DHC Working Group O. Troan
Internet-Draft R. Droms
Expires: August 11, 2003 Cisco Systems
February 10, 2003
IPv6 Prefix Options for DHCPv6
draft-ietf-dhc-dhcpv6-opt-prefix-delegation-02.txt
Status of this Memo
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This Internet-Draft will expire on August 11, 2003.
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 DHCP. This mechanism is intended
for delegating 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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Model and Applicability . . . . . . . . . . . . . . . . . . 4
5. Identity Association for Prefix Delegation . . . . . . . . . 6
6. Overview of DHCP with Prefix Delegation . . . . . . . . . . 7
7. Interface Selection . . . . . . . . . . . . . . . . . . . . 7
8. Identity Association for Prefix Delegation Option . . . . . 8
9. IA_PD Prefix option . . . . . . . . . . . . . . . . . . . . 9
10. Delegating Router Solicitation . . . . . . . . . . . . . . . 11
10.1 Requesting router behaviour . . . . . . . . . . . . . . . . 11
10.2 Delegating router behaviour . . . . . . . . . . . . . . . . 11
11. Requesting router initiated prefix delegation . . . . . . . 12
11.1 Requesting router behaviour . . . . . . . . . . . . . . . . 13
11.2 Delegating Router behaviour . . . . . . . . . . . . . . . . 14
12. Prefix Delegation reconfiguration . . . . . . . . . . . . . 15
12.1 Delegating Router behaviour . . . . . . . . . . . . . . . . 15
12.2 Requesting Router behaviour . . . . . . . . . . . . . . . . 15
13. Relay agent behaviour . . . . . . . . . . . . . . . . . . . 15
14. Security Considerations . . . . . . . . . . . . . . . . . . 15
15. IANA Considerations . . . . . . . . . . . . . . . . . . . . 16
16. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 16
17. Changes since revision-01 . . . . . . . . . . . . . . . . . 16
Normative References . . . . . . . . . . . . . . . . . . . . 16
Informative References . . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 17
Full Copyright Statement . . . . . . . . . . . . . . . . . . 18
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1. Introduction
This document describes new options for DHCP, which provide a
mechanism for the delegation of IPv6 prefixes. Through these
options, a delegating router can delegate prefixes to authorised
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 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.
2. Terminology
This document uses the terminology defined in RFC2460 [2] and the
DHCP specification [6]. 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.
3. 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 RFC 2119 [1].
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4. Model and Applicability
The model of operation for prefix delegation is as follows. A
delegating router is provided DHCPv6 prefixes to be delegated to
requesting routers. Examples of ways in which the delegating router
may be provided these prefixes are given in Section 11.2. A
requesting router requests prefix(es) from the delegating router, as
described in Section 11.1. The delegating router chooses prefix(es)
for delegation, and returns the 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.
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.
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Figure 1 illustrates a network architecture in which prefix
delegation would be used.
+--------+ \
| AAA | \
| server | \
+---+----+ |
___|__________________ |
/ \ |
| ISP core network | |
\__________ ___________/ |
| | ISP
+-------+-------+ | network
| Aggregation | |
| device | |
| (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.
In this example an AAA server is configured with a prefix assigned to
the customer at the time of subscription 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 consults the AAA server to authenticate and authorise the
requesting router. The AAA server returns the subscriber's
prefix(es) in a Framed-IPv6-Prefix attribute as described in RFC 3162
[7], and the delegating router returns them to the requesting router.
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The requesting router assigns longer prefixes from the delegated
prefix for assignment 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.
5. 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 DHCPv6 specification [6]) 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 IDs 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
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 Section 8 for the representation of an IA_PD in a DHCP
message.
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6. 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.
7. 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 the section "Client Source Address and Interface
Selection" of the DHCP specification [6] 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
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address from the upstream interface and which is suitable for use by
the delegating router in responding to the requesting router.
8. 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 (TBD)
option-length: 12 + length of IA_PD-options field.
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 contacts
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 contacts
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.
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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 recontacts 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 0 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
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, 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.
9. 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.
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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 .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code: OPTION_IAPREFIX (TBD)
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
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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 "Router Configuration
Variables" of RFC2461 [3], unless administratively configured.
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.
10. Delegating Router Solicitation
The requesting router locates and selects a delegating router in the
same way as described in section "DHCP Server Solicitation" of the
DHCP specification [6]. The details of the solicitation process are
described in this section.
10.1 Requesting router behaviour
The requesting router creates and transmits a Solicit message as
described in sections "Creation of Solicit Messages" and
"Transmission of Solicit Messages" of the DHCP specification [6].
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 "Receipt of Advertise Messages" in the DHCP
specification [6]. 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.
10.2 Delegating router behaviour
The delegating router processes Solicit messages from requesting
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routers in the same way as described in section "Receipt of Solicit
messages" of the DHCP specification [6]. 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 [7].
If the delegating router cannot delegate any prefixes to an IA_PD in
the message from the requesting router, the delegating router MUST
include the IA_PD in the Reply message with no prefixes in the IA_PD
and a Status Code option in the IA_PD containing status code
NoPrefixAvail.
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.
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" in the DHCP specification [6].
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 a Status Code option with 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.
11. Requesting router initiated prefix delegation
A requesting router uses the same message exchanges as described in
section "DHCP Client-Initiated Configuration Exchange" of the DHCP
specification [6] 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.
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11.1 Requesting router behaviour
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 include 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.
If such verification is needed the requesting router MUST initiate a
Rebind/Reply message exchange as described in the section "Creation
and Transmission of Rebind Messages" of the DHCP specification [6],
with the exception that the retransmission parameters should be set
as for the Confirm message, described in the section "Creation and
Transmission of Confirm Messages" of the DHCP specification [6]. 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 duration is
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
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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:1::/48, it might assign 3FFE:FFFF:0:1::/64 and
3FFE:FFFF:1:1::/64 to one of the links, and 3FFE:FFFF:0:2::/64 and
3FFE:FFFF:1:2::/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.
11.2 Delegating Router behaviour
When a delegating router receives a Request message from a requesting
router that contains an IA_PD option, and the delegating router is
authorised 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 10.2 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 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. 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.
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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.
12. Prefix Delegation reconfiguration
This section describes prefix delegation in Reconfigure message
exchanges.
12.1 Delegating Router behaviour
The delegating router initiates a configuration message exchange with
a requesting router, as described in the section "DHCP Server-
Initiated Configuration Exchange" of the DHCP specification [6]. 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).
12.2 Requesting Router behaviour
The requesting router responds to a Reconfigure message received from
a delegating router as described in the DHCP specification [6]. 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.
13. Relay agent behaviour
A relay agent forwards messages containing Prefix Delegation options
in the same way as described in section "Relay Behaviour" of the DHCP
specification [6].
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.
14. Security Considerations
Security considerations in DHCP are described in the section
"Security Considerations" of the DHCP specification [6].
A rogue delegating router can issue bogus prefixes to a requesting
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router. This may cause denial of service due to unreachability.
An intruder 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 "Authentication of DHCP messages" in the DHCP
specification [6]. 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.
15. IANA Considerations
IANA is requested to assign option codes to these options from the
option-code space as defined in section "DHCPv6 Options" of the
DHCPv6 specification [6].
IANA is requested to assign a status code to the NoPrefixAvail status
code from the status-code space as defined in section "Status Codes"
of the DHCPv6 specification [6].
16. 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.
17. Changes since revision-01
o Clarified the usage of how Preferred/Valid lifetimes should be
used in Router Advertisements.
o Clarified the use of NoPrefixAvail in the case were the delegating
router cannot delegate any prefixes.
o Use Rebind/Reply message exchange for binding confirmation rather
than Renew/Reply.
Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
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[2] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC 2460, December 1998.
[3] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery for
IP Version 6 (IPv6)", RFC 2461, December 1998.
[4] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 2373, July 1998.
[5] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[6] Droms, R., "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", draft-ietf-dhc-dhcpv6-28 (work in progress), November
2002.
[7] Aboba, B., Zorn, G. and D. Mitton, "RADIUS and IPv6", RFC 3162,
August 2001.
Informative References
[8] Miyakawa, S., "Requirements for IPv6 prefix delegation", draft-
ietf-ipv6-prefix-delegation-requirement-00 (work in progress),
November 2002.
Authors' Addresses
Ole Troan
Cisco Systems
250 Longwater Avenue
Reading RG2 6GB
United Kingdom
Phone: +44 20 8824 8666
EMail: ot@cisco.com
Ralph Droms
Cisco Systems
300 Apollo Drive
Chelmsford, MA 01824
USA
Phone: +1 978 497 4733
EMail: rdroms@cisco.com
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