Network Working Group R. Gellens Internet-Draft QUALCOMM Incorporated Obsoletes: RFC5721 (if approved) C. Newman Updates: RFC1939 (if approved) Oracle Intended status: Standards Track J. Yao Expires: October 1, 2011 CNNIC K. Fujiwara JPRS March 30, 2011 POP3 Support for UTF-8 draft-ietf-eai-rfc5721bis-01.txt Abstract This specification extends the Post Office Protocol version 3 (POP3) to support un-encoded international characters in user names, passwords, mail addresses, message headers, and protocol-level textual error strings. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on October 1, 2011. Copyright Notice Copyright (c) 2011 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 Gellens, et al. Expires October 1, 2011 [Page 1] Internet-Draft POP3 Support for UTF-8 March 2011 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. 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 1.1. Conventions Used in This Document . . . . . . . . . . . . 3 2. LANG Capability . . . . . . . . . . . . . . . . . . . . . . . 4 3. UTF8 Capability . . . . . . . . . . . . . . . . . . . . . . . 6 3.1. The UTF8 Command . . . . . . . . . . . . . . . . . . . . . 7 3.2. USER Argument to UTF8 Capability . . . . . . . . . . . . . 8 4. Native UTF-8 Maildrops . . . . . . . . . . . . . . . . . . . . 9 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 6. Security Considerations . . . . . . . . . . . . . . . . . . . 9 7. Change History . . . . . . . . . . . . . . . . . . . . . . . . 10 7.1. draft-ietf-eai-rfc5721bis: Version 00 . . . . . . . . . . 10 7.2. draft-ietf-eai-rfc5721bis: Version 01 . . . . . . . . . . 10 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10 8.1. Normative References . . . . . . . . . . . . . . . . . . . 10 8.2. Informative References . . . . . . . . . . . . . . . . . . 11 Appendix A. Design Rationale . . . . . . . . . . . . . . . . . . 12 Appendix B. Acknowledgments . . . . . . . . . . . . . . . . . . . 12 Gellens, et al. Expires October 1, 2011 [Page 2] Internet-Draft POP3 Support for UTF-8 March 2011 1. Introduction This document forms part of the Email Address Internationalization (EAI) protocols described in the EAI Framework document [I-D.ietf-eai-frmwrk-4952bis]. As part of the overall EAI work, email messages may be transmitted and delivered containing un-encoded UTF-8 characters, and mail drops that are accessed using POP3 [RFC1939] might natively store UTF-8. This specification extends POP3 [RFC1939] using the POP3 extension mechanism [RFC2449] to permit un-encoded UTF-8 [RFC3629] in headers, as described in "Internationalized Email Headers" [I-D.ietf-eai-rfc5335bis]. It also adds a mechanism to support login names and passwords outside the ASCII character set, and a mechanism to support UTF-8 protocol-level error strings in a language appropriate for the user. Within this specification, the term "down-conversion" refers to the process of modifying a message containing UTF-8 headers [I-D.ietf-eai-rfc5335bis] or body parts with 8bit content-transfer- encoding, as defined in MIME Section 2.8 [RFC2045], into conforming 7-bit Internet Message Format [RFC5322] with message header extensions for non-ASCII text [RFC2047] and other 7-bit encodings. Down-conversion is specified by "Post-delivery Message Downgrading for Internationalized Email Messages" [popimap-downgrade]. 1.1. Conventions Used in This Document The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in "Key words for use in RFCs to Indicate Requirement Levels" [RFC2119]. 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. Note that examples always use 7-bit ASCII characters due to limitations of this document format; in particular, some examples for the "LANG" command may appear silly as a result. Gellens, et al. Expires October 1, 2011 [Page 3] Internet-Draft POP3 Support for UTF-8 March 2011 2. LANG Capability Per "POP3 Extension Mechanism" [RFC2449], this document adds a new capability response tag to indicate support for a new command: LANG. The capability tag and new command are described below. CAPA tag: LANG Arguments with CAPA tag: none Added Commands: LANG Standard commands affected: All Announced states / possible differences: both / no Commands valid in states: AUTHENTICATION, TRANSACTION Specification reference: this document Discussion: POP3 allows most +OK and -ERR server responses to include human- readable text that, in some cases, might be presented to the user. But that text is limited to ASCII by the POP3 specification [RFC1939]. The LANG capability and command permit a POP3 client to negotiate which language the server should use when sending human- readable text. A server that advertises the LANG extension MUST use the language "i-default" as described in [RFC2277] as its default language until another supported language is negotiated by the client. A server MUST include "i-default" as one of its supported languages. The LANG command requests that human-readable text included in all subsequent +OK and -ERR responses be localized to a language matching the language range argument (the "Basic Language Range" as described by [RFC4647]). If the command succeeds, the server returns a +OK response followed by a single space, the exact language tag selected, another space, and the rest of the line is human-readable text in the appropriate language. This and subsequent protocol-level human- Gellens, et al. Expires October 1, 2011 [Page 4] Internet-Draft POP3 Support for UTF-8 March 2011 readable text is encoded in the UTF-8 charset. If the command fails, the server returns an -ERR response and subsequent human-readable response text continues to use the language that was previously active (typically i-default). The special "*" language range argument indicates a request to use a language designated as preferred by the server administrator. The preferred language MAY vary based on the currently active user. If no argument is given and the POP3 server issues a positive response, then the response given is multi-line. After the initial +OK, for each language tag the server supports, the POP3 server responds with a line for that language. This line is called a "language listing". In order to simplify parsing, all POP3 servers are required to use a certain format for language listings. A language listing consists of the language tag [RFC5646] of the message, optionally followed by a single space and a human-readable description of the language in the language itself, using the UTF-8 charset. Examples: < Note that some examples do not include the correct character accents due to limitations of this document format. > < The server defaults to using English i-default responses until the client explicitly changes the language. > C: USER karen S: +OK Hello, karen C: PASS password S: +OK karen's maildrop contains 2 messages (320 octets) < Client requests deprecated MUL language. Server replies with -ERR response. > C: LANG MUL S: -ERR invalid language MUL < A LANG command with no parameters is a request for a language listing. > C: LANG S: +OK Language listing follows: S: en English S: en-boont English Boontling dialect Gellens, et al. Expires October 1, 2011 [Page 5] Internet-Draft POP3 Support for UTF-8 March 2011 S: de Deutsch S: it Italiano S: es Espanol S: sv Svenska S: i-default Default language S: . < A request for a language listing might fail. > C: LANG S: -ERR Server is unable to list languages < Once the client changes the language, all responses will be in that language, starting with the response to the LANG command. > C: LANG es S: +OK es Idioma cambiado < If a server does not support the requested primary language, responses will continue to be returned in the current language the server is using. > C: LANG uga S: -ERR es Idioma <> no es conocido C: LANG sv S: +OK sv Kommandot "LANG" lyckades C: LANG * S: +OK es Idioma cambiado 3. UTF8 Capability Per "POP3 Extension Mechanism" [RFC2449], this document adds a new capability response tag to indicate support for new server functionality, including a new command: UTF8. The capability tag and new command and functionality are described below. CAPA tag: UTF8 Arguments with CAPA tag: USER Added Commands: UTF8 Gellens, et al. Expires October 1, 2011 [Page 6] Internet-Draft POP3 Support for UTF-8 March 2011 Standard commands affected: USER, PASS, APOP, LIST, TOP, RETR Announced states / possible differences: both / no Commands valid in states: AUTHORIZATION Specification reference: this document Discussion: This capability adds the "UTF8" command to POP3. The UTF8 command switches the session from ASCII to UTF-8 mode. 3.1. The UTF8 Command The UTF8 command enables UTF-8 mode. The UTF8 command has no parameters. Maildrops can natively store UTF-8 or be limited to ASCII. UTF-8 mode has no effect on messages in an ASCII-only maildrop. Messages in native UTF-8 maildrops can be ASCII or UTF-8 using internationalized headers [I-D.ietf-eai-rfc5335bis] and/or 8bit content-transfer-encoding, as defined in MIME Section 2.8 [RFC2045]. In UTF-8 mode, both UTF-8 and ASCII messages are sent to the client as-is (without conversion). When not in UTF-8 mode, UTF-8 messages in a native UTF-8 maildrop MUST NOT be sent to the client as-is. The UTF8 Commands MAY fail. UTF-8 messages in a native UTF-8 maildrop MAY be down-converted (downgraded) to comply with unextended POP and Internet Mail Format without UTF-8 mode support. Note that even in UTF-8 mode, MIME binary content-transfer-encoding is still not permitted. The octet count (size) of a message reported in a response to the LIST command SHOULD match the actual number of octets sent in a RETR response (not counting byte-stuffing). Sizes reported elsewhere, such as in STAT responses and non-standardized, free-form text in positive status indicators (following "+OK") need not be accurate, but it is preferable if they are. Mail stores are either ASCII or native UTF-8, and clients either issue the UTF8 command or not. The message needs converting only when it is native UTF-8 and the client has not issued the UTF8 command, in which case the server MAY choose to down-convert it. The Gellens, et al. Expires October 1, 2011 [Page 7] Internet-Draft POP3 Support for UTF-8 March 2011 down-converted message may be larger. The server may choose various strategies regarding down-conversion, which include when to down- convert, whether to cache or store the down-converted form of a message (and if so, for how long), and whether to calculate or retain the size of a down-converted message independently of the down- converted content. If the server does not have immediate access to the accurate down-converted size, it may be faster to estimate rather than calculate it. Servers are expected to normally follow the RFC 1939 [RFC1939] text on using the "exact size" in a scan listing, but there may be situations with maildrops containing very large numbers of messages in which this might be a problem. If the server does estimate, reporting a scan listing size smaller than what it turns out to be could be a problem for some clients. In summary, it is better for servers to report accurate sizes, but if this is not possible, high guesses are better than small ones. Some POP servers include the message size in the non-standardized text response following "+OK" (the 'text' production of RFC 2449 [RFC2449]), in a RETR or TOP response (possibly because some examples in POP3 [RFC1939] do so). There has been at least one known case of a client relying on this to know when it had received all of the message rather than following the POP3 [RFC1939] rule of looking for a line consisting of a termination octet (".") and a CRLF pair. While any such client is non-compliant, if a server does include the size in such text, it is better if it is accurate. Clients MUST NOT issue the STLS command [RFC2595] after issuing UTF8; servers MAY (but are not required to) enforce this by rejecting with an "-ERR" response an STLS command issued subsequent to a successful UTF8 command. (Because this is a protocol error as opposed to a failure based on conditions, an extended response code [RFC2449] is not specified.) 3.2. USER Argument to UTF8 Capability If the USER argument is included with this capability, it indicates that the server accepts UTF-8 user names and passwords. Servers that include the USER argument in the UTF8 capability response SHOULD apply SASLprep [RFC4013] to the arguments of the USER and PASS commands. A client or server that supports APOP and permits UTF-8 in user names or passwords MUST apply SASLprep [RFC4013] to the user name and password used to compute the APOP digest. When applying SASLprep [RFC4013], servers MUST reject UTF-8 user names or passwords that contain a Unicode character listed in Section 2.3 of SASLprep [RFC4013]. When applying SASLprep to the USER Gellens, et al. Expires October 1, 2011 [Page 8] Internet-Draft POP3 Support for UTF-8 March 2011 argument, the PASS argument, or the APOP username argument, a compliant server or client MUST treat them as a query string (i.e., unassigned Unicode code points are allowed). When applying SASLprep to the APOP password argument, a compliant server or client MUST treat them as a stored string (i.e., unassigned Unicode code points are prohibited). The client does not need to issue the UTF8 command prior to using UTF-8 in authentication. However, clients MUST NOT use UTF-8 characters in USER, PASS, or APOP commands unless the USER argument is included in the UTF8 capability response. The server MUST reject UTF-8 user names or passwords that fail to comply with the formal syntax in UTF-8 [RFC3629]. Use of UTF-8 characters in the AUTH command is governed by the POP3 SASL [RFC5034] mechanism. 4. Native UTF-8 Maildrops When a POP3 server uses a native UTF-8 maildrop, it is the responsibility of the server to comply with the POP3 base specification [RFC1939] and Internet Message Format [RFC5322] when not in UTF-8 mode. Mechanisms for 7-bit downgrading to help comply with the standards are described in [popimap-downgrade]. 5. IANA Considerations This specification updates two capabilities ("UTF8" and "LANG") to the POP3 capability registry [RFC2449]. 6. 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. The "LANG *" command might reveal the existence and preferred language of a user to an active attacker probing the system if the active language changes in response to the USER, PASS, or APOP commands prior to validating the user's credentials. Servers MUST implement a configuration to prevent this exposure. It is possible for a man-in-the-middle attacker to insert a LANG command in the command stream, thus making protocol-level diagnostic responses unintelligible to the user. A mechanism to integrity- protect the session, such as Transport Layer Security (TLS) [RFC2595] can be used to defeat such attacks. Gellens, et al. Expires October 1, 2011 [Page 9] Internet-Draft POP3 Support for UTF-8 March 2011 Modifying server authentication code (in this case, to support UTF8 command) needs to be done with care to avoid introducing vulnerabilities (for example, in string parsing). The UTF8 command description (Section 3.1) contains a discussion on reporting inaccurate sizes. An additional risk to doing so is that, if a client allocates buffers based on the reported size, it may overrun the buffer, crash, or have other problems if the message data is larger than reported. 7. Change History 7.1. draft-ietf-eai-rfc5721bis: Version 00 following the new charter 7.2. draft-ietf-eai-rfc5721bis: Version 01 refine the texts 8. References 8.1. Normative References [I-D.ietf-eai-frmwrk-4952bis] Klensin, J. and Y. Ko, "Overview and Framework for Internationalized Email", draft-ietf-eai-frmwrk-4952bis-10 (work in progress), September 2010. [I-D.ietf-eai-rfc5335bis] Yang, A. and S. Steele, "Internationalized Email Headers", draft-ietf-eai-rfc5335bis-10 (work in progress), March 2011. [RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3", STD 53, RFC 1939, May 1996. [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- Gellens, et al. Expires October 1, 2011 [Page 10] Internet-Draft POP3 Support for UTF-8 March 2011 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. [RFC2277] Alvestrand, H., "IETF Policy on Character Sets and Languages", BCP 18, RFC 2277, January 1998. [RFC2449] Gellens, R., Newman, C., and L. Lundblade, "POP3 Extension Mechanism", RFC 2449, November 1998. [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. [RFC4647] Phillips, A. and M. Davis, "Matching of Language Tags", BCP 47, RFC 4647, September 2006. [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, October 2008. [RFC5646] Phillips, A. and M. Davis, "Tags for Identifying Languages", BCP 47, RFC 5646, September 2009. 8.2. Informative References [RFC2595] Newman, C., "Using TLS with IMAP, POP3 and ACAP", RFC 2595, June 1999. [RFC5034] Siemborski, R. and A. Menon-Sen, "The Post Office Protocol (POP3) Simple Authentication and Security Layer (SASL) Authentication Mechanism", RFC 5034, July 2007. [popimap-downgrade] Fujiwara, K., "Post-delivery Message Downgrading for Internationalized Email Messages", draft-ietf-eai-popimap-downgrade-00 Gellens, et al. Expires October 1, 2011 [Page 11] Internet-Draft POP3 Support for UTF-8 March 2011 (work in progress), October 2010. Appendix A. Design Rationale This non-normative section discusses the reasons behind some of the design choices in the above specification. Due to interoperability problems with RFC 2047 and limited deployment of RFC 2231, it is hoped these 7-bit encoding mechanisms can be deprecated in the future when UTF-8 header support becomes prevalent. USER is optional because the implementation burden of SASLprep [RFC4013] is not well understood, and mandating such support in all cases could negatively impact deployment. While it is possible to provide useful examples for language negotiation without support for non-ASCII characters, it is difficult to provide useful examples for commands specifically designed to use the UTF-8 charset un-encoded when the document format is limited to ASCII. As a result, there are no plans to provide examples for that part of the specification as long as this remains an experimental proposal. However, implementers of this specification are encouraged to provide examples to the document authors for a future revision. Appendix B. Acknowledgments Thanks to John Klensin, Tony Hansen, and other EAI working group participants who provided helpful suggestions and interesting debate that improved this specification. Authors' Addresses Randall Gellens QUALCOMM Incorporated 5775 Morehouse Drive San Diego, CA 92651 US EMail: rg+ietf@qualcomm.com Gellens, et al. Expires October 1, 2011 [Page 12] Internet-Draft POP3 Support for UTF-8 March 2011 Chris Newman Oracle 800 Royal Oaks Monrovia, CA 91016-6347 US EMail: chris.newman@oracle.com Jiankang YAO CNNIC No.4 South 4th Street, Zhongguancun Beijing Phone: +86 10 58813007 EMail: yaojk@cnnic.cn Kazunori Fujiwara Japan Registry Services Co., Ltd. Chiyoda First Bldg. East 13F, 3-8-1 Nishi-Kanda Tokyo Phone: +81 3 5215 8451 EMail: fujiwara@jprs.co.jp Gellens, et al. Expires October 1, 2011 [Page 13]