Network Working Group H. Singh
Internet-Draft W. Beebee
Intended status: Standards Track Cisco Systems, Inc.
Expires: August 28, 2008 E. Nordmark
Sun Microsystems
February 25, 2008
IPv6 Subnet Model
draft-wbeebee-on-link-and-off-link-determination-02
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Copyright Notice
Copyright (C) The IETF Trust (2008).
Abstract
IPv6 specifies a model of a subnet that is different than the IPv4
subnet model. The subtlety of the differences has turned out to
cause interoperability problems. This note spells out the most
important difference; that an IPv6 address isn't automatically
associated with an IPv6 on-link subnet prefix.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Host Behavior Rules . . . . . . . . . . . . . . . . . . . . . . 4
3. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
6.1. Normative References . . . . . . . . . . . . . . . . . . . 5
6.2. Informative References . . . . . . . . . . . . . . . . . . 5
Appendix A. CHANGE HISTORY . . . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 7
Intellectual Property and Copyright Statements . . . . . . . . . . 9
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1. Introduction
In many, if not all, IPv4 implementations when an IPv4 address is
assigned to an interface there is always a netmask associated with
the address. That netmask together with the IPv4 address designates
an on-link prefix. Addresses that match this prefix are viewed as
local i.e., traffic to such addresses is not sent to a router. See
section 3.3.1 in [RFC1122].
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
corresponding on-link subnet prefixes, and conversely, can have on-
link subnet prefixes that are not related to any of the IPv6
addresses that are assigned to the hosts.
In IPv6, by default, a host treats only the link-local subnet as on-
link.
The reception of a Prefix Information Option (PIO) with the L-bit set
and a non-zero valid lifetime creates (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 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 such packets MUST be dropped.
(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 this can
result in lack of IPv6 connectivity. One example, included in
draft-wbeebee-nd-implementation-problems-00
[I-D.wbeebee-nd-implementation-problems], follows: a host receives a
Router Advertisement Message with no on-link prefix advertised. The
host incorrectly decides to perform address resolution when the host
should send all traffic to a default router. Neither the router nor
any other host may respond to the address resolution, preventing this
host from sending IPv6 traffic.
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Correct implementation of the on-link determination is critically
important in some deployments. For instance, with certain layer 2
technologies it is not possible or very inefficient for hosts to
perform address resolution. It is much more efficient for the hosts
to send all packets for non-link-local addresses to one of the
default routers, and have the default routers forward those packets.
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 does 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].
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
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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
send an ICMPv6 Destination Unreachable message. 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. Security Considerations
As this document merely restates and clarifies RFC 4861, it does not
introduce any new security issues.
4. IANA Considerations
None.
5. Acknowledgements
Thanks (in alphabetical order) to Adeel Ahmed, Jari Arkko, Ralph
Droms, Alun Evans, Dave Forster, Prashanth Krishnamurthy, Suresh
Krishnan, Josh Littlefield, Madhu Sudan, Jinmei Tatuya, Bernie Volz,
and Vlad Yasevich for their consistent input, ideas and review during
the production of this document.
6. References
6.1. Normative References
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
6.2. Informative References
[I-D.wbeebee-nd-implementation-problems]
Singh, H. and W. Beebee, "Known ND Implementation
Problems", draft-wbeebee-nd-implementation-problems-00
(work in progress), September 2007.
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[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.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862, September 2007.
[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 since draft-wbeebee-on-and-off-link-determination-00.txt are:
o Made global changes in document to replace RFC 2461 and RFC 2462
with RFC 4861 and RFC 4862 respectively. Removed text related to
2461bis-11 and 2462bis-08.
o Inserted new bullet item to section 2 that explains off-link and
on-link default behavior.
o On-link behavior has been replaced with on-link determination.
o At the end of sections 2.1 and 2.2.1, the last paragraph related
to Redirects has been reworded to place more details in the
Redirect section.
o Section 2.2 has all text removed and then new text has been added.
o The Redirect Clarifications section has been rewritten to explain
an extra case when the Redirect does not include the Target Link-
Layer Address Option. This section has been revised to restrict
the scope of the Redirects sent from aggregation routers mentioned
to those with on-link destinations.
o Jinmei Tatuya has been added to the list of people in the
Acknowledged section for his valuable feedback on the -00 draft.
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o Two bis draft references in the References section have been
removed.
Changes since draft-wbeebee-on-and-off-link-determination-01.txt are:
o Added a new author in Erik Nordmark to the draft.
o Changed title of draft from "ND On-link and Off-link
Determination" to "IPv6 Subnet Model". Also changed Abstract and
Introduction sections to reflect new title.
o Changed text of the example in Introduction section from "follows:
a host receives an RA with no prefix advertised and incorrectly
decides to perform address resolution when the host should have
sent all traffic to the default router. The router does not
respond to the address resolution and the layer 2 driver of the
host stops transmitting IPv6 packets." to "follows: a host
receives a Router Advertisement Message with no on-link prefix
advertised. The host incorrectly decides to perform address
resolution when the host should send all traffic to a default
router. Neither the router nor any other host may respond to the
address resolution, preventing this host from sending IPv6
traffic."
o Removed sections 2.1-2.3 - folded information from these sections
into Introduction section and bullets of section 2.
o Removed sections 3 and 4 - folded subnet model and on-link
determination related information from these sections into
Introduction section and bullets of section 2.
o Made changes to References sections. Removed RFC2472 from
Normative References. Moved RFC4861 from Informative to Normative
References. Added RFC1122 and RFC3315 to Informative References.
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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