Internet Engineering Task Force (IETF) R. Gellens

Internet Engineering Task Force (IETF) R. Gellens
Request for Comments: 5721 QUALCOMM Incorporated
Category: Experimental C. Newman
ISSN: 2070-1721 Sun Microsystems
February 2010

POP3 Support for UTF-8

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 document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.

This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Engineering
Task Force (IETF). It represents the consensus of the IETF
community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Not
all documents approved by the IESG are a candidate for any level of
Internet Standard; see Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5721.

Copyright Notice

Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

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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 . . . . . . . . . . . . . 9
4. Native UTF-8 Maildrops . . . . . . . . . . . . . . . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. Normative References . . . . . . . . . . . . . . . . . . . 10
7.2. Informative References . . . . . . . . . . . . . . . . . . 11
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) experiment described in the EAI Framework 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 [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 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.

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. In an attempt to reduce confusion, this
Experimental document does not contain an "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].

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.

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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.

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

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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 [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. >

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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

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

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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 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 [RFC5335] 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 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
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
UTF-8 and, hence, will issue the UTF8 command in the first place.
The counter argument to this is that clients that do not support
UTF-8 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.

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

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.

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 UTF-8 and the client has not issued the UTF-8
command, in which case the server must 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

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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
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
UTF-8 in authentication. However, clients MUST NOT use UTF-8 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 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 "Downgrading Mechanism for Email
Address Internationalization" [RFC5504].

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5. IANA Considerations

This specification 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 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.

Modifying server authentication code (in this case, to support UTF-8)
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. 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.

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RFC 5721 POP3 Support for UTF-8 February 2010

[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.

[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.

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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 UTF-8 is tempting, since this is an
area that clients often fail to correctly implement. However, after
much discussion, the EAI 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 authors for a future revision.

While down-conversion of native UTF-8 messages is mandatory in the
absence of the UTF8 command, servers are not required to use
"Downgrading Mechanism for Email Address Internationalization"
[RFC5504] to do so. As clients are upgraded with UTF-8 support and
the ability to intelligently handle (e.g., display and reply to)
UTF-8 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: [email protected]

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

EMail: [email protected]

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