Network Working Group H. Singh
Internet-Draft W. Beebee
Intended status: Standards Track Cisco Systems, Inc.
Expires: January 11, 2009 E. Nordmark
Sun Microsystems
July 10, 2008
IPv6 Subnet Model: the Relationship between Links and Subnet Prefixes
draft-ietf-6man-ipv6-subnet-model-01
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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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Host Behavior and Rules . . . . . . . . . . . . . . . . . . . . 4
3. Observed Incorrect Implementation Behavior . . . . . . . . . . 5
4. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
8.1. Normative References . . . . . . . . . . . . . . . . . . . 6
8.2. Informative References . . . . . . . . . . . . . . . . . . 6
Appendix A. CHANGE HISTORY . . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 7
Intellectual Property and Copyright Statements . . . . . . . . . . 9
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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 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 an entry (or updates
the valid lifetime for an existing entry) in the Prefix List. All
the prefixes that are on the Prefix List, i.e., have not yet timed
out, are on-link.
The on-link definition in the Terminology section of [RFC4861]
defines the complete list of cases where an address is considered on-
link. Note, in particular, that Redirect Messages can also indicate
an address is off-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 Rules section. (Note that
RFC 4861 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.
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Host behavior is clarified in the Host Behavior Rules section.
Finally, this document mainly restates and clarifies [RFC4861].
2. Host Behavior and Rules
A correctly implemented IPv6 host MUST adhere to the following rules:
1. By default only the link-local prefix is on-link.
2. The configuration of an IPv6 address, whether through IPv6
stateless address autoconfiguration [RFC4862], DHCPv6 [RFC3315],
or manual configuration MUST NOT implicitely cause a prefix
derived from that address to be treated as on-link. 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] or via manual configuration.
Note that the requirement for manually configured addresses is
not explicitly mentioned in [RFC4861].
3. If on-link determination persists across IPv6 interface
initializations, then lack of IPv6 connectivity can result. For
example, a host receives an RA from a router with on-link prefix
A. The host reboots. During the reboot, 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 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 connectivity.
4. 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].
5. 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.
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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.
3. 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 the 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.
4. 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].
5. Security Considerations
This document mainly restates and clarifies [RFC4861]. It does not
introduce any new security issues.
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6. IANA Considerations
None.
7. Acknowledgements
Thanks (in alphabetical order) to Adeel Ahmed, Jari Arkko, Ralph
Droms, Alun Evans, Dave Forster, Prashanth Krishnamurthy, Suresh
Krishnan, Josh Littlefield, Thomas Narten, Madhu Sudan, Jinmei
Tatuya, Dave Thaler, Bernie Volz, and Vlad Yasevich for their
consistent input, ideas and review during the production of this
document.
8. References
8.1. Normative References
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
8.2. 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.
[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,
June 2007.
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[RFC4943] Roy, S., Durand, A., and J. Paugh, "IPv6 Neighbor
Discovery On-Link Assumption Considered Harmful",
RFC 4943, September 2007.
Appendix A. CHANGE HISTORY
[NOTE TO RFC EDITOR: PLEASE REMOVE THIS SECTION UPON PUBLICATION.]
Changes in draft-ietf-6man-ipv6-subnet-model-01.txt since -00.txt
are:
o Changed Introduction section to remove any mention of src address
of ND message as a means for on-link determination. Also reworded
first paragrpah of Introduction section.
o Reworded bullet 2 of section 2 and added text to clarify on-link
definition.
o Added text to bullet 3 of section 2 to make explicit that this is
a new rule.
o Reworded bullet 5 of section 2 to clearly explain where ICMPv6
Destination Unreachable is sent to.
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/
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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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