IPv6 Operations Working Group A. Matsumoto
Internet-Draft T. Fujisaki
Intended status: Informational NTT
Expires: July 31, 2008 R. Hiromi
K. Kanayama
Intec Netcore
January 28, 2008
Problem Statement of Default Address Selection in Multi-prefix
Environment: Operational Issues of RFC3484 Default Rules
draft-ietf-v6ops-addr-select-ps-03.txt
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Copyright (C) The IETF Trust (2008).
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 communication. RFC 3484 defines both the source and
destination address selection algorithms, but the multi-prefix
environment considered here needs additional rules beyond those of
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 . . . . . . . . . . . . . 5
2.1.4. Combined Use of Global and ULA . . . . . . . . . . . . 7
2.1.5. Site Renumbering . . . . . . . . . . . . . . . . . . . 7
2.1.6. Multicast Source Address Selection . . . . . . . . . . 8
2.1.7. Temporary Address Selection . . . . . . . . . . . . . 8
2.2. Destination Address Selection . . . . . . . . . . . . . . 8
2.2.1. IPv4 or IPv6 prioritization . . . . . . . . . . . . . 8
2.2.2. ULA and IPv4 dual-stack environment . . . . . . . . . 9
2.2.3. ULA or Global Prioritization . . . . . . . . . . . . . 10
3. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4. Security Considerations . . . . . . . . . . . . . . . . . . . 11
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
6.1. Normative References . . . . . . . . . . . . . . . . . . . 12
6.2. Informative References . . . . . . . . . . . . . . . . . . 12
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 a ULA [RFC4193] and
Global scope networks, an end-host has multiple IP addresses. These
are examples of 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, filtering by a router or discarding of
packets bacause 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 both source and destination address
selection algorithms. In most cases, the host will be able to
communicate with the targeted host using the algorithms. However,
there are still problematic cases such as when multiple default
routes are supplied. This document describes such possibilities of
incorrect address selection, which leads to dropping packets and
communication failure.
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 address policy provisions are not mentioned.
RFC 3484 only defines how to configure the address policy table
manually.
1.1. Scope of this document
There has been a lot of discussion about "multiple addresses/
prefixes" but the multi-homing techniques for achieving 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:db8:1000::/36 | | 2001:db8:8000::/36
+----+-+ +-+----+
| ISP1 | | ISP2 |
+----+-+ +-+----+
| |
2001:db8:1000:::/48 | | 2001:db8:8000::/48
+------+---+ +----+-----+
| Gateway1 | | Gateway2 |
+--------+-+ +-+--------+
| |
2001:db8:1000:1::/64 | | 2001:db8:8000:1::/64
| |
-----+-+-----+------
|
+-+----+ 2001:db8:1000:1::EUI64
| Host | 2001:db8:8000:1::EUI64
+------+
[Fig. 1]
Generally speaking, there is no interaction between next-hop
determination and address selection. In this example, when a Host
sends a packet via Gateway1, the Host does not necessarily choose
address 2001:db8:1000:1::EUI64 given by Gateway1 as the source
address. This causes the same problem as described in the next
section 'Ingress Filtering Problem'.
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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
++-------++
| Gateway |
+----+----+
| 2001:db8:1000:1::/64
| 2001:db8:8000:1::/64
------+---+----------
|
+-+----+ 2001:db8:1000:1::EUI64
| Host | 2001:db8:8000:1::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
popular among ISPs to mitigate the damage of DoS attacks.
In this example, when the Gateway chooses the default route to ISP2
and the Host chooses 2001:db8:1000:1::EUI64 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.
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.
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+--------+
| 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
++-------++
| Gateway |
+----+----+
| 2001:db8:1000:1::/64
| 2001:db8:8000:1::/64
------+---+----------
|
+--+-----+ 2001:db8:1000:1::EUI64
| Host-A | 2001:db8:8000:1::EUI64
+--------+
[Fig. 3]
You do not need two physical network connections here. The
connection from the 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 packet that has been sent will be the one delegated from
ISP2, that is 2001:db8:8000:1::EUI64, 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::EUI64 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::EUI64, which is closed from the Internet.
The important point is that each host chooses a correct source
address for a given destination address as long as NAT does not exist
in the IPv6 world.
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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 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 Gateway.
There is no serious problem related to address selection in this
case, 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 a few years, however, 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.
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.
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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.
+-----+---+
| 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 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 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 and CoA in a 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.
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+---------+
| 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.
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-C 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-C | AAAA = 2001:db8::80
+-----+--+ A = 192.0.2.1
|
============
| Internet |
============
| no IPv6 connectivity
+----+----+
| Gateway |
+----+----+
|
| 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 | 192.0.2.245
+------+
[Fig. 7]
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 |
+-+--|-+
| |
===========|==
| 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. 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.
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,
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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.
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, the malicious host can know 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 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. IANA Considerations
This document has no actions for IANA.
6. References
6.1. Normative References
[RFC3484] Draves, R., "Default Address Selection for Internet
Protocol version 6 (IPv6)", RFC 3484, February 2003.
[RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, October 2005.
6.2. Informative References
[I-D.ietf-v6ops-nap]
Velde, G., "Local Network Protection for IPv6",
draft-ietf-v6ops-nap-06 (work in progress), January 2007.
[RFC3041] Narten, T. and R. Draves, "Privacy Extensions for
Stateless Address Autoconfiguration in IPv6", RFC 3041,
January 2001.
[RFC4192] Baker, F., Lear, E., and R. Droms, "Procedures for
Renumbering an IPv6 Network without a Flag Day", RFC 4192,
September 2005.
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[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
Appendix A. Appendix. Revision History
01:
IP addresse notations changed to docmentation address.
Descriptoin of solutions deleted.
02:
Security considerations section rewritten according to comments
from SECDIR.
03:
Intended status changed to Informational.
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
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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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