Network Working Group S. Roy
Internet-Draft A. Durand
Expires: April 19, 2004 J. Paugh
Sun Microsystems, Inc.
October 20, 2003
IPv6 Neighbor Discovery On-Link Assumption Considered Harmful
draft-ietf-v6ops-onlinkassumption-00.txt
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document proposes a change to the IPv6 Neighbor Discovery
conceptual host sending algorithm. According to the algorithm, when
a host's default router list is empty, the host assumes that all
destinations are on-link. 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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1 First Rule of Destination Address Selection . . . . . . . . . 5
3.2 Delays Associated with Address Resolution . . . . . . . . . . 5
3.3 Multi-homing Ambiguity . . . . . . . . . . . . . . . . . . . . 6
4. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
Normative References . . . . . . . . . . . . . . . . . . . . . 9
Informative References . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 10
A. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11
Intellectual Property and Copyright Statements . . . . . . . . 12
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1. Introduction
Neighbor Discovery for IPv6 [ND] defines a conceptual sending
algorithm for hosts. This algorithm states that if a host's default
router list is empty, then the host assumes that all destinations are
on-link.
This assumption creates problems for destination address selection as
defined in [ADDRSEL], and adds connection delays associated with
unnecessary address resolution and neighbor unreachability detection.
The behavior associated with the assumption is undefined in
multihomed scenarios, and has some subtle security implications. All
of these issues are discussed in this document.
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2. Background
This part of Neighbor Discovery's [ND] conceptual sending algorithm
was created to facilitate communication on a single link between
systems manually configured with different global prefixes. For
example, two systems that are manually configured with global
addresses while on separate links 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 as
seamlessly. One workaround would be to use link-local addresses for
this communication. Another is to configure new global addresses
using the same /64 prefix on these systems, either by manually
configuring such addresses, or by placing a router on-link that
advertises this prefix.
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3. Problems
The on-link assumption causes the following problems.
3.1 First Rule of Destination Address Selection
Default Address Selection for IPv6 [ADDRSEL] defines a destination
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 unreachability determination for a destination as it pertains to
this rule is an implementation detail. One implementable method is
to do a simple forwarding table lookup on the destination, and to
deem the destination as reachable if the lookup succeeds. The
Neighbor Discovery on-link assumption makes this method somewhat
irrelevant, however, as an implementation of the assumption could
simply be to insert an IPv6 default on-link route into the system's
forwarding table when the default router list is empty. The
side-effect is that the rule would always determine that all IPv6
destinations are reachable.
On a network where there is no IPv6 router (all off-link IPv6
destinations are unreachable) and there is off-link IPv4
connectivity, the on-link assumption causes the rule to not
necessarily prefer reachable IPv4 destinations over unreachable IPv6
destinations. This results in unreachable destinations being placed
at the front of the sorted list.
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
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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)
(amplified by transport protocol delays). 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 NUD of
each IPv6 addresses before successful communication with the IPv4
address is unacceptable.
3.3 Multi-homing Ambiguity
There is no defined way to implement this aspect of the sending
algorithm on a multi-homed node. 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-homed
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 may not
succeed since the wrong link could be picked. The second option
would always succeed in reaching the destination (assuming that it's
reachable) but is more complex to implement. 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.
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4. Conclusion
This document suggests the following changes to the Neighbor
Discovery [ND] specification:
The last sentence of the second paragraph of section 5.2
("Conceptual Sending Algorithm") should be removed. This sentence
is currently, "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")
should be removed. The item currently reads, "If the Default
Router List is empty, assume that all destinations are on-link as
specified in Section 5.2."
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.
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5. Security Considerations
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 [PSREQ]
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 [PSREQ].
Another security related side-effect of the on-link assumption has to
do with 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 second
party could then spoof the destination on-link.
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Normative References
[ADDRSEL] Draves, R., "Default Address Selection for Internet
Protocol version 6 (IPv6)", RFC 3484, February 2003.
[ND] Narten, T., Nordmark, E. and W. Simpson, "Neighbor
Discovery for IP Version 6 (IPv6)", RFC 2461, December
1998.
[PSREQ] Nikander, P., Kempf, J. and E. Nordmark, "IPv6 Neighbor
Discovery trust models and threats",
draft-ietf-send-psreq-04, October 2003.
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Informative References
[AUTOCONF]
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
Sun Microsystems, Inc.
17 Network Circle
UMPK17-202
Menlo Park, CA 94025
EMail: alain.durand@sun.com
James Paugh
Sun Microsystems, Inc.
17 Network Circle
UMPK17-202
Menlo Park, CA 94025
EMail: james.paugh@sun.com
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Appendix A. Acknowledgments
The authors gratefully acknowledge the contributions of Jim Bound,
Mika Liljeberg, Erik Nordmark, Pekka Savola, and Ronald van der Pol.
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