Network Working Group R. Gellens
Internet-Draft QUALCOMM Incorporated
Obsoletes: 5721 (if approved) C. Newman
Intended status: Standards Track Oracle
Expires: February 1, 2013 J. Yao
CNNIC
K. Fujiwara
JPRS
July 31, 2012
POP3 Support for UTF-8
draft-ietf-eai-rfc5721bis-07.txt
Abstract
This specification extends the Post Office Protocol version 3 (POP3)
to support UTF-8 encoded international string 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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
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This Internet-Draft will expire on February 1, 2013.
Copyright Notice
Copyright (c) 2012 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
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publication of this document. Please review these documents
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to this document. Code Components extracted from this document must
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described in the Simplified BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
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than English.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions Used in This Document . . . . . . . . . . . . 3
2. UTF8 Capability . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. The UTF8 Command . . . . . . . . . . . . . . . . . . . . . 4
2.2. USER Argument to UTF8 Capability . . . . . . . . . . . . . 6
3. LANG Capability . . . . . . . . . . . . . . . . . . . . . . . 6
4. Non-ASCII character Maildrops . . . . . . . . . . . . . . . . 9
5. UTF8 Response Code . . . . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. Change History . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. draft-ietf-eai-rfc5721bis: Version 00 . . . . . . . . . . 10
8.2. draft-ietf-eai-rfc5721bis: Version 01 . . . . . . . . . . 10
8.3. draft-ietf-eai-rfc5721bis: Version 02 . . . . . . . . . . 11
8.4. draft-ietf-eai-rfc5721bis: Version 03 . . . . . . . . . . 11
8.5. draft-ietf-eai-rfc5721bis: Version 04 . . . . . . . . . . 11
8.6. draft-ietf-eai-rfc5721bis: Version 05 . . . . . . . . . . 11
8.7. draft-ietf-eai-rfc5721bis: Version 06 . . . . . . . . . . 11
8.8. draft-ietf-eai-rfc5721bis: Version 07 . . . . . . . . . . 11
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
9.1. Normative References . . . . . . . . . . . . . . . . . . . 11
9.2. Informative References . . . . . . . . . . . . . . . . . . 13
Appendix A. Design Rationale . . . . . . . . . . . . . . . . . . 13
Appendix B. Acknowledgments . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
This document forms part of the Email Address Internationalization
protocols described in the Email Address Internationalization
Framework document [RFC6530]. As part of the overall Email Address
Internationalization work, email messages could be transmitted and
delivered containing Unicode string encoded in UTF-8 in the header
and/or body, and maildrops 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,
and bodies (e.g., transferred using 8-bit Content Transfer Encoding)
as described in "Internationalized Email Headers" [RFC6532]. It also
adds a mechanism to support login names and passwords containing
UTF-8 string and a mechanism to support UTF-8 string in protocol
level response strings as well as the ability to negotiate a language
for such response strings.
This specification also adds a new response code to indicate that a
message was not delivered because it required UTF-8 mode discussed in
section 2 and the server was unable or unwilling to create and
deliver a variant form of the message as discussed in Section 7 of
[I-D.ietf-eai-5738bis].
This specification replaces an earlier, experimental, approach to the
same problem RFC 5721 [RFC5721].
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 terms "UTF-8 string" or "UTF-8 character" are used to refer to
Unicode characters, which may or may not be members of the ASCII
subset, in UTF-8 RFC3629 [RFC3629], a standard Unicode Encoding Form.
All other specialized terms used in this specification are defined in
the Email Address Internationalization framework document.
In examples, "C:" and "S:" indicate lines sent by the client and
server, respectively. If a single "C:" or "S:" label applies to
multiple lines, then the line breaks between those lines are for
editorial clarity only and are not part of the actual protocol
exchange.
Note that examples always use ASCII characters due to limitations of
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this document format; otherwise, some examples for the "LANG" command
may appear incorrectly.
2. UTF8 Capability
This specification adds a new POP3 Extension [RFC2449] capability
response tag and command to specify support for header field
information in UTF-8 rather than only ASCII. 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 the ASCII-only mode of RFC 1939 to UTF-8
mode. The UTF-8 mode means that, all messages transmitted between
servers and clients are UTF-8 strings, and both servers and clients
can send and accept UTF-8 string.
2.1. The UTF8 Command
The UTF8 command enables UTF-8 mode. The UTF8 command has no
parameters.
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 [RFC6532] and/or 8bit content-transfer-
encoding, as defined in MIME Section 2.8 [RFC2045]. The character
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encoding format of maildrops may not be UTF-8 or ASCII. In UTF-8
mode, if the character encoding format of maildrops is UTF-8 or
ASCII, the messages are sent to the client as-is; if the character
encoding format of maildrops is format other than UTF-8 or ASCII, the
messages' encoding format SHOULD be converted to be UTF-8 before they
are sent to the client. When not in UTF-8 mode, non-ASCII string
messages including UTF-8 string messages in the maildrop MUST NOT be
sent to the client as-is. If a client requests a UTF-8 message when
not in UTF-8 mode, the server MUST either create the message content
variant (discussed in Section 7 of [I-D.ietf-eai-5738bis]) it sends
to the client to comply with unextended POP and Internet Mail Format
without UTF-8 mode support, or fail the request with a -ERR response
containing the UTF-8 response code (see section 5). The UTF8 command
MAY fail.
Note that even in UTF-8 mode, MIME binary content-transfer-encoding
as defined in MIME Section 6.2 [RFC2045] 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.
Normal operation for maildrops that natively support non-ASCII
characters will be for both servers and clients to support the
extension discussed in this specification. Upgrading of both clients
and servers is the only fully satisfactory way to support the
capabilities offered by the "UTF8" extension and SMTPUTF8 mail more
generally. Servers must, however, anticipate the possibility of a
client attempting to access a message that requires this extension
without having issued the "UTF8" command. There are no completely
satisfactory responses for that case other than upgrading the client
to support this specification. One solution, unsatisfactory because
the user may be confused by being able to access the message through
some means and not others, is that a server MAY choose to reject the
command to retrieve the message as discussed in Section 5. Other
alternatives, including the possibility of creating and delivering
variant form of the message, are discussed in Section 7 of
[I-D.ietf-eai-5738bis].
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.)
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2.2. 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] or one of its standards-
track successors 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] or one of its standards-
track successors 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 UTF-8 character listed in Section
2.3 of SASLprep. 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]. When applying
SASLprep to the APOP password argument, a compliant server or client
MUST treat them as a stored string [RFC3454].
The client does not need to issue the UTF8 command prior to using
UTF-8 in authentication. However, clients MUST NOT use UTF-8 string
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 string in the AUTH command is governed by the POP3 SASL
[RFC5034] mechanism.
3. LANG Capability
This document adds a new POP3 Extension [RFC2449] 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
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Added Commands:
LANG
Standard commands affected:
All
Announced states / possible differences:
both / no
Commands valid in states:
AUTHORIZATION, 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 uses when sending human-readable
text.
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. Human-readable text in the appropriate language then
appears in the rest of the line. 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 used.
If the client issues a LANG command with the special "*" language
range argument, it 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, that response will usually consist of multi-lines. 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".
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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. There are no specific
listing order of languages, which may depend on configuration or
implementation.
Examples:
Note that some examples do not include the correct character
accents due to limitations of this document format.
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
S: de Deutsch
S: it Italiano
S: es Espanol
S: sv Svenska
S: .
A request for a language listing might fail.
C: LANG
S: -ERR Server is unable to list languages
Once the client selects 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
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If a server returns an -ERR response to a LANG command
that specifies a primary language, the current language
for responses remains in effect.
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
4. Non-ASCII character Maildrops
When a POP3 server uses a native non-ASCII character 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. When the server is not in UTF-8 mode and the
message requires that mode, requests to download the message MAY be
rejected (as specified in the next section) or the various other
alternatives outlined in Section 2.1 above, including creation and
delivery of variations on the original message, MAY be considered.
5. UTF8 Response Code
Per "POP3 Extension Mechanism" [RFC2449], this document adds a new
response code: UTF8, described below.
Complete response code:
UTF8
Valid for responses:
-ERR
Valid for commands:
LIST, TOP, RETR
Response code meaning and expected client behavior:
The UTF8 response code indicates that a failure is due to a request
when not in UTF-8 mode for message content containing UTF-8 string.
The client MAY reissue the command after entering UTF-8 mode.
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6. IANA Considerations
Section 2 and 3 of this specification update two capabilities ("UTF8"
and "LANG") to the POP3 capability registry [RFC2449].
Section 5 of this specification also adds one new response code
("UTF8") to the POP3 response codes registry [RFC2449].
7. Security Considerations
The security considerations of UTF-8 [RFC3629], SASLprep [RFC4013]
and Unicode Format for Network Interchange [RFC5198] 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 are
strongly advised to 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 protect the
integrity of the session, such as Transport Layer Security (TLS)
[RFC2595] can be used to defeat such attacks.
As with other internationalization upgrades, modifications to server
authentication code (in this case, to support non-ASCII strings)
needs to be done with care to avoid introducing vulnerabilities (for
example, in string parsing or matching). This is particularly
important if the native databases or mailstore of the operating
system use some character set or encoding other than Unicode in
UTF-8.
8. Change History
8.1. draft-ietf-eai-rfc5721bis: Version 00
following the new charter
8.2. draft-ietf-eai-rfc5721bis: Version 01
refine the texts
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8.3. draft-ietf-eai-rfc5721bis: Version 02
update the texts based on Joseph's comments
8.4. draft-ietf-eai-rfc5721bis: Version 03
improve the texts
text instructing servers to either downconvert or reject
new UTF-8 response code for use
8.5. draft-ietf-eai-rfc5721bis: Version 04
improve the texts
8.6. draft-ietf-eai-rfc5721bis: Version 05
updated according to jabber interim meeting result
updated according to john and apparea's review comments
8.7. draft-ietf-eai-rfc5721bis: Version 06
improve the texts, updated section 3.2 to provide for SASL successor
specs.
8.8. draft-ietf-eai-rfc5721bis: Version 07
updated according to John's comments
9. References
9.1. Normative References
[I-D.ietf-eai-5738bis] Resnick, P., Newman, C., and S. Shen, "IMAP
Support for UTF-8", draft-ietf-eai-5738bis-03
(work in progress), December 2011.
[RFC1939] Myers, J. and M. Rose, "Post Office Protocol
- Version 3", STD 53, RFC 1939, May 1996.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose
Internet Mail Extensions (MIME) Part One:
Format of Internet Message Bodies", RFC 2045,
November 1996.
[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail
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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.
[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.
[RFC5198] Klensin, J. and M. Padlipsky, "Unicode Format
for Network Interchange", RFC 5198,
March 2008.
[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.
[RFC6530] Klensin, J. and Y. Ko, "Overview and
Framework for Internationalized Email",
RFC 6530, February 2012.
[RFC6532] Yang, A., Steele, S., and N. Freed,
"Internationalized Email Headers", RFC 6532,
February 2012.
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9.2. 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.
[RFC5721] Gellens, R. and C. Newman, "POP3 Support for
UTF-8", RFC 5721, February 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.
The USER capability (Section 2.2) and hence the upgraded USER command
and additional support for non-ASCII credentials, are 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, Joseph Yee, Tony Hansen, Alexey Melnikov and
other Email Address Internationalization working group participants
who provided helpful suggestions and interesting debate that improved
this specification.
Authors' Addresses
Randall Gellens
QUALCOMM Incorporated
5775 Morehouse Drive
San Diego, CA 92651
US
EMail: rg+ietf@qualcomm.com
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Chris Newman
Oracle
800 Royal Oaks
Monrovia, CA 91016-6347
US
EMail: chris.newman@oracle.com
Jiankang YAO
CNNIC
No.4 South 4th Street, Zhongguancun
Beijing
Phone: +86 10 58813007
EMail: yaojk@cnnic.cn
Kazunori Fujiwara
Japan Registry Services Co., Ltd.
Chiyoda First Bldg. East 13F, 3-8-1 Nishi-Kanda
Tokyo
Phone: +81 3 5215 8451
EMail: fujiwara@jprs.co.jp
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