One document matched: draft-ietf-6man-ipv6-subnet-model-00.txt
Network Working Group H. Singh
Internet-Draft W. Beebee
Intended status: Standards Track Cisco Systems, Inc.
Expires: November 8, 2008 E. Nordmark
Sun Microsystems
May 7, 2008
IPv6 Subnet Model: the Relationship between Links and Subnet Prefixes
draft-ietf-6man-ipv6-subnet-model-00
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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 Rules . . . . . . . . . . . . . . . . . . . . . . 4
3. Observed Incorrect Implementation Behavior . . . . . . . . . . 5
4. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
8.1. Normative References . . . . . . . . . . . . . . . . . . . 6
8.2. Informative References . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 7
Intellectual Property and Copyright Statements . . . . . . . . . . 8
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1. Introduction
IPv4 implementations associate a netmask when an IPv4 address is
assigned to an interface. That netmask together with the IPv4
address designates an on-link prefix. Addresses that match 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]. Further, note
that implementations of IPv4 point-to-point interfaces might not have
an associated IPv4 subnet prefix.
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.
In addition to the Prefix List, individual addresses are on-link if
they are the target of a Redirect Message indicating on-link, or the
source of a valid Neighbor Solicitation or Neighbor Advertisement
message. Note 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.
Host behavior is clarified in the Host Behavior Rules section.
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Finally, this document merely restates and clarifies [RFC4861].
2. Host Behavior 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 imply that any prefix is on-
link. A host is explicitly told that prefixes or addresses are
on-link through the means specified in [RFC4861]. Note that this
requirement for manually configured addresses is not explicitly
mentioned in [RFC4861].
3. On-link determination SHOULD NOT persist across IPv6 interface
initializations. Note that section 5.7 of [RFC4862] describes
the use of stable storage for addresses acquired with stateless
address autoconfiguration with a note that the Preferred and
Valid Lifetimes must be retained if this approach is used.
However no RFC suggests or recommends retaining the on-link
prefixes.
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.
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 analogous to the behavior
specified in the last paragraph of section 7.2.2 of
[RFC4861]: when address resolution fails, the host SHOULD
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send an ICMPv6 Destination Unreachable indication as
specified in [RFC4861]. The specified behavior MAY be
extended to cover this case where address resolution cannot
be performed.
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
As this document merely restates and clarifies [RFC4861], it does not
introduce any new security issues.
6. IANA Considerations
None.
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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.
[RFC4943] Roy, S., Durand, A., and J. Paugh, "IPv6 Neighbor
Discovery On-Link Assumption Considered Harmful",
RFC 4943, September 2007.
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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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