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Versions: 00 01 02 03 04 05 06 07 08 09 RFC 5721

Network Working Group                                         R. Gellens
Internet-Draft                                     QUALCOMM Incorporated
Intended status: Experimental                                  C. Newman
Expires: April 28, 2010                                 Sun Microsystems
                                                        October 25, 2009


                         POP3 Support for UTF-8
                       draft-ietf-eai-pop-09.txt

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.  This document may contain material
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Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   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 (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

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.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Conventions Used in this Document  . . . . . . . . . . . .  3
     1.2.  Change History . . . . . . . . . . . . . . . . . . . . . .  4
       1.2.1.  Changes from -08 to -09  . . . . . . . . . . . . . . .  4
       1.2.2.  Changes from -07 to -08  . . . . . . . . . . . . . . .  4
       1.2.3.  Changes from -06 to -07  . . . . . . . . . . . . . . .  5
       1.2.4.  Changes from -05 to -06  . . . . . . . . . . . . . . .  5
       1.2.5.  Changes from -04 to -05  . . . . . . . . . . . . . . .  6
       1.2.6.  Changes from -03 to -04  . . . . . . . . . . . . . . .  6
       1.2.7.  Changes from -02 to -03  . . . . . . . . . . . . . . .  6
       1.2.8.  Changes from -01 to -02  . . . . . . . . . . . . . . .  7
       1.2.9.  Changes from -00 to -01  . . . . . . . . . . . . . . .  7
       1.2.10. Changes from draft-newman-ima-pop  . . . . . . . . . .  7
     1.3.  Open Issues  . . . . . . . . . . . . . . . . . . . . . . .  8
   2.  LANG Capability  . . . . . . . . . . . . . . . . . . . . . . .  8
   3.  UTF8 Capability  . . . . . . . . . . . . . . . . . . . . . . . 10
     3.1.  The UTF8 Command . . . . . . . . . . . . . . . . . . . . . 11
     3.2.  USER Argument to UTF8 Capability . . . . . . . . . . . . . 13
   4.  Issues with UTF-8 Header maildrop  . . . . . . . . . . . . . . 13
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 14
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 15
   Appendix A.  Design Rationale  . . . . . . . . . . . . . . . . . . 16
   Appendix B.  Acknowledgments . . . . . . . . . . . . . . . . . . . 16
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16








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1.  Introduction

   This document forms part of the Email Address Internationalization
   (EAI) experiment described in the EAI Framework [RFC4952] document
   (for background, please see the charter of the EAI working group) and
   should be evaluated within the context of EAI.  As part of the
   overall EAI work, email messages may be transmitted and delivered
   containing un-encoded UTF-8 characters, and mail drops 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 [RFC5335].  It also
   adds a mechanism to support login names outside the ASCII character
   set, and a mechanism to support UTF-8 protocol-level error strings in
   a language appropriate for the user.

   This document updates POP3 [RFC1939], and the fact that an
   Experimental specification updates a Standards-Track specification
   means that people who participate in the experiment have to consider
   the standard updated.  Note that, as an Experimental document, there
   is no "Updates" header.  If and when a version of this document moves
   to the standards track, an "Updates: 1939" header should be added.

   Within this specification, the term down-conversion refers to the
   process of modifying a message containing UTF8 headers [RFC5335] 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
   Downgrading mechanism for Email Address Internationalization
   [RFC5504].

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].

   The formal syntax uses the Augmented Backus-Naur Form (ABNF)
   [RFC5234] notation including the core rules defined in Appendix B of
   RFC 5234.

   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



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   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.

1.2.  Change History

   This section describes the change history of this Internet draft and
   will be removed when/if this is published as an RFC.

1.2.1.  Changes from -08 to -09

   o  Added new paragraph to start of Introduction to more clearly
      explain this document within the larger EAI context.

   o  Added informative reference to EAI Framework (RFC 4952).

   o  Removed "Updates: 1939" header and added a note that one should be
      added if and when it is published on the standards track.

   o  Added clarifying text that the "language range" argument of RFC
      4647 is the "Basic Language Range".

1.2.2.  Changes from -07 to -08

   o  Changed wording on applying SASLprep to APOP digest inputs.

   o  Added mandatory rejection of user names or passwords which fail to
      comply with formal syntax of RFC 3629.

   o  Added text that, when applying SASLprep, servers MUST reject user
      names or passwords which contain characters listed in section 2.3
      of RFC 4013.

   o  Added normative reference to RFC 3629.

   o  Changed SASLprep language so that both clients and servers MUST
      apply SASLprep to user names and passwords used to compute APOP
      digest, and servers SHOULD apply SASLprep to arguments of USER and
      PASS.

   o  Fixed typo ("ACII" instead of "ASCII").

   o  Clarified that size doesn't include byte-stuffing.

   o  Added explanation to Introduction regarding updating RFC 1939.




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   o  Added more prominent text on examples which are silly because they
      use 7-bit ASCII.

   o  Replaced French examples with Spanish to try and make them
      slightly less embarrassing.

   o  Replaced French-Canadian (fr-ca) example with Swedish, to try and
      avoid accented characters.  Also added Swedish to language
      listing.

   o  Added "Examples" to LANG command examples.

   o  Added introductory text to sections on LANG and UTF8 capability
      tags.

1.2.3.  Changes from -06 to -07

   o  Added discussion about accuracy of size.

   o  Added mention of potential buffer overflow problems because of
      inaccurate sizes to the Security Considerations.

   o  Added informative reference to SASL for POP3 (RFC 5034).

   o  Removed text making changes to AUTH, as this is handled by POP3
      SASL.

   o  Fixed typo ("depricated" instead of "deprecated").

   o  Reworded Design Rationale appendix.

1.2.4.  Changes from -05 to -06

   o  Removed LIST and TOP as possible arguments to the UTF8 tag in the
      CAPA response.

   o  Clarified that the UTF8 command has no parameters.

   o  Changed "arguments" to "arguments with CAPA tag" to clarify that
      these are possible arguments to the tag in the CAPA response and
      not command parameters.

   o  Clarified use of "argument" to refer to CAPA tag and "parameter"
      to refer to commands.

   o  Clarified that free-form text is non-standard.





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   o  Removed open issue (downgrading).

   o  Added discussion of downgrading to Appendix A.

   o  Updated downgrade reference to RFC 5504.

   o  Tweaked RFC 2119 text to satisfy I-D nit checker.

1.2.5.  Changes from -04 to -05

   o  Downgrading is back to an informative, not normative reference,
      and is suggested as a good idea but explicitly not required.

   o  Language listing now specifies that the human-readable description
      of a language is in the language itself.

   o  Updated 2822 reference to 5322, made text "Internet Message
      Format".

   o  Updated reference to utf8headers draft to RFC5335.

   o  Updated reference to RFC4234 to RFC5234.

1.2.6.  Changes from -03 to -04

   o  Specified that it is an error to issue STLS after UTF8.

   o  Removed prior open issues.

   o  Downgrading added as open issue.

1.2.7.  Changes from -02 to -03

   o  Updated references.

   o  Replaced US-ASCII with ASCII.

   o  Added comment to language listing failure example.

   o  Replaced RET8, LST8, and TOP8 commands with a single mode-switch
      UTF8 command issued before authentication.  This simplifies the
      protocol, and allows servers to optionally down-convert a cache of
      the maildrop prior to issuing the +OK response entering
      TRANSACTION state.

   o  Removed most up-conversion material.





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   o  Removed definition of up-conversion.

   o  Removed IMAP4 reference.

   o  Added AUTH command to those affected by UTF8 capability.

   o  Removed LST8 and TOP8 capability parameters and commands.

   o  Removed NO-RETR capability.  POP servers are now unconditionally
      required to support down-conversion of UTF8-native maildrops.

   o  Added sentence about modifying authentication code to Security
      Considerations.

   o  eai-downgrade draft is now normative and required.

   o  Deleted references to RFCs 1341, 1847, 2049, 2183, 3501, 3516, and
      3490.

1.2.8.  Changes from -01 to -02

   o  Minor grammatical tweaks.

   o  Add passwords to Abstract.

   o  Removed new editor's name from Acknowledgments.

1.2.9.  Changes from -00 to -01

   o  Update references

1.2.10.  Changes from draft-newman-ima-pop

   o  Change title to make this a WG document.

   o  Add LANG command and extension.

   o  Rename RET8 capability to UTF8 and add sub-sections for arguments.

   o  Add TOP8 command.

   o  Add definition of up-conversion and down-conversion.

   o  Some grammar fix-ups and section re-ordering based on RFC editor
      style.






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1.3.  Open Issues

   1.   none


2.  LANG Capability

   Per the 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 needs to 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



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   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
   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 [RFC4646] 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. >



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      C: LANG
      S: +OK Language listing follows:
      S: en English
      S: en-boont English Boontling dialect
      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

                                 Examples


3.  UTF8 Capability

   Per the 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.







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   CAPA tag:
      UTF8

   Arguments with CAPA tag:
      USER

   Added Commands:
      UTF8

   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 UTF8 mode.

3.1.  The UTF8 Command

   The UTF8 command enables UTF8 mode.  The UTF8 command has no
   parameters.

   Maildrops can natively store UTF8 or be limited to ASCII.  UTF8 mode
   has no effect on messages in an ASCII-only maildrop.  Messages in
   native-UTF8 maildrops can be ASCII or UTF8 using internationalized
   headers [RFC5335] and/or 8bit content-transfer-encoding as defined in
   MIME section 2.8 [RFC2045].  In UTF8 mode, both UTF8 and ASCII
   messages are sent to the client as-is (without conversion).  When not
   in UTF8 mode, UTF8 messages in a native UTF8 maildrop MUST be down-
   converted (downgraded) to comply with unextended POP and Internet
   Mail Format.  POP servers (unlike SMTP and Submit servers) are not
   required to use Downgrading mechanism for Email Address
   Internationalization [RFC5504].

   Discussion: The main argument against a single required mechanism for
   downgrade by a POP server is that the only clients that have any use
   for a standardized downgraded message (because they wish to interpret
   downgrade headers, for example) are ones that can support UTF8 and
   hence will issue the UTF8 command in the first place.  The counter



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   argument to this is that non-UTF8 clients might be upgraded in the
   future; it's desirable for an upgraded client to be capable of
   interpreting prior downgraded messages in the local mail store, which
   is most likely if the messages were downgraded using one standardized
   procedure.

   Therefore, while POP servers are not required to use the Downgrading
   mechanism for Email Address Internationalization [RFC5504], there are
   advantages to them doing so.

   Note that even in UTF8 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.

   Discussion: 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 UTF8 and the client has not issued the UTF8
   command, in which case the server must downconvert it.  The
   downconverted message may be larger.  The server may choose various
   strategies regarding downconversion, which include when to
   downconvert, whether to cache or store the downconverted form of a
   message (and if so, for how long), and whether to calculate or retain
   the size of a downconverted message independently of the
   downconverted content.  If the server does not have immediate access
   to the accurate downconverted 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 not, 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.



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   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 which 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 which contain a Unicode character listed in
   section 2.3 of SASLprep [RFC4013].

   The client does not need to issue the UTF8 command prior to using
   UTF8 in authentication.  However, clients MUST NOT use UTF8 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 which fail to
   comply with the formal syntax in UTF-8 [RFC3629].

   Use of UTF8 in the AUTH command is governed by the POP3 SASL
   [RFC5034] mechanism.


4.  Issues with UTF-8 Header maildrop

   When a POP3 server uses a UTF8-native maildrop, it is the
   responsibility of the server to comply with the POP3 base
   specification [RFC1939] and Internet Message Format [RFC5322] when
   not in UTF8 mode.  Mechanisms for 7-bit downgrading to help comply
   with the standards are described in Downgrading mechanism for Email
   Address Internationalization [RFC5504].







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5.  IANA Considerations

   This adds two new 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 can 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 TLS [RFC2595] can be used to defeat such
   attacks.

   Modifying server authentication code (in this case, to support UTF8)
   needs to be done with care to avoid introducing vulnerabilities (for
   example, in string parsing).

   The UTF8 Command (Section 3.1) description 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.  References

7.1.  Normative References

   [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-ASCII Text",



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              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.

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              October 2008.

   [RFC4646]  Phillips, A. and M. Davis, "Tags for Identifying
              Languages", BCP 47, RFC 4646, September 2006.

   [RFC4647]  Phillips, A. and M. Davis, "Matching of Language Tags",
              BCP 47, RFC 4647, September 2006.

   [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.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC5335]  Abel, Y., "Internationalized Email Headers", RFC 5335,
              September 2008.

7.2.  Informative References

   [RFC2595]  Newman, C., "Using TLS with IMAP, POP3 and ACAP",
              RFC 2595, June 1999.

   [RFC4952]  Klensin, J. and Y. Ko, "Overview and Framework for
              Internationalized Email", RFC 4952, July 2007.

   [RFC5034]  Siemborski, R. and A. Menon-Sen, "The Post Office Protocol
              (POP3) Simple Authentication and Security Layer (SASL)
              Authentication Mechanism", RFC 5034, July 2007.

   [RFC5504]  Fujiwara, K. and Y. Yoneya, "Downgrading Mechanism for
              Email Address Internationalization", RFC 5504, March 2009.





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Appendix A.  Design Rationale

   This non-normative section discusses the reasons behind some of the
   design choices in the above specification.

   Having servers perform up-conversion so that, at a minimum, RFC2047-
   encoded words are decoded into UTF8 is tempting, since this is an
   area that clients often fail to correctly implement.  However, after
   much discussion the group felt that the benefits did not justify the
   burden.

   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 author for a future revision.

   While down-conversion of native-UTF8 messages is mandatory in the
   absence of the UTF8 command, servers are not required to do so as
   specified in Downgrading Mechanism [RFC5504].  As clients are
   upgraded with UTF8 support and the ability to intelligently handle
   (e.g., display and reply to) UTF8 messages that were downgraded in
   transit, it is better if they are also able to handle messages in the
   local mail store that were downgraded by the POP server.  This is
   more likely if the POP server downgrades messages using the same
   mechanism as an SMTP server.


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.







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Authors' Addresses

   Randall Gellens
   QUALCOMM Incorporated
   5775 Morehouse Drive
   San Diego, CA  92651
   US

   Email: rg+ietf@qualcomm.com


   Chris Newman
   Sun Microsystems
   800 Royal Oaks
   Monrovia, CA  91016-6347
   US

   Email: chris.newman@sun.com

































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