Network Working Group                                           H. Singh
Internet-Draft                                                 W. Beebee
Intended status: Standards Track                     Cisco Systems, Inc.
Expires: November 8, 2008                                    E. Nordmark
                                                        Sun Microsystems
                                                             May 7, 2008

 IPv6 Subnet Model: the Relationship between Links and Subnet Prefixes

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   IPv6 specifies a model of a subnet that is different than the IPv4
   subnet model.  The subtlety of the differences has resulted in
   incorrect implementations that do not interoperate.  This document
   spells out the most important difference; that an IPv6 address isn't
   automatically associated with an IPv6 on-link prefix.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Host Behavior Rules . . . . . . . . . . . . . . . . . . . . . . 4
   3.  Observed Incorrect Implementation Behavior  . . . . . . . . . . 5
   4.  Conclusion  . . . . . . . . . . . . . . . . . . . . . . . . . . 5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 6
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 6
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 6
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 7
   Intellectual Property and Copyright Statements  . . . . . . . . . . 8

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1.  Introduction

   IPv4 implementations associate a netmask when an IPv4 address is
   assigned to an interface.  That netmask together with the IPv4
   address designates an on-link prefix.  Addresses that match this
   prefix are viewed as on-link i.e., traffic to these addresses is not
   sent to a router.  See section 3.3.1 in [RFC1122].  Further, note
   that implementations of IPv4 point-to-point interfaces might not have
   an associated IPv4 subnet prefix.

   The behavior of IPv6 as specified in Neighbor Discovery [RFC4861] is
   quite different.  The on-link determination is separate from the
   address assignment.  A host can have IPv6 addresses without any
   related on-link prefixes or have on-link prefixes that are not
   related to any IPv6 addresses that are assigned to the host.  Any
   assigned address on an interface should initially be considered as
   having no internal structure as shown in [RFC4291].

   In IPv6, by default, a host treats only the link-local prefix as on-

   The reception of a Prefix Information Option (PIO) with the L-bit set
   [RFC4861] and a non-zero valid lifetime creates an entry (or updates
   the valid lifetime for an existing entry) in the Prefix List.  All
   the prefixes that are on the Prefix List, i.e., have not yet timed
   out, are on-link.

   In addition to the Prefix List, individual addresses are on-link if
   they are the target of a Redirect Message indicating on-link, or the
   source of a valid Neighbor Solicitation or Neighbor Advertisement
   message.  Note that Redirect Messages can also indicate an address is
   off-link.  Individual address entries can be expired by the Neighbor
   Unreachability Detection mechanism.

   A host only performs address resolution for IPv6 addresses that are
   on-link.  Packets to any other address are sent to a default router.
   If there is no default router, then the node should send an ICMPv6
   Destination Unreachable indication as specified in [RFC4861] - more
   details are provided in the Host Behavior Rules section.  (Note that
   RFC 4861 changed the behavior when the Default Router List is empty.
   The behavior in the old version of Neighbor Discovery [RFC2461] was
   different when there were no default routers.)

   Failure of host implementations to correctly implement the IPv6
   subnet model can result in lack of IPv6 connectivity.  See the
   Observed Incorrect Implementation Behavior section for details.

   Host behavior is clarified in the Host Behavior Rules section.

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   Finally, this document merely restates and clarifies [RFC4861].

2.  Host Behavior Rules

   A correctly implemented IPv6 host MUST adhere to the following rules:

   1.  By default only the link-local prefix is on-link.

   2.  The configuration of an IPv6 address, whether through IPv6
       stateless address autoconfiguration [RFC4862], DHCPv6 [RFC3315],
       or manual configuration MUST NOT imply that any prefix is on-
       link.  A host is explicitly told that prefixes or addresses are
       on-link through the means specified in [RFC4861].  Note that this
       requirement for manually configured addresses is not explicitly
       mentioned in [RFC4861].

   3.  On-link determination SHOULD NOT persist across IPv6 interface
       initializations.  Note that section 5.7 of [RFC4862] describes
       the use of stable storage for addresses acquired with stateless
       address autoconfiguration with a note that the Preferred and
       Valid Lifetimes must be retained if this approach is used.
       However no RFC suggests or recommends retaining the on-link

   4.  In the absence of other sources of on-link information, including
       Redirects, if the RA advertises a prefix with the on-link(L) bit
       set and later the Valid Lifetime expires, the host MUST then
       consider addresses of the prefix to be off-link, as specified by
       the PIO paragraph of section 6.3.4 of [RFC4861].

   5.  Newer implementations, which are compliant with [RFC4861] MUST
       adhere to the following rules.  Older implementations, which are
       compliant with [RFC2461] but not [RFC4861] may remain as is.  If
       the Default Router List is empty and there is no other source of
       on-link information about any address or prefix:

       1.  The host MUST NOT assume that all destinations are on-link.

       2.  The host MUST NOT perform address resolution for non-link-
           local addresses.

       3.  Since the host cannot assume the destination is on-link, and
           off-link traffic cannot be sent to a default router (since
           the Default Router List is empty), address resolution cannot
           be performed.  This case is analogous to the behavior
           specified in the last paragraph of section 7.2.2 of
           [RFC4861]: when address resolution fails, the host SHOULD

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           send an ICMPv6 Destination Unreachable indication as
           specified in [RFC4861].  The specified behavior MAY be
           extended to cover this case where address resolution cannot
           be performed.

       On-link information concerning particular addresses and prefixes
       can make those specific addresses and prefixes on-link, but does
       not change the default behavior mentioned above for addresses and
       prefixes not specified.  [RFC4943] provides justification for
       these rules.

3.  Observed Incorrect Implementation Behavior

   One incorrect implementation behavior illustrates the severe
   consequences when the IPv6 subnet model is not understood by the
   implementers of several popular host operating systems.  In an access
   concentrator network ([RFC4388]), a host receives a Router
   Advertisement Message with no on-link prefix advertised.  The host
   incorrectly assumes the prefix is on-link and performs address
   resolution when the host should send all non-link-local traffic to a
   default router.  Neither the router nor any other host will respond
   to the address resolution, preventing this host from sending IPv6

4.  Conclusion

   This document clarifies and summarizes the relationship between links
   and subnet prefixes described in [RFC4861].  Configuration of an IPv6
   address does not imply the existence of corresponding on-link
   prefixes.  One should also look at API considerations for prefix
   length as described in last paragraph of section 4.2 of [RFC4903].

5.  Security Considerations

   As this document merely restates and clarifies [RFC4861], it does not
   introduce any new security issues.

6.  IANA Considerations


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7.  Acknowledgements

   Thanks (in alphabetical order) to Adeel Ahmed, Jari Arkko, Ralph
   Droms, Alun Evans, Dave Forster, Prashanth Krishnamurthy, Suresh
   Krishnan, Josh Littlefield, Thomas Narten, Madhu Sudan, Jinmei
   Tatuya, Dave Thaler, Bernie Volz, and Vlad Yasevich for their
   consistent input, ideas and review during the production of this

8.  References

8.1.  Normative References

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

8.2.  Informative 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.

   [RFC3315]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
              and M. Carney, "Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 3315, July 2003.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, February 2006.

   [RFC4388]  Woundy, R. and K. Kinnear, "Dynamic Host Configuration
              Protocol (DHCP) Leasequery", RFC 4388, February 2006.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862, September 2007.

   [RFC4903]  Thaler, D., "Multi-Link Subnet Issues", RFC 4903,
              June 2007.

   [RFC4943]  Roy, S., Durand, A., and J. Paugh, "IPv6 Neighbor
              Discovery On-Link Assumption Considered Harmful",
              RFC 4943, September 2007.

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Authors' Addresses

   Hemant Singh
   Cisco Systems, Inc.
   1414 Massachusetts Ave.
   Boxborough, MA  01719

   Phone: +1 978 936 1622

   Wes Beebee
   Cisco Systems, Inc.
   1414 Massachusetts Ave.
   Boxborough, MA  01719

   Phone: +1 978 936 2030

   Erik Nordmark
   Sun Microsystems
   17 Network Circle
   Menlo Park, CA 94025

   Phone: +1 650 786 2921

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