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IETF RFC 4943
IPv6 Neighbor Discovery On-Link Assumption Considered Harmful
Last modified on Tuesday, September 4th, 2007
 Permanent link to RFC 4943
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Network Working Group S. Roy
Request for Comments: 4943 Sun Microsystems, Inc.
Category: Informational A. Durand
Comcast
J. Paugh
Nominum, Inc.
September 2007
IPv6 Neighbor Discovery On-Link Assumption Considered Harmful
 Status of This Memo
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
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
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 how these problems outweigh the
benefits of this part of the conceptual sending algorithm.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Background on the On-link Assumption . . . . . . . . . . . . . 2
3. Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. First Rule of Destination Address Selection . . . . . . . . 3
3.2. Delays Associated with Address Resolution . . . . . . . . . 3
3.3. Multi-interface Ambiguity . . . . . . . . . . . . . . . . . 4
3.4. Security-Related Issues . . . . . . . . . . . . . . . . . . 4
4. Changes to RFC 2461 . . . . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
6. Normative References . . . . . . . . . . . . . . . . . . . . . 6
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 7
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RFC 4943 On-Link Assumption Harmful September 2007
1. Introduction
Neighbor Discovery for IPv6 [RFC 4861] defines a conceptual sending
algorithm for hosts. The version of the algorithm described in
[RFC 2461] 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 [RFC 4861] 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 they may attempt to
connect to unreachable IPv6 off-link nodes.
The on-link assumption creates problems for destination address
selection as defined in [RFC 3484], and it 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 [RFC 2461] conceptual sending
algorithm was created to facilitate communication on a single link
between systems configured with different global prefixes in the
absence of an IPv6 router. 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. On the other hand, the on-link assumption
introduces several problems to various parts of the networking stack
described in Section 3. As such, this document points out that the
problems introduced by the on-link assumption outweigh the benefit
that the assumption lends to this scenario. It is more beneficial to
the end user to remove the on-link assumption from Neighbor Discovery
and declare this scenario illegitimate (or a misconfiguration).
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RFC 4943 On-Link Assumption Harmful September 2007
3. Problems
The on-link assumption causes the following problems.
3.1. First Rule of Destination Address Selection
Default Address Selection for IPv6 [RFC 3484] 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 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 3 seconds (MAX_MULTICAST_SOLICIT * RETRANS_TIMER). This is
greatly amplified by transport protocol delays. For example,
[RFC 1122] Section 4.2.3.5 requires that TCP retransmit for at least 3
minutes before aborting the connection attempt.
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RFC 4943 On-Link Assumption Harmful September 2007
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
address before successful communication with the IPv4 address is
unacceptable.
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.
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
the sending node is attached to multiple links. With the on-link
assumption, this node assumes that the destination is on-link, but on
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
the on-link assumption on a multi-interface node:
1. Attempt to send the packet on a single link (either
administratively pre-defined or using some algorithm).
2. Attempt to send the packet 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 the destination but is more complex to
implement and isn't guaranteed to pick the correct destination. For
example, there could be more than one node configured with the same
address, each reachable through a different link. The address by
itself does not disambiguate which destination the sender actually
wanted to reach, so attempting to send the packet to every link is
not guaranteed to reach the anticipated destination. The third
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
destination, don't attempt to send the packet. An implementation
that behaves this way would require an administrator to configure
routes to the destination in order to have reachability to the
destination, thus eliminating the ambiguity.
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 [RFC 3756]
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
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RFC 4943 On-Link Assumption Harmful September 2007
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 [RFC 3756].
Another security-related side-effect of the on-link assumption has to
do with virtual private networks (VPNs). 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.
4. Changes to RFC 2461
The following changes have been made to the Neighbor Discovery
specification between [RFC 2461] and [RFC 4861]:
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
addresses all of the security-related vulnerabilities of the protocol
as described in Section 3.4.
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RFC 4943 On-Link Assumption Harmful September 2007
6. Normative References
[RFC 1122] Braden, R., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989.
[RFC 2461] Narten, T., Nordmark, E., and W. Simpson, "Neighbor
Discovery for IP Version 6 (IPv6)", RFC 2461,
December 1998.
[RFC 3484] Draves, R., "Default Address Selection for Internet
Protocol version 6 (IPv6)", RFC 3484, February 2003.
[RFC 3756] Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor
Discovery (ND) Trust Models and Threats", RFC 3756,
May 2004.
[RFC 4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
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RFC 4943 On-Link Assumption Harmful September 2007
Appendix A. Acknowledgments
The authors gratefully acknowledge the contributions of Jim Bound,
Spencer Dawkins, Tony Hain, Mika Liljeberg, Erik Nordmark, Pekka
Savola, and Ronald van der Pol.
Authors' Addresses
Sebastien Roy
Sun Microsystems, Inc.
1 Network Drive
UBUR02-212
Burlington, MA 01803
EMail: sebastien.roy@sun.com
Alain Durand
Comcast
1500 Market Street
Philadelphia, PA 19102
EMail: alain_durand@cable.comcast.com
James Paugh
Nominum, Inc.
2385 Bay Road
Redwood City, CA 94063
EMail: jim.paugh@nominum.com
Roy, et al. Informational PAGE 7
RFC 4943 On-Link Assumption Harmful September 2007
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Copyright © The IETF Trust (2007).
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IPv6 Neighbor Discovery On-Link Assumption Considered Harmful
RFC TOTAL SIZE: 16719 bytes
PUBLICATION DATE: Tuesday, September 4th, 2007
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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