|Title||IPv6 Subnet Model: The Relationship between Links and Subnet
|Author||H. Singh, W. Beebee, E. Nordmark
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.
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 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
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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
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
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
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"
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
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
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
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.
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
a. The host MUST NOT assume that all destinations are on-link.
b. The host MUST NOT perform address resolution for non-link-
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
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
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
default router. Neither the router nor any other host will respond
to the address resolution, preventing this host from sending IPv6
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)
o any Neighbor Discovery message is received from the address.
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
Thomas Narten contributed significant text and provided substantial
guidance to the production of this document.
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.
11.1. 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,
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,
[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,
[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,
[RFC4943] Roy, S., Durand, A., and J. Paugh, "IPv6 Neighbor
Discovery On-Link Assumption Considered Harmful",
RFC 4943, September 2007.
Cisco Systems, Inc.
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