One document matched: draft-ietf-v6ops-onlinkassumption-02.txt
Differences from draft-ietf-v6ops-onlinkassumption-01.txt
Network Working Group S. Roy
Internet-Draft A. Durand
Expires: November 5, 2004 J. Paugh
Sun Microsystems, Inc.
May 7, 2004
IPv6 Neighbor Discovery On-Link Assumption Considered Harmful
draft-ietf-v6ops-onlinkassumption-02.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document describes 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 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 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 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-homing Ambiguity . . . . . . . . . . . . . . . . . . 4
3.4 Security Related Issues . . . . . . . . . . . . . . . . . 5
4. Proposed Changes to RFC2461 . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
6.1 Normative References . . . . . . . . . . . . . . . . . . . . 6
6.2 Informative References . . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 7
A. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7
B. Changes from draft-ietf-v6ops-onlinkassumption-01 . . . . . . 7
C. Changes from draft-ietf-v6ops-onlinkassumption-00 . . . . . . 8
Intellectual Property and Copyright Statements . . . . . . . . 9
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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 is problematic with IPv6-capable nodes that do not
have off-link IPv6 connectivity. Specifically, it 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.
A revision of Neighbor Discovery [NDBIS] is removing the on-link
assumption from the specification, but this memo gives historical
reference and background to why this is has been a good decision.
2. Background on the On-link Assumption
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, 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 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, however users may not have appropriate
privileges or knowledge to implement this workaround.
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
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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. This causes the rule to not necessarily prefer reachable
IPv4 destinations over unreachable IPv6 destinations, resulting 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
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 Neighbor
Unreachability Detection (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
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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 [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 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 second party could then spoof the
destination on-link.
4. Proposed Changes to RFC2461
This document suggests the following changes to the Neighbor
Discovery [ND] specification:
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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.
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
[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", October 2003.
draft-ietf-send-psreq-04
6.2 Informative References
[AUTOCONF]
Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[NDBIS] Narten, T., Nordmark, E., Simpson, W., Soliman, H. and J.
Tatuya, "Neighbor Discovery for IP Version 6 (IPv6)",
February 2004.
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draft-soliman-ipv6-2461-bis-01
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
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-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
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Section 3.1.
o Miscellaneous editorial changes.
Appendix C. 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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