IPv6 Operations Working Group A. Matsumoto
Internet-Draft T. Fujisaki
Intended status: Standards Track NTT
Expires: August 5, 2007 R. Hiromi
K. Kanayama
Intec Netcore
Feb 2007
Requirements for the address selection mechanisms
draft-ietf-v6ops-addr-select-req-01.txt
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Abstract
RFC3484 defines source and destination address selection algorithms
that are commonly deployed in current popular OSs. Meanwhile, there
is a possibility to provide multiple addresses in one physical
network. In such a multi-prefix environment, end-hosts could
encounter some troubles in the communication because of default use
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of the RFC3484 mechanism. Some mechanism for the address selection
problems are proposed including RFC3484 policy table distribution and
RFC3484-update. This document describes the requirements for these
address selection mechanisms.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Scope of this document . . . . . . . . . . . . . . . . . . 3
2. Requirements of Address Selection . . . . . . . . . . . . . . 3
2.1. Contents of Policy Table . . . . . . . . . . . . . . . . . 3
2.2. Timing . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.3. Redistribution of changed Policy Table . . . . . . . . . . 4
2.4. Sections . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.5. Generating Policy Table per CPE/Node . . . . . . . . . . . 4
2.6. Security . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Possible Solutions for Address Selection Problem . . . . . . . 4
3.1. Routing System Assistance for Address Selection by
Fred Baker . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. 3484-update . . . . . . . . . . . . . . . . . . . . . . . 5
3.3. shim6 . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.4. policy distribution mechanism . . . . . . . . . . . . . . 6
4. Discussion at 67th IETF . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
Appendix A. Solutions for RFC3484 policy distribution . . . . . . 9
A.1. Policy distribution with router advertisement (RA)
message option . . . . . . . . . . . . . . . . . . . . . . 10
A.2. Policy distribution in DHCPv6 . . . . . . . . . . . . . . 10
A.3. Using other protocols . . . . . . . . . . . . . . . . . . 11
A.4. Defining a new protocol . . . . . . . . . . . . . . . . . 11
A.5. Converting routing information to policy table . . . . . . 11
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7.1. Normative References . . . . . . . . . . . . . . . . . . . 12
7.2. Informative References . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . . . . 14
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1. Introduction
One physical network can have 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 documented in PS. [I-D.arifumi-v6ops-addr-select-ps]
RFC 3484 [RFC3484] defines both source and destination address
selection algorithms. RFC 3484 defines a default address table, and
enables adding other entries to this table. Flexible address
selection can be carried out.
In addition, the distribution 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
autoconfiguration.
To make a smooth connection with the appropriate source and
destination address selection inside a multi-prefix environment,
nodes must be informed about routing policies of their upstream
networks and possible source address selection policies. Then, those
nodes must put those policies into individual policy tables.
On the other hand, the RFC3484 mechanism is commonly deployed.
However, manual configuration of the policy table is not a feasible
idea and some automatic mechanism is needed.
In this document, requirements for distribution of the address
selection policy are described for promotional use of the RFC3484
mechanism.
1.1. Scope of this document
The routing information from an upstream network is necessary, but in
this document, we are focused on how to select source and destination
addresses at the RFC3484 address policy table of the end-host.
2. Requirements of Address Selection
2.1. Contents of Policy Table
A Policy Table is a set of Policies described in RFC 3484. Each
Policy consists of four elements: prefix value, precedence value,
label value, and zone index value. The Policy distribution mechanism
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should be able to distribute a Policy Table that has one or more
Policies to Nodes.
2.2. Timing
The Policy Table should be distributed to Nodes by a Policy broker at
any time when Nodes send a request for the Policy.
2.3. Redistribution of changed Policy Table
When a Policy broker has any change in a Policy Table that is
distributed to Nodes, the Policy broker should redistribute the
latest Policy Table to Nodes.
2.4. Sections
The Policy distribution mechanism should support being performed in
two kinds of sections: from PE to CPE and from CPE to Node. Policy
distribution mechanisms provided in each section may or many not be
the same.
2.5. Generating Policy Table per CPE/Node
The Policy distribution mechanism should allow for generating an
appropriate Policy Table per Node. For example, in some cases, each
Node may have a different set of assigned prefixes. In such a case,
the appropriate Policy Table for each Node may also be different, and
a Policy broker may be needed to generate the Policy Table according
to the identity of the Node.
2.6. Security
The Policy distribution mechanism should provide for reliable, secure
distribution of the Policy distribution from a Policy broker to
Nodes.
3. Possible Solutions for Address Selection Problem
A few mechanisms for address selection problems are proposed. This
section quickly reviews each proposal including a policy distribution
mechanism.
3.1. Routing System Assistance for Address Selection by Fred Baker
Fred Baker proposed to us about this mechanism. A host asks the DMZ
routers or the local router which is the best pair of source and
destination addresses when the host has a set of addresses A and
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destination host has a set of addresses B. And then, the host uses
the policy provided by the server/routing system as a guide in
applying the response. He also proposed a mechanism that utilizes
ICMP error message to change the source address of the existing
session. This point resembles 5.2 3484-update mechanism, so the
following evaluation is based only on the first part of his proposal.
Advantages:
- A host can choose the best address pair that reflects the dynamic
changing routing status.
- The destination address selection can be handled in this
mechanisim as well as source address selection.
Disadvantages:
- A host can choose the best address pair that reflects the dynamic
- A host has to consult the routing system every time it starts a
connection if the host doesn't have address selection information
for the destination host or the information lifetime is expired.
This could be a possible scalability problem.
- A host has to wait until the response is received from the routing
system.
- The existing host/router OS implementation has to be changed a
lot. In the existing TCP/IP protocol stack implementation,
destination address selection is mainly the role of the
application and not that of the kernel unlike source address
selection. Therefore, implementing this model without causing any
affects on applications is not so easy.
3.2. 3484-update
M. Bagnulo proposed a new method of address selection in his draft.
[I-D.bagnulo-rfc3484-update] When the host notices that a network
failure occurs or packets are dropped somewhere in the network by for
example, an ingress filter, the host changes the source address of
the connection to another source address. The host stores a cache of
address selection information so that the host can select an
appropriate source address for new connections.
Advantages:
- A host can choose the best address that reflects the dynamic
changing routing status.
Disadvantages:
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- A host has to learn address selection information per destination
host. The number of cache entries can be too big.
- The existing host/router OS implementation has to be changed a
lot. In particular, changing the source address of the existing
connection is not so easy and has a big impact on the existing
TCP/IP protocol stack implementation.
- There is not so much experience with this kind of address
selection cache mechanism.
- The host tries every address one-by-one, so the user has to wait
for a long time before the appropriate address pair is found.
3.3. shim6
shim6 is designed for site-multihoming. This mechanism introduces a
new method of address selection for session initiation and session
survivability, which is documented in
[I-D.ietf-shim6-locator-pair-selection] and
[I-D.ietf-shim6-failure-detection].
The shim6 host detects connection failures and changes the source
address during the session.
Advantages:
- The shim6 host performs address selection that reflects network
failures in the source and destination end-to-end link. Moreover,
network failure avoidance can be achieved by end hosts themselves.
Disadvantages:
- A host has to learn address selection information per destination
host. The number of cache entry can be too big.
- The existing host/router OS implementation has to be changed
significantly.
- The host tries every address one-by-one, so the user has to wait
for a long time before the appropriate address pair is found.
3.4. policy distribution mechanism
This mechanism takes advantages of RFC 3484 Policy Table that is
widely deployed already. By distributing policies for Policy Table,
you can auto-configure a host's address selection policy.
Advantages:
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- A host can receive and understand address selection information
before the host starts a connection. Therefore, the amount of
traffic and connection overhead time can be minimized.
- A host does not need any other address-selection-related
information once that host receives the address selection policy
set. This can also reduce the amount of traffic.
- The existing OS implementation does not need to be changed
significantly on the OS that implements the RFC 3484 policy table.
Only the delivery mechanism to the table has to be prepared.
- Destination address selection can also be controlled by this
mechanism.
Disadvantages:
- No other address selection rule that is beyond the RFC 3484 policy
table framework can be implemented.
- The OS implementation has to be changed, and the policy
distribution server, such as a gateway router, has to be prepared.
- When DHCP or RA is used for transport mechanism of policy table,
frequently changing policy cannot be delivered to hosts quickly
because of the nature of these protocols.
4. Discussion at 67th IETF
Here listed some points that was raised at San Diego and comments
below. These points are classified into 3 classes from the aspect of
RFC3484. It seems to be better to settle the basis for this
discussion. That is, we can assume RFC3484 as it is now, we should
modify RFC3484 or we should start from nothing.
1) Issues that don't need RFC3484 modification">
- The ability to deliver specific set of policies to a specific host
This issue is already in the requiremnt draft.
2) Issues that may need slight RFC3484 change.
- The address type dependent preference.
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There was a thread "address selection and DHCPv6" by James Carlson
at IPv6 ML about address type dependent preference, such as
DHCPv6, RA, manual and also privacy extension(RFC3041) address.
http://www1.ietf.org/mail-archive/web/ipv6/current/msg06910.html
It is hard to define default preferences for these address types,
because it depends on the usage of these addresses, but not on
address types themselves. It is the policy table where you can
control host's address selection behavior. At this time, however,
I cannot say policy table is the perfect way to fulfill this
requirement.
For example, You can set priority on 3041 address by putting a
line in policy table specifying 3041 address by 128-bit prefixlen
and continuing to update policy table according to 3041 address
changes. But, this is surely troublesome for users and
implementers.
One idea is to update RFC3484 policy table definition so that it
can handle alias addresses like privacy, DHCPv6 generated, RA
generated, manually generated (and even Home Address ?)
To prefer privacy address by default, and to prefer RA-generated
address for site internal, the policy table will look like this.
Prefix Pref Label
2001:db8:1234::(PRIVACY)/128 30 2
::/0 10 2
2001:db8:1234::(RA):/128 30 1
2001:db8::/48 20 1
3) Issues that need big RFC 3484 change.
- Multiple Interfaces Issues
Dave Thaler gave us comments that multiple-interface hosts may
face policy collision and distribution of dst address selection
policy and src address selection policy should be separated.
Also, per-interface policy table was proposed.
After all, this is a policy collision problem. To make a host
have one policy table per network interface doesn't solve policy
collision issue. Source address selection is performed after
output interface is selected, but destination address selection is
before output interface selection. In this case, destination
address selection uses all the policy tables a host has, so here
collision can happen.
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Separating destination address selection and source address
selection will have a big change on RFC3484 policy table
definition. Though it may be a good idea to avoid source address
selection policy collision.
- application specific address selection should be considered.
Also, XML was proposed for the right format to describe those
policies.
This issue is so much application dependent. Even if policy table
supports application specific policies, the application doesn't
necessarily follow the policy table. It seems to me a better idea
to use address selection APIs or application specific
configuration file for it.
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.
Appendix A. Solutions for RFC3484 policy distribution
In this section, several mechanisms for distributing RFC3484 policy
are compared and evaluated. The reason why this section is in
appendix is that these discussions should be after address selection
mechanism selection is finished and policy distribution mechanism is
selected. solution.
As described in section 3.1, the address selection policy table
consists of four elements: prefix value, precedence, label, and zone-
index. The policy distribution mechanism will deliver lists of these
elements.
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A.1. Policy distribution with router advertisement (RA) message option
The RA message can be used to deliver a policy table by adding a new
ND option. Existing ND transport mechanisms (i.e., advertisements
and solicitations) are used. Advantages and disadvantages are almost
the same as those described in [DNS configuration RFC, RA section].
In addition, an advantage and disadvantages of distributing a policy
table are as follows.
Advantages:
- The RA message is used to deliver IPv6 address prefixes.
Therefore, delivering policies for selecting addresses with the
address attached to the host would be natural.
Disadvantages:
- The RA message is limited in size, and the RA may not be
sufficient to deliver full policies. The same compression
techniques, which were adopted in RFC4191 [RFC4191] can be used to
increase the number of policies delivered by RA messages.
- Currently, RA messages are not used between a PE and CPE. Other
protocols may be necessary to deliver a policy table.
- Configuring a policy table in each router that advertises RA
messages with an address prefix is necessary, so if a site has a
lot of routers, there will be a higher management cost.
- Delivering a specific policy table to one node is impossible
because RA messages are multicast.
A.2. Policy distribution in DHCPv6
By defining a new DHCPv6 option like
[I-D.fujisaki-dhc-addr-select-opt], a policy table can be delivered.
The advantages and disadvantages are almost the same as those
described in [DNS configuration RFC, DHCPv6 section].
In addition, there are the following advantages and disadvantages.
Advantages:
- Currently, DHCPv6 prefix delegation is mainly used between a PE
and CPE. Delivering a policy table with prefixes is possible.
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- A DHCPv6 server can deliver a host-specific policy table.
- By using a DHCPv6 relay mechanism, managing a policy table from a
central server is possible.
Disadvantages:
- The DHCPv6 message size is limited to the maximum UDP transmission
size, so delivering complex policies by DHCPv6 may be impossible.
A.3. Using other protocols
Using other protocols (i.e., http and ftp) to deliver the policy
table is possible.
Advantages:
- No new transport mechanisms are necessary.
Disadvantages:
- Other service discovery mechanisms will be necessary.
- The procedure to distribute information should be defined (e.g.,
when to distribute and where the information is stored).
- Existing protocols may not have a mechanism to inform clients
about policy changes.
A.4. Defining a new protocol
Defining a new protocol to deliver a policy table will have the
following advantages and disadvantages.
Advantages:
- Defining a protocol suitable for policy distribution may be
possible.
Disadvantages:
- In addition to the disadvantages of 4.3, a new transport mechanism
needs to be defined.
A.5. Converting routing information to policy table
In an environment in which routing information and network links are
separated (e.g., between PE and CPE), converting routing information
to a policy table is possible. However, when intermediate routers
and nodes receive next-hop information, that is aggregated as a
default route or neighbor router, and cannot generate policy table [a
policy table cannot be generated].
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Advantages:
- No new distribution mechanism is necessary.
Disadvantages:
- This mechanism can be used only in a limited environment.
7. References
7.1. Normative References
[I-D.arifumi-v6ops-addr-select-ps]
Matsumoto, A., "Problem Statement of Default Address
Selection in Multi-prefix Environment: Operational Issues
of RFC3484 Default Rules",
draft-arifumi-v6ops-addr-select-ps-01 (work in progress),
October 2006.
[RFC3484] Draves, R., "Default Address Selection for Internet
Protocol version 6 (IPv6)", RFC 3484, February 2003.
7.2. Informative References
[I-D.bagnulo-rfc3484-update]
Bagnulo, M., "Updating RFC 3484 for multihoming support",
draft-bagnulo-rfc3484-update-00 (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-03 (work in progress),
January 2007.
[I-D.ietf-shim6-failure-detection]
Arkko, J. and I. Beijnum, "Failure Detection and Locator
Pair Exploration Protocol for IPv6 Multihoming",
draft-ietf-shim6-failure-detection-07 (work in progress),
December 2006.
[I-D.ietf-shim6-locator-pair-selection]
Bagnulo, M., "Default Locator-pair selection algorithm for
the SHIM6 protocol",
draft-ietf-shim6-locator-pair-selection-01 (work in
progress), October 2006.
[RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and
More-Specific Routes", RFC 4191, November 2005.
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[RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, October 2005.
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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