Network Working Group                                         R. Gellens
Internet-Draft                                     QUALCOMM Incorporated
Obsoletes: RFC5721 (if approved)                               C. Newman
Intended status: Standards Track                                  Oracle
Expires: January 12, 2012                                         J. Yao
                                                             K. Fujiwara
                                                           July 11, 2011

                         POP3 Support for UTF-8


   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 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
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   Drafts is at

   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 January 12, 2012.

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
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect

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   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
     7.3.  draft-ietf-eai-rfc5721bis:  Version 02 . . . . . . . . . . 10
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 12
   Appendix A.  Design Rationale  . . . . . . . . . . . . . . . . . . 12
   Appendix B.  Acknowledgments . . . . . . . . . . . . . . . . . . . 12

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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 with UTF-8 charset beyond ASCII, and a mechanism
   to support UTF-8 charset in protocol level response strings for
   languages beyond English.

   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",
   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

   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.

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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:

   Arguments with CAPA tag:

   Added Commands:

   Standard commands affected:

   Announced states / possible differences:
      both / no

   Commands valid in states:

   Specification reference:
      this document


   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-

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   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.


      < 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

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      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 <<UGA>> 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:

   Arguments with CAPA tag:

   Added Commands:

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   Standard commands affected:

   Announced states / possible differences:
      both / no

   Commands valid in states:

   Specification reference:
      this document


   This capability adds the "UTF8" command to POP3.  The UTF8 command
   switches the session from ASCII to UTF-8 mode.  In UTF-8 mode, it
   means that both servers and clients can send and accept the UTF-8

3.1.  The UTF8 Command

   The UTF8 command enables UTF-8 mode.  The UTF8 command has no

   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 were.

   Mail stores are either ASCII or native UTF-8, and clients either
   issue the UTF8 command or not.  The message needs converting only

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   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
   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

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   names or passwords that contain a Unicode character listed in Section
   2.3 of SASLprep [RFC4013].  When applying SASLprep to the USER
   argument, the PASS argument, or the APOP username argument, a
   compliant server or client MUST treat them as a query string
   [RFC3454](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 [RFC3454] (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-

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   protect the session, such as Transport Layer Security (TLS) [RFC2595]
   can be used to defeat such attacks.

   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

7.3.  draft-ietf-eai-rfc5721bis:  Version 02

   update the texts based on Joseph's comments

8.  References

8.1.  Normative References

   [I-D.ietf-eai-frmwrk-4952bis]  Klensin, J. and Y. Ko, "Overview and
                                  Framework for Internationalized
                                  draft-ietf-eai-frmwrk-4952bis-10 (work
                                  in progress), September 2010.

   [I-D.ietf-eai-rfc5335bis]      Yang, A., Steele, S., and N. Freed,
                                  "Internationalized Email Headers",
                                  draft-ietf-eai-rfc5335bis-11 (work in
                                  progress), July 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

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                                  (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.

   [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.

   [RFC3454]                      Hoffman, P. and M. Blanchet,
                                  "Preparation of Internationalized
                                  Strings ("stringprep")", RFC 3454,
                                  December 2002.

   [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.

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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",
                                  (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.

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


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   Chris Newman
   800 Royal Oaks
   Monrovia, CA  91016-6347


   Jiankang YAO
   No.4 South 4th Street, Zhongguancun

   Phone: +86 10 58813007

   Kazunori Fujiwara
   Japan Registry Services Co., Ltd.
   Chiyoda First Bldg. East 13F, 3-8-1 Nishi-Kanda

   Phone: +81 3 5215 8451

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