One document matched: draft-ietf-6man-addr-select-opt-10.xml
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<rfc category="std" ipr="pre5378Trust200902" docName="draft-ietf-6man-addr-select-opt-10.txt">
<front>
<title abbrev='DHCPv6 Address Selection Policy Opt'>
Distributing Address Selection Policy using DHCPv6
</title>
<author initials='A.M.' surname="Matsumoto" fullname='Arifumi Matsumoto'>
<organization abbrev='NTT'>NTT NT Lab</organization>
<address>
<postal>
<street>3-9-11 Midori-Cho</street>
<city>Musashino-shi</city>
<region>Tokyo</region>
<code>180-8585</code>
<country>Japan</country>
</postal>
<phone>+81 422 59 3334</phone>
<email>arifumi@nttv6.net</email>
</address>
</author>
<author initials='T.F.' surname="Fujisaki" fullname='Tomohiro Fujisaki'>
<organization abbrev='NTT'>NTT NT Lab</organization>
<address>
<postal>
<street>3-9-11 Midori-Cho</street>
<city>Musashino-shi</city>
<region>Tokyo</region>
<code>180-8585</code>
<country>Japan</country>
</postal>
<phone>+81 422 59 7351</phone>
<email>fujisaki@nttv6.net</email>
</address>
</author>
<author initials='T.C.' surname="Chown" fullname='Tim Chown'>
<organization abbrev='University of Southampton'>University of Southampton</organization>
<address>
<postal>
<street>Southampton, Hampshire SO17 1BJ</street>
<country>United Kingdom</country>
</postal>
<email>tjc@ecs.soton.ac.uk</email>
</address>
</author>
<date month="April" year="2013"/>
<area>Internet</area>
<workgroup>6man Working Group</workgroup>
<keyword>Internet-Draft</keyword>
<abstract>
<t>
RFC 6724 defines default address selection mechanisms for IPv6 that allow nodes to select an appropriate address when faced with multiple source and/or destination addresses to choose between.
The RFC 6724 allowed for the future definition of methods to administratively configure the address selection policy information.
This document defines a new DHCPv6 option for such configuration, allowing a site administrator to distribute address selection policy overriding the default address selection parameters and policy table, and thus control the address selection behavior of nodes in their site.
</t>
</abstract>
</front>
<middle>
<section title="Introduction"> <!-- 1 -->
<t>
RFC 3484 <xref target="RFC3484"/> describes default algorithms for selecting an address when a node has multiple destination and/or source addresses to choose from by using an address selection policy.
In Section 2 of RFC 6724, it is suggested that the default policy table may be administratively configured to suit the specific needs of a site.
This specification defines a new DHCPv6 option for such configuration.
</t>
<t>
Some problems have been identified with the default RFC 3484 address selection policy <xref target="RFC5220"/>.
It is unlikely that any default policy will suit all scenarios, and thus mechanisms to control the source address selection policy will be necessary.
Requirements for those mechanisms are described in <xref target="RFC5221"/>, while solutions are discussed in <xref target="I-D.ietf-6man-addr-select-considerations"/>.
Those documents have helped shape the improvements in the default address selection algorithm <xref target="RFC6724"/> as well as the DHCPv6 option defined in this specification.
</t>
<section title="Conventions Used in This Document">
<t>
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in <xref target="RFC2119"/>.
</t>
</section>
<section title="Terminology">
<t>
This document uses the terminology defined in <xref target="RFC2460"/> and the DHCPv6 specification defined in <xref target="RFC3315"/>
</t>
</section>
</section>
<section title="Address Selection options"> <!-- 2 -->
<t>The Address Selection option provides the address selection policy table, and some other configuration parameters.</t>
<t>
An Address Selection option contains zero or more policy table options.
Multiple Policy Table options in an Address Selection option constitute a single policy table.
When it does not contain policy table option, it is used to convey the A and P flags.
</t>
<t>The format of the Address Selection option is given below.</t>
<figure><artwork>
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPTION_ADDRSEL | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |A|P| |
+-+-+-+-+-+-+-+-+ POLICY TABLE OPTIONS |
| (variable length) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Address Selection option format
</artwork></figure>
<t>
<list style='hanging' hangIndent='5'>
<t hangText="option-code:"> OPTION_ADDRSEL (TBD).
<vspace blankLines='1' />
</t>
<t hangText="option-len:">
The total length of the Reserved field, A, P flags, and POLICY TABLE OPTIONS in octets.
<vspace blankLines='1' />
</t>
<t hangText="Reserved:">
Reserved field.
Server MUST set this value to zero and client MUST ignore its content.
<vspace blankLines='1' />
</t>
<t hangText="A:">
Automatic Row Addition flag.
This flag toggles the Automatic Row Addition flag at client hosts, which is described in the section 2.1 in RFC 6724 <xref target="RFC6724"></xref>.
If this flag is set to 1, it does not change client host behavior, that is, a client MAY automatically add additional site-specific rows to the policy table.
If set to 0, the Automatic Row Addition flag is disabled, and a client SHOULD NOT automatically add rows to the policy table.
<vspace blankLines='1' />
</t>
<t hangText="P:">
Privacy Preference flag.
This flag toggles the Privacy Preference flag at client hosts, which is described in the section 5 in RFC 6724 <xref target="RFC6724"></xref>.
If this flag is set to 1, it does not change client host behavior, that is, a client will prefer temporary addresses.
If set to 0, the Privacy Preference flag is disabled, and a client will prefer public addresses.
<vspace blankLines='1' />
</t>
<t hangText="POLICY TABLE OPTIONS:">
Zero or more Address Selection Policy Table options described below.
This option corresponds to a row in the policy table defined in the section 2.1 in RFC 6724 <xref target="RFC6724"></xref>.
<vspace blankLines='1' />
</t>
</list>
</t>
<figure><artwork>
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPTION_ADDRSEL_TABLE | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| label | precedence | prefix-len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| prefix (variable length) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Address Selection Policy Table option format
</artwork></figure>
<t>
<list style='hanging' hangIndent='5'>
<t hangText="option-code:"> OPTION_ADDRSEL_TABLE (TBD).
<vspace blankLines='1' />
</t>
<t hangText="option-len:">
The total length of the label field, precedence field, prefix-len field, and prefix field.
<vspace blankLines='1' />
</t>
<t hangText="label:">
An 8-bit unsigned integer; this value is for correlation of source address prefixes and destination address prefixes.
This field is used to deliver a label value in RFC 6724 policy table.
<vspace blankLines='1' />
</t>
<t hangText="precedence:">
An 8-bit unsigned integer; this value is used for sorting destination addresses.
This field is used to to deliver a precedence value in RFC 6724 policy table.
<vspace blankLines='1' />
</t>
<t hangText="prefix-len:">
An 8-bit unsigned integer; the number of leading bits in the prefix that are valid. The value ranges from 0 to 128.
<vspace blankLines='1' />
</t>
<t hangText="prefix:">
A variable-length field containing an IP address or the prefix of an IP address.
An IPv4-mapped address <xref target="RFC4291"/> must be used to represent an IPv4 address as a prefix value.
This field is padded with zeros up to the nearest octet boundary when prefix6-len is not divisible by 8.
This can be expressed using the following equation: (prefix-len+7)/8
So the length of this field should be between 0 and 16 bytes.
For example, the prefix 2001:db8::/60 would be encoded with an prefix-len of 60, the prefix would be 8 octets and would contains octets 20 01 0d b8 00 00 00 00.
<vspace blankLines='1' />
</t>
</list>
</t>
</section>
<section title="Processing the Policy Table option"> <!-- 3 -->
<t>This section describes how to process received Policy Table option at the DHCPv6 client.</t>
<t>
This option's concept is to serve as a hint for a node about how to behave in the network.
Ultimately, it can be controlled by the node's administrator how to deal with the received policy information, but the implementation SHOULD follow the way described below uniformly to ease diagnose brokenness and to reduce operational costs.
</t>
<section title="Handling of the local policy table">
<t>
RFC 6724 defines the default policy table.
Also, users are usually able to configure the policy table to satisfy their own requirements.
</t>
<t>
The client implementation SHOULD provide the following choices to the user.
The choice a SHOULD be default, as far as the policy table is not configured by the user.
</t>
<t>
<list style="hanging">
<t hangText="a)">
replace the existing active policy table with the DHCPv6 distributed policy table.
</t>
<t hangText="b)">
preserve the existing active policy table, whether this be the default policy table, or user configured policy.
</t>
</list>
</t>
</section>
<section title="Handling of the stale policy table">
<t>
When the information from the DHCP server goes stale, the policy received form the DHCP server SHOULD be deprecated.
</t>
<t>
The received information can be considered stale in several cases, such as, when the interface goes down, the DHCP server does not respond for a certain amount of time, and the Information Refresh Time is expired.
</t>
</section>
<section title="Multi-interface situation">
<t>
The policy table, and other parameters specified in this document are node-global information by their nature.
One reason being that the outbound interface is usually chosen after destination address selection.
So, a host cannot make use of multiple address selection policies even if they are stored per interface.
</t>
<t>
Even if the received policy from one source is merged with one from another source, the effect of both policy are more or less changed.
The policy table is defined as a whole, so the slightest addition/deletion from the policy table brings a change in semantics of the policy.
</t>
<t>
It also should be noted that absence of the distributed policy from a certain network interface should not be treated as absence of policy itself, because it may mean preference for the default address selection policy.
</t>
<t>
Under the above assumptions, how to handle received policy is specified below.
</t>
<t>
A node SHOULD use Address Selection options by default in any of the following two cases:
</t>
<t>
<list style="hanging">
<t hangText="1:">
The host is single-homed, where the host belongs to one administrative network domain exclusively usually through one active network interface.
</t>
<t hangText="2:">
The host implements some advanced heuristics to deal with multiple received policy, which is outside the scope of this document.
</t>
</list>
</t>
<t>
The above restrictions do not preclude implementations from providing configuration options to enable this option on a certain network interface.
</t>
<t>
Nor, they do not preclude implementations from storing distributed address selection policies per interface.
They can be used effectively on such implementations that adopt per-application interface selection.
</t>
</section>
</section>
<section title="Implementation Considerations"> <!-- 4 -->
<t>
<list style='symbols'>
<t>
The value 'label' is passed as an unsigned integer, but there is no special meaning for the value, that is whether it is a large or small number.
It is used to select a preferred source address prefix corresponding to a destination address prefix by matching the same label value within the DHCP message.
DHCPv6 clients SHOULD convert this label to a representation appropriate for the local implementation (e.g., string).
<vspace blankLines='1' />
</t>
<t>
Currently, the label and precedence values are defined as 8-bit unsigned integers.
In almost all cases, this value will be enough.
<vspace blankLines='1' />
</t>
<t>
The maximum number of address selection rules that may be conveyed in one DHCPv6 message depends on the prefix length of each rule and the maximum DHCPv6 message size defined in RFC 3315.
It is possible to carry over 3,000 rules in one DHCPv6 message (maximum UDP message size).
However, it should not be expected that DHCP clients, servers and relay agents can handle UDP fragmentation.
Network adiministrators SHOULD consider local limitations to the maximum DHCPv6 message size that can be reliably transported via their specific local infrastructure to end nodes; and therefore they SHOULD consider the number of options, the total size of the options, and the resulting DHCPv6 message size, when defining their Policy Table.
<vspace blankLines='1' />
</t>
</list>
</t>
</section>
<section title="Security Considerations"> <!-- 5 -->
<t>
A rogue DHCPv6 server could issue bogus address selection policies to a client.
This might lead to incorrect address selection by the client, and the affected packets might be blocked at an outgoing ISP because of ingress filtering, incur additional network charges, or be misdirected to an attacker's machine.
Alternatively, an IPv6 transition mechanism might be preferred over native IPv6, even if it is available.
To guard against such attacks, a legitimate DHCPv6 server should communicate through a secure, trusted channel, such as a channel protected by IPsec, SEND and DHCP authentication, as described in section 21 of RFC 3315,
</t>
<t>
Another threat is about privacy concern.
As in the security consideration section of RFC 6724, at least a part of, the address selection policy stored in a host can be leaked by a packet from a remote host.
This issue will not be modified by the introduction of this option, regardless of whether the host is multihomed or not.
</t>
</section>
<section title="IANA Considerations"> <!-- 6 -->
<t>
IANA is requested to assign option codes to OPTION_ADDRSEL and OPTION_ADDRSEL_TABLE from the "DHCP Option Codes" registry (http://www.iana.org/assignments/dhcpv6-parameters/dhcpv6-parameters.xml).
</t>
</section>
</middle>
<back>
<references title="Normative References">
&rfc2119;
&rfc3315;
&rfc3484;
&rfc6724;
</references>
<references title="Informative References">
&DTCONS;
&rfc2460;
&rfc3493;
&rfc4291;
&rfc4941;
&rfc5220;
&rfc5221;
</references>
<section title="Acknowledgements">
<t>
Authors would like to thank to Dave Thaler, Pekka Savola, Remi Denis-Courmont, Francois-Xavier Le Bail, Ole Troan, Bob Hinden, Dmitry Anipko, Ray Hunter, Rui Paulo, Brian E Carpenter, Tom Petch, and the members of 6man's address selection design team for their invaluable contributions to this document.
</t>
</section>
</back>
</rfc>
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