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IPv6 Neighbor Discovery On-Link Assumption Considered Harmful
RFC 4943

Document Type RFC - Informational (September 2007)
Authors Alain Durand , Sebastien C. Roy , James M. Paugh
Last updated 2015-10-14
RFC stream Internet Engineering Task Force (IETF)
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RFC 4943
Network Working Group                                             S. Roy
Request for Comments: 4943                        Sun Microsystems, Inc.
Category: Informational                                        A. Durand
                                                                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.


   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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1.  Introduction

   Neighbor Discovery for IPv6 [RFC4861] 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 [RFC4861] 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 [RFC3484], 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

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 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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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
   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,
   [RFC1122] Section requires that TCP retransmit for at least 3
   minutes before aborting the connection attempt.

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   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

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 [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

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   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 (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 [RFC2461] and [RFC4861]:

      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

      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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6.  Normative References

   [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.

   [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.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.

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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
   Burlington, MA  01803


   Alain Durand
   1500 Market Street
   Philadelphia, PA  19102


   James Paugh
   Nominum, Inc.
   2385 Bay Road
   Redwood City, CA  94063


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