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rfc:rfc5942

Internet Engineering Task Force (IETF) H. Singh Request for Comments: 5942 W. Beebee Updates: 4861 Cisco Systems, Inc. Category: Standards Track E. Nordmark ISSN: 2070-1721 Oracle, Inc.

                                                             July 2010

IPv6 Subnet Model: The Relationship between Links and Subnet Prefixes

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 RFC 4861.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc5942.

Copyright Notice

 Copyright (c) 2010 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Singh, et al. Standards Track [Page 1] RFC 5942 IPv6 Subnet Model July 2010

 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Table of Contents

 1. Introduction ....................................................2
 2. Requirements Language ...........................................4
 3. Host Behavior ...................................................4
 4. Host Rules ......................................................7
 5. Observed Incorrect Implementation Behavior ......................8
 6. Updates to RFC 4861 .............................................9
 7. Conclusion ......................................................9
 8. Security Considerations .........................................9
 9. Contributors ....................................................9
 10. Acknowledgements ...............................................9
 11. References ....................................................10
    11.1. Normative References .....................................10
    11.2. Informative References ...................................10

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.  Nodes
 consider addresses covered by an on-link prefix to be directly
 attached to the same link as the sending node, i.e., they send
 traffic for such addresses directly rather than to a router.  See
 Section 3.3.1 of [RFC1122].  Prior to the development of subnetting
 [RFC0950] and Classless Inter-Domain Routing (CIDR) [RFC4632], an
 address's netmask could be derived directly from the address simply
 by determining whether it was a Class A, B, or C address.  Today,
 assigning an address to an interface also requires specifying a
 netmask to use.  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 (ND)
 [RFC4861] is quite different.  The on-link determination is separate
 from the address assignment.  A host can have IPv6 addresses without

Singh, et al. Standards Track [Page 2] RFC 5942 IPv6 Subnet Model July 2010

 any related on-link prefixes or can 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 prefixes on a host's Prefix List (i.e.,
 those prefixes that have not yet timed out) are considered to be
 on-link by that host.
 The on-link definition in the Terminology section of [RFC4861], as
 modified by this document, defines the complete list of cases in
 which a host considers an address to be 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
 the sender considers to be 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 "Host Rules" sections of this document.  (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 (NS) responses are assumed to be sent out
 the same interface on which the corresponding query was received
 without using the Conceptual Sending Algorithm.
 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" in [RFC4861] to address security concerns arising from
 particular ND implementations.
 Host behavior is clarified in the "Host Behavior" and "Host Rules"
 sections.

Singh, et al. Standards Track [Page 3] RFC 5942 IPv6 Subnet Model July 2010

2. Requirements Language

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in RFC 2119 [RFC2119].

3. 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 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.  However, in order for a node to know that a destination
     is on-link, it must obtain configuration information to that
     effect.  In IPv6, there are two main ways of maintaining
     information about on-link destinations.  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
     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.  It should be noted that ND does not have a way to indicate a
     destination is "off-link".  Rather, a destination is assumed to
     be off-link, unless there is explicit information indicating that
     it is on-link.  Such information may later expire or be changed,

Singh, et al. Standards Track [Page 4] RFC 5942 IPv6 Subnet Model July 2010

     in which case a destination may revert back to being considered
     off-link, but that is different than there being an explicit
     mechanism for signaling that a destination is off-link.  Redirect
     messages do not contain sufficient information to signal that an
     address is off-link.  Instead, Redirect messages indicate a
     preferred next hop that is a more appropriate choice to use than
     the originator of the Redirect.
 3.  IPv6 also defines the term "neighbor" 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 (NA) 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, Destination Cache, and Default Router List.  The
     combination of Prefix List, Destination Cache, and Default Router
     List form what many implementations consider to be the IP data
     forwarding 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
        be set to STALE.

Singh, et al. Standards Track [Page 5] RFC 5942 IPv6 Subnet Model July 2010

     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 6.2.6 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 updating the Destination 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 Router Solicitation (RS).  The Neighbor Cache can
     still be updated through receipt of an NS, NA, or RS.

Singh, et al. Standards Track [Page 6] RFC 5942 IPv6 Subnet Model July 2010

 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 an 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 address only assigned
     to an interface on a different link.  Although implementations
     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.

4. 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].
 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.  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
     update its Prefix List with respect to the entry.  In most cases,
     this will result in the addresses covered by the prefix
     defaulting back to being considered off-link, as specified by the
     PIO paragraph of Section 6.3.4 of [RFC4861].  However, there are
     cases where an address could be covered by multiple entries in
     the Prefix List, where expiration of one prefix would result in
     destinations then being covered by a different entry.

Singh, et al. Standards Track [Page 7] RFC 5942 IPv6 Subnet Model July 2010

 4.  Implementations compliant with [RFC4861] MUST adhere to the
     following rules.  If the Default Router List is empty and there
     is no other source of on-link information about any address or
     prefix:
     a.  The host MUST NOT assume that all destinations are on-link.
     b.  The host MUST NOT perform address resolution for non-link-
         local addresses.
     c.  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 the
         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.

5. 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.  An address
 could be acquired through the DHCPv6 identity association for non-
 temporary addresses (IA_NA) option from [RFC3315] (which does not
 include a prefix length), or through manual configuration (if no
 prefix length is specified).  The host incorrectly assumes an
 invented prefix is on-link.  This invented prefix typically is a /64
 that was written by the developer of the operating system network
 module API to any IPv6 application as a "default" prefix length when
 a length isn't specified.  This may cause the API to seem to work in
 the case of a network interface initiating stateless address
 autoconfiguration (SLAAC); however, it can cause connectivity
 problems in Non-Broadcast Multi-Access (NBMA) networks.  Having
 incorrectly assumed an invented prefix, the host performs address
 resolution when the host should send all non-link-local traffic to a

Singh, et al. Standards Track [Page 8] RFC 5942 IPv6 Subnet Model July 2010

 default router.  Neither the router nor any other host will respond
 to the address resolution, preventing this host from sending IPv6
 traffic.

6. Updates to RFC 4861

 This document deprecates the following two bullets from the on-link
 definition in Section 2.1 of [RFC4861]:
 o  a Neighbor Advertisement message is received for the (target)
    address, or
 o  any Neighbor Discovery message is received from the address.

7. 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 the last paragraph of Section 4.2 of
 [RFC4903].  This document also updates the definition of "on-link"
 from [RFC4861] by deprecating the last two bullets.

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

9. Contributors

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

10. 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.

Singh, et al. Standards Track [Page 9] RFC 5942 IPv6 Subnet Model July 2010

11. References

11.1. Normative References

 [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC4861]   Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
             "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
             September 2007.

11.2. Informative References

 [RFC0950]   Mogul, J. and J. Postel, "Internet Standard Subnetting
             Procedure", STD 5, RFC 950, August 1985.
 [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.
 [RFC4632]   Fuller, V. and T. Li, "Classless Inter-domain Routing
             (CIDR): The Internet Address Assignment and Aggregation
             Plan", BCP 122, RFC 4632, August 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.

Singh, et al. Standards Track [Page 10] RFC 5942 IPv6 Subnet Model July 2010

 [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
 Oracle, Inc.
 17 Network Circle
 Menlo Park, CA 94025
 USA
 Phone: +1 650 786 2921
 EMail: erik.nordmark@oracle.com

Singh, et al. Standards Track [Page 11]

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