Network Working Group C. Newman
Internet-Draft Sun Microsystems
Updates: 1939 (if approved) June 13, 2006
Expires: December 15, 2006
POP3 Support for UTF-8
draft-ietf-eai-pop-00.txt
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This specification extends the Post Office Protocol version 3 (POP3)
to support un-encoded international characters in user names, mail
addresses, message headers, and protocol-level textual error strings.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions Used in this Document . . . . . . . . . . . . 3
1.2. Change History . . . . . . . . . . . . . . . . . . . . . . 3
1.2.1. Changes from draft-newman-ima-pop . . . . . . . . . . 3
1.3. Open Issues . . . . . . . . . . . . . . . . . . . . . . . 4
2. LANG Capability . . . . . . . . . . . . . . . . . . . . . . . 4
3. UTF8 Capability . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. USER Argument to UTF8 Capability . . . . . . . . . . . . . 7
3.2. LST8 Argument to UTF8 Capability . . . . . . . . . . . . . 7
3.3. TOP8 Argument to UTF8 Capability . . . . . . . . . . . . . 8
4. NO-RETR Capability . . . . . . . . . . . . . . . . . . . . . . 8
5. Up-Conversion Server Requirements . . . . . . . . . . . . . . 9
6. Issues with UTF-8 Header Mail Drop . . . . . . . . . . . . . . 10
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
9.1. Normative References . . . . . . . . . . . . . . . . . . . 11
9.2. Informative References . . . . . . . . . . . . . . . . . . 12
Appendix A. Design Rationale . . . . . . . . . . . . . . . . . . 12
Appendix B. Acknowledgments . . . . . . . . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 15
Intellectual Property and Copyright Statements . . . . . . . . . . 16
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1. Introduction
This specification extends POP3 [RFC1939] using the POP3 Extension
Mechanism [RFC2449] to permit un-encoded UTF-8 [RFC3629] in headers
as described in Transmission of Email Headers in UTF-8 Encoding
[I-D.yeh-ima-utf8headers]. It also adds a mechanism to support login
names outside the US-ASCII character set, and a mechanism to support
UTF-8 protocol-level error strings in a language appropriate for the
user.
Within this specification, the term up-conversion refers to
converting a traditional 7-bit Internet message [RFC2822] with
Message Header Extensions for Non-ASCII Text [RFC2047] and other
7-bit encodings to a message with UTF-8 headers [I-D.yeh-ima-
utf8headers] and minimal use of 7-bit encodings. Down-conversion
refers to the inverse process. One mechanism to perform down-
conversion is described by Downgrading mechanism for
Internationalized eMail Address [I-D.ietf-eai-downgrade].
1.1. Conventions Used in this Document
The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
in this document are to be interpreted as defined in "Key words for
use in RFCs to Indicate Requirement Levels" [RFC2119].
The formal syntax use the Augmented Backus-Naur Form (ABNF) [RFC4234]
notation including the core rules defined in Appendix B of RFC 4234.
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.
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 draft-newman-ima-pop
o Change title to make this a WG document.
o Add LANG command and extension.
o Rename RET8 capability to UTF8 and add sub-sections for arguments.
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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
The decision on how to handle UTF-8 in Received headers will impact
the up-conversion requirements section.
2. LANG Capability
CAPA tag:
LANG
Arguments:
none
Added Commands:
LANG
Standard commands affected:
All
Announced states / possible differences:
both / no
Commands valid in states:
AUTHENTICATION, TRANSACTION
Specification reference:
this document
Discussion:
POP3 allows most +OK and -ERR server responses to include human-
readable text that in some cases needs to be presented to the user.
But that text is limited to US-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
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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 as described by section 2.5 of [RFC3066].
If the command succeeds, the server returns a +OK response followed
by a single space, the exact RFC 3066 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 a -ERR response and
subsequent human-readable response text continues to use the language
that was previously active (typically i-default).
The client MUST NOT use MUL (Multiple languages) or UND
(Undetermined) language tags and the server MUST return -ERR if
either tag is used. 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 was 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 RFC 3066 language tag of the message, optionally followed by a
single space and a human readable description of that language using
the UTF-8 charset.
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< The server defaults to using English i-default responses until
the user 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 requested MUL language. Server MUST reply with -ERR >
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: .
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
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3. UTF8 Capability
CAPA tag:
UTF8
Arguments:
USER, LST8, TOP8
Added Commands:
RET8, LST8, TOP8
Standard commands affected:
USER, PASS, APOP
Announced states / possible differences:
both / no
Commands valid in states:
TRANSACTION
Specification reference:
this document
Discussion:
This capability adds UTF-8 content support to POP3. This capability
always adds the "RET8" command to POP3. The RET8 command is
identical to the RETR command, except that the retrieved message uses
UTF-8 in headers [I-D.yeh-ima-utf8headers]. In addition, the 8bit
content-transfer-encoding as defined in MIME section 2.8 [RFC2045] is
explicitly permitted. The retrieved message MUST still be textual
and otherwise formatted according to RFC 2822 [RFC2822] and MIME
[RFC2045]. The MIME binary content-transfer-encoding is not
permitted. Clients wishing to use binary MIME should implement IMAP4
[RFC3501] with the IMAP4 Binary Content Extension [RFC3516].
3.1. USER Argument to UTF8 Capability
If the USER argument is included with this capability, that indicates
the server accepts UTF-8 user names and passwords and applies
SASLprep [RFC4013] to the arguments of the USER, PASS and APOP
commands. A client that supports APOP and permits UTF-8 in user
names or passwords MUST also implement SASLprep [RFC4013] on the user
name and password used to compute the APOP digest.
3.2. LST8 Argument to UTF8 Capability
If the LST8 argument is included with this capability, that indicates
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the server implements the LST8 command. The LST8 command is
identical to the LIST command except that the octet counts are the
exact octet counts returned by the RET8 command. A POP3 client that
uses RET8 MUST use LST8 instead of LIST if LST8 is advertised.
3.3. TOP8 Argument to UTF8 Capability
If the TOP8 argument is included with this capability, that indicates
the server implements the TOP8 command. TOP8 is identical to TOP,
except the headers are UTF-8.
4. NO-RETR Capability
CAPA tag:
NO-RETR
Arguments:
none
Added Commands:
none
Standard commands affected:
RETR, LIST, TOP
Announced states / possible differences:
both / no
Commands valid in states:
N/A
Specification reference:
this document
Discussion:
This capability permits a POP3 server to advertise that it does not
support the RETR, LIST or TOP commands. Any attempt to use any of
these three commands results in an error response. As this is an
incompatible change to POP3, a clear warning is necessary. POP3
clients that find implementation of the UTF8 capability problematic
are encouraged to at least detect the NO-RETR capability and provide
an informative error message to the end-user.
When a POP3 server runs on a UTF-8 header native mail drop, the down-
conversion step necessary to implement RETR in a backwards compatible
fashion becomes more difficult to support. Although it is hoped
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deployed POP3 servers do not advertise NO-RETR for some years, this
capability is intended to minimize the disruption when legacy support
finally goes away.
A server that advertises NO-RETR MUST advertise UTF8 with at least
the LST8 argument and MUST NOT advertise TOP.
5. Up-Conversion Server Requirements
When a POP3 server uses a traditional mail drop that supports only
7-bit headers, it MUST support message header up-conversion for the
RET8, LST8, and TOP8 commands. As POP3 clients are best when simple,
the more up-conversion the server performs, the better. Minimal up-
conversion is described in this section.
The server MUST support up-conversion of the following address
header-fields in the message header: From, Sender, To, CC, Bcc,
Resent-From, Resent-Sender, Resent-To, Resent-CC, Resent-Bcc, and
Reply-To. This up-conversion MUST include address local-parts
encoded according to [TBD], address domains encoded according to IDNA
[RFC3490], and MIME header encoding [RFC2047] of display-names and
any RFC 2822 comments.
The following charsets MUST be supported for up-conversion of MIME
header encoding [RFC2047]: UTF-8, US-ASCII, ISO-8859-1, ISO-8859-2,
ISO-8859-3, ISO-8859-4, ISO-8859-5, ISO-8859-6, ISO-8859-7,
ISO-8859-8, ISO-8859-9, ISO-8859-10, ISO-8859-14, and ISO-8859-15.
Other widely deployed MIME charsets SHOULD be supported.
Up-conversion of MIME header encoding of the following headers MUST
also be implemented: Subject, Date (RFC 2822 comments only),
Comments, Keywords, Content-Description.
While this specification does not require it, server implementations
are encouraged to up-convert all MIME body headers, and particularly
the deprecated (and misused) name parameter [RFC1341] on Content-Type
and the Content-Disposition [RFC2183] filename parameter. These may
be encoded using the standard MIME parameter encoding [RFC2231]
mechanism, or via non-standard use of MIME header encoding [RFC2047]
in quoted strings.
Servers are also encouraged to up-convert the headers on embedded
message/rfc822 body parts [TBD-ref]. Servers MAY convert the charset
on MIME body parts to UTF-8, and MAY remove quoted-printable or
base64 encodings as long as the resulting text complies with the
requirements of the 8-bit content-transfer-encoding [RFC2045].
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The POP3 server MUST NOT perform up-conversion of headers and content
of multipart/signed [RFC1847], as well as Original-Recipient and
Return-Path.
6. Issues with UTF-8 Header Mail Drop
When a POP3 server uses a mail drop that supports UTF-8 headers and
it does not advertise the NO-RETR capability, it is the
responsibility of the server to comply with the POP3 base
specification [RFC1939] and RFC 2822 [RFC2822] with respect to the
RETR, LIST, and TOP commands. Mechanisms for 7-bit downgrading to
help comply with the standards are discussed in Downgrading mechanism
for Internationalized eMail Address (IMA) [I-D.ietf-eai-downgrade].
A POP3 server with a mail drop that supports UTF-8 headers MUST
comply with the RET8 protocol requirements implicit from Section 5.
However, the code necessary for such compliance need not be part of
the POP3 server itself in this case. For example, the minimal
required up-conversion could be performed when a message is inserted
into the POP3-accessible mail drop.
7. IANA Considerations
This adds three new capabilities ("UTF8", "LANG", and "NO-RETR") to
the POP3 capability registry [RFC2449].
8. Security Considerations
The security considerations of UTF-8 [RFC3629] and SASLprep [RFC4013]
apply to this specification, particularly with respect to use of
UTF-8 in user names and passwords.
The "LANG *" command can reveal the existence and preferred language
of a user to an active attacker probing the system if the active
language changes in response to the USER, PASS, or APOP commands
prior to validating the user's credentials. Servers MUST implement a
configuration to prevent this exposure.
It is possible for a man-in-the-middle attacker to insert a LANG
command in the command stream thus making protocol-level diagnostic
responses unintelligible to the user. A mechanism to integrity
protect the session, such as TLS [RFC2595] can be used to defeat such
attacks.
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9. References
9.1. 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.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822,
April 2001.
[RFC3066] Alvestrand, H., "Tags for the Identification of
Languages", BCP 47, RFC 3066, January 2001.
[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, March 2003.
[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.
[RFC4234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
[]
Yeh, J., "Transmission of Email Headers in UTF-8
Encoding", draft-yeh-ima-utf8headers-01 (work in
progress), February 2006.
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9.2. Informative References
[RFC1341] Borenstein, N. and N. Freed, "MIME (Multipurpose Internet
Mail Extensions): Mechanisms for Specifying and Describing
the Format of Internet Message Bodies", RFC 1341,
June 1992.
[RFC1847] Galvin, J., Murphy, S., Crocker, S., and N. Freed,
"Security Multiparts for MIME: Multipart/Signed and
Multipart/Encrypted", RFC 1847, October 1995.
[RFC2049] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Five: Conformance Criteria and
Examples", RFC 2049, November 1996.
[RFC2183] Troost, R., Dorner, S., and K. Moore, "Communicating
Presentation Information in Internet Messages: The
Content-Disposition Header Field", RFC 2183, August 1997.
[RFC2231] Freed, N. and K. Moore, "MIME Parameter Value and Encoded
Word Extensions: Character Sets, Languages, and
Continuations", RFC 2231, November 1997.
[RFC2595] Newman, C., "Using TLS with IMAP, POP3 and ACAP",
RFC 2595, June 1999.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, March 2003.
[RFC3516] Nerenberg, L., "IMAP4 Binary Content Extension", RFC 3516,
April 2003.
[I-D.ietf-eai-downgrade]
Yoneya, Y. and K. Fujiwara, "Downgrading mechanism for
Internationalized eMail Address (IMA)",
draft-ietf-eai-downgrade-00 (work in progress), May 2006.
Appendix A. Design Rationale
This non-normative section discusses the reasons behind some of the
design choices in the above specification.
The basic approach of advertising a parallel command set and
permitting graceful migration of both client and server with minimal
disruption is a deliberate choice. While a mechanism that makes RETR
"just-send-UTF-8" might deploy faster, it would also create
interoperability problems. The approach used prevents
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interoperability problems until the NO-RETR mechanism is deployed. A
client command to cause a model switch could also work, but the
parallel command approach is cleaner given the small number of
commands.
The choice to make RET8 nearly identical to RETR is important to
minimize the code changes necessary in a client. An alternative
approach that permits binary MIME and uses a length-counted argument
would be architecturally superior but is dismissed due to the
migration problems it would cause. The IMAP4 Binary extension should
be sufficient for cases where binary MIME support is deemed
necessary.
LST8 is optional to minimize the cost of deploying UTF-8 support on a
legacy mail drop. The server load necessary to perform up-conversion
on every message in the mail drop to determine the LST8 octet-counts
would be prohibitively expensive when there's no way to cache those
counts. The octet counts from the LIST command should be close
enough to the RET8 size for most POP3 user interfaces, and robust
POP3 clients already have to deal with LIST octet counts that don't
match the actual size of the RETR result.
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.
The NO-RETR mechanism simplifies diagnosis of interoperability
problems when legacy support goes away. In the situation where
backwards compatibility is broken anyway, just-send-8 RETR has the
advantage that it might work with some legacy clients. However, the
difficulty of diagnosing interoperability problems caused by a just-
send-8 RETR mechanism is the reason the NO-RETR mechanism was chosen.
The up-conversion requirements are designed to balance the desire to
deprecate and eventually eliminate complicated encodings (like MIME
header encodings) without creating a significant deployment burden
for servers. While it would be desirable to require up-conversion of
attachment file names, the erroneous perception that MIME parsing is
difficult in combination with multiple deployed mechanisms for such
file names tip the balance.
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.
Aggressive conversion of these encodings to UTF-8 will help simplify
the infrastructure and improve interoperability in the future.
The set of mandatory charsets comes from two sources: MIME
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requirements [RFC2049] and IETF Policy on Character Sets [RFC2277].
Including a requirement to up-convert widely deployed encoded
ideographic charsets to UTF-8 would be reasonable for most scenarios,
but may require unacceptable table sizes for some embedded devices.
The open-ended recommendation to support widely deployed charsets
avoids the political ramifications of attempting to list such
charsets. The author believes market forces, existing open-source
software, and public conversion tables are sufficient to deploy the
appropriate charsets. Specifically, use of an open-source charset
conversion library (such as ICU) is likely sufficient to fulfill this
recommendation.
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
US-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.
This was deliberately written so the down-conversion specification is
not a normative reference. While this specification does reiterate
the requirements of the base POP3 specification with respect to
message format, no specific mechanism to achieve those requirements
is mandated.
Appendix B. Acknowledgments
Thanks to Randy Gellens, 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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Author's Address
Chris Newman
Sun Microsystems
3401 Centrelake Dr., Suite 410
Ontario, CA 91761
US
Email: chris.newman@sun.com
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