Network Working Group S. Roy
Internet-Draft Sun Microsystems, Inc.
Expires: October 3, 2005 A. Durand
Comcast Corporation
J. Paugh
April 2005
IPv6 Neighbor Discovery On-Link Assumption Considered Harmful
draft-ietf-v6ops-onlinkassumption-03.txt
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Copyright (C) The Internet Society (2005).
Abstract
This document describes the historical and background information
behind the removal of the "on-link assumption" from the conceptual
host sending algorithm defined in Neighbor Discovery for IP Version 6
(IPv6). According to the algorithm as originally described, when a
host's default router list is empty, the host assumes that all
destinations are on-link. This is particularly problematic with
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IPv6-capable nodes that do not have off-link IPv6 connectivity (e.g.,
no default router). This document describes how making this
assumption causes problems, and describes how these problems outweigh
the benefits of this part of the conceptual sending algorithm.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Background on the On-link Assumption . . . . . . . . . . . . . 3
3. Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1 First Rule of Destination Address Selection . . . . . . . 3
3.2 Delays Associated with Address Resolution . . . . . . . . 4
3.3 Multi-interface Ambiguity . . . . . . . . . . . . . . . . 5
3.4 Security Related Issues . . . . . . . . . . . . . . . . . 5
4. Changes to RFC2461 . . . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
6.1 Normative References . . . . . . . . . . . . . . . . . . . 6
6.2 Informative References . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 7
A. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7
B. Changes from draft-ietf-v6ops-onlinkassumption-02 . . . . . . 7
C. Changes from draft-ietf-v6ops-onlinkassumption-01 . . . . . . 8
D. Changes from draft-ietf-v6ops-onlinkassumption-00 . . . . . . 8
Intellectual Property and Copyright Statements . . . . . . . . 9
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1. Introduction
Neighbor Discovery for IPv6 [I-D.ietf-ipv6-2461bis] defines a
conceptual sending algorithm for hosts. The version of the algorithm
described in [RFC2461] states that if a host's default router list is
empty, then the host assumes that all destinations are on-link. This
memo documents the removal of this assumption in the updated Neighbor
Discovery specification [I-D.ietf-ipv6-2461bis], and describes the
reasons why this assumption was removed.
This assumption is problematic with IPv6-capable nodes that do not
have off-link IPv6 connectivity. This is typical when systems that
have IPv6 enabled on their network interfaces (either on by default
or administratively configured that way) are attached to networks
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
connect to unreachable IPv6 off-link nodes.
The on-link assumption creates problems for destination address
selection as defined in [RFC3484], and adds connection delays
associated with unnecessary address resolution and neighbor
unreachability detection. The behavior associated with the
assumption is undefined on multi-interface nodes, and has some subtle
security implications. All of these issues are discussed in this
document.
2. Background on the On-link Assumption
This part of Neighbor Discovery's [RFC2461] conceptual sending
algorithm was created to facilitate communication on a single link
between systems manually configured with different global prefixes.
For example, consider the case where two systems on separate links
are manually configured with global addresses, and are then plugged
in back-to-back. They can still communicate with each other via
their global addresses because they'll correctly assume that each is
on-link.
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
as the link-local prefix.
3. Problems
The on-link assumption causes the following problems.
3.1 First Rule of Destination Address Selection
Default Address Selection for IPv6 [RFC3484] defines a destination
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address selection algorithm that takes an unordered list of
destination addresses as input, and produces a sorted list of
destination addresses as output. The algorithm consists of
destination address sorting rules, the first of which is "Avoid
unusable destinations". The idea behind this rule is to place
unreachable destinations at the end of the sorted list so that
applications will be least likely to try to communicate with those
addresses first.
The on-link assumption could potentially cause false positives when
attempting unreachability determination for this rule. On a network
where there is no IPv6 router (all off-link IPv6 destinations are
unreachable), the on-link assumption states that destinations are
assumed to be on-link. An implementation could interpret that as, if
the default router list is empty, then all destinations are reachable
on-link. This may cause the rule to prefer an unreachable IPv6
destination over a reachable IPv4 destination.
3.2 Delays Associated with Address Resolution
Users expect that applications quickly connect to a given destination
regardless of the number of IP addresses assigned to that
destination. If a destination name resolves to multiple addresses
and the application attempts to communicate to each address until one
succeeds, this process shouldn't take an unreasonable amount of time.
It is therefore important that the system quickly determine if IPv6
destinations are unreachable so that the application can try other
destinations when those IPv6 destinations are unreachable.
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
address resolution must be performed on all IPv6 addresses to which
the host sends packets. The Application will not receive
acknowledgment of the unreachability of destinations that are not on-
link until at least address resolution has failed, which is no less
than three seconds (MAX_MULTICAST_SOLICIT * RETRANS_TIMER). This is
greatly amplified by transport protocol delays. For example,
[RFC1122] requires that TCP retransmit for at least 3 minutes before
aborting the connection attempt.
When the application has a large list of off-link unreachable IPv6
addresses followed by at least one reachable IPv4 address, the delay
associated with Neighbor Unreachability Detection (NUD) of each IPv6
addresses before successful communication with the IPv4 address is
unacceptable.
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3.3 Multi-interface Ambiguity
There is no defined way to implement this aspect of the sending
algorithm on a node that is attached to multiple links. From an
implementor's point of view, there are three ways to handle sending
an IPv6 packet to a destination in the face of the on-link assumption
on a multi-interface node:
1. Attempt to resolve the destination on a single link.
2. Attempt to resolve the destination on every link.
3. Drop the packet.
If the destination is indeed on-link, the first option might not
succeed since the wrong link could be picked. The second option
might succeed in reaching a destination (assuming that one is
reachable) but is more complex to implement, and isn't guaranteed to
pick the correct destination. For example, there is still ambiguity
about which link to use if more than one node answers the
solicitations on multiple links. Dropping the packet is equivalent
to not making the on-link assumption at all. In other words, if
there is no route to the destination, don't attempt to send the
packet.
3.4 Security Related Issues
The on-link assumption discussed here introduces a security
vulnerability to the Neighbor Discovery protocol described in section
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
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
nodes by sending packets on the same link as the host. The
vulnerability is discussed in detail in [RFC3756].
Another security related side-effect of the on-link assumption has to
do with virtual private networks (VPN's). It has been observed that
some commercially available VPN software solutions that don't support
IPv6 send IPv6 packets to the local media in the clear (their
security policy doesn't simply drop IPv6 packets). Consider a
scenario where a system has a single Ethernet interface with VPN
software that encrypts and encapsulates certain packets. The system
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
local media. A malicious third party could then spoof the
destination on-link.
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4. Changes to RFC2461
The following changes have been made to the Neighbor Discovery
specification between [RFC2461] and [I-D.ietf-ipv6-2461bis]:
The last sentence of the second paragraph of section 5.2
("Conceptual Sending Algorithm") was removed. This sentence was,
"If the Default Router List is empty, the sender assumes that the
destination is on-link."
Bullet item 3) in section 6.3.6 ("Default Router Selection") was
removed. The item read, "If the Default Router List is empty,
assume that all destinations are on-link as specified in Section
5.2."
APPENDIX A was modified to remove on-link assumption related text
in bullet item 1) under the discussion on what happens when a
multihomed host fails to receive Router Advertisements.
The result of these changes is that destinations are considered
unreachable when there is no routing information for that destination
(through a default router or otherwise). Instead of attempting link-
layer address resolution when sending to such a destination, a node
should send an ICMPv6 Destination Unreachable message (code 0 - no
route to destination) message up the stack.
5. Security Considerations
The removal of the on-link assumption from Neighbor Discovery removes
some security related vulnerabilities of the protocol as described in
Section 3.4.
6. References
6.1 Normative References
[I-D.ietf-ipv6-2461bis]
Narten, T., "Neighbor Discovery for IP version 6 (IPv6)",
draft-ietf-ipv6-2461bis-02 (work in progress),
February 2005.
[RFC1122] Braden, R., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989.
[RFC2461] Narten, T., Nordmark, E., and W. Simpson, "Neighbor
Discovery for IP Version 6 (IPv6)", RFC 2461,
December 1998.
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[RFC3484] Draves, R., "Default Address Selection for Internet
Protocol version 6 (IPv6)", RFC 3484, February 2003.
[RFC3756] Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor
Discovery (ND) Trust Models and Threats", RFC 3756,
May 2004.
6.2 Informative References
[RFC2462] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
Authors' Addresses
Sebastien Roy
Sun Microsystems, Inc.
1 Network Drive
UBUR02-212
Burlington, MA 01801
Email: sebastien.roy@sun.com
Alain Durand
Comcast Corporation
1500 Market St.
Philadelphia, PA 09102
Email: alain_durand@cable.comcast.com
James Paugh
Email: jpaugh@speakeasy.net
Appendix A. Acknowledgments
The authors gratefully acknowledge the contributions of Jim Bound,
Tony Hain, Mika Liljeberg, Erik Nordmark, Pekka Savola, and Ronald
van der Pol.
Appendix B. Changes from draft-ietf-v6ops-onlinkassumption-02
o Changed abstract to reflect the historical nature of this
document.
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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 C. 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 D. 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.
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