Network Working Group A. Matsumoto
Internet-Draft T. Fujisaki
Intended status: Standards Track NTT
Expires: February 16, 2007 R. Hiromi
K. Kanayama
Intec Netcore
August 15, 2006
Problem Statement of Default Address Selection in Multi-prefix
Environment: Operational Issues of RFC3484 Default Rules
draft-arifumi-v6ops-addr-select-ps-01.txt
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
One physical network can carry multiple logical networks. Moreover,
we can use multiple physical networks at the same time in a host. In
that environment, end-hosts might have multiple IP addresses and be
required to use them selectively. Without an appropriate source/
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destination address selection mechanism, the host will experience
some trouble in the communication. RFC 3484 defines both the source
and destination address selection algorithms, but the multi-prefix
environment considered here needs additional rules beyond the default
operation. This document describes the possible problems that end-
hosts could encounter in an environment with multiple logical
networks.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Scope of this document . . . . . . . . . . . . . . . . . . 3
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Source Address Selection . . . . . . . . . . . . . . . . . 3
2.1.1. Multiple Routers on Single Interface . . . . . . . . . 4
2.1.2. Ingress Filtering Problem . . . . . . . . . . . . . . 5
2.1.3. Half-Closed Network Problem . . . . . . . . . . . . . 6
2.1.4. Combined Use of Global and ULA . . . . . . . . . . . . 7
2.1.5. Site Renumbering . . . . . . . . . . . . . . . . . . . 8
2.1.6. Multicast Source Address Selection . . . . . . . . . . 8
2.1.7. Temporary Address Selection . . . . . . . . . . . . . 8
2.2. Destination Address Selection . . . . . . . . . . . . . . 9
2.2.1. IPv4 or IPv6 prioritization . . . . . . . . . . . . . 9
2.2.2. ULA and IPv4 dual-stack environment . . . . . . . . . 10
2.2.3. ULA or Global Prioritization . . . . . . . . . . . . . 10
3. Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.1. More Specific Routes (RFC 4191) . . . . . . . . . . . . . 11
3.2. Policy Table Manipulation . . . . . . . . . . . . . . . . 11
3.3. Revising RFC 3484 . . . . . . . . . . . . . . . . . . . . 12
4. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 12
5. Security Considerations . . . . . . . . . . . . . . . . . . . 12
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7.1. Normative References . . . . . . . . . . . . . . . . . . . 12
7.2. Informative References . . . . . . . . . . . . . . . . . . 13
Appendix A. Appendix. Revision History . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . . . . 15
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1. Introduction
One physical network can carry multiple logical networks. In that
case, an end-host has multiple IP addresses. In the IPv4-IPv6 dual
stack environment or in a site connected to both ULA [RFC4193] and
Global scope networks, an end-host has multiple IP addresses. These
are examples of the networks that we focus on in this document. In
such an environment, an end-host will encounter some communication
trouble.
Inappropriate source address selection at the end-host causes
unexpected asymmetric routing or filtering by a router on the way
back or discard due to there being no route to the host.
Considering a multi-prefix environment, the destination address
selection is also important for correct communication establishment.
The key to the appropriate process will come from the way to
configure the source address and destination address to the
interfaces at the end-hosts by the network policy of the site.
RFC 3484 [RFC3484] defines both source and destination address
selection algorithms. In most cases, the host will be able to
communicate with the targeted host using the algorithms. But there
are still problematic cases such as when multiple default routes are
supplied. This document describes such possibilities of false
dropping during address selection.
In addition, the provision of an address policy table is an important
matter. RFC 3484 describes all the algorithms for setting the
address policy table but it makes no mention of the provisions of
address policy and does not define how to set it except manually.
1.1. Scope of this document
There has been a lot of discussion about "multiple addresses/
prefixes" but the multi-homing issues for redundancy are out of our
scope. Cooperation with a mechanism like shim6 is rather desirable.
We focus on an end-site network environment. The scope of this
document is to sort out problematic cases of false dropping of the
address selection within a multi-prefix environment.
2. Problem Statement
2.1. Source Address Selection
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2.1.1. Multiple Routers on Single Interface
==================
| Internet |
==================
| |
2001:db8::/32 | | 3ffe:1800::/32
+----+-+ +-+----+
| ISP1 | | ISP2 |
+----+-+ +-+----+
| |
2001:db8:a::/48 | | 3ffe:1800:a::/48
+------+---+ +----+-----+
| Gateway1 | | Gateway2 |
+--------+-+ +-+--------+
| |
2001:db8:a:1::/64 | | 3ffe:1800:a:1::/64
| |
-----+-+-----+------
|
+-+----+ 2001:db8:a:1:EUI64
| Host | 3ffe:1800:a:1:EUI64
+------+
[Fig. 1]
Generally speaking, there is no interaction between next-hop
determination and address selection. In this example, when Host
sends a packet via Gateway1, the Host does not necessarily choose the
address 2001:db8:a:1::EUI64 given by Gateway1 as the source address.
This causes the same problem as described in the next section
'Ingress Filtering Problem'.
To solve this case, one approach is to configure correctly both the
routing configuration and address selection policy at Host. You can
use RFC 4191 [RFC4191] to deliver routing information to hosts.
Another approach is to configure the gateways to make use of packet
redirection between the gateways.
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2.1.2. Ingress Filtering Problem
==================
| Internet |
==================
| |
2001:db8::/32 | | 3ffe:1800::/32
+----+-+ +-+----+
| ISP1 | | ISP2 |
+----+-+ +-+----+
| |
2001:db8:a::/48 | | 3ffe:1800:a::/48
++-------++
| Gateway |
+----+----+
| 2001:db8:a:1::/64
| 3ffe:1800:a:1::/64
------+---+----------
|
+-+----+ 2001:db8:a:1:EUI64
| Host | 3ffe:1800:a:1:EUI64
+------+
[Fig. 2]
When a relatively small site, which we call a "customer network", is
attached to two upstream ISPs, each ISP delegates a network address
block, which is usually /48, and a host has multiple IPv6 addresses.
When the source address of an outgoing packet is not the one that is
delegated by an upstream ISP, there is a possibility that the packet
will be dropped at the ISP by its Ingress Filter. Ingress
filtering(uRPF: unicast Reverse Path Forwarding) is becoming more and
more popular among ISPs in order to mitigate the damage of DoS
attacks.
In this example, when the Gateway chooses the default route to ISP2
and the Host chooses 2001:db8:a:1::EUI64 as the source address for
packets sent to a host(2001:fa8::1) somewhere in the Internet, the
packets may be dropped at ISP2 because of Ingress Filtering.
One possible solution for this problem is to adopt source-address-
based routing at the customer site's gateway, but this manner of
routing is not very popular at the moment.
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2.1.3. Half-Closed Network Problem
You can see a second typical source address selection problem in a
multihome site with global-closed mixed connectivity like the figure
below. In this case, Host-A is in a multihomed network and has two
IPv6 addresses, one delegated from each of the upstream ISPs. Note
that ISP2 is a closed network and does not have connectivity to the
Internet.
+--------+
| Host-C | 3ffe:503:c:1:EUI64
+-----+--+
|
============== +--------+
| Internet | | Host-B | 3ffe:1800::EUI64
============== +--------+
| |
2001:db8::/32 | | 3ffe:1800::/32
+----+-+ +-+---++
| ISP1 | | ISP2 | (Closed Network/VPN tunnel)
+----+-+ +-+----+
| |
2001:db8:a::/48 | | 3ffe:1800:a::/48
++-------++
| Gateway |
+----+----+
| 2001:db8:a:1::/64
| 3ffe:1800:a:1::/64
------+---+----------
|
+--+-----+ 2001:db8:a:1:EUI64
| Host-A | 3ffe:1800:a:1:EUI64
+--------+
[Fig. 3]
You don't need two physical network connection here. The connection
from Gateway to ISP2 can be a logical link over ISP1 and the
Internet.
When Host-A starts the connection to Host-B in ISP2, the source
address of a sending packet will be the one delegated from ISP2, that
is 3ffe:1800:a:1:EUI64, because of rule 8 (longest matching prefix)
in RFC 3484.
Host-C is located somewhere in the Internet and has an IPv6 address
3ffe:503:c:1:EUI64. When Host-A sends a packet to Host-C, the
longest matching algorithm chooses 3ffe:1800:a:1:EUI64 for the source
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address. In this case, the packet goes through ISP1 and may be
filtered by ISP1's ingress filter. Even if the packet is fortunately
not filtered by ISP1, a return packet from Host-C cannot possibly be
delivered to Host-A because the return packet is destined for 3ffe:
1800:a:1:EUI64, which is closed from the Internet.
In this case, source-address-based routing alone described in the
previous section does not solve the problem. What is important is
that each host chooses a correct source address for a given
destination address as far as NAT does not exist in the IPv6 world.
2.1.4. Combined Use of Global and ULA
============
| Internet |
============
|
|
+----+----+
| ISP |
+----+----+
|
2001:db8:a::/48 |
+----+----+
| Gateway |
+-+-----+-+
| | 2001:db8:a:100::/64
fd01:2:3:200:/64 | | fd01:2:3:100:/64
-----+--+- -+--+----
| |
fd01:2:3:200:EUI64 | | 2001:db8:a:100:EUI64
+----+----+ +-+----+ fd01:2:3:100:EUI64
| Printer | | Host |
+---------+ +------+
[Fig. 4]
As NAP [I-D.ietf-v6ops-nap] describes, using ULA may be beneficial in
some scenarios. If ULA is used for internal communication, packets
with ULA addresses need to be filtered at Gateway.
There is no serious problem related to address selection in this
case, thanks to the unlikeness of ULA and Global Unicast Address for
now. RFC 3484's longest matching rule chooses the correct address
for both intra-site and extra-site communication.
In a few years, however, the longest matching rule will not be able
to choose the correct address anymore: the moment the assignment of
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those Global Unicast Addresses whose beginning bit is 1 starts. In
RFC 4291 [RFC4291], almost all the space of IPv6, including those
with beginning bit 1, is assigned as Global Unicast Addresses.
2.1.5. Site Renumbering
RFC 4192 [RFC4192] describes a recommended procedure for renumbering
a network from one prefix to another. An auto-configured address has
a lifetime, so by stopping advertisement of the old prefix it is
eventually invalidated.
However, it takes a long time to invalidate the old prefix. You
cannot stop routing to the old prefix as long as the old prefix is
not deprecated. This issue can be a tough issue for ISP network
administrator.
+-----+---+
| Gateway |
+----+----+
| 2001:db8:b::/64 (new)
| 2001:db8:a::/64 (old)
------+---+----------
|
+--+-----+ 2001:db8:b::EUI64 (new)
| Host-A | 2001:db8:a::EUI64 (old)
+--------+
[Fig. 5]
2.1.6. Multicast Source Address Selection
This case is an example of Site-local or Global prioritization. When
you send a multicast packet across site-borders, the source address
of the multicast packet must be a global scope address. The longest
matching algorithm, however, selects a ULA address if the sending
host has both a ULA and a global address.
2.1.7. Temporary Address Selection
RFC 3041 [RFC3041] defines a Temporary Address. The usage of
Temporary Address has both pros and cons. It is good for viewing
web-pages or communicating with the general public, but it is bad for
a service that uses address-based authentication and for logging
purpose.
It would be better if you could turn the temporary address on and
off. It would also be better if you could switch its usage per
service(destination address). The same situation can be found when
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using HA and CoA in MobileIP network.
2.2. Destination Address Selection
2.2.1. IPv4 or IPv6 prioritization
The default policy table gives IPv6 addresses higher precedence than
IPv4 addresses. There seem to be many cases, however, where network
administrators want to control the address selection policy of end-
hosts the other way around.
+---------+
| Tunnel |
| Service |
+--+---++-+
| ||
| ||
===========||==
| Internet || |
===========||==
| ||
192.0.2.0/24 | ||
+----+-+ ||
| ISP | ||
+----+-+ ||
| ||
IPv4 (Native) | || IPv6 (Tunnel)
192.0.2.0/26 | ||
++-----++-+
| Gateway |
+----+----+
| 2001:db8:a:1::/64
| 192.0.2.0/28
|
------+---+----------
|
+-+----+ 2001:db8:a:1:EUI64
| Host | 192.0.2.2
+------+
[Fig. 6]
In the figure above, a site has native IPv4 and tunneled IPv6
connectivity. Therefore, the administrator may want to set a higher
priority for using IPv4 than using IPv6 because the quality of the
tunnel network seems to be worse than that of the native transport.
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2.2.2. ULA and IPv4 dual-stack environment
This is a special form of IPv4 and IPv6 prioritization. When an
enterprise has IPv4 Internet connectivity but does not yet have IPv6
Internet connectivity, and the enterprise wants to provide site-local
IPv6 connectivity, ULA is the best choice for site-local IPv6
connectivity. Each employee host will have both an IPv4 global or
private address and a ULA. Here, when this host tries to connect to
Host-C that has registered both A and AAAA records in the DNS, the
host will choose AAAA as the destination address and ULA for the
source address. This will clearly result in a connection failure.
+--------+
| Host-C | AAAA = 2001:db8::80
+-----+--+ A = 192.47.163.1
|
============
| Internet |
============
| no IPv6 connectivity
+----+----+
| Gateway |
+----+----+
|
| fd01:2:3::/48 (ULA)
| 192.0.2.0/24
++--------+
| Router |
+----+----+
| fd01:2:3:4::/64 (ULA)
| 192.0.2.240/28
------+---+----------
|
+-+----+ fd01:2:3:4::100 (ULA)
| Host | 192.0.2.245
+------+
[Fig. 7]
2.2.3. ULA or Global Prioritization
It is very common to differentiate services by the client's source
address. IP-address-based authentication is an extreme example of
this. Another typical example is a web service that has pages for
the public and internal pages for employees or involved parties. Yet
another example is DNS zone splitting.
However, ULA and IPv6 global address both have global scope, and RFC
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3484 default rules do not specify which address should be given
priority. This point makes IPv6 implementation of address-based
service differentiation a bit harder.
+------+
| Host |
+-+--|-+
| |
===========|==
| Internet | |
===========|==
| |
| |
+----+-+ +-->+------+
| ISP +------+ DNS | 2001:db8:a::80
+----+-+ +-->+------+ fc12:3456:789a::80
| |
2001:db8:a::/48 | |
fc12:3456:789a::/48 | |
+----+----|+
| Gateway ||
+---+-----|+
| | 2001:db8:a:100::/64
| | fc12:3456:789a:100:/64
--+-+---|-----
| |
+-+---|+ 2001:db8:a:100:EUI64
| Host | fc12:3456:789a:100:EUI64
+------+
[Fig. 7]
3. Solutions
3.1. More Specific Routes (RFC 4191)
This method enables network administrator to distribute routing
information to end-hosts. It can solve only two problems in this
document, that is 2.1.1, 2.2.2. Routing information doesn't
determine the source address when multiple addresses are attached to
the outgoing network interface. So, it cannot be used for every
cases here.
3.2. Policy Table Manipulation
Almost all the problem cases raised in this document can be solved by
configuring the policy table at end-hosts. The problem for a site-
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administrator is that he does not have the means to deliver policies
to end-hosts. Therefore, we proposed a method for policy
distribution in the form of DHCPv6 option
[I-D.fujisaki-dhc-addr-select-opt]. The usage of this mechanisim is
illustrated in another I-D [I-D.arifumi-ipv6-policy-dist].
3.3. Revising RFC 3484
Revising address selection rules defined in RFC 3484 in another idea.
These problems are, however, too network-environment-specific, so
it's not easy to have all-purpose rule set.
4. Conclusion
We have covered problems related to destination or source address
selection. These problems have their roots in the situation where
end-hosts have multiple IP addresses. In this situation, every end-
host must choose an appropriate destination and source address, which
cannot be achieved only by routers.
It should be noted that end-hosts must be informed about routing
policies of their upstream networks for appropriate address
selection. A site administrator must consider every possible address
false-selection problem and take countermeasures beforehand.
5. Security Considerations
Address false-selection can lead to serious security problem, such as
session hijack. However, it should be noted that address selection
is eventually up to end-hosts. We have no means to enforce one
specific address selection policy to every end-host. So, a network
administrator has to take countermeasures for unexpected address
selection.
6. IANA Considerations
This document has no actions for IANA.
7. References
7.1. Normative References
[RFC3484] Draves, R., "Default Address Selection for Internet
Protocol version 6 (IPv6)", RFC 3484, February 2003.
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[RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, October 2005.
7.2. Informative References
[I-D.arifumi-ipv6-policy-dist]
Matsumoto, A., "Practical Usages of Address Selection
Policy Distribution", draft-arifumi-ipv6-policy-dist-01
(work in progress), June 2006.
[I-D.fujisaki-dhc-addr-select-opt]
Fujisaki, T., "Distributing Default Address Selection
Policy using DHCPv6",
draft-fujisaki-dhc-addr-select-opt-02 (work in progress),
June 2006.
[I-D.ietf-v6ops-nap]
Velde, G., "IPv6 Network Architecture Protection",
draft-ietf-v6ops-nap-03 (work in progress), July 2006.
[RFC3041] Narten, T. and R. Draves, "Privacy Extensions for
Stateless Address Autoconfiguration in IPv6", RFC 3041,
January 2001.
[RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and
More-Specific Routes", RFC 4191, November 2005.
[RFC4192] Baker, F., Lear, E., and R. Droms, "Procedures for
Renumbering an IPv6 Network without a Flag Day", RFC 4192,
September 2005.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
Appendix A. Appendix. Revision History
01:
Authors' addresses corrected.
Solutions section added.
Security Considerations section fully rewritten.
Some editorial changes.
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Authors' Addresses
Arifumi Matsumoto
NTT PF Lab
Midori-Cho 3-9-11
Musashino-shi, Tokyo 180-8585
Japan
Phone: +81 422 59 3334
Email: arifumi@nttv6.net
Tomohiro Fujisaki
NTT PF Lab
Midori-Cho 3-9-11
Musashino-shi, Tokyo 180-8585
Japan
Phone: +81 422 59 7351
Email: fujisaki@syce.net
Ruri Hiromi
Intec Netcore, Inc.
Shinsuna 1-3-3
Koto-ku, Tokyo 136-0075
Japan
Phone: +81 3 5665 5069
Email: hiromi@inetcore.com
Ken-ichi Kanayama
Intec Netcore, Inc.
Shinsuna 1-3-3
Koto-ku, Tokyo 136-0075
Japan
Phone: +81 3 5665 5069
Email: kanayama@inetcore.com
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