wdiff rfc5721.original rfc5721.txt

Network Working Group R. Gellens
Internet-Draft
Request for Comments: 5721 QUALCOMM Incorporated
Intended status:
Category: Experimental C. Newman
Expires: April 28, 2010
Sun Microsystems
October 25,
December 2009

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

This Internet-Draft memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.

This document is submitted subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in full conformance with effect on the
provisions date of BCP 78
publication of this document. Please review these documents
carefully, as they describe your rights and BCP 79. 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 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.

Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.

Internet-Drafts are draft documents valid for a maximum

Table 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 Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions Used in progress."

The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.

The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.

This Internet-Draft will expire on April 28, 2010.

Copyright (c) 2009 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 (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.

Abstract

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions Used in this Document . . . . . . . . . . . . 3
1.2. Change History . . . . . . . . . . . . . . . . . . . . . 2
2. LANG Capability . 4
1.2.1. Changes from -08 to -09 . . . . . . . . . . . . . . . 4
1.2.2. Changes from -07 to -08 . . . . . . . 3
3. UTF8 Capability . . . . . . . . 4
1.2.3. Changes from -06 to -07 . . . . . . . . . . . . . . . 5
1.2.4. Changes from -05 to -06 . . . . . . . . .
3.1. The UTF8 Command . . . . . . 5
1.2.5. Changes from -04 to -05 . . . . . . . . . . . . . . . 6
1.2.6. Changes from -03
3.2. USER Argument to -04 . . . . . . . . . UTF8 Capability . . . . . . 6
1.2.7. Changes from -02 to -03 . . . . . . . 8
4. Issues with UTF-8 Header Maildrop . . . . . . . . 6
1.2.8. Changes from -01 to -02 . . . . . . 8
5. IANA Considerations . . . . . . . . . 7
1.2.9. Changes from -00 to -01 . . . . . . . . . . . . 9
6. Security Considerations . . . 7
1.2.10. Changes from draft-newman-ima-pop . . . . . . . . . . 7
1.3. Open Issues . . . . . . 9
7. References . . . . . . . . . . . . . . . . . 8
2. LANG Capability . . . . . . . . . 9
7.1. Normative References . . . . . . . . . . . . . . 8
3. UTF8 Capability . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . . . 10
3.1. The UTF8 Command . . .
Appendix A. Design Rationale . . . . . . . . . . . . . . . . . . 11
3.2. USER Argument to UTF8 Capability . . . . . . . . . . . . . 13
4. Issues with UTF-8 Header maildrop . . . . . .
Appendix B. Acknowledgments . . . . . . . . 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 11

1. Introduction

This document forms part of the Email Address Internationalization
(EAI) experiment described in the EAI Framework [RFC4952] document [RFC4952]
(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 that are
accessed using POP3 [RFC1939] might natively store UTF-8.

This specification extends POP3 [RFC1939] using the POP3 Extension
Mechanism extension
mechanism [RFC2449] to permit un-encoded UTF-8 [RFC3629] in headers headers,
as described in Internationalized "Internationalized Email Headers Headers" [RFC5335]. It also
adds a mechanism to support login names outside the ASCII character
set,
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 Standards Track specification
means that people who participate in the experiment have to consider
the standard 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, Standards Track, an "Updates: 1939" header should be added.

Within this specification, the term down-conversion "down-conversion" refers to the
process of modifying a message containing UTF8 headers [RFC5335] or
body parts with 8bit content-transfer-encoding content-transfer-encoding, as defined in MIME
section
Section 2.8 [RFC2045] [RFC2045], into conforming 7-bit Internet Message Format
[RFC5322] with Message Header Extensions message header extensions for Non-ASCII Text non-ASCII text [RFC2047]
and other 7-bit encodings. Down-conversion is specified by
Downgrading mechanism
"Downgrading Mechanism for Email Address Internationalization Internationalization"
[RFC5504].

1.1. Conventions Used in this 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 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
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.

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.

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.

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.

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 use 7-bit ASCII characters due to support down-conversion
limitations 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 document format; in particular, some examples 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.

1.3. Open Issues

1. none
the "LANG" command may appear silly as a result.

2. LANG Capability

Per the POP3 "POP3 Extension Mechanism 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 that, in some cases 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
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 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] [RFC5646] of the message, optionally followed by a
single space and a human readable 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 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
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 fail. >

C: LANG
S: -ERR Server is unable to list languages

< Once the client changes the language, all responses will be in
that language 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
functionality, including a new command, command: UTF8. The capability tag and
new command and functionality are described below.

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 content-transfer-encoding, as defined
in MIME section 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 "Downgrading Mechanism for Email Address
Internationalization
Internationalization" [RFC5504].

Discussion: The main argument against a single required mechanism for
downgrade
downgrading 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 and, hence, will issue the UTF8 command in the first place. The
counter 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 "Downgrading
Mechanism for Email Address Internationalization 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 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 down-convert it. The
downconverted down-
converted message may be larger. The server may choose various
strategies regarding downconversion, down-conversion, which include when to
downconvert, down-
convert, whether to cache or store the downconverted down-converted form of a
message (and if so, for how long), and whether to calculate or retain
the size of a downconverted down-converted message independently of the
downconverted down-
converted content. If the server does not have immediate access to
the accurate downconverted 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 not, 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 which 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 which that contain a Unicode character listed in
section 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 (i.e.,
unassigned Unicode codepoints 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 codepoints
are prohibited).

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 that 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 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 "Downgrading Mechanism for Email
Address Internationalization Internationalization" [RFC5504].

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 stream, thus making protocol-level diagnostic
responses unintelligible to the user. A mechanism to integrity integrity-
protect the session, such as TLS Transport Layer Security (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 command description (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",
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.

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

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

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

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

[RFC5646] Phillips, A. and M. Davis, "Tags for Identifying
Languages", BCP 47, RFC 5646, September 2009.

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.

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 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 so, as
specified in Downgrading "Downgrading Mechanism for Email Address
Internationalization" [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 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:

EMail: rg+ietf@qualcomm.com

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

Email: chris.newman@sun.com

EMail: chis.newman@sun.com