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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: September 7, 2009                                   E. Nordmark
                                                        Sun Microsystems
                                                           March 6, 2009


 IPv6 Subnet Model: the Relationship between Links and Subnet Prefixes
                  draft-ietf-6man-ipv6-subnet-model-03

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Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.



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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

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 updates (partially due to security concerns caused by incorrect
   implementations) a part of the definition of on-link from [RFC4861].


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Host Behavior  . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Host Rules . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   4.  Observed Incorrect Implementation Behavior . . . . . . . . . .  9
   5.  Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   8.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . .  9
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     10.1.  Normative References  . . . . . . . . . . . . . . . . . . 10
     10.2.  Informative References  . . . . . . . . . . . . . . . . . 10
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11



















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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 Inter-
   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 (or updates) an entry
   in the Prefix List.  All the prefixes that are on the Prefix List,
   i.e., have not yet timed out, are considered to be 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.  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
   Observed Incorrect Implementation Behavior section for details.

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



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2.  Host Behavior

   1.  The original ND specification [RFC4861] was unclear in its usage
       of the term on-link in a few places.  In IPv6, an address is
       considered to be on-link (with respect to a specific link), if
       the address has been assigned to an interface attached to that
       link.  Any node attached to the link can send a datagram directly
       to an on-link address without forwarding the datagram through a
       router.  In IPv6, there are two ways to indicate an address is
       on-link.  First, a host maintains a Prefix List that identifies
       ranges of addresses that are to be considered on-link.  Second,
       Redirects can identify individual destinations that are on-link;
       such Redirects update the Destination Cache.



       The Prefix List is populated via the following means:



       *  Receipt of a Valid RA that specifies a prefix with the L-bit
          set.  Such a prefix is considered on-link for a period
          specified in the Valid Lifetime and is added to the Prefix
          List.  (The link-local prefix is effectively considered a
          permanent entry on the Prefix List.)

       *  Indication of an on-link prefix (which may be a /128) via
          manual configuration, or some other yet-to-be specified
          configuration mechanism.



       A Redirect can also signal whether an address is on-link.  If a
       host originates a packet, but the first-hop router routes the
       received packet back out onto the same link, the router also
       sends the host a Redirect.  If the Target and Destination Address
       of the Redirect are the same, the Target Address is to be treated
       as on-link as specified in Section 8 of [RFC4861].  That is, the
       host updates its Destination Cache (but not its Prefix List --
       though the impact is similar).



   2.  Note that Redirects cannot signal that an address is off-link.
       In section 8.1 of [RFC4861], a Redirect message is silently
       discarded if it does not have an IP source address that is the
       same as the current first-hop router for the specified ICMP
       Destination Address.  An ICMP Destination Address on the same



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       link would have no current first-hop router.  Any Redirect
       message received could not have an IP source address that is the
       same as the current (null) first-hop router, so the Redirect MUST
       be dropped.



   3.  IPv6 also defines the term "neighbor" and "link" to refer to
       nodes attached to the same link and that can send packets
       directly to each other.  Received ND packets that pass the
       required validation tests can only come from a neighbor attached
       to the link on which the ND packet was received.  Unfortunately,
       [RFC4861] is imprecise in its definition of on-link and states
       that a node considers an address to be on-link if:

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

          - any Neighbor Discovery message is received from the address.

       Neither of these tests are acceptable definitions for an address
       to be considered as on-link as defined above, and this document
       deprecates and removes both of them from the formal definition of
       on-link.  Neither of these tests should be used as justification
       for modifying the Prefix List or Destination Cache for an
       address.



       The conceptual sending algorithm of [RFC4861] defines a Prefix
       List and Neighbor Cache.  The combination of Prefix List and
       Neighbor Cache form what many implementations consider to be the
       "IP routing table" for a host.  Note that the Neighbor Cache is a
       separate data structure referenced by the Destination Cache, but
       entries in the Neighbor Cache are not necessarily in the
       Destination Cache.  It is quite possible (and intentional) that
       entries be added to the Neighbor Cache for addresses that would
       not be considered on-link as-defined above.  For example, upon
       receipt of a valid NS, Section 7.2.3 of [RFC4861] states:



          If an entry does not already exist, the node SHOULD create a
          new one and set its reachability state to STALE as specified
          in Section 7.3.3.  If an entry already exists, and the cached
          link-layer address differs from the one in the received Source
          Link-Layer option, the cached address should be replaced by
          the received address, and the entry's reachability state MUST



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          be set to STALE.

       The intention of the above feature is to add an address to the
       Neighbor Cache, even though it might not be considered on- link
       per the Prefix List.  The benefit of such a step is to have the
       receiver populate the Neighbor Cache with an address it will
       almost certainly be sending packets to shortly, thus avoiding the
       need for an additional round of ND to perform address resolution.
       But because there is no validation of the address being added to
       the Neighbor Cache, an intruder could spoof the address and cause
       a receiver to add an address for a remote site to its Neighbor
       Cache.  This vulnerability is a specific instance of the broad
       set of attacks that are possible by an on-link neighbor
       [RFC3756].This causes no problems in practice, so long as the
       entry only exists in the Neighbor Cache and the address is not
       considered to be on-link by the IP forwarding code (i.e., the
       address is not added to the Prefix List and is not marked as on-
       link in the Destination Cache).



   4.  After the update to the on-link definition in [RFC4861], certain
       text from section 7.2.3 of [RFC4861] may appear, upon a cursory
       examination, to be inconsistent with the updated definition of
       on-link because the text does not ensure that the source address
       is already deemed on-link through other methods:

          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.

          Similarly, the following text from section 6.2.5 of [RFC4861]
          may also seem inconsistent: If there is no existing Neighbor
          Cache entry for the solicitation's sender, the router creates
          one, installs the link- layer address and sets its
          reachability state to STALE as specified in Section 7.3.3.

       However, the text in the aforementioned sections of [RFC4861],
       upon closer inspection, is actually consistent with the
       deprecation of the last two bullets of the on-link definition
       because there are two different ways in which on-link
       determination can affect the state of ND: through updating the
       Prefix List or the Neighbor Cache.  Through deprecating the last
       two bullets of the on-link definition, the Prefix List is
       explicitly not to be changed when a node receives an NS, NA, or
       RS.  The Neighbor Cache can still be updated through receipt of



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       an NS, NA, or RS.



   5.  [RFC4861] is written from the perspective of a host with a single
       interface on which Neighbor Discovery is run.  All ND traffic
       (whether sent or received) traverses the single interface.  On
       hosts with multiple interfaces, care must be taken to ensure that
       the scope of ND processing from one link stays local to that
       link.  That is, when responding to a NS, the NA would be sent out
       on the same link on which it was received.  Likewise, a host
       would not respond to a received NS for an an address assigned to
       an interface on a different link.  Although implementions may
       choose to implement Neighbor Discovery using a single data
       structure that merges the Neighbor Caches of all interfaces, an
       implementation's behavior must be consistent with the above
       model.



   6.  Note that the receipt of a link-local IPv6 multicast packet which
       is not an ND packet indicates direct reachability on a link, but
       is not specifically treated by [RFC4861].



   7.  Note that the receipt of a packet with the Hop Limit field
       unchanged (the Hop Limit could be specified in a packet-type
       specific document) which is not an ND packet indicates direct
       reachability on a link, but is not specifically treated by
       [RFC4861].


3.  Host Rules

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

   1.  The assignment 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 and added to the Prefix
       List.  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].




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   2.  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].



   3.  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.

   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 powers down.  During the power off, 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 powers on
   and 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



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   connectivity.


4.  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.


5.  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 updates the definition of on-link from [RFC4861]
   by retracting the last two bullets.


6.  Security Considerations

   This document addresses a security concern present in [RFC4861].  As
   a result, the last bullet of the on-link definition in [RFC4861] has
   been retracted.


7.  IANA Considerations

   None.


8.  Contributors

   Thomas Narten contributed significant text and provided substantial
   guidance to the production of this document.







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9.  Acknowledgements

   Thanks (in alphabetical order) to Adeel Ahmed, Jari Arkko, Ralph
   Droms, Alun Evans, Dave Forster, Prashanth Krishnamurthy, Suresh
   Krishnan, Josh Littlefield, David Miles, 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 related to an NS message that provides one reason
   for invalidating a part of the on-link definition was found by David
   Miles.  Jinmei Tatuya found the security problem to also exist with
   an RS message.


10.  References

10.1.  Normative References

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

10.2.  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.


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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