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Versions: (draft-arifumi-v6ops-addr-select-ps)
00 01 02 03 04 05 06 07 08 09 RFC 5220
IPv6 Operations Working Group A. Matsumoto
Internet-Draft T. Fujisaki
Intended status: Informational NTT
Expires: December 19, 2008 R. Hiromi
Intec Netcore
K. Kanayama
INTEC Systems
June 17, 2008
Problem Statement of Default Address Selection in Multi-prefix
Environment: Operational Issues of RFC3484 Default Rules
draft-ietf-v6ops-addr-select-ps-09.txt
Status of this Memo
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This Internet-Draft will expire on December 19, 2008.
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 OS's. But, it has been too difficult to use operationally
for several reasons. In some environment where multiple prefixes are
assigned on a single physical link, the host using the default
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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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Scope of this document . . . . . . . . . . . . . . . . . . 3
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Source Address Selection . . . . . . . . . . . . . . . . . 4
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 . . . . . . . . . . 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. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
6.1. Normative References . . . . . . . . . . . . . . . . . . . 14
6.2. Informative References . . . . . . . . . . . . . . . . . . 15
Appendix A. Appendix. Revision History . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16
Intellectual Property and Copyright Statements . . . . . . . . . . 17
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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 ULA [RFC4193] 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 [RFC3484] defines default source and destination address
selection algorithms and is implemented in a variety of OS's. But,
it has been too difficult to use operationally for several reasons,
such as lack of autoconfiguration method. There are some problematic
cases where the hosts using the default address selection rules
encounter communication troubles.
This document describes such possibilities of incorrect address
selection which leads to dropping packets and communication failure.
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
this kind 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
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the case can be solved in the framework of RFC 3484 or not. 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
an solution, configuration pain is yet another common problem.
2. Problem Statement
2.1. Source Address Selection
2.1.1. Multiple Routers on 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
+------+
[Fig. 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:
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As this case depends on next hop selection, controling the address
selection behavior at 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 Host, and using the
corresponding static address selection policy at Host.
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
+------+
[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
is becoming more popular among ISPs to mitigate the damage of 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:
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One possible solution for this case is adopting source address
based routing at Router. Another solution may be using static
routing at Router, and using the corresponding static address
selection policy at Host.
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 in 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 | 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
+--------+
[Fig. 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
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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
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 problems, it is likely that
delivering additional information to a node fits better than
algorithmic solutions 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 |
+---------+ +------+
[Fig. 4]
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As RFC 4864 [RFC4864] describes, using a ULA may be beneficial in
some scenarios. If the ULA is used for internal communication,
packets with ULA need to be filtered at the Router.
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 [RFC4291],
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::1 and fd0:1::1, the source
address will be fd00:1::1, because the longest matching bit length is
0 for 2001:db8::1 and 1 for fd0:1::1 respectively.
Solution analysis:
This problem can be solved in the RFC 3484 framework. For
example, configuring some address selection policies into 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 [RFC4192] 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 [RFC4862] 5.5.4 does not absolutely
prohibit the use of a deprecated address for a new outgoing
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connection due to limitations relating to what applications are
capable of doing."
+-----+---+
| Router |
+----+----+
| 2001:db8:b::/64 (new)
| 2001:db8:a::/64 (old)
------+---+----------
|
+--+---+ 2001:db8:b::100 (new)
| Host | 2001:db8:a::100 (old)
+------+
[Fig. 5]
Solution analysis:
This problem can be mitigated in the RFC 3484 framework. For
example, configuring some address selection policies into 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 [RFC3041] defines a Temporary Address. The usage of a
Temporary Address has both pros and cons. That is good for viewing
web pages or communicating with the general public, but that 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 HA (home address) and CoA (care-of address)in a Mobile
IPv6 [RFC3775] network.
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The Future Work section in RFC 3041 discusses that an API extension
might be necessary to achieve a better address selection mechanism
with finer granularity.
Solution analysis:
This problem can not 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 [RFC5014].
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 | ||
++-----++-+
| Router |
+----+----+
| 2001:db8:a:1::/64
| 192.0.2.0/28
|
------+---+----------
|
+-+----+ 2001:db8:a:1::100
| Host | 192.0.2.2
+------+
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[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.
Solution analysis:
This problem can be solved in the RFC 3484 framework. For
example, configuring some address selection policies into 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
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.
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+--------+
| 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
+--------+
[Fig. 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 an 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.
However, a ULA and IPv6 global address both have global scope, and
RFC3484 default rules do not specify which address should be given
priority. This point makes IPv6 implementation of address-based
service differentiation a bit harder.
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+--------+
| 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
+--------+
[Fig. 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.
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, 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.
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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 2.1.3,
when Host-A uses a default address selection policy and chooses an
inappropriate address, a packet sent to 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 consideration section in 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 a anycast or a multicast packet,
the malicious host can get knowledge multiple addresses attached to
the target host. In a case like 2.1.4, if an attacker can make Host
to send a multicast packet and 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. IANA Considerations
This document has no actions for IANA.
6. References
6.1. Normative References
[RFC3041] Narten, T. and R. Draves, "Privacy Extensions for
Stateless Address Autoconfiguration in IPv6", RFC 3041,
January 2001.
[RFC3484] Draves, R., "Default Address Selection for Internet
Protocol version 6 (IPv6)", RFC 3484, February 2003.
[RFC3775] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support
in IPv6", RFC 3775, June 2004.
[RFC4192] Baker, F., Lear, E., and R. Droms, "Procedures for
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Renumbering an IPv6 Network without a Flag Day", RFC 4192,
September 2005.
[RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, October 2005.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862, September 2007.
[RFC4864] Van de Velde, G., Hain, T., Droms, R., Carpenter, B., and
E. Klein, "Local Network Protection for IPv6", RFC 4864,
May 2007.
[RFC5014] Nordmark, E., Chakrabarti, S., and J. Laganier, "IPv6
Socket API for Source Address Selection", RFC 5014,
September 2007.
6.2. Informative References
Appendix A. Appendix. Revision History
01:
IP address notations changed to documentation address.
Description of solutions deleted.
02:
Security considerations section rewritten according to comments
from SECDIR.
03:
Intended status changed to Informational.
04:
This version reflects comments from IESG members.
05:
This version reflects comments from IESG members and Bob Hinden.
06:
This version reflects comments from Thomas Narten.
07:
This version reflects comments from Alfred Hoenes.
08:
Solution analysis for the section 2.1.6 was added.
09:
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Typos were fixed, thanks to Jari Arrko.
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
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Full Copyright Statement
Copyright (C) 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
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
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Matsumoto, et al. Expires December 19, 2008 [Page 17]
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