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

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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

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.

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.

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,

   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.

   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.

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.

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.

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.

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

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

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.

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.

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.

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.

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.

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: [email protected] URI: http://www.cisco.com/

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

Phone: +1 978 936 2030 EMail: [email protected] URI: http://www.cisco.com/

Erik Nordmark Oracle, Inc. 17 Network Circle Menlo Park, CA 94025 USA

Phone: +1 650 786 2921 EMail: [email protected]