draft-ietf-v6ops-onlinkassumption-04.txt   rfc4943.txt 
Network Working Group S. Roy Network Working Group S. Roy
Internet-Draft Sun Microsystems, Inc. Request for Comments: 4943 Sun Microsystems, Inc.
Expires: July 13, 2006 A. Durand Category: Informational A. Durand
Comcast Corporation Comcast
J. Paugh J. Paugh
Nominum, Inc. Nominum, Inc.
January 9, 2006 September 2007
IPv6 Neighbor Discovery On-Link Assumption Considered Harmful IPv6 Neighbor Discovery On-Link Assumption Considered Harmful
draft-ietf-v6ops-onlinkassumption-04.txt
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Copyright Notice
Copyright (C) The Internet Society (2006). This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Abstract Abstract
This document describes the historical and background information This document describes the historical and background information
behind the removal of the "on-link assumption" from the conceptual behind the removal of the "on-link assumption" from the conceptual
host sending algorithm defined in Neighbor Discovery for IP Version 6 host sending algorithm defined in Neighbor Discovery for IP Version 6
(IPv6). According to the algorithm as originally described, when a (IPv6). According to the algorithm as originally described, when a
host's default router list is empty, the host assumes that all host's default router list is empty, the host assumes that all
destinations are on-link. This is particularly problematic with destinations are on-link. This is particularly problematic with
IPv6-capable nodes that do not have off-link IPv6 connectivity (e.g., IPv6-capable nodes that do not have off-link IPv6 connectivity (e.g.,
no default router). This document describes how making this no default router). This document describes how making this
assumption causes problems, and describes how these problems outweigh assumption causes problems and how these problems outweigh the
the benefits of this part of the conceptual sending algorithm. benefits of this part of the conceptual sending algorithm.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Background on the On-link Assumption . . . . . . . . . . . . . 3 2. Background on the On-link Assumption . . . . . . . . . . . . . 2
3. Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. First Rule of Destination Address Selection . . . . . . . 4 3.1. First Rule of Destination Address Selection . . . . . . . . 3
3.2. Delays Associated with Address Resolution . . . . . . . . 4 3.2. Delays Associated with Address Resolution . . . . . . . . . 3
3.3. Multi-interface Ambiguity . . . . . . . . . . . . . . . . 5 3.3. Multi-interface Ambiguity . . . . . . . . . . . . . . . . . 4
3.4. Security Related Issues . . . . . . . . . . . . . . . . . 5 3.4. Security-Related Issues . . . . . . . . . . . . . . . . . . 4
4. Changes to RFC2461 . . . . . . . . . . . . . . . . . . . . . . 6 4. Changes to RFC 2461 . . . . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6 5. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7 6. Normative References . . . . . . . . . . . . . . . . . . . . . 6
6.1. Normative References . . . . . . . . . . . . . . . . . . . 7
6.2. Informative References . . . . . . . . . . . . . . . . . . 7
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 7 Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 7
Appendix B. Changes from draft-ietf-v6ops-onlinkassumption-03 . . 7
Appendix C. Changes from draft-ietf-v6ops-onlinkassumption-02 . . 8
Appendix D. Changes from draft-ietf-v6ops-onlinkassumption-01 . . 8
Appendix E. Changes from draft-ietf-v6ops-onlinkassumption-00 . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . . . . 11
1. Introduction 1. Introduction
Neighbor Discovery for IPv6 [I-D.ietf-ipv6-2461bis] defines a Neighbor Discovery for IPv6 [RFC4861] defines a conceptual sending
conceptual sending algorithm for hosts. The version of the algorithm algorithm for hosts. The version of the algorithm described in
described in [RFC2461] states that if a host's default router list is [RFC2461] states that if a host's default router list is empty, then
empty, then the host assumes that all destinations are on-link. This the host assumes that all destinations are on-link. This memo
memo documents the removal of this assumption in the updated Neighbor documents the removal of this assumption in the updated Neighbor
Discovery specification [I-D.ietf-ipv6-2461bis], and describes the Discovery specification [RFC4861] and describes the reasons why this
reasons why this assumption was removed. assumption was removed.
This assumption is problematic with IPv6-capable nodes that do not This assumption is problematic with IPv6-capable nodes that do not
have off-link IPv6 connectivity. This is typical when systems that have off-link IPv6 connectivity. This is typical when systems that
have IPv6 enabled on their network interfaces (either on by default have IPv6 enabled on their network interfaces (either on by default
or administratively configured that way) are attached to networks or administratively configured that way) are attached to networks
that have no IPv6 services such as off-link routing. Such systems that have no IPv6 services such as off-link routing. Such systems
will resolve DNS names to AAAA and A records, and may attempt to will resolve DNS names to AAAA and A records, and they may attempt to
connect to unreachable IPv6 off-link nodes. connect to unreachable IPv6 off-link nodes.
The on-link assumption creates problems for destination address The on-link assumption creates problems for destination address
selection as defined in [RFC3484], and adds connection delays selection as defined in [RFC3484], and it adds connection delays
associated with unnecessary address resolution and neighbor associated with unnecessary address resolution and neighbor
unreachability detection. The behavior associated with the unreachability detection. The behavior associated with the
assumption is undefined on multi-interface nodes, and has some subtle assumption is undefined on multi-interface nodes and has some subtle
security implications. All of these issues are discussed in this security implications. All of these issues are discussed in this
document. document.
2. Background on the On-link Assumption 2. Background on the On-link Assumption
This part of Neighbor Discovery's [RFC2461] conceptual sending This part of Neighbor Discovery's [RFC2461] conceptual sending
algorithm was created to facilitate communication on a single link algorithm was created to facilitate communication on a single link
between systems configured with different global prefixes in the between systems configured with different global prefixes in the
absence of an IPv6 router. For example, consider the case where two absence of an IPv6 router. For example, consider the case where two
systems on separate links are manually configured with global systems on separate links are manually configured with global
addresses, and are then plugged in back-to-back. They can still addresses and are then plugged in back-to-back. They can still
communicate with each other via their global addresses because communicate with each other via their global addresses because
they'll correctly assume that each is on-link. they'll correctly assume that each is on-link.
Without the on-link assumption, the above scenario wouldn't work, and Without the on-link assumption, the above scenario wouldn't work, and
the systems would need to be configured to share a common prefix such the systems would need to be configured to share a common prefix such
as the link-local prefix. On the other hand, the on-link assumption as the link-local prefix. On the other hand, the on-link assumption
introduces several problems to various parts of the networking stack introduces several problems to various parts of the networking stack
described in Section 3. As such, this document points out that the described in Section 3. As such, this document points out that the
problems introduced by the on-link assumption outweigh the benefit problems introduced by the on-link assumption outweigh the benefit
that the assumption lends to this scenario. It is more beneficial to that the assumption lends to this scenario. It is more beneficial to
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and declare this scenario illegitimate (or a misconfiguration). and declare this scenario illegitimate (or a misconfiguration).
3. Problems 3. Problems
The on-link assumption causes the following problems. The on-link assumption causes the following problems.
3.1. First Rule of Destination Address Selection 3.1. First Rule of Destination Address Selection
Default Address Selection for IPv6 [RFC3484] defines a destination Default Address Selection for IPv6 [RFC3484] defines a destination
address selection algorithm that takes an unordered list of address selection algorithm that takes an unordered list of
destination addresses as input, and produces a sorted list of destination addresses as input and produces a sorted list of
destination addresses as output. The algorithm consists of destination addresses as output. The algorithm consists of
destination address sorting rules, the first of which is "Avoid destination address sorting rules, the first of which is "Avoid
unusable destinations". The idea behind this rule is to place unusable destinations". The idea behind this rule is to place
unreachable destinations at the end of the sorted list so that unreachable destinations at the end of the sorted list so that
applications will be least likely to try to communicate with those applications will be least likely to try to communicate with those
addresses first. addresses first.
The on-link assumption could potentially cause false positives when The on-link assumption could potentially cause false positives when
attempting unreachability determination for this rule. On a network attempting unreachability determination for this rule. On a network
where there is no IPv6 router (all off-link IPv6 destinations are where there is no IPv6 router (all off-link IPv6 destinations are
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Users expect that applications quickly connect to a given destination Users expect that applications quickly connect to a given destination
regardless of the number of IP addresses assigned to that regardless of the number of IP addresses assigned to that
destination. If a destination name resolves to multiple addresses destination. If a destination name resolves to multiple addresses
and the application attempts to communicate to each address until one and the application attempts to communicate to each address until one
succeeds, this process shouldn't take an unreasonable amount of time. succeeds, this process shouldn't take an unreasonable amount of time.
It is therefore important that the system quickly determine if IPv6 It is therefore important that the system quickly determine if IPv6
destinations are unreachable so that the application can try other destinations are unreachable so that the application can try other
destinations when those IPv6 destinations are unreachable. destinations when those IPv6 destinations are unreachable.
For an IPv6 enabled host deployed on a network that has no IPv6 For an IPv6-enabled host deployed on a network that has no IPv6
routers, the result of the on-link assumption is that link-layer routers, the result of the on-link assumption is that link-layer
address resolution must be performed on all IPv6 addresses to which address resolution must be performed on all IPv6 addresses to which
the host sends packets. The Application will not receive the host sends packets. The application will not receive
acknowledgment of the unreachability of destinations that are not on- acknowledgment of the unreachability of destinations that are not on-
link until at least address resolution has failed, which is no less link until at least address resolution has failed, which is no less
than three seconds (MAX_MULTICAST_SOLICIT * RETRANS_TIMER). This is than 3 seconds (MAX_MULTICAST_SOLICIT * RETRANS_TIMER). This is
greatly amplified by transport protocol delays. For example, greatly amplified by transport protocol delays. For example,
[RFC1122] section 4.2.3.5 requires that TCP retransmit for at least 3 [RFC1122] Section 4.2.3.5 requires that TCP retransmit for at least 3
minutes before aborting the connection attempt. minutes before aborting the connection attempt.
When the application has a large list of off-link unreachable IPv6 When the application has a large list of off-link unreachable IPv6
addresses followed by at least one reachable IPv4 address, the delay addresses followed by at least one reachable IPv4 address, the delay
associated with Neighbor Unreachability Detection (NUD) of each IPv6 associated with Neighbor Unreachability Detection (NUD) of each IPv6
addresses before successful communication with the IPv4 address is address before successful communication with the IPv4 address is
unacceptable. unacceptable.
3.3. Multi-interface Ambiguity 3.3. Multi-interface Ambiguity
There is no defined way to implement this aspect of the sending There is no defined way to implement this aspect of the sending
algorithm on a node that is attached to multiple links. algorithm on a node that is attached to multiple links.
Specifically, a problem arises when a node is faced with sending a Specifically, a problem arises when a node is faced with sending a
packet to an IPv6 destination address to which it has no route, and packet to an IPv6 destination address to which it has no route, and
the sending node is attached to multiple links. With the on-link the sending node is attached to multiple links. With the on-link
assumption, this node assumes that the destination is on-link, but on assumption, this node assumes that the destination is on-link, but on
which link? From an implementor's point of view, there are three which link? From an implementor's point of view, there are three
ways to handle sending an IPv6 packet to a destination in the face of ways to handle sending an IPv6 packet to a destination in the face of
the on-link assumption on a multi-interface node: the on-link assumption on a multi-interface node:
1. Attempt to send the packet on a single link (either 1. Attempt to send the packet on a single link (either
administratively pre-defined or using some algorithm.) administratively pre-defined or using some algorithm).
2. Attempt to send the packet on every link. 2. Attempt to send the packet on every link.
3. Drop the packet. 3. Drop the packet.
If the destination is indeed on-link, the first option might not If the destination is indeed on-link, the first option might not
succeed since the wrong link could be picked. The second option succeed since the wrong link could be picked. The second option
might succeed in reaching the destination but is more complex to might succeed in reaching the destination but is more complex to
implement, and isn't guaranteed to pick the correct destination. For implement and isn't guaranteed to pick the correct destination. For
example, there could be more than one node configured with the same example, there could be more than one node configured with the same
address, each reachable through a different link. The address by address, each reachable through a different link. The address by
itself does not disambiguate which destination the sender actually itself does not disambiguate which destination the sender actually
wanted to reach, so attempting to send the packet to every link is wanted to reach, so attempting to send the packet to every link is
not guaranteed to reach the anticipated destination. The third not guaranteed to reach the anticipated destination. The third
option, dropping the packet, is equivalent to not making the on-link option, dropping the packet, is equivalent to not making the on-link
assumption at all. In other words, if there is no route to the assumption at all. In other words, if there is no route to the
destination, don't attempt to send the packet. An implementation destination, don't attempt to send the packet. An implementation
that behaves this way would require an administrator to configure that behaves this way would require an administrator to configure
routes to the destination in order to have reachability to the routes to the destination in order to have reachability to the
destination, thus eliminating the ambiguity. destination, thus eliminating the ambiguity.
3.4. Security Related Issues 3.4. Security-Related Issues
The on-link assumption discussed here introduces a security The on-link assumption discussed here introduces a security
vulnerability to the Neighbor Discovery protocol described in section vulnerability to the Neighbor Discovery protocol described in Section
4.2.2 of IPv6 Neighbor Discovery Trust Models and Threats [RFC3756] 4.2.2 of IPv6 Neighbor Discovery Trust Models and Threats [RFC3756]
titled "Default router is 'killed'". There is a threat that a host's titled "Default router is 'killed'". There is a threat that a host's
router can be maliciously killed in order to cause the host to start router can be maliciously killed in order to cause the host to start
sending all packets on-link. The attacker can then spoof off-link sending all packets on-link. The attacker can then spoof off-link
nodes by sending packets on the same link as the host. The nodes by sending packets on the same link as the host. The
vulnerability is discussed in detail in [RFC3756]. vulnerability is discussed in detail in [RFC3756].
Another security related side-effect of the on-link assumption has to Another security-related side-effect of the on-link assumption has to
do with virtual private networks (VPN's). It has been observed that do with virtual private networks (VPNs). It has been observed that
some commercially available VPN software solutions that don't support some commercially available VPN software solutions that don't support
IPv6 send IPv6 packets to the local media in the clear (their IPv6 send IPv6 packets to the local media in the clear (their
security policy doesn't simply drop IPv6 packets). Consider a security policy doesn't simply drop IPv6 packets). Consider a
scenario where a system has a single Ethernet interface with VPN scenario where a system has a single Ethernet interface with VPN
software that encrypts and encapsulates certain packets. The system software that encrypts and encapsulates certain packets. The system
attempts to send a packet to an IPv6 destination that it obtained by attempts to send a packet to an IPv6 destination that it obtained by
doing a DNS lookup, and the packet ends up going in the clear to the doing a DNS lookup, and the packet ends up going in the clear to the
local media. A malicious third party could then spoof the local media. A malicious third party could then spoof the
destination on-link. destination on-link.
4. Changes to RFC2461 4. Changes to RFC2461
The following changes have been made to the Neighbor Discovery The following changes have been made to the Neighbor Discovery
specification between [RFC2461] and [I-D.ietf-ipv6-2461bis]: specification between [RFC2461] and [RFC4861]:
The last sentence of the second paragraph of section 5.2 The last sentence of the second paragraph of Section 5.2
("Conceptual Sending Algorithm") was removed. This sentence was, ("Conceptual Sending Algorithm") was removed. This sentence was,
"If the Default Router List is empty, the sender assumes that the "If the Default Router List is empty, the sender assumes that the
destination is on-link." destination is on-link."
Bullet item 3) in section 6.3.6 ("Default Router Selection") was Bullet item 3) in Section 6.3.6 ("Default Router Selection") was
removed. The item read, "If the Default Router List is empty, removed. The item read, "If the Default Router List is empty,
assume that all destinations are on-link as specified in Section assume that all destinations are on-link as specified in Section
5.2." 5.2."
APPENDIX A was modified to remove on-link assumption related text APPENDIX A was modified to remove on-link assumption related text
in bullet item 1) under the discussion on what happens when a in bullet item 1) under the discussion on what happens when a
multihomed host fails to receive Router Advertisements. multihomed host fails to receive Router Advertisements.
The result of these changes is that destinations are considered The result of these changes is that destinations are considered
unreachable when there is no routing information for that destination unreachable when there is no routing information for that destination
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layer address resolution when sending to such a destination, a node layer address resolution when sending to such a destination, a node
should send an ICMPv6 Destination Unreachable message (code 0 - no should send an ICMPv6 Destination Unreachable message (code 0 - no
route to destination) message up the stack. route to destination) message up the stack.
5. Security Considerations 5. Security Considerations
The removal of the on-link assumption from Neighbor Discovery The removal of the on-link assumption from Neighbor Discovery
addresses all of the security-related vulnerabilities of the protocol addresses all of the security-related vulnerabilities of the protocol
as described in Section 3.4. as described in Section 3.4.
6. References 6. Normative References
6.1. Normative References
[I-D.ietf-ipv6-2461bis]
Narten, T., "Neighbor Discovery for IP version 6 (IPv6)",
draft-ietf-ipv6-2461bis-05 (work in progress),
October 2005.
[RFC1122] Braden, R., "Requirements for Internet Hosts - [RFC1122] Braden, R., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989. Communication Layers", STD 3, RFC 1122, October 1989.
[RFC2461] Narten, T., Nordmark, E., and W. Simpson, "Neighbor [RFC2461] Narten, T., Nordmark, E., and W. Simpson, "Neighbor
Discovery for IP Version 6 (IPv6)", RFC 2461, Discovery for IP Version 6 (IPv6)", RFC 2461,
December 1998. December 1998.
[RFC3484] Draves, R., "Default Address Selection for Internet [RFC3484] Draves, R., "Default Address Selection for Internet
Protocol version 6 (IPv6)", RFC 3484, February 2003. Protocol version 6 (IPv6)", RFC 3484, February 2003.
[RFC3756] Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor [RFC3756] Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor
Discovery (ND) Trust Models and Threats", RFC 3756, Discovery (ND) Trust Models and Threats", RFC 3756,
May 2004. May 2004.
6.2. Informative References [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
[RFC2462] Thomson, S. and T. Narten, "IPv6 Stateless Address September 2007.
Autoconfiguration", RFC 2462, December 1998.
Appendix A. Acknowledgments Appendix A. Acknowledgments
The authors gratefully acknowledge the contributions of Jim Bound, The authors gratefully acknowledge the contributions of Jim Bound,
Spencer Dawkins, Tony Hain, Mika Liljeberg, Erik Nordmark, Pekka Spencer Dawkins, Tony Hain, Mika Liljeberg, Erik Nordmark, Pekka
Savola, and Ronald van der Pol. Savola, and Ronald van der Pol.
Appendix B. Changes from draft-ietf-v6ops-onlinkassumption-03
o Clarified that the scenario described in the background section
(section 2) is considered a misconfiguration.
o In section 3.2, specified the section number of RFC1122 that
specifies the 3 minute TCP retransmission period.
o Clarified section 3.3 (Multi-interface Ambiguity) to make explicit
that it's talking about an interface selection problem, and not an
address selection problem. The change also clarifies that the
third behavior eliminates the problematic ambiguity of the
described scenario.
o Modified section 5 (Security Considerations) to state that the
removal of the on-link assumption addresses all security concerns
described in section 3.4.
o Changed Jim Paugh's (co-author) organization and mailing address.
Appendix C. Changes from draft-ietf-v6ops-onlinkassumption-02
o Changed abstract to reflect the historical nature of this
document.
o Changed the introduction to stress that this is historical
information documenting the removal of the on-link assumption from
the ND spec.
o Added text to the introduction stating that the assumption is a
problem for nodes with IPv6 on by default.
o Added mention to RFC1122 in section 3.2.
o Changed use of the term multi-homed nodes to "nodes that are
attached to multiple links".
o Changed section 4 from "Proposed Changes" to "Changes" and
adjusted included text to reflect that the changes have been made.
Appendix D. Changes from draft-ietf-v6ops-onlinkassumption-01
o Added text in the Introduction stating that rfc2461bis has removed
the on-link assumption, and that this memo gives the historical
reference and background for its removal.
o Stated in Section 2 that users may not have sufficient privileges
or knowledge to manually configure addresses or routers in order
to work-around the lack of an on-link assumption.
o Removed implementation details of the on-link assumption from
Section 3.1.
o Miscellaneous editorial changes.
Appendix E. Changes from draft-ietf-v6ops-onlinkassumption-00
o Clarified in the abstract and introduction that the problem is
with systems that are IPv6 enabled but have no off-link
connectivity.
o In Section 3.3, clarified that soliciting on all links could have
ambiguous results.
o The old Security Considerations section was moved to Section 3.4,
and the new Security Considerations section refers to that new
section.
o Miscellaneous editorial changes.
Authors' Addresses Authors' Addresses
Sebastien Roy Sebastien Roy
Sun Microsystems, Inc. Sun Microsystems, Inc.
1 Network Drive 1 Network Drive
UBUR02-212 UBUR02-212
Burlington, MA 01803 Burlington, MA 01803
Email: sebastien.roy@sun.com EMail: sebastien.roy@sun.com
Alain Durand Alain Durand
Comcast Corporation Comcast
1500 Market Street 1500 Market Street
Philadelphia, PA 09102 Philadelphia, PA 19102
Email: alain_durand@cable.comcast.com EMail: alain_durand@cable.comcast.com
James Paugh James Paugh
Nominum, Inc. Nominum, Inc.
2385 Bay Road 2385 Bay Road
Redwood City, CA 94063 Redwood City, CA 94063
Email: jim.paugh@nominum.com EMail: jim.paugh@nominum.com
Intellectual Property Statement Full Copyright Statement
Copyright (C) The IETF Trust (2007).
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 are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
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WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
The IETF takes no position regarding the validity or scope of any The IETF takes no position regarding the validity or scope of any
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skipping to change at page 11, line 28 skipping to change at line 324
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The IETF invites any interested party to bring to its attention any The IETF invites any interested party to bring to its attention any
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This document and the information contained herein are provided on an
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OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM 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.
Copyright Statement
Copyright (C) The Internet Society (2006). 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.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
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