Network Working Group H. Singh
Internet-Draft W. Beebee
Intended status: Standards Track Cisco Systems, Inc.
Expires: September 7, 2009 E. Nordmark
Sun Microsystems
March 6, 2009
IPv6 Subnet Model: the Relationship between Links and Subnet Prefixes
draft-ietf-6man-ipv6-subnet-model-03
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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 [RFC4861].
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 . . . . . . . . . . . . . . . . . . . . . . . . . 9
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 a 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-
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 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.
A host only performs 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. The behavior in the old version of Neighbor Discovery
[RFC2461] was different when there were no default routers.)
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.
Host behavior is clarified in the Host Behavior and Rules section.
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2. Host Behavior
1. The original 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 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. Note that Redirects cannot signal that an address is off-link.
In section 8.1 of [RFC4861], a Redirect message is silently
discarded if it does not have an IP source address that is the
same as the current first-hop router for the specified ICMP
Destination Address. An ICMP Destination Address on the same
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link would have no current first-hop router. Any Redirect
message received could not have an IP source address that is the
same as the current (null) first-hop router, so the Redirect MUST
be dropped.
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
address.
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 routing 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
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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 6.2.5 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
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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
model.
6. Note that the receipt of a link-local IPv6 multicast packet which
is not an ND packet indicates direct reachability on a link, but
is not specifically treated by [RFC4861].
7. Note that the receipt of a packet with the Hop Limit field
unchanged (the Hop Limit could be specified in a packet-type
specific document) which is not an ND packet indicates direct
reachability on a link, but is not specifically treated by
[RFC4861].
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
for manually configured addresses is not explicitly mentioned in
[RFC4861].
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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
[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.
Using cached on-link determination information without first
verifying that the information is still valid after IPv6 interface
re-initialization 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 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
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connectivity.
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.
7. IANA Considerations
None.
8. Contributors
Thomas Narten contributed significant text and provided substantial
guidance to the production of this document.
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9. Acknowledgements
Thanks (in alphabetical order) to Adeel Ahmed, Jari Arkko, Ralph
Droms, Alun Evans, Dave Forster, Prashanth Krishnamurthy, Suresh
Krishnan, Josh Littlefield, 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
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,
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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: 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
Sun Microsystems
17 Network Circle
Menlo Park, CA 94025
USA
Phone: +1 650 786 2921
Email: erik.nordmark@sun.com
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