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IETF RFC 5220
Problem Statement for Default Address Selection in Multi-Prefix Environments: Operational Issues of RFC 3484 Default Rules
Last modified on Wednesday, July 9th, 2008
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Network Working Group A. Matsumoto
Request for Comments: 5220 T. Fujisaki
Category: Informational NTT
R. Hiromi
Intec Netcore
K. Kanayama
INTEC Systems
July 2008
Problem Statement for Default Address Selection in Multi-Prefix
Environments: Operational Issues of RFC 3484 Default Rules
 Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Abstract
A single physical link can have multiple prefixes assigned to it. In
that environment, end hosts might have multiple IP addresses and be
required to use them selectively. RFC 3484 defines default source
and destination address selection rules and is implemented in a
variety of OSs. But, it has been too difficult to use operationally
for several reasons. In some environments where multiple prefixes
are assigned on a single physical link, the host using the default
address selection rules will experience some trouble in
communication. This document describes the possible problems that
end hosts could encounter in an environment with multiple prefixes.
Matsumoto, et al. Informational PAGE 1 
RFC 5220 Address Selection PS July 2008
Table of Contents
1. Introduction ....................................................2
1.1. Scope of This Document .....................................3
2. Problem Statement ...............................................4
2.1. Source Address Selection ...................................4
2.1.1. Multiple Routers on a 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 ..................9
2.1.7. Temporary Address Selection .........................9
2.2. Destination Address Selection .............................10
2.2.1. IPv4 or IPv6 Prioritization ........................10
2.2.2. ULA and IPv4 Dual-Stack Environment ................11
2.2.3. ULA or Global Prioritization .......................12
3. Conclusion .....................................................13
4. Security Considerations ........................................14
5. Normative References ...........................................14
1. Introduction
In IPv6, a single physical link can have multiple prefixes assigned
to it. In such cases, an end host may have multiple IP addresses
assigned to an interface on that link. In the IPv4-IPv6 dual-stack
environment or in a site connected to both a Unique Local Address
(ULA) [RFC 4193] and globally routable networks, an end host typically
has multiple IP addresses. These are examples of the networks that
we focus on in this document. In such an environment, an end host
may encounter some communication troubles.
Inappropriate source address selection at the end host causes
unexpected asymmetric routing, filtering by a router, or discarding
of packets because there is no route to the host.
Considering a multi-prefix environment, destination address selection
is also important for correct or better communication establishment.
RFC 3484 [RFC 3484] defines default source and destination address
selection algorithms and is implemented in a variety of OSs. But, it
has been too difficult to use operationally for several reasons, such
as lack of an autoconfiguration method. There are some problematic
cases where the hosts using the default address selection rules
encounter communication troubles.
This document describes the possibilities of incorrect address
selection that lead to dropping packets and communication failure.
Matsumoto, et al. Informational PAGE 2
RFC 5220 Address Selection PS July 2008
1.1. Scope of This Document
As other mechanisms already exist, the multi-homing techniques for
achieving redundancy are basically out of our scope.
We focus on an end-site network environment and unmanaged hosts in
such an environment. This is because address selection behavior at
these kinds of hosts is difficult to manipulate, owing to the users'
lack of knowledge, hosts' location, or massiveness of the hosts.
The scope of this document is to sort out problematic cases related
to address selection. It includes problems that can be solved in the
framework of RFC 3484 and problems that cannot. For the latter, RFC
3484 might be modified to meet their needs, or another address
selection solution might be necessary. For the former, an additional
mechanism that mitigates the operational difficulty might be
necessary.
This document also includes simple solution analysis for each
problematic case. This analysis basically just focuses on whether or
not the case can be solved in the framework of RFC 3484. If not,
some possible solutions are described. Even if a case can be solved
in the framework of RFC 3484, as mentioned above, it does not
necessarily mean that there is no operational difficulty. For
example, in the environment stated above, it is not a feasible
solution to configure each host's policy table by hand. So, for such
a solution, the difficulty of configuration is yet another common
problem.
Matsumoto, et al. Informational PAGE 3
RFC 5220 Address Selection PS July 2008
2. Problem Statement
2.1. Source Address Selection
2.1.1. Multiple Routers on a Single Interface
==================
| Internet |
==================
| |
2001:db8:1000::/36 | | 2001:db8:8000::/36
+----+-+ +-+----+
| ISP1 | | ISP2 |
+----+-+ +-+----+
| |
2001:db8:1000:::/48 | | 2001:db8:8000::/48
+-----+---+ +----+----+
| Router1 | | Router2 |
+-------+-+ +-+-------+
| |
2001:db8:1000:1::/64 | | 2001:db8:8000:1::/64
| |
-----+-+-----+------
|
+-+----+ 2001:db8:1000:1::100
| Host | 2001:db8:8000:1::100
+------+
Figure 1
Generally speaking, there is no interaction between next-hop
determination and address selection. In this example, when a host
starts a new connection and sends a packet via Router1, the host does
not necessarily choose address 2001:db8:1000:1::100 given by Router1
as the source address. This causes the same problem as described in
the next section, "Ingress Filtering Problem".
Solution analysis:
As this case depends on next-hop selection, controlling the
address selection behavior at the Host alone doesn't solve the
entire problem. One possible solution for this case is adopting
source-address-based routing at Router1 and Router2. Another
solution may be using static routing at Router1, Router2, and the
Host, and using the corresponding static address selection policy
at the Host.
Matsumoto, et al. Informational PAGE 4
RFC 5220 Address Selection PS July 2008
2.1.2. Ingress Filtering Problem
==================
| Internet |
==================
| |
2001:db8:1000::/36 | | 2001:db8:8000::/36
+----+-+ +-+----+
| ISP1 | | ISP2 |
+----+-+ +-+----+
| |
2001:db8:1000:::/48 | | 2001:db8:8000::/48
++-------++
| Router |
+----+----+
| 2001:db8:1000:1::/64
| 2001:db8:8000:1::/64
------+---+----------
|
+-+----+ 2001:db8:1000:1::100
| Host | 2001:db8:8000:1::100
+------+
Figure 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
is becoming more popular among ISPs to mitigate the damage of
denial-of-service (DoS) attacks.
In this example, when the router chooses the default route to ISP2
and the host chooses 2001:db8:1000:1::100 as the source address for
packets sent to a host (2001:db8:2000::1) somewhere on the Internet,
the packets may be dropped at ISP2 because of ingress filtering.
Solution analysis:
One possible solution for this case is adopting source-address-
based routing at the Router. Another solution may be using static
routing at the Router, and using the corresponding static address
selection policy at the Host.
Matsumoto, et al. Informational PAGE 5
RFC 5220 Address Selection PS July 2008
2.1.3. Half-Closed Network Problem
You can see a second typical source address selection problem in a
multi-homed site with global half-closed connectivity, as shown in
the figure below. In this case, Host-A is in a multi-homed 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 | 2001:db8:a000::1
+-----+--+
|
============== +--------+
| Internet | | Host-B | 2001:db8:8000::1
============== +--------+
| |
2001:db8:1000:/36 | | 2001:db8:8000::/36
+----+-+ +-+---++
| ISP1 | | ISP2 | (Closed Network/VPN tunnel)
+----+-+ +-+----+
| |
2001:db8:1000:/48 | | 2001:db8:8000::/48
++-------++
| Router |
+----+----+
| 2001:db8:1000:1::/64
| 2001:db8:8000:1::/64
------+---+----------
|
+--+-----+ 2001:db8:1000:1::100
| Host-A | 2001:db8:8000:1::100
+--------+
Figure 3
You do not need two physical network connections here. The
connection from the Router 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 packet that has been sent will be the one delegated from
ISP2 (that is, 2001:db8:8000:1::100) because of rule 8 (longest
matching prefix) in RFC 3484.
Host-C is located somewhere on the Internet and has IPv6 address
2001:db8:a000::1. When Host-A sends a packet to Host-C, the longest
matching algorithm chooses 2001:db8:8000:1::100 for the source
Matsumoto, et al. Informational PAGE 6
RFC 5220 Address Selection PS July 2008
address. In this case, the packet goes through ISP1 and may be
filtered by ISP1's ingress filter. Even if the packet is not
filtered by ISP1, a return packet from Host-C cannot possibly be
delivered to Host-A because the return packet is destined for 2001:
db8:8000:1::100, which is closed from the Internet.
The important point is that each host chooses a correct source
address for a given destination address. To solve this kind of
network-policy-based address selection problem, it is likely that
delivering additional information to a node provides a better
solution than using algorithms that are local to the node.
Solution analysis:
This problem can be solved in the RFC 3484 framework. For
example, configuring some address selection policies into Host-A's
RFC 3484 policy table can solve this problem.
2.1.4. Combined Use of Global and ULA
============
| Internet |
============
|
|
+----+----+
| ISP |
+----+----+
|
2001:db8:a::/48 |
+----+----+
| Router |
+-+-----+-+
| | 2001:db8:a:100::/64
fd01:2:3:200:/64 | | fd01:2:3:100:/64
-----+--+- -+--+----
| |
fd01:2:3:200::101 | | 2001:db8:a:100::100
+----+----+ +-+----+ fd01:2:3:100::100
| Printer | | Host |
+---------+ +------+
Figure 4
As RFC 4864 [RFC 4864] describes, using a ULA may be beneficial in
some scenarios. If the ULA is used for internal communication,
packets with the ULA need to be filtered at the Router.
Matsumoto, et al. Informational PAGE 7
RFC 5220 Address Selection PS July 2008
This case does not presently create an address selection problem
because of the dissimilarity between the ULA and the global unicast
address. The longest matching rule of RFC 3484 chooses the correct
address for both intra-site and extra-site communication.
In the future, however, there is a possibility that the longest
matching rule will not be able to choose the correct address anymore.
That is the moment when the assignment of those global unicast
addresses starts, where the first bit is 1. In RFC 4291 [RFC 4291],
almost all address spaces of IPv6, including those whose first bit is
1, are assigned as global unicast addresses.
Namely, when we start to assign a part of the address block 8000::/1
as the global unicast address and that part is used somewhere in the
Internet, the longest matching rule ceases to function properly for
the people trying to connect to the servers with those addresses.
For example, when the destination host has an IPv6 address 8000::1,
and the originating host has 2001:db8:a:100::100 and
fd01:2:3:100::100, the source address will be fd01:2:3:100::100,
because the longest matching bit length is 0 for 2001:db8:a:100::100
and 1 for fd01:2:3:100::100, respectively.
Solution analysis:
This problem can be solved in the RFC 3484 framework. For
example, configuring some address selection policies into the
Host's RFC 3484 policy table can solve this problem. Another
solution is to modify RFC 3484 and define ULA's scope smaller than
the global scope.
2.1.5. Site Renumbering
RFC 4192 [RFC 4192] describes a recommended procedure for renumbering
a network from one prefix to another. An autoconfigured address has
a lifetime, so by stopping advertisement of the old prefix, the
autoconfigured address is eventually invalidated.
However, invalidating the old prefix takes a long time. You cannot
stop routing to the old prefix as long as the old prefix is not
removed from the host. This can be a tough issue for ISP network
administrators.
There is a technique of advertising the prefix with the preferred
lifetime zero; however, RFC 4862 [RFC 4862], Section 5.5.4, does not
absolutely prohibit the use of a deprecated address for a new
outgoing connection due to limitations on the capabilities of
applications.
Matsumoto, et al. Informational PAGE 8
RFC 5220 Address Selection PS July 2008
+-----+---+
| Router |
+----+----+
| 2001:db8:b::/64 (new)
| 2001:db8:a::/64 (old)
------+---+----------
|
+--+---+ 2001:db8:b::100 (new)
| Host | 2001:db8:a::100 (old)
+------+
Figure 5
Solution analysis:
This problem can be mitigated in the RFC 3484 framework. For
example, configuring some address selection policies into the
Host's RFC 3484 policy table can solve this problem.
2.1.6. Multicast Source Address Selection
This case is an example of site-local or global unicast
prioritization. When you send a multicast packet across site
borders, the source address of the multicast packet should be a
globally routable address. The longest matching algorithm, however,
selects a ULA if the sending host has both a ULA and a Global Unicast
Address.
Solution analysis:
This problem can be solved in the RFC 3484 framework. For
example, configuring some address selection policies into the
sending host's RFC 3484 policy table can solve this problem.
2.1.7. Temporary Address Selection
RFC 3041 [RFC 3041] defines a Temporary Address. The usage of a
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
purposes.
If you could turn the temporary address on and off, that would be
better. If you could switch its usage per service (destination
address), that would also be better. The same situation can be found
when using an HA (home address) and a CoA (care-of address) in a
Mobile IPv6 [RFC 3775] network.
Matsumoto, et al. Informational PAGE 9
RFC 5220 Address Selection PS July 2008
Section 6 ("Future Work") of RFC 3041 discusses that an API extension
might be necessary to achieve a better address selection mechanism
with finer granularity.
Solution analysis:
This problem cannot be solved in the RFC 3484 framework. A
possible solution is to make applications to select desirable
addresses by using the IPv6 Socket API for Source Address
Selection defined in RFC 5014 [RFC 5014].
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 so that it is the other way around.
Matsumoto, et al. Informational PAGE 10
RFC 5220 Address Selection PS July 2008
+---------+
| Tunnel |
| Service |
+--+---++-+
| ||
| ||
===========||==
| Internet || |
===========||==
| ||
192.0.2.0/24 | ||
+----+-+ ||
| ISP | ||
+----+-+ ||
| ||
IPv4 (Native) | || IPv6 (Tunnel)
192.0.2.0/26 | ||
++-----++-+
| Router |
+----+----+
| 2001:db8:a:1::/64
| 192.0.2.0/28
|
------+---+----------
|
+-+----+ 2001:db8:a:1::100
| Host | 192.0.2.2
+------+
Figure 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 for using IPv6 because the quality of
the tunnel network seems to be worse than that of the native
transport.
Solution analysis:
This problem can be solved in the RFC 3484 framework. For
example, configuring some address selection policies into the
Host's RFC 3484 policy table can solve this problem.
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, a ULA is the best choice for site-local IPv6
Matsumoto, et al. Informational PAGE 11
RFC 5220 Address Selection PS July 2008
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-B that has registered both A and AAAA records in the DNS, the
host will choose AAAA as the destination address and the ULA for the
source address. This will clearly result in a connection failure.
+--------+
| Host-B | AAAA = 2001:db8::80
+-----+--+ A = 192.0.2.1
|
============
| Internet |
============
| no IPv6 connectivity
+----+----+
| Router |
+----+----+
|
| fd01:2:3::/48 (ULA)
| 192.0.2.128/25
++--------+
| Router |
+----+----+
| fd01:2:3:4::/64 (ULA)
| 192.0.2.240/28
------+---+----------
|
+-+------+ fd01:2:3:4::100 (ULA)
| Host-A | 192.0.2.245
+--------+
Figure 7
Solution analysis:
This problem can be solved in the RFC 3484 framework. For
example, configuring some address selection policies into Host-A's
RFC 3484 policy table can solve this problem.
2.2.3. ULA or Global Prioritization
Differentiating services by the client's source address is very
common. IP-address-based authentication is a typical 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.
Matsumoto, et al. Informational PAGE 12
RFC 5220 Address Selection PS July 2008
However, a ULA and an IPv6 global address both have global scope, and
RFC 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-B |
+-+--|---+
| |
===========|==
| Internet | |
===========|==
| |
| |
+----+-+ +-->+------+
| ISP +------+ DNS | 2001:db8:a::80
+----+-+ +-->+------+ fc12:3456:789a::80
| |
2001:db8:a::/48 | |
fc12:3456:789a::/48 | |
+----+----|+
| Router ||
+---+-----|+
| | 2001:db8:a:100::/64
| | fc12:3456:789a:100::/64
--+-+---|-----
| |
+-+---|--+ 2001:db8:a:100::100
| Host-A | fc12:3456:789a:100::100
+--------+
Figure 8
Solution analysis:
This problem can be solved in the RFC 3484 framework. For
example, configuring some address selection policies into Host-A's
RFC 3484 policy table can solve this problem.
3. 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; this
choice cannot be achieved only by routers.
Matsumoto, et al. Informational PAGE 13
RFC 5220 Address Selection PS July 2008
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.
4. Security Considerations
When an intermediate router performs policy routing (e.g., source-
address-based routing), inappropriate address selection causes
unexpected routing. For example, in the network described in Section
2.1.3, when Host-A uses a default address selection policy and
chooses an inappropriate address, a packet sent to a VPN can be
delivered to a location via the Internet. This issue can lead to
packet eavesdropping or session hijack. However, sending the packet
back to the correct path from the attacker to the node is not easy,
so these two risks are not serious.
As documented in the Security Considerations section of RFC 3484,
address selection algorithms expose a potential privacy concern.
When a malicious host can make a target host perform address
selection (for example, by sending an anycast or multicast packet),
the malicious host can get knowledge of multiple addresses attached
to the target host. In a case like Section 2.1.4, if an attacker can
make the Host to send a multicast packet and the Host performs the
default address selection algorithm, the attacker may be able to
determine the ULAs attached to the host.
These security risks have roots in inappropriate address selection.
Therefore, if a countermeasure is taken, and hosts always select an
appropriate address that is suitable to a site's network structure
and routing, these risks can be avoided.
5. Normative References
[RFC 3041] Narten, T. and R. Draves, "Privacy Extensions for
Stateless Address Autoconfiguration in IPv6", RFC 3041,
January 2001.
[RFC 3484] Draves, R., "Default Address Selection for Internet
Protocol version 6 (IPv6)", RFC 3484, February 2003.
[RFC 3775] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support
in IPv6", RFC 3775, June 2004.
[RFC 4192] Baker, F., Lear, E., and R. Droms, "Procedures for
Renumbering an IPv6 Network without a Flag Day", RFC 4192,
September 2005.
Matsumoto, et al. Informational PAGE 14
RFC 5220 Address Selection PS July 2008
[RFC 4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, October 2005.
[RFC 4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC 4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862, September 2007.
[RFC 4864] Van de Velde, G., Hain, T., Droms, R., Carpenter, B., and
E. Klein, "Local Network Protection for IPv6", RFC 4864,
May 2007.
[RFC 5014] Nordmark, E., Chakrabarti, S., and J. Laganier, "IPv6
Socket API for Source Address Selection", RFC 5014,
September 2007.
Matsumoto, et al. Informational PAGE 15
RFC 5220 Address Selection PS July 2008
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@nttv6.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 Systems Institute, Inc.
Shimoshin-machi 5-33
Toyama-shi, Toyama 930-0804
Japan
Phone: +81 76 444 8088
EMail: kanayama_kenichi@intec-si.co.jp
Matsumoto, et al. Informational PAGE 16
RFC 5220 Address Selection PS July 2008
Full Copyright Statement
Copyright © The IETF Trust (2008).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
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OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
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Matsumoto, et al. Informational PAGE 17
Problem Statement for Default Address Selection in Multi-Prefix Environments: Operational Issues of RFC 3484 Default Rules
RFC TOTAL SIZE: 33661 bytes
PUBLICATION DATE: Wednesday, July 9th, 2008
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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