Network Working Group                                           H. Singh
Internet-Draft                                                 W. Beebee
Updates: 4861 (if approved)                          Cisco Systems, Inc.
Intended status: Standards Track                             E. Nordmark
Expires: May 18, 2010                                   Sun Microsystems
                                                       November 14, 2009

 IPv6 Subnet Model: the Relationship between Links and Subnet Prefixes


   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].

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
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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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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 . . . . . . . . . . . . . . . . . . . . . . . . . 10
   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 an 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-

   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.

   IPv6 packets sent using the Conceptual Sending Algorithm as described
   in [RFC4861] only trigger 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.  In the old version of Neighbor Discovery [RFC2461],
   if the Default router List is empty, rather than sending the ICMPv6
   Destination Unreachable indication, the [RFC2461] node assumed that
   the destination was on-link.")  Note that ND is scoped to a single
   link.  All Neighbor Solicitation responses are assumed to be sent out
   the same interface on which the corresponding query was received
   without using the Conceptual Sending Algorithm.

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   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.

   This document deprecates the last two bullets from the definition of
   on-link from [RFC4861] to address security concerns arising from
   particular ND implementations.

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

2.  Host Behavior

   1.  The original Neighbor Discovery (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 Router Advertisement (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

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       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 Redirect Messages do not contain sufficient information
       to signal that an address is off-link.  Rather, they indicate a
       preferred next-hop that is a more appropriate choice to use than
       the originator of the Redirect.  That alternate next-hop may be
       the destination itself (in which case packets would flow directly
       to a neighbor), or a router closer to the destination than the
       current next-hop router (which is the originator of the
       Redirect).  Note, however, that the Redirect message itself does
       not contain sufficient information to distinguish these cases.
       But that does not matter, because the receiver of such a message
       does the same in either case, updating its Neighbor Cache as
       defined in Section 8.1 of [RFC4861].

   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

       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 data forwarding table for a host.  Note that the Neighbor

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       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
          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.

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       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
       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

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

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       for manually configured addresses is not explicitly mentioned in

   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

       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.

   4.  Hosts MUST verify that on-link information is still valid after
       IPv6 interface re-initialization before using cached on-link
       determination information.  Failure to do so 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

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       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

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.  US-CERT Vulnerability Note VU#472363 lists the
   implementations affected.

7.  IANA Considerations


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8.  Contributors

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

9.  Acknowledgements

   Thanks (in alphabetical order) to Adeel Ahmed, Jari Arkko, Ralph
   Droms, Alun Evans, Dave Forster, Prashanth Krishnamurthy, Suresh
   Krishnan, Josh Littlefield, Bert Manfredi, 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

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              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.

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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