One document matched: draft-ietf-eai-pop-03.xml

<?xml version="1.0"?>
<!DOCTYPE rfc SYSTEM "rfc2629.dtd">
<?rfc toc="yes"?>
<?rfc compact="yes"?>
<?rfc subcompact="no"?>
<?rfc strict="yes"?>
<?rfc symrefs="yes"?>
<?rfc linkmailto="no"?>
<?rfc comments="yes"?>
<?rfc inline="yes"?>
<rfc ipr="full3978" docName="draft-ietf-eai-pop-03.txt" updates="1939" category="exp">
<front>
<title>POP3 Support for UTF-8</title>
<?rfc include="author.cnewman"?>
<?rfc include="author.rgellens"?>
<date month="February" year="2008" />
<area>Applications</area>
<keyword>I-D</keyword>
<keyword>Internet-Draft</keyword>
<abstract><t>
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.
</t></abstract>
</front>
<middle>
<section title="Introduction">
<t>This specification extends <xref target="RFC1939">POP3</xref> using the
<xref target="RFC2449">POP3 Extension Mechanism</xref> to
permit un-encoded <xref target="RFC3629">UTF-8</xref> in headers as described in <xref target="I-D.ietf-eai-utf8headers">Internationalized Email Headers</xref>.  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.</t>

<t>Within this specification, the term down-conversion refers to the  process of modifying a message containing <xref target="I-D.ietf-eai-utf8headers">UTF8 headers</xref> or body parts with 8bit content-transfer-encoding as defined in <xref
target="RFC2045">MIME section 2.8</xref> into conforming 7-bit <xref target="RFC2822">Internet message format</xref> with <xref target="RFC2047">Message Header Extensions for Non-ASCII Text</xref> and other 7-bit encodings.  Down-conversion is specified by <xref target="I-D.ietf-eai-downgrade">Downgrading mechanism for Email Address Internationalization</xref>.</t>

<section title="Conventions Used in this Document">
<t>The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
in this document are to be interpreted as defined in
<xref target="RFC2119">"Key words for use in RFCs to Indicate
Requirement Levels"</xref>.</t>

<t>The formal syntax uses the <xref target="RFC4234">Augmented
Backus-Naur Form (ABNF)</xref> notation including the core rules
defined in Appendix B of RFC 4234.</t>

<t>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.</t>
</section>

<section title="Change History">
<t>This section describes the change history of this Internet draft and will be removed when/if this is published as an RFC.</t>
<section title="Changes from -02 to -03">
<t><list style="symbols">
<t>Updated references.</t>
<t>Replaced US-ASCII with ASCII.</t>
<t>Added comment to language listing failure example.</t>
<t>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.</t>
<t>Removed most up-conversion material.</t>
<t>Removed definition of up-conversion.</t>
<t>Removed IMAP4 reference.</t>
<t>Added AUTH command to those affected by UTF8 capability.</t>
<t>Removed LST8 and TOP8 capability parameters and commands.</t>
<t>Removed NO-RETR capability.  POP servers are now unconditionally required to support down-conversion of UTF8-native maildrops.</t>
<t>Added sentence about modifying authentication code to Security Considerations.</t>
<t>eai-downgrade draft is now normative and required.</t>
<t>Deleted references to RFCs 1341, 1847, 2049, 2183, 3501, 3516, and 3490.</t>
</list>
</t>
</section>
<section title="Changes from -01 to -02">
<t><list style="symbols">
<t>Minor grammatical tweaks.</t>
<t>Add passwords to Abstract.</t>
<t>Removed new editor's name from Acknowledgments.</t>
</list>
</t>
</section>
<section title="Changes from -00 to -01">
<t><list style="symbols">
<t>Update references</t>
</list>
</t>
</section>
<section title="Changes from draft-newman-ima-pop">
<t><list style="symbols">
<t>Change title to make this a WG document.</t>
<t>Add LANG command and extension.</t>
<t>Rename RET8 capability to UTF8 and add sub-sections for arguments.</t>
<t>Add TOP8 command.</t>
<t>Add definition of up-conversion and down-conversion.</t>
<t>Some grammar fix-ups and section re-ordering based on RFC editor style.</t>
</list>
</t>
</section>
</section>
<section title="Open Issues">
<t><list style="format %d. ">
<t>POP servers are now unconditionally required to down-convert messages in a UTF8-native maildrop when not in UTF8 mode.  Previous versions of this document permitted servers to decline to do so.  This should be verified as group consensus.</t>
<t>Servers do not up-convert messages.  While this makes sense for RETR (since doing so would break digital signatures), it might make sense for TOP, as it would provide a smaller and more clear representation of the
message.  However, the usefulness of this seems to be not worth the extra burden.</t>
<t>The current version allows clients to use UTF8 in authentication commands without sending the UTF8 command.</t>
</list>
</t>
</section>
</section>
<?rfc needLines="8"?>
<section title="LANG Capability">
<t><list style="hanging">
<t hangText="CAPA tag:"><vspace/>LANG</t>
<t hangText="Arguments:"><vspace/>none</t>
<t hangText="Added Commands:"><vspace/>LANG</t>
<t hangText="Standard commands affected:"><vspace/>All</t>
<t hangText="Announced states / possible differences:"><vspace/>both / no</t>
<t hangText="Commands valid in states:"><vspace/>AUTHENTICATION, TRANSACTION</t>
<t hangText="Specification reference:"><vspace/>this document</t>
<t hangText="Discussion:"></t>
</list>
</t>
<t>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 <xref
target="RFC1939">POP3 specification</xref>.  The LANG capability and
command permit a POP3 client to negotiate which language the server
should use when sending human-readable text.</t>
    
<t>A server that advertises the LANG extension MUST use the language
"i-default" as described in <xref target="RFC2277"></xref> 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.</t>

<t>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 as described by <xref
target="RFC4647"></xref>.  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.</t>

<t>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).</t>

<t>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.</t>

<t>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".</t>

<t>In order to simplify parsing, all POP3 servers are required to use a certain format for language listings.  A language listing consists of the <xref
target="RFC4646">language tag</xref> of the message, optionally followed by a single space and a human readable description of that language using the UTF-8 charset.</t>

<figure title="Examples"><artwork><![CDATA[
   < 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 depricated MUL language. Server replies
   with -ERR response >
    
   C: LANG MUL 
   S: -ERR invalid language MUL
    
   < A LANG command with no arguments 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 German
   S: it Italian
   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.
   Note: the example does not include the correct character accents
   due to limitations of this document format. > 
    
   C: LANG fr 
   S: +OK fr La Language commande a ete execute avec success
    
   < 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 Ce Language n'est pas supporte
   
   C: LANG fr-ca 
   S: +OK fr La Language commande a ete execute avec success
   
   C: LANG *
   S: +OK fr La Language commande a ete execute avec success
]]></artwork></figure>
</section>

<section title="UTF8 Capability">
<t><list style="hanging">
<t hangText="CAPA tag:"><vspace/>UTF8</t>
<t hangText="Arguments:"><vspace/>USER, LIST, TOP</t>
<t hangText="Added Commands:"><vspace/>UTF8</t>
<t hangText="Standard commands affected:"><vspace/>AUTH, USER, PASS, APOP, LIST, TOP, RETR</t>
<t hangText="Announced states / possible differences:"><vspace/>both / no</t>
<t hangText="Commands valid in states:"><vspace/>AUTHORIZATION</t>
<t hangText="Specification reference:"><vspace/>this document</t>
<t hangText="Discussion:"></t>
</list>
</t>
<t>This capability adds the "UTF8" command to POP3.  The UTF8 command
switches the session from ASCII to UTF8 mode.</t>

<section title="The UTF8 Command">
<t>The UTF8 command enables UTF8 mode.  Maildrops can natively store UTF8 or be limited to ASCII.  UTF8 mode has no effect on messages in an
ACII-only maildrop.  Messages in native-UTF8 maildrops can be ASCII
or UTF8 using <xref
target="I-D.ietf-eai-utf8headers">internationalized headers</xref>
and/or 8bit content-transfer-encoding as defined in <xref
target="RFC2045">MIME section 2.8</xref>.
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 downconverted using the <xref target="I-D.ietf-eai-downgrade">Downgrading mechanism for Email Address Internationalization</xref>.</t>

<t>Note that even in UTF8 mode, MIME binary content-transfer-encoding is still not permitted.</t>

<t>The octet count (size) of a message reported in a response to
the LIST command SHOULD match what the server sends in a RETR
response.  Sizes reported elsewhere, such as in STAT responses and
free-form text in positive status indicators (following "+OK") need
not be accurate, but it is preferable if they are.</t>

</section>

<section title="USER Argument to UTF8 Capability">
<t>If the USER argument is included with this capability, it indicates
that the server accepts UTF-8 user names and passwords and applies <xref
target="RFC4013">SASLprep</xref> to the arguments of the AUTH, USER, PASS and APOP commands.  A client that supports APOP and permits UTF-8 in user names or passwords MUST also implement <xref
target="RFC4013">SASLprep</xref> on the user name and password used to compute the APOP digest.</t>
<t>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 with the UTF8 capability.</t>
<t>Use of UTF8 in the AUTH command is governed by the SASL mechanism.</t>
</section>

</section>

<section title="Issues with UTF-8 Header maildrop">
<t>When a POP3 server uses a UTF8-native maildrop, it is the responsibility
of the server to comply with the <xref target="RFC1939">POP3 base
specification</xref> and <xref target="RFC2822">RFC 2822</xref> when not in UTF8 mode.  Mechanisms for 7-bit
downgrading to help comply with the standards are specified in <xref
target="I-D.ietf-eai-downgrade">Downgrading mechanism for Email Address Internationalization</xref>.</t>

</section>

<?rfc needLines="4"?>
<section title="IANA Considerations">
<t>This adds two new capabilities ("UTF8" and "LANG") to the <xref target="RFC2449">POP3 capability registry</xref>.</t>
</section>

<section title="Security Considerations">
<t>The security considerations of <xref target="RFC3629">UTF-8</xref>
and <xref target="RFC4013">SASLprep</xref> apply to this specification,
particularly with respect to use of UTF-8 in user names and passwords.</t>

<t>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.</t>

<t>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 <xref target="RFC2595">TLS</xref> can be used to defeat such attacks.</t>

<t>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).</t>
</section>

</middle>
<back>
<references title="Normative References">
<?rfc include="reference.RFC.1939"?> <!-- POP3 -->
<?rfc include="reference.RFC.2045"?> <!-- MIME Bodies -->
<?rfc include="reference.RFC.2047"?> <!-- MIME Headers -->
<?rfc include="reference.RFC.2119"?> <!-- Keywords -->
<?rfc include="reference.RFC.2277"?> <!-- Policy -->
<?rfc include="reference.RFC.2449"?> <!-- POP3 CAPA -->
<?rfc include="reference.RFC.2822"?> <!-- Message Format -->
<?rfc include="reference.RFC.4646"?> <!-- language tags -->
<?rfc include="reference.RFC.4647"?> <!-- language range -->
<?rfc include="reference.RFC.3629"?> <!-- UTF-8 -->
<?rfc include="reference.RFC.4013"?> <!-- SASLprep -->
<?rfc include="reference.RFC.4234"?> <!-- ABNF -->
<reference anchor='I-D.ietf-eai-utf8headers'>
 <front>
  <title>Internationalized Email Headers</title>
  <author initials='J' surname='Yeh' fullname='Jeff Yeh'>
    <organization />
  </author>
  <date month='Feb' day='5' year='2008' />
 </front>
 <seriesInfo name='Internet-Draft' value='draft-ietf-eai-utf8headers-09' />
 <format type='TXT'
        target='http://www.ietf.org/internet-drafts/draft-ietf-eai-utf8headers-09.txt' />
</reference>
</references>
<references title="Informative References">
<?rfc include="reference.RFC.2595"?> <!-- IMAP/POP/ACAP TLS -->
<reference anchor='I-D.ietf-eai-downgrade'>
 <front>
  <title>Downgrading mechanism for Email Address Internationalization (EAI)</title>
  <author initials='Y' surname='Yoneya' fullname='Yoshiro Yoneya'>
    <organization />
  </author>
  <author initials='K' surname='Fujiwara' fullname='Kazunori Fujiwara'>
    <organization />
  </author>
  <date month='Feb' day='15' year='2008' />
 </front>

 <seriesInfo name='Internet-Draft' value='draft-ietf-eai-downgrade-06'/>
 <format type='TXT' target='http://www.ietf.org/internet-drafts/draft-ietf-eai-downgrade-06.txt'/>
</reference>
</references>
<section title="Design Rationale">
<t>This non-normative section discusses the reasons behind some of the design choices in the above specification.</t>
<t>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, modifying messages breaks digital signatures, and would require servers to support arbitrary charset conversion.</t> 
<t>USER is optional because the implementation burden of <xref target="RFC4013">SASLprep</xref> is not well understood and mandating such support in all cases could negatively impact deployment.</t>
<t>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. </t>
<t>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.</t>
</section>
<section title="Acknowledgments">
<t>Thanks to John Klensin, Tony Hansen and other EAI
working group participants who provided helpful suggestions and
interesting debate that improved this specification.</t>
</section>
</back>
</rfc>