Network Working Group O. Troan

Network Working Group O. Troan
Request for Comments: 3633 R. Droms
Category: Standards Track Cisco Systems
December 2003

IPv6 Prefix Options for
Dynamic Host Configuration Protocol (DHCP) version 6

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2003). All Rights Reserved.

Abstract

The Prefix Delegation options provide a mechanism for automated
delegation of IPv6 prefixes using the Dynamic Host Configuration
Protocol (DHCP). This mechanism is intended for delegating a long-
lived prefix from a delegating router to a requesting router, across
an administrative boundary, where the delegating router does not
require knowledge about the topology of the links in the network to
which the prefixes will be assigned.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. DHCPv6 specification dependency . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 3
5. Model and Applicability . . . . . . . . . . . . . . . . . . 3
5.1. Example network architecture . . . . . . . . . . . . . 4
6. Identity Association for Prefix Delegation . . . . . . . . . 5
7. Overview of DHCP with Prefix Delegation . . . . . . . . . . 6
8. Interface Selection . . . . . . . . . . . . . . . . . . . . 7
9. Identity Association for Prefix Delegation Option . . . . . 7
10. IA_PD Prefix option . . . . . . . . . . . . . . . . . . . . 9
11. Delegating Router Solicitation . . . . . . . . . . . . . . . 11
11.1. Requesting router behavior . . . . . . . . . . . . . . 11
11.2. Delegating router behavior . . . . . . . . . . . . . . 11
12. Requesting router initiated prefix delegation . . . . . . . 12

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12.1. Requesting router behavior . . . . . . . . . . . . . . 12
12.2. Delegating Router behavior . . . . . . . . . . . . . . 14
13. Prefix Delegation reconfiguration . . . . . . . . . . . . . 15
13.1. Delegating Router behavior . . . . . . . . . . . . . . 15
13.2. Requesting Router behavior . . . . . . . . . . . . . . 15
14. Relay agent behavior . . . . . . . . . . . . . . . . . . . . 15
15. Security Considerations . . . . . . . . . . . . . . . . . . 16
16. IANA Considerations . . . . . . . . . . . . . . . . . . . . 16
17. Intellectual Property Statement. . . . . . . . . . . . . . . 17
18. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
18.1. Normative References . . . . . . . . . . . . . . . . . 17
18.2. Informative References . . . . . . . . . . . . . . . . 17
19. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 18
20. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 18
21. Full Copyright Statement . . . . . . . . . . . . . . . . . . 19

1. Introduction

This document describes new options for Dynamic Host Configuration
Protocol (DHCP) that provide a mechanism for the delegation of IPv6
prefixes [1]. Through these options, a delegating router can
delegate prefixes to authorized requesting routers.

The prefix delegation mechanism described in this document is
intended for simple delegation of prefixes from a delegating router
to requesting routers. It is appropriate for situations in which the
delegating router does not have knowledge about the topology of the
networks to which the requesting router is attached, and the
delegating router does not require other information aside from the
identity of the requesting router to choose a prefix for delegation.
For example, these options would be used by a service provider to
assign a prefix to a Customer Premise Equipment (CPE) device acting
as a router between the subscriber's internal network and the service
provider's core network.

Many applications expect stable addresses. Even though this
mechanism makes automatic renumbering easier, it is expected that
prefixes have a long lifespan. During renumbering it is expected
that the old and the new prefix co-exist for some time.

The design of this prefix delegation mechanism meets the requirements
for prefix delegation in Requirements for IPv6 prefix delegation [6].

Note that this use of DHCP is not bound to the assignment of IP
addresses or other configuration information to hosts, and that no
mechanism is currently available to communicate delegated prefixes to
a DHCP server that serves such a function. This may be an item of
future work, should usage warrant.

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2. DHCPv6 specification dependency

This document describes new DHCPv6 options for IPv6 prefix
delegation. This document should be read in conjunction with the
DHCPv6 specification, RFC 3315 [2], for a complete specification of
the Prefix Delegation options and mechanism. Definitions for terms
and acronyms not specifically defined in this document are defined in
RFC 3315.

3. Terminology

This document uses the terminology defined in RFC 2460 [1] and RFC
3315. In addition, this document uses the following terms:

requesting router: The router that acts as a DHCP client and is
requesting prefix(es) to be assigned.

delegating router: The router that acts as a DHCP server, and is
responding to the prefix request.

Identity Association for Prefix Delegation (IA_PD): A collection of
prefixes assigned to the requesting router. Each
IA_PD has an associated IAID. A requesting
router may have more than one IA_PD assigned to
it; for example, one for each of its interfaces.

4. Requirements

The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in BCP 14, RFC 2119 [3].

5. Model and Applicability

The model of operation for prefix delegation is as follows. A
delegating router is provided IPv6 prefixes to be delegated to
requesting routers. Examples of ways in which the delegating router
may be provided these prefixes are given in Section 12.2. A
requesting router requests prefix(es) from the delegating router, as
described in Section 12.1. The delegating router chooses prefix(es)
for delegation, and responds with prefix(es) to the requesting
router. The requesting router is then responsible for the delegated
prefix(es). For example, the requesting router might assign a subnet
from a delegated prefix to one of its interfaces, and begin sending
router advertisements for the prefix on that link.

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Each prefix has an associated valid and preferred lifetime, which
constitutes an agreement about the length of time over which the
requesting router is allowed to use the prefix. A requesting router
can request an extension of the lifetimes on a delegated prefix and
is required to terminate the use of a delegated prefix if the valid
lifetime of the prefix expires.

This prefix delegation mechanism would be appropriate for use by an
ISP to delegate a prefix to a subscriber, where the delegated prefix
would possibly be subnetted and assigned to the links within the
subscriber's network.

5.1. Example network architecture

Figure 1 illustrates a network architecture in which prefix
delegation could be used.

______________________ \
/ \ \
| ISP core network | \
\__________ ___________/ |
| |
+-------+-------+ |
| Aggregation | | ISP
| device | | network
| (delegating | |
| router) | |
+-------+-------+ |
| /
|DSL to subscriber /
|premises /
|
+------+------+ \
| CPE | \
| (requesting | \
| router) | |
+----+---+----+ |
| | | Subscriber
---+-------------+-----+- -+-----+-------------+--- | network
| | | | |
+----+-----+ +-----+----+ +----+-----+ +-----+----+ |
|Subscriber| |Subscriber| |Subscriber| |Subscriber| /
| PC | | PC | | PC | | PC | /
+----------+ +----------+ +----------+ +----------+ /

Figure 1: An example of prefix delegation.

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In this example, the delegating router is configured with a set of
prefixes to be used for assignment to customers at the time of each
customer's first connection to the ISP service. The prefix
delegation process begins when the requesting router requests
configuration information through DHCP. The DHCP messages from the
requesting router are received by the delegating router in the
aggregation device. When the delegating router receives the request,
it selects an available prefix or prefixes for delegation to the
requesting router. The delegating router then returns the prefix or
prefixes to the requesting router.

The requesting router subnets the delegated prefix and assigns the
longer prefixes to links in the subscriber's network. In a typical
scenario based on the network shown in Figure 1, the requesting
router subnets a single delegated /48 prefix into /64 prefixes and
assigns one /64 prefix to each of the links in the subscriber
network.

The prefix delegation options can be used in conjunction with other
DHCP options carrying other configuration information to the
requesting router. The requesting router may, in turn, then provide
DHCP service to hosts attached to the internal network. For example,
the requesting router may obtain the addresses of DNS and NTP servers
from the ISP delegating router, and then pass that configuration
information on to the subscriber hosts through a DHCP server in the
requesting router.

6. Identity Association for Prefix Delegation

An IA_PD is a construct through which a delegating router and a
requesting router can identify, group and manage a set of related
IPv6 prefixes. Each IA_PD consists of an IAID and associated
configuration information. An IA_PD for prefixes is the equivalent
of an IA (described in RFC 3315) for addresses.

An IA_PD is different from an IA, in that it does not need to be
associated with exactly one interface. One IA_PD can be associated
with the requesting router, with a set of interfaces or with exactly
one interface. A requesting router must create at least one distinct
IA_PD. It may associate a distinct IA_PD with each of its downstream
network interfaces and use that IA_PD to obtain a prefix for that
interface from the delegating router.

The IAID uniquely identifies the IA_PD and must be chosen to be
unique among the IA_PD IAIDs on the requesting router. The IAID is
chosen by the requesting router. For any given use of an IA_PD by
the requesting router, the IAID for that IA_PD MUST be consistent
across restarts of the requesting router. The requesting router may

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maintain consistency either by storing the IAID in non-volatile
storage or by using an algorithm that will consistently produce the
same IAID as long as the configuration of the requesting router has
not changed. If the requesting router uses only one IAID, it can use
a well-known value, e.g., zero.

The configuration information in an IA_PD consists of one or more
IPv6 prefixes along with the times T1 and T2 for the IA_PD. See
section 9 for the representation of an IA_PD in a DHCP message.

7. Overview of DHCP with Prefix Delegation

Prefix delegation with DHCP is independent of address assignment with
DHCP. A requesting router can use DHCP for just prefix delegation or
for prefix delegation along with address assignment and other
configuration information.

A requesting router first creates an IA_PD and assigns it an IAID.
The requesting router then transmits a Solicit message containing an
IA_PD option describing the IA_PD. Delegating routers that can
delegate prefixes to the IA_PD respond to the requesting router with
an Advertise message.

The requesting router may include prefixes in the IA_PDs as a hint to
the delegating router about specific prefixes for which the
requesting router has a preference.

When the requesting router has identified a delegating router, the
requesting router uses a Request message to populate the IA_PDs with
prefixes. The requesting router includes one or more IA_PD options
in the Request message. The delegating router returns prefixes and
other information about the IA_PDs to the requesting router in IA_PD
options in a Reply message. The requesting router records the
lifetimes for the delegated prefix(es) and uses the prefix(es) as
described in the previous section.

Before the valid lifetime on each delegated prefix expires, the
requesting router includes the prefix in an IA_PD option sent in a
Renew message to the delegating router. The delegating router
responds by returning the prefix with updated lifetimes to the
requesting router.

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8. Interface Selection

Delegated prefixes are not associated with a particular interface in
the same way as addresses are for address assignment, and the rules
described in section 16, "Client Source Address and Interface
Selection" of RFC 3315 do not apply.

When a requesting router sends a DHCP message, it SHOULD be sent on
the interface associated with the upstream router (ISP network). The
upstream interface is typically determined by configuration. This
rule applies even in the case where a separate IA_PD is used for each
downstream interface.

When a requesting router sends a DHCP message directly to a
delegating router using unicast (after receiving the Server Unicast
option from that delegating router), the source address SHOULD be an
address from the upstream interface and which is suitable for use by
the delegating router in responding to the requesting router.

9. Identity Association for Prefix Delegation Option

The IA_PD option is used to carry a prefix delegation identity
association, the parameters associated with the IA_PD and the
prefixes associated with it.

The format of the IA_PD option is:

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_IA_PD | option-length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IAID (4 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| T1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| T2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. IA_PD-options .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

option-code: OPTION_IA_PD (25)

option-length: 12 + length of IA_PD-options field.

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IAID: The unique identifier for this IA_PD; the IAID must
be unique among the identifiers for all of this
requesting router's IA_PDs.

T1: The time at which the requesting router should
contact the delegating router from which the
prefixes in the IA_PD were obtained to extend the
lifetimes of the prefixes delegated to the IA_PD;
T1 is a time duration relative to the current time
expressed in units of seconds.

T2: The time at which the requesting router should
contact any available delegating router to extend
the lifetimes of the prefixes assigned to the
IA_PD; T2 is a time duration relative to the
current time expressed in units of seconds.

IA_PD-options: Options associated with this IA_PD.

The IA_PD-options field encapsulates those options that are specific
to this IA_PD. For example, all of the IA_PD Prefix Options carrying
the prefixes associated with this IA_PD are in the IA_PD-options
field.

An IA_PD option may only appear in the options area of a DHCP
message. A DHCP message may contain multiple IA_PD options.

The status of any operations involving this IA_PD is indicated in a
Status Code option in the IA_PD-options field.

Note that an IA_PD has no explicit "lifetime" or "lease length" of
its own. When the valid lifetimes of all of the prefixes in a IA_PD
have expired, the IA_PD can be considered as having expired. T1 and
T2 are included to give delegating routers explicit control over when
a requesting router should contact the delegating router about a
specific IA_PD.

In a message sent by a requesting router to a delegating router,
values in the T1 and T2 fields indicate the requesting router's
preference for those parameters. The requesting router sets T1 and
T2 to zero if it has no preference for those values. In a message
sent by a delegating router to a requesting router, the requesting
router MUST use the values in the T1 and T2 fields for the T1 and T2
parameters. The values in the T1 and T2 fields are the number of
seconds until T1 and T2.

The delegating router selects the T1 and T2 times to allow the
requesting router to extend the lifetimes of any prefixes in the

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IA_PD before the lifetimes expire, even if the delegating router is
unavailable for some short period of time. Recommended values for T1
and T2 are .5 and .8 times the shortest preferred lifetime of the
prefixes in the IA_PD that the delegating router is willing to
extend, respectively. If the time at which the prefixes in an IA_PD
are to be renewed is to be left to the discretion of the requesting
router, the delegating router sets T1 and T2 to 0.

If a delegating router receives an IA_PD with T1 greater than T2, and
both T1 and T2 are greater than 0, the delegating router ignores the
invalid values of T1 and T2 and processes the IA_PD as though the
delegating router had set T1 and T2 to 0.

If a requesting router receives an IA_PD with T1 greater than T2, and
both T1 and T2 are greater than 0, the client discards the IA_PD
option and processes the remainder of the message as though the
delegating router had not included the IA_PD option.

10. IA_PD Prefix option

The IA_PD Prefix option is used to specify IPv6 address prefixes
associated with an IA_PD. The IA_PD Prefix option must be
encapsulated in the IA_PD-options field of an IA_PD option.

The format of the IA_PD Prefix option is:

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_IAPREFIX | option-length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| preferred-lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| valid-lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| prefix-length | |
+-+-+-+-+-+-+-+-+ IPv6 prefix |
| (16 octets) |
| |
| |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | .
+-+-+-+-+-+-+-+-+ .
. IAprefix-options .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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option-code: OPTION_IAPREFIX (26)

option-length: 25 + length of IAprefix-options field

preferred-lifetime: The recommended preferred lifetime for the IPv6
prefix in the option, expressed in units of
seconds. A value of 0xFFFFFFFF represents
infinity.

valid-lifetime: The valid lifetime for the IPv6 prefix in the
option, expressed in units of seconds. A value of
0xFFFFFFFF represents infinity.

prefix-length: Length for this prefix in bits

IPv6-prefix: An IPv6 prefix

IAprefix-options: Options associated with this prefix

In a message sent by a requesting router to a delegating router, the
values in the fields can be used to indicate the requesting router's
preference for those values. The requesting router may send a value
of zero to indicate no preference. A requesting router may set the
IPv6 prefix field to zero and a given value in the prefix-length
field to indicate a preference for the size of the prefix to be
delegated.

In a message sent by a delegating router the preferred and valid
lifetimes should be set to the values of AdvPreferredLifetime and
AdvValidLifetime as specified in section 6.2.1, "Router Configuration
Variables" of RFC 2461 [4], unless administratively configured.

A requesting router discards any prefixes for which the preferred
lifetime is greater than the valid lifetime. A delegating router
ignores the lifetimes set by the requesting router if the preferred
lifetime is greater than the valid lifetime and ignores the values
for T1 and T2 set by the requesting router if those values are
greater than the preferred lifetime.

The values in the preferred and valid lifetimes are the number of
seconds remaining for each lifetime.

An IA_PD Prefix option may appear only in an IA_PD option. More than
one IA_PD Prefix Option can appear in a single IA_PD option.

The status of any operations involving this IA_PD Prefix option is
indicated in a Status Code option in the IAprefix-options field.

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11. Delegating Router Solicitation

The requesting router locates and selects a delegating router in the
same way as described in section 17, "DHCP Server Solicitation" of
RFC 3315. The details of the solicitation process are described in
this section.

11.1. Requesting router behavior

The requesting router creates and transmits a Solicit message as
described in sections 17.1.1, "Creation of Solicit Messages" and
17.1.2, "Transmission of Solicit Messages" of RFC 3315. The
requesting router creates an IA_PD and assigns it an IAID. The
requesting router MUST include the IA_PD option in the Solicit
message.

The requesting router processes any received Advertise messages as
described in section 17.1.3, "Receipt of Advertise Messages" of RFC
3315. The requesting router MAY choose to consider the presence of
advertised prefixes in its decision about which delegating router to
respond to.

The requesting router MUST ignore any Advertise message that includes
a Status Code option containing the value NoPrefixAvail, with the
exception that the requesting router MAY display the associated
status message to the user.

11.2. Delegating router behavior

The delegating router sends an Advertise message to the requesting
router in the same way as described in section 17.2.2, "Creation and
transmission of Advertise messages" of RFC 3315. If the message
contains an IA_PD option and the delegating router is configured to
delegate prefix(es) to the requesting router, the delegating router
selects the prefix(es) to be delegated to the requesting router. The
mechanism through which the delegating router selects prefix(es) for
delegation is not specified in this document. Examples of ways in
which the delegating router might select prefix(es) for a requesting
router include: static assignment based on subscription to an ISP;
dynamic assignment from a pool of available prefixes; selection based
on an external authority such as a RADIUS server using the Framed-
IPv6-Prefix option as described in RFC 3162 [5].

If the requesting router includes an IA_PD Prefix option in the IA_PD
option in its Solicit message, the delegating router MAY choose to
use the information in that option to select the prefix(es) or prefix
size to be delegated to the requesting router.

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The delegating router sends an Advertise message to the requesting
router in the same way as described in section, "Creation and
transmission of Advertise messages" of RFC 3315. The delegating
router MUST include an IA_PD option, identifying any prefix(es) that
the delegating router will delegate to the requesting router.

If the delegating router will not assign any prefixes to any IA_PDs
in a subsequent Request from the requesting router, the delegating
router MUST send an Advertise message to the requesting router that
includes the IA_PD with no prefixes in the IA_PD and a Status Code
option in the IA_PD containing status code NoPrefixAvail and a status
message for the user, a Server Identifier option with the delegating
router's DUID and a Client Identifier option with the requesting
router's DUID.

12. Requesting router initiated prefix delegation

A requesting router uses the same message exchanges as described in
section 18, "DHCP Client-Initiated Configuration Exchange" of RFC
3315 to obtain or update prefix(es) from a delegating router. The
requesting router and the delegating router use the IA_PD Prefix
option to exchange information about prefix(es) in much the same way
IA Address options are used for assigned addresses.

12.1. Requesting router behavior

The requesting router uses a Request message to populate IA_PDs with
prefixes. The requesting router includes one or more IA_PD options
in the Request message. The delegating router then returns the
prefixes for the IA_PDs to the requesting router in IA_PD options in
a Reply message.

The requesting router includes IA_PD options in any Renew, or Rebind
messages sent by the requesting router. The IA_PD option includes
all of the prefixes the requesting router currently has associated
with that IA_PD.

In some circumstances the requesting router may need verification
that the delegating router still has a valid binding for the
requesting router. Examples of times when a requesting router may
ask for such verification include:

o The requesting router reboots.

o The requesting router's upstream link flaps.

o The requesting router is physically disconnected from a wired
connection.

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If such verification is needed the requesting router MUST initiate a
Rebind/Reply message exchange as described in section 18.1.4,
"Creation and Transmission of Rebind Messages" of RFC 3315, with the
exception that the retransmission parameters should be set as for the
Confirm message, described in section 18.1.2, "Creation and
Transmission of Confirm Messages" of RFC 3315. The requesting router
includes any IA_PDs, along with prefixes associated with those IA_PDs
in its Rebind message.

Each prefix has valid and preferred lifetimes whose durations are
specified in the IA_PD Prefix option for that prefix. The requesting
router uses Renew and Rebind messages to request the extension of the
lifetimes of a delegated prefix.

The requesting router uses a Release message to return a delegated
prefix to a delegating router. The prefixes to be released MUST be
included in the IA_PDs.

The Confirm and Decline message types are not used with Prefix
Delegation.

Upon the receipt of a valid Reply message, for each IA_PD the
requesting router assigns a subnet from each of the delegated
prefixes to each of the links to which the associated interfaces are
attached, with the following exception: the requesting router MUST
NOT assign any delegated prefixes or subnets from the delegated
prefix(es) to the link through which it received the DHCP message
from the delegating router.

When a requesting router subnets a delegated prefix, it must assign
additional bits to the prefix to generate unique, longer prefixes.
For example, if the requesting router in Figure 1 were delegated
3FFE:FFFF:0::/48, it might generate 3FFE:FFFF:0:1::/64 and
3FFE:FFFF:0:2::/64 for assignment to the two links in the subscriber
network. If the requesting router were delegated 3FFE:FFFF:0::/48
and 3FFE:FFFF:5::/48, it might assign 3FFE:FFFF:0:1::/64 and
3FFE:FFFF:5:1::/64 to one of the links, and 3FFE:FFFF:0:2::/64 and
3FFE:FFFF:5:2::/64 for assignment to the other link.

If the requesting router assigns a delegated prefix to a link to
which the router is attached, and begins to send router
advertisements for the prefix on the link, the requesting router MUST
set the valid lifetime in those advertisements to be no later than
the valid lifetime specified in the IA_PD Prefix option. A
requesting router MAY use the preferred lifetime specified in the
IA_PD Prefix option.

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Handling of Status Codes options in received Reply messages is
described in section 18.1.8, "Receipt of Reply Messages" of RFC 3315.
The NoPrefixAvail Status Code is handled in the same manner as the
NoAddrsAvail Status Code.

12.2. Delegating Router behavior

When a delegating router receives a Request message from a requesting
router that contains an IA_PD option, and the delegating router is
authorized to delegate prefix(es) to the requesting router, the
delegating router selects the prefix(es) to be delegated to the
requesting router. The mechanism through which the delegating router
selects prefix(es) for delegation is not specified in this document.
Section 11.2 gives examples of ways in which a delegating router
might select the prefix(es) to be delegated to a requesting router.

A delegating router examines the prefix(es) identified in IA_PD
Prefix options (in an IA_PD option) in Renew and Rebind messages and
responds according to the current status of the prefix(es). The
delegating router returns IA_PD Prefix options (within an IA_PD
option) with updated lifetimes for each valid prefix in the message
from the requesting router. If the delegating router finds that any
of the prefixes are not in the requesting router's binding entry, the
delegating router returns the prefix to the requesting router with
lifetimes of 0.

The delegating router behaves as follows when it cannot find a
binding for the requesting router's IA_PD:

Renew message: If the delegating router cannot find a binding
for the requesting router's IA_PD the delegating
router returns the IA_PD containing no prefixes
with a Status Code option set to NoBinding in the
Reply message.

Rebind message: If the delegating router cannot find a binding
for the requesting router's IA_PD and the
delegating router determines that the prefixes in
the IA_PD are not appropriate for the link to
which the requesting router's interface is
attached according to the delegating routers
explicit configuration, the delegating router MAY
send a Reply message to the requesting router
containing the IA_PD with the lifetimes of the
prefixes in the IA_PD set to zero. This Reply
constitutes an explicit notification to the
requesting router that the prefixes in the IA_PD
are no longer valid. If the delegating router is

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unable to determine if the prefix is not
appropriate for the link, the Rebind message is
discarded.

A delegating router may mark any prefix(es) in IA_PD Prefix options
in a Release message from a requesting router as "available",
dependent on the mechanism used to acquire the prefix, e.g., in the
case of a dynamic pool.

The delegating router MUST include an IA_PD Prefix option or options
(in an IA_PD option) in Reply messages sent to a requesting router.

13. Prefix Delegation reconfiguration

This section describes prefix delegation in Reconfigure message
exchanges.

13.1. Delegating Router behavior

The delegating router initiates a configuration message exchange with
a requesting router, as described in section 19, "DHCP Server-
Initiated Configuration Exchange" of RFC 3315, by sending a
Reconfigure message (acting as a DHCP server) to the requesting
router, as described in section 19.1, "Server Behavior" of RFC 3315.
The delegating router specifies the IA_PD option in the Option
Request option to cause the requesting router to include an IA_PD
option to obtain new information about delegated prefix(es).

13.2. Requesting Router behavior

The requesting router responds to a Reconfigure message, acting as a
DHCP client, received from a delegating router as described in
section 19.4, "Client Behavior" of RFC 3315. The requesting router
MUST include the IA_PD Prefix option(s) (in an IA_PD option) for
prefix(es) that have been delegated to the requesting router by the
delegating router from which the Reconfigure message was received.

14. Relay agent behavior

A relay agent forwards messages containing Prefix Delegation options
in the same way as described in section 20, "Relay Agent Behavior" of
RFC 3315.

If a delegating router communicates with a requesting router through
a relay agent, the delegating router may need a protocol or other
out-of-band communication to add routing information for delegated
prefixes into the provider edge router.

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15. Security Considerations

Security considerations in DHCP are described in section 23,
"Security Considerations" of RFC 3315.

A rogue delegating router can issue bogus prefixes to a requesting
router. This may cause denial of service due to unreachability.

A malicious requesting router may be able to mount a denial of
service attack by repeated requests for delegated prefixes that
exhaust the delegating router's available prefixes.

To guard against attacks through prefix delegation, requesting
routers and delegating routers SHOULD use DHCP authentication as
described in section 21, "Authentication of DHCP messages" of RFC
3315. For point to point links, where one trusts that there is no
man in the middle, or one trusts layer two authentication, DHCP
authentication or IPsec may not be necessary. Because a requesting
router and delegating routers must each have at least one assigned
IPv6 address, the routers may be able to use IPsec for authentication
of DHCPv6 messages. The details of using IPsec for DHCPv6 are under
development.

Networks configured with delegated prefixes should be configured to
preclude intentional or inadvertent inappropriate advertisement of
these prefixes.

16. IANA Considerations

IANA has assigned option codes to:

OPTION_IA_PD (25)

OPTION_IAPREFIX (26)

from the option-code space as defined in section 24.3, "DHCP Options"
of RFC 3315.

IANA has assigned status code 6 to:

NoPrefixAvail: Delegating router has no prefixes available to
assign to the IAPD(s)

from the status-code space as defined in section 24.4, "Status Codes"
of RFC 3315.

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17. Intellectual Property Statement

The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.

The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.

18. References

18.1. Normative References

[1] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC 2460, December 1998.

[2] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C. and M.
Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
RFC 3315, July 2003.

[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.

[4] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery for
IP Version 6 (IPv6)", RFC 2461, December 1998.

[5] Aboba, B., Zorn, G. and D. Mitton, "RADIUS and IPv6", RFC 3162,
August 2001.

18.2. Informative References

[6] Miyakawa, S. and R. Droms, "Requirements for IPv6 prefix
delegation", Work in Progress, August 2003.

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RFC 3633 IPv6 Prefix Options for DHCPv6 December 2003

19. Acknowledgements

Thanks for the input and review by (in alphabetical order) Steve
Deering, Dave Forster, Brian Haberman, Tatuya Jinmei, Shin Miyakawa,
Pekka Savola, Bernie Volz, Trevor Warwick and Toshi Yamasaki.

20. Authors' Addresses

Ole Troan
Cisco Systems
250 Longwater Avenue
Reading RG2 6GB
United Kingdom

Phone: +44 20 8824 8666
EMail: [email protected]

Ralph Droms
Cisco Systems
1414 Massachusetts Avenue
Boxborough, MA 01719
USA

Phone: +1 978 936 1674
EMail: [email protected]

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RFC 3633 IPv6 Prefix Options for DHCPv6 December 2003

21. Full Copyright Statement

Copyright (C) The Internet Society (2003). All Rights Reserved.

This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.

The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assignees.

This document and the information contained herein is provided on an
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TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Acknowledgement

Funding for the RFC Editor function is currently provided by the
Internet Society.

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