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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 RFC 5942

Network Working Group                                           H. Singh
Internet-Draft                                                 W. Beebee
Intended status: Standards Track                     Cisco Systems, Inc.
Expires: April 9, 2009                                       E. Nordmark
                                                        Sun Microsystems
                                                         October 6, 2008


 IPv6 Subnet Model: the Relationship between Links and Subnet Prefixes
                  draft-ietf-6man-ipv6-subnet-model-02

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Abstract

   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.  This document
   also invalidates (partially due to security concerns) a part of the
   definition of on-link from [RFC4861].






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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Host Behavior and Rules  . . . . . . . . . . . . . . . . . . .  4
   3.  Observed Incorrect Implementation Behavior . . . . . . . . . .  6
   4.  Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  6
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  6
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  7
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     8.1.  Normative References . . . . . . . . . . . . . . . . . . .  7
     8.2.  Informative References . . . . . . . . . . . . . . . . . .  7
   Appendix A.  CHANGE HISTORY  . . . . . . . . . . . . . . . . . . .  8
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . .  9
   Intellectual Property and Copyright Statements . . . . . . . . . . 10




































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1.  Introduction

   IPv4 implementations typically associate a netmask with an address
   when an IPv4 address is assigned to an interface.  That netmask
   together with the IPv4 address designates an on-link prefix.
   Addresses that are covered by 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].  Prior to the deployment of Classless Intern-
   Domain Routing (CIDR), an address's netmask could be derived directly
   from the address.  In the absence of specifying a specific netmask
   when assigning a address, some implementations would fall back to
   deriving the netmask from the class of the address.

   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-
   link.

   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.

   The on-link definition in the Terminology section of [RFC4861], as
   modified by this document, defines the complete list of cases where
   an address is considered on-link.  Note, in particular, 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 and Rules section.  (Note
   that [RFC4861] 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



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   Observed Incorrect Implementation Behavior section for details.

   Host behavior is clarified in the Host Behavior and Rules section.


2.  Host Behavior and 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 implicitly cause a prefix
       derived from that address to be treated as on-link.  A host
       considers a prefix to be on-link only through explicit means,
       such as those specified in the on-link definition in the
       Terminology section of [RFC4861], as modified by this document,
       or via manual configuration.  Note that the requirement for
       manually configured addresses is not explicitly mentioned in
       [RFC4861].

   3.  Note that the following items (from the definition of on-link in
       [RFC4861]):

          - a Neighbor Advertisement message is received for the
          (target) address, or

          - any Neighbor Discovery message is received from the address.

       are not sufficient to consider an address to be on-link and will
       be removed in a future update to [RFC4861].  A literal reading of
       the second test would allow a neighboring intruder to generate
       bogus ND messages that result in a spoofed address being
       improperly treated as on-link.  This vulnerability is a specific
       instance of the broad set of attacks that are possible by an on-
       link neighbor [RFC3756].  The threat is particularly problematic
       in the case of routers which allow such a spoofed message to
       update their forwarding tables (which can happen if a neighbor
       cache entry can update the forwarding table).  Only addresses
       that are covered by the modified on-link definition should be
       treated as on-link from a sending or forwarding perspective, and
       it should be noted that routers should generally obtain on-link
       information from sources other than RAs and Redirects.

   4.  To maintain consistency with the invalidation of the last two
       bullets of the on-link definition in [RFC4861], the following
       text from section 7.2.3 of [RFC4861] will also be augmented:



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          If the Source Address is not the unspecified address and, on
          link layers that have addresses, the solicitation includes a
          Source Link-Layer Address option, then the recipient SHOULD
          create or update the Neighbor Cache entry for the IP Source
          Address of the solicitation.

       changes to:

          If the Source Address is not the unspecified address and, on
          link layers that have addresses, the solicitation includes a
          Source Link-Layer Address option, then the recipient SHOULD
          create or update the Neighbor Cache entry for the IP Source
          Address of the solicitation provided that the source address
          of the NS is deemed on-link through other indications.

   5.  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].

   6.  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 specified in the last paragraph
           of section 4 of [RFC4943]: when there is no route to
           destination, the host should send an ICMPv6 Destination
           Unreachable indication (for example, a locally delivered
           error message) as specified in the Terminology section of
           [RFC4861].

       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.




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   Using cached on-link determination information without first
   verifying that the information is still valid after IPv6 interface
   re-initialization may lead to lack of IPv6 network connectivity.  For
   example, a host receives an RA from a router with on-link prefix A.
   The host reboots.  During the reboot, the router sends out prefix A
   with on-link bit set and a zero lifetime to indicate a renumbering.
   The host misses the renumbering.  The host comes online.  Then, the
   router sends an RA with no PIO.  The host uses cached on-link prefix
   A and issues NS's instead of sending traffic to a default router.
   The "Observed Incorrect Implementation Behavior" section below
   describes how this can result in lack of IPv6 connectivity.


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 an invented 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 traffic.


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].
   This document also invalidates a part of the definition of on-link
   from [RFC4861].


5.  Security Considerations

   This document addresses a security concern present in [RFC4861].  As
   a result, the last two bullets of the on-link definition in [RFC4861]
   have been invalidated.


6.  IANA Considerations

   None.



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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, David Miles, 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
   document.  The security problem that provides one reason for
   invalidating a part of the on-link definition was found by David
   Miles.  Thomas Narten has provided substantial guidance to the
   production of this document.


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.

   [RFC3756]  Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor
              Discovery (ND) Trust Models and Threats", RFC 3756,
              May 2004.

   [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,



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              June 2007.

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


Appendix A.  CHANGE HISTORY

   [NOTE TO RFC EDITOR: PLEASE REMOVE THIS SECTION UPON PUBLICATION.]

   Changes in draft-ietf-6man-ipv6-subnet-model-02.txt since -01.txt
   are:

   o  Augmented Abstract to say an important change to [RFC4861] is
      being made by this document.

   o  Removed the following sentence at the end of the Introduction
      section: "Finally, this document mainly restates and clarifies
      [RFC4861]."

   o  Added new bullet three to the "Host Behavior and Rules" section
      where the bullet invalidates bullets three and four from the on-
      link definition from [RFC4861].

   o  Added new bullet four to the "Host Behavior and Rules" section
      where the bullet proposes changes to text in section 7.2.3 of
      [RFC4861].

   o  The security section has been modified to reflect the important
      invalidation proposed by this document.

   o  Modified minor text in the "Observed Incorrect Implementation
      Behavior" section to explain what the prefix is in the second
      sentence.

   o  Changed bullet 3 from a new rule with normative language to just a
      paragraph of text describing behavior for a host blindly caching
      on-link determination and a possible severe consequence of that.
      The text also includes a solution for the problem.  The new text
      lies at the end of section 2 as a new paragraph.

   o  The title of section 2 has been changed to Host Behavior and
      Rules.  Also changed Host Behavior Rules to Host Behavior and
      Rules in two places in the Introduction section.

   Changes in draft-ietf-6man-ipv6-subnet-model-01.txt since -00.txt
   are:



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   o  Changed Introduction section to remove any mention of src address
      of ND message as a means for on-link determination.  Also reworded
      first paragraph of Introduction section.

   o  Reworded bullet 2 of section 2 and added text to clarify on-link
      definition.

   o  Added text to bullet 3 of section 2 to make explicit that this is
      a new rule.

   o  Reworded bullet 5 of section 2 to clearly explain where ICMPv6
      Destination Unreachable is sent to.


Authors' Addresses

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

   Phone: +1 978 936 1622
   Email: shemant@cisco.com
   URI:   http://www.cisco.com/


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

   Phone: +1 978 936 2030
   Email: wbeebee@cisco.com
   URI:   http://www.cisco.com/


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

   Phone: +1 650 786 2921
   Email: erik.nordmark@sun.com





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