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

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

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Copyright Notice

   Copyright (C) The Internet Society (2004). All Rights Reserved.


   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

   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

   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

   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

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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   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

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

   [PSREQ]    Nikander, P., Kempf, J. and E. Nordmark, "IPv6 Neighbor
              Discovery trust models and threats", October 2003.


6.2  Informative References

              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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Authors' Addresses

   Sebastien Roy
   Sun Microsystems, Inc.
   1 Network Drive
   Burlington, MA  01801


   Alain Durand
   Sun Microsystems, Inc.
   17 Network Circle
   Menlo Park, CA  94025


   James Paugh
   Sun Microsystems, Inc.
   17 Network Circle
   Menlo Park, CA  94025


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

   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

   o  Miscellaneous editorial changes.

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