WDIFF

draft-ietf-nntpext-tls-nntp-01.txt --> draft-ietf-nntpext-tls-nntp-02.txt

Network Working Group J. Vinocur
INTERNET DRAFT Cornell University
Document: draft-ietf-nntpext-tls-nntp-01.txt draft-ietf-nntpext-tls-nntp-02.txt C. Newman
Sun Microsystems
October 2003
K. Murchison
Oceana Matrix Ltd.
September 2004

Using TLS with NNTP

Status of this memo

This document is an Internet-Draft

By submitting this Internet-Draft, I certify that any applicable
patent or other IPR claims of which I am aware have been disclosed,
and is any of which I become aware will be disclosed, in full conformance accordance
with
all provisions of Section 10 of RFC 2026. 3668.

Internet-Drafts are working documents of the Internet Engineering
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Internet-Drafts are draft documents valid for a maximum of six
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as reference material or to cite them other than as "work in
progress."

The list of current Internet-Drafts can be accessed at
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Abstract

This memo defines an extension to the Network News Transport
Protocol [NNTP] to provide connection-based encryption (via
Transport Layer Security [TLS]). The primary goal is to provide
encryption for single-link confidentiality purposes, but data
integrity and (optional) certificate-based peer entity
authentication are also possible.

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Table of Contents

0. Changes from Previous Version ............................ 2
1. Introduction ............................................. 3
1.1. Conventions Used in this Document ................... 3
2. Advertising Capabilities with the Extensions Mechanism ... 3
3. STARTTLS Command ......................................... 4
3.1. Usage ............................................... 4
3.2. Description ......................................... 4
3.2.1. Processing After the STARTTLS Command .......... 5
3.2.2. Result of the STARTTLS Command ................. 6
3.3. Examples ............................................ 7
4. Augmented BNF Syntax for the STARTTLS ........................ Extension .......... 8
4.1. Commands ............................................ 8
4.2. LIST EXTENSIONS responses ........................... 8
5. Summary of Response Codes ................................ 8
6. Security Considerations .................................. 8
6. Acknowledgements ......................................... 10 9
7. Normative References ..................................... IANA Considerations ...................................... 10
8. References ............................................... 11
8.1. Normative References ................................ 11
8.2. Informative References ................................... 10 .............................. 12
9. Authors' Addresses ....................................... 11 12
10. Acknowledgements ........................................ 12
11. Intellectual Property Rights ............................ 13
12. Copyright ............................................... 13

0. Changes from Previous Version

New:
o Text needed to comply with extensions framework guidelines:
- Allows 483 to be returned for most commands
- No pipelining
- Not impacted by MODE READER Example of 580 response
o Examples Summary of Response Codes section

Changed:
o Welcome banner is *not* reissued after STARTTLS
o STARTTLS on an already-secure link gives 502 (not 580) IANA Considerations section

Changed:
o Failed negotiation gives 580 on the reestablished insecure link Augmented BNF section follows base document
o Removed MULTIDOMAIN, need is resolved by RFC 3546 (a SHOULD) unused references

Clarified:
o Removed definition of 483, which is now included in base spec STARTTLS has initial responses and one subsequent response
o Use HDR instead of PAT in the LIST EXTENSIONS example

Clarified: Client should just go ahead and send a command after STARTTLS,
since there is no greeting
o When the capability can Addressed why STARTTLS should not be advertised
o The specifc octet where encrypted session begins after authentication

Other:
o Reformatting to match base spec style
o Assorted updates of phrasing and typographical varieties
o Updated several references per new versions No spaces before output of documents

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1. Introduction

Historically, unencrypted NNTP [NNTP] connections were satisfactory
for most purposes. However, sending passwords unencrypted over the
network is no longer appropriate, and sometimes strong encryption
is desired for the entire connection.

The STARTTLS extension provides a way to use the popular TLS [TLS]
service with the existing NNTP protocol. The current
(unstandardized) use of TLS for NNTP is most commonly on a
dedicated TCP port; this practice is discouraged for the reasons
documented in section 7 of "Using TLS with IMAP, POP3 and ACAP"
[TLS-IMAPPOP]. Therefore, this specification formalizes and
extends the STARTTLS command already in occasional use by the
installed base.

1.1. Conventions Used in this Document

The key words "REQUIRED", "MUST", "MUST NOT", "SHOULD", "SHOULD
NOT", "MAY", and "OPTIONAL" in this document are to be interpreted
as described in "Key words for use in RFCs to Indicate Requirement
Levels" [KEYWORDS].

Terms related to authentication are defined in "On Internet
Authentication" [AUTH].

This document assumes you are familiar with NNTP [NNTP] and TLS
[TLS].

In the examples, commands from the client are indicated with [C],
and responses from the server are indicated with [S].

2. Advertising Capabilities with the Extensions Mechanism

The LIST EXTENSIONS command, documented in section 8 of [NNTP],
provides a mechanism for clients to discover what extensions are
available.

A server supporting the STARTTLS command as defined in section 4 3
will advertise the "STARTTLS" capability in response to the LIST
EXTENSIONS command. However, this capability is not advertised
after successful authentication [NNTP-AUTH], nor is it advertised
once a TLS layer is active (see section 4.2.2). 3.2.2). This capability
may be advertised both before and after any use of MODE READER,
with the same semantics.

As the STARTTLS command is related to security, cached results of
LIST EXTENSIONS from a previous session MUST NOT be used, as per

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section 11.6 of [NNTP].

Example:

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section 11.6 of [NNTP].

Example: September 2004

[C] LIST EXTENSIONS
[S] 202 Extensions supported:
[S] OVER
[S] HDR
[S] LISTGROUP
[S] STARTTLS
[S] .

Note that the STARTTLS command constitutes a mode change and thus
clients MUST wait for completion prior to sending additional
commands.

3. STARTTLS Command

3.1. Usage

This command MUST NOT be pipelined.

Syntax
STARTTLS

Responses

Initial responses
382 Continue with TLS negotiation
403 TLS temporarily not available
501 Command not supported or command syntax error
502 Command unavailable [1]

Subsequent response
580 TLS negotiation failed

[1] If a TLS layer is already active, or authentication has
occurred, STARTTLS is not a valid command (see sections 4.2 3.2 and
4.2.2).

Clients MUST support other response codes by processing them based
on the first digit. However,
3.2.2).

Notwithstanding section 3.2.1 of [NNTP], the server MUST NOT return
483 in response to STARTTLS. (See section 3.2.1 of [NNTP].)

3.2. Description

A client issues the STARTTLS command to request negotiation of TLS.
The client MUST NOT send any additional commands on the socket
until after it has received the server response to the command;
this command
This command MUST NOT be pipelined as per section 3.2 3.3 of [NNTP].
The STARTTLS command is usually used to request session encryption,
although it can be used for client certificate authentication.

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An NNTP server returns the 483 response to indicate that a secure
or encrypted connection is required for the command sent by the
client. Use of the STARTTLS command as described below is one way
to establish a connection with these properties. The client MAY

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An NNTP server MAY require September 2004

therefore send STARTTLS after receiving a 483 response; the client
also MAY decide to perform send STARTTLS without previously receiving a TLS negotiation
before accepting any commands. In this case, the server SHOULD
return the 483 encryption-required response code to every command
other than HELP, LIST EXTENSIONS, QUIT, and any commands that
establish encryption, such as STARTTLS;
response. Additionally, the server MUST NOT return 483 in response
to these commands. Additionally, the client MAY
decide to establish a security layer without first receiving a 483
response. STARTTLS command.

If the client receives a failure response to STARTTLS, the client
must decide whether or not to continue the NNTP session. Such a
decision is based on local policy. For instance, if TLS was being
used for client authentication, the client might try to continue
the session, in case the server allows it to do so even with no
authentication. However, if TLS was being negotiated for
encryption, a client that gets a failure response needs to decide
whether to continue without TLS encryption, to wait and try again
later, or to give up and notify the user of the error.

After receiving a 382 response to a STARTTLS command, the client
MUST start the TLS negotiation before giving any other NNTP
commands. The TLS negotiation begins with the first octet
following the CRLF of the 382 response. If, after having issued
the STARTTLS command, the client finds out that some failure
prevents it from actually starting a TLS handshake, then it SHOULD
immediately close the connection.

Servers MUST be able to understand backwards-compatible TLS Client
Hello messages (provided that client_version is TLS 1.0 or later),
and clients MAY use backwards-compatible Client Hello messages.
Neither clients or servers are required to actually support Client
Hello messages for anything other than TLS 1.0. However, the TLS
extension for Server Name Indication [TLS-EXT] SHOULD be
implemented by all clients; it also SHOULD be implemented by any
server implementing STARTTLS that is known by multiple names
(otherwise it is not possible for a server with several hostnames
to present the correct certificate to the client).

Although current use of TLS most often involves the dedication of
port 563 for NNTP over TLS, the continued use of TLS on a separate
port is discouraged for the reasons documented in section 7 of
"Using TLS with IMAP, POP3 and ACAP" [TLS-IMAPPOP].

3.2.1. Processing After the STARTTLS Command

After the TLS handshake has been completed successfully, both
parties MUST immediately decide whether or not to continue based on
the authentication and privacy achieved. The NNTP client and

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server may decide to move ahead even if the TLS negotiation ended
with no authentication and/or no privacy because NNTP services are
often performed without authentication or privacy, but some NNTP
clients or servers may want to continue only if a particular level

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of authentication and/or privacy was achieved.

If the NNTP client decides that the level of authentication or
privacy is not high enough for it to continue, it SHOULD issue a
QUIT command immediately after the TLS negotiation is complete. If
the NNTP server decides that the level of authentication or privacy
is not high enough for it to continue, it SHOULD do at least one of
(1) close the connection, being aware that the client may interpret
this behavior as a network problem and immediately reconnect and
issue the same command sequence, or (2) keep the connection open
and reply to NNTP commands from the client with the 483 response
code (with a possible text string such as "Command refused due to
lack of security"), however this behavior may tie up resources
unacceptably.

The decision of whether or not to believe the authenticity of the
other party in a TLS negotiation is a local matter. However, some
general rules for the decisions are:

o The client MAY check that the identity presented in the server's
certificate matches the intended server hostname or domain.
This check is not required (and may fail in the absence of the
TLS server_name extension [TLS-EXT], as described above), but if
it is implemented and the match fails, the client SHOULD either
request explicit user confirmation, or terminate the connection
but allow the user to disable the check in the future.
o Generally an NNTP server would want to accept any verifiable
certificate from a client, however authentication can be done
using the client certificate (perhaps in combination with the
SASL EXTERNAL mechanism [NNTP-AUTH], although an implementation
supporting STARTTLS is not required to support SASL in general
or that mechanism in particular). The server MAY use
information about the client certificate for identification of
connections or posted articles (either in its logs or directly
in posted articles).

3.2.2. Result of the STARTTLS Command

If the TLS handshake fails in such a way that recovery it is possible, possible to
recover the underlying NNTP session, the server will send a 580
response (without encryption), beginning with the first post-handshake post-
handshake octet.

Upon successful completion of the TLS handshake, the NNTP protocol
is reset to the initial state (the state in NNTP directly immediately after the

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connection initial greeting
response (see 5.1 of [NNTP]) has been sent. In this case, as no
greeting is sent, the next step is established). for the client to send a
command. The server MUST discard any knowledge obtained from the

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client, such as the current newsgroup and article number, that was
not obtained from the TLS negotiation itself;
immediately after the TLS handshake, the server MUST NOT issue a
welcome banner and MUST be prepared to accept commands from the
client. itself. The client MUST
discard any knowledge obtained from the server, such as the list of
NNTP service extensions, which was not obtained from the TLS
negotiation itself.

The extensions returned in response to a LIST EXTENSIONS command
received after the TLS handshake MAY be different than the list
returned before the TLS handshake. For example, an NNTP server
supporting SASL [NNTP-AUTH] might not want to advertise support for
a particular mechanism unless a client has sent an appropriate
client certificate during a TLS handshake.

Both the client and the server MUST know if there is a TLS session
active. A client MUST NOT attempt to start a TLS session if a TLS
session is already active. A server MUST NOT return the STARTTLS
extension in response to a LIST EXTENSIONS command received after a
TLS handshake has completed, and a server MUST respond with a 502
response code if a STARTTLS command is received while a TLS session
is already active.

3.3. Examples

Example of a client being prompted to use encryption and
negotiating it successfully (showing the removal of STARTTLS from
the extensions list once a TLS layer is active), followed by an
(inappropriate) attempt by the client to initiate another TLS
negotiation:

[C] LIST EXTENSIONS
[S] 202 Extensions supported:
[S] STARTTLS
[S] OVER
[S] .
[C] GROUP local.confidential
[S] 483 Encryption or stronger authentication required
[C] STARTTLS
[S] 382 Continue with TLS negotiation
[TLS negotiation occurs here]
[Following successful negotiation, traffic is via the TLS layer]
[C] LIST EXTENSIONS
[S] 202 Extensions supported:
[S] OVER
[S] .
[C] STARTTLS
[S] 502 STARTTLS not allowed with active TLS layer

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Example of a request to begin TLS negotiation declined by the
server:

[C] STARTTLS
[S] 403 TLS temporarily not available

4. Augmented BNF Syntax for STARTTLS

This amends

Example of a failed attempt to negotiate TLS, with recovery of the formal syntax for
underlying NNTP [NNTP] to add the session:

[C] GROUP local.confidential
[S] 483 Encryption or stronger authentication required
[C] STARTTLS
command. The syntax
[S] 382 Continue with TLS negotiation
[TLS negotiation is defined using ABNF [ABNF], including attempted here]
[Following failed negotiation, traffic resumes without TLS]
[S] 580 TLS negotiation failed
[C] GROUP local.public
[S] 211 1234 3000234 3002322 local.public

4. Augmented BNF Syntax for the
core rules from STARTTLS Extension

This section 6 describes the syntax of [ABNF]. the STARTTLS extension. It
extends the syntax in [NNTP], and non-terminals not defined in this
document are defined there.

4.1. Commands

This syntax extends the non-terminal "command", which represents an
NNTP command.

command /= =/ starttls-command

starttls-command = "STARTTLS" *WSP CRLF

4.2. LIST EXTENSIONS responses

This syntax defines the specific LIST EXTENSIONS responses for the
STARTTLS extension.

extension-descriptor =/ starttls-extension
starttls-extension = %x53.54.41.52.54.54.4C.53 ; WSP and CRLF are "STARTTLS"

5. Summary of Response Codes

This section contains a list of every new response code defined in sec. 13 of [NNTP]

5.
this document, whether it is multi-line, which commands can
generate it, what arguments it has, and what its meaning is.

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Response code 382
Generated by: STARTTLS
Meaning: continue with TLS negotiation

Response code 580
Generated by: STARTTLS
Meaning: TLS negotiation failed

6. Security Considerations

In general, the security considerations of the TLS protocol [TLS]
and any implemented extensions [TLS-EXT] are applicable here; only
the most important are highlighted specifically below. Also, this
extension is not intended to cure the security considerations
described in section 14 11 of [NNTP]; those considerations remain
relevant to any NNTP implementation.

Use of STARTTLS cannot protect protocol exchanges conducted prior
to authentication. For this reason, the LIST EXTENSIONS command
SHOULD be re-issued after successful negotiation of a security
layer, and other protocol state SHOULD be re-negotiated as well.

It should be noted that NNTP is not an end-to-end mechanism. Thus,
if an NNTP client/server pair decide to add TLS privacy, they are
securing the transport only for that link. Further, because
delivery of a single piece of news may go between more than two
NNTP servers, adding TLS privacy to one pair of servers does not
mean that the entire NNTP chain has been made private. Further,
just because an NNTP server can authenticate an NNTP client, it
does not mean that the articles from the NNTP client were
authenticated by the NNTP client when the client received them.

Both the NNTP client and server must check the result of the TLS
negotiation to see whether an acceptable degree of authentication
and privacy was achieved. Ignoring this step completely
invalidates using TLS for security. The decision about whether
acceptable authentication or privacy was achieved is made locally,
is implementation-dependent, and is beyond the scope of this
document.

The NNTP client and server should note carefully the result of the

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TLS negotiation. If the negotiation results in no privacy, or if
it results in privacy using algorithms or key lengths that are
deemed not strong enough, or if the authentication is not good
enough for either party, the client may choose to end the NNTP
session with an immediate QUIT command, or the server may choose
not to accept any more NNTP commands.

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The client and server should also be aware that the TLS protocol
permits privacy and security capabilities to be renegotiated mid-
connection (see section 7.4.1 of [TLS]). For example, one of the
parties may desire minimal encryption after any authentication
steps have been performed. This underscores the fact that security
is not present simply because TLS has been negotiated; the nature
of the established security layer must be considered.

A man-in-the-middle attack can be launched by deleting the 382
response from the server. This would cause the client not to try to
start a TLS session. Another man-in-the-middle attack is to allow
the server to announce its STARTTLS capability, but to alter the
client's request to start TLS and the server's response. An NNTP
client can partially protect against these attacks by recording the
fact that a particular NNTP server offers TLS during one session
and generating an alarm if it does not appear in the LIST
EXTENSIONS response for a later session (of course, the STARTTLS
extension would not be listed after a security layer is in place).

If the TLS negotiation fails or if the client receives a 483
response, the client has to decide what to do next. The client has
to choose among three main options: to go ahead with the rest of
the NNTP session, to retry TLS at a later time, or to give up and
postpone newsreading activity. If a failure or error occurs, the
client can assume that the server may be able to negotiate TLS in
the future, and should try to negotiate TLS in a later session.
However, if the client and server were only using TLS for
authentication and no previous 480 response was received, the
client may want to proceed with the NNTP session, in case some of
the operations the client wanted to perform are accepted by the
server even if the client is unauthenticated.

Before the TLS handshake has begun, any protocol interactions are
performed in the clear and may be modified by an active attacker.
For this reason, clients and servers MUST discard any sensitive
knowledge obtained prior to the start of the TLS handshake upon
completion of the TLS handshake.

Vinocur & Newman

7. IANA Considerations

This section gives a formal definition of the STARTTLS extension as
required by Section 8 of [NNTP] for the IANA registry.

o The STARTTLS extension provides connection-based encryption via
Transport Layer Security (TLS).

o The extension-label is "STARTTLS".

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6. Acknowledgements

A significant amount of September 2004

o The extension-label has no arguments.

o The extension defines one new command, STARTTLS, whose
behaviour, arguments, and responses are defined in Section 3.

o The extension does not associate any new responses with pre-
existing NNTP commands.

o The extension does affect the overall behaviour of both server
and client, in that after successful use of the new command, all
communication is transmitted with the TLS layer as an
intermediary.

o The extension does not affect the maximum length of commands and
initial response lines.

o The extension does not alter pipelining, but the STARTTLS text was lifted
command cannot be pipelined.

o Use of this extension does alter the output from RFC 3207 LIST
EXTENSIONS; once the new command has been used successfully,
this extension can no longer be advertised by Paul Hoffman.

Special acknowledgement goes also LIST EXTENSIONS.

o The extension does not cause any pre-existing command to produce
a 401, 480, or 483 response.

o The STARTTLS command can be used before or after the people who commented
privately on intermediate revisions of this document, as well as MODE READER
command, with the members same semantics.

o Published Specification: This document.

o Author, Change Controller, and Contact for Further Information:
Author of the IETF NNTP Working Group for continual insight in
discussion.

7. this document.

8. References

8.1. Normative References

[ABNF] Crocker, D., Overell, P., "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.

[AUTH] Haller, N., Atkinson, R., "On Internet Authentication", RFC 1704,
October 1994.

[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.

[NNTP] Feather, C., "Network News Transport Protocol"
(draft-ietf-nntpext-base-20.txt).

[SMTP] Klensin, J., "Simple Mail Transport Protocol", RFC 2821, April
2001.
draft-ietf-nntpext-base-*.txt, Work in Progress.

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[TLS] Dierks, T., Allen, C., "The TLS Protocol Version 1.0", RFC 2246,
January 1999.

[TLS-EXT] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
Wright, T., "Transport Layer Security (TLS) Extensions", RFC 3546, June
2003.

[TLS-IMAPPOP] Newman, C., "Using TLS with IMAP, POP3 and ACAP", RFC
2595, June 1999.

8.2. Informative References

[HTTP] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter,
L., Leach, P., Berners-Lee, T., "Hypertext Transfer Protocol --
HTTP/1.1", RFC 2616, June 1999.

[NNTP-AUTH] Vinocur, J., Newman, C., Murchison, K., "NNTP Extension
for Authentication", draft-ietf-nntpext-auth-*.txt, Work in
Progress.

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9. Authors' Addresses

Jeffrey M. Vinocur
Department of Computer Science
Upson Hall
Cornell University
Ithaca, NY 14853

EMail: vinocur@cs.cornell.edu

Chris Newman
Sun Microsystems
1050 Lakes Drive, Suite 250
West Covina, CA 91790

EMail: cnewman@iplanet.com

Full Copyright Statement

This document and translations

Kenneth Murchison
Oceana Matrix Ltd.
21 Princeton Place
Orchard Park, NY 14127 USA

Email: ken@oceana.com

10. Acknowledgements

A significant amount of it may be copied and furnished the STARTTLS text was lifted from RFC 3207
by Paul Hoffman.

Special acknowledgement goes also to
others, and derivative works that comment the people who commented

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privately on or otherwise explain
it or assist in its implementation may be prepared, copied,
published and distributed, intermediate revisions of this document, as well as
the members of the IETF NNTP Working Group for continual insight in whole
discussion.

11. Intellectual Property Rights

The IETF takes no position regarding the validity or in part, without restriction scope of any kind, provided
intellectual property or other rights that might be claimed to
pertain to the above copyright notice and this
paragraph are included on all such copies and derivative works.
However, implementation or use of the technology described in
this document itself may or the extent to which any license under such rights
might or might not be modified in available; neither does it represent that it
has made any effort to identify any way, such
as by removing rights. Information on
the copyright notice or references IETF's procedures with respect to the Internet
Society or other Internet organizations, except as needed rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for the
purpose publication and any assurances
of developing Internet standards in which case licenses to be made available, or the
procedures result of an attempt made
to obtain a general license or permission for copyrights defined in the Internet Standards process
must be followed, use of such
proprietary rights by implementers or as required users of this specification
can be obtained from the IETF Secretariat.

The IETF invites any interested party to translate it into languages bring to its attention any
copyrights, patents or patent applications, or other than English.

The limited permissions granted above are perpetual and will not proprietary
rights which may cover technology that may be
revoked by required to practice
this standard. Please address the information to the IETF
Executive Director.

12. Copyright

Copyright (C) The Internet Society or its successors or assigns. (2004). This document is
subject to the rights, licenses and restrictions contained in BCP
78, and except as set forth therein, the authors retain all their
rights."

This document and the information contained herein is provided on
an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Vinocur & Newman

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