RFC 5942: IPv6 Subnet Model: The Relationship between Links and Subnet Prefixes

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.



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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



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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.




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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,



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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.




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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.




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 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.






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 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





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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.




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RFC 5942 IPv6 Subnet Model July 2010


11. References

11.1. Normative References

 [] Bradner, S., "Key words for use in RFCs to Indicate
 Requirement Levels", BCP 14, RFC 2119, March 1997.

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

11.2. Informative References

 [] Mogul, J. and J. Postel, "Internet Standard Subnetting
 Procedure", STD 5, RFC 950, August 1985.

 [] Braden, R., "Requirements for Internet Hosts -
 Communication Layers", STD 3, RFC 1122, October 1989.

 [] Narten, T., Nordmark, E., and W. Simpson, "Neighbor
 Discovery for IP Version 6 (IPv6)", RFC 2461,
 December 1998.

 [] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
 and M. Carney, "Dynamic Host Configuration Protocol for
 IPv6 (DHCPv6)", RFC 3315, July 2003.

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

 [] Hinden, R. and S. Deering, "IP Version 6 Addressing
 Architecture", RFC 4291, February 2006.

 [] Woundy, R. and K. Kinnear, "Dynamic Host Configuration
 Protocol (DHCP) Leasequery", RFC 4388, February 2006.

 [] Fuller, V. and T. Li, "Classless Inter-domain Routing
 (CIDR): The Internet Address Assignment and Aggregation
 Plan", BCP 122, RFC 4632, August 2006.

 [] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
 Address Autoconfiguration", RFC 4862, September 2007.

 [] Thaler, D., "Multi-Link Subnet Issues", RFC 4903,
 June 2007.





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RFC 5942 IPv6 Subnet Model July 2010


 [] 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
















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RFC 5942: IPv6 Subnet Model: The Relationship between Links and Subnet Prefixes