTF-8 [RFC5721]. + + The set of mandatory charsets comes from two sources: MIME + requirements [RFC2049] and IETF Policy on Character Sets [RFC2277]. + Including a requirement to up-convert widely deployed encoded + ideographic charsets to UTF-8 would be reasonable for most scenarios, + but may require unacceptable table sizes for some embedded devices. + The open-ended recommendation to support widely deployed charsets + avoids the political ramifications of attempting to list such + charsets. The authors believe market forces, existing open-source + software, and public conversion tables are sufficient to deploy the + appropriate charsets. + + + + + + +Resnick & Newman Experimental [Page 14] + +RFC 5738 IMAP Support for UTF-8 March 2010 + + +Appendix B. Examples Demonstrating Relationships between UTF8= + Capabilities + + + UTF8=ACCEPT UTF8=USER UTF8=APPEND + UTF8=ACCEPT UTF8=ALL + UTF8=ALL ; Note, same as above + UTF8=ACCEPT UTF8=USER UTF8=APPEND UTF8=ALL UTF8=ONLY + UTF8=USER UTF8=ONLY ; Note, same as above + +Appendix C. Acknowledgments + + The authors wish to thank the participants of the EAI working group + for their contributions to this document with particular thanks to + Harald Alvestrand, David Black, Randall Gellens, Arnt Gulbrandsen, + Kari Hurtta, John Klensin, Xiaodong Lee, Charles Lindsey, Alexey + Melnikov, Subramanian Moonesamy, Shawn Steele, Daniel Taharlev, and + Joseph Yee for their specific contributions to the discussion. + +Authors' Addresses + + Pete Resnick + Qualcomm Incorporated + 5775 Morehouse Drive + San Diego, CA 92121-1714 + US + + Phone: +1 858 651 4478 + EMail: presnick@qualcomm.com + URI: http://www.qualcomm.com/~presnick/ + + Chris Newman + Sun Microsystems + 800 Royal Oaks + Monrovia, CA 91016 + US + + EMail: chris.newman@sun.com + + + + + + + + + + + + + +Resnick & Newman Experimental [Page 15] + |