Network Working Group O. Troan
Internet-Draft R. Droms
Expires: October 30, 2002 Cisco Systems
May 1, 2002
IPv6 Prefix Options for DHCPv6
draft-troan-dhcpv6-opt-prefix-delegation-01.txt
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Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
The Prefix Delegation option and the Prefix Request option provide a
mechanism for delegation of IPv6 prefixes using DHCP. This prefix
delegation mechanism is intended for simple prefix delegation 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 . . . . . . . . . . . . . . . . . . 3
5. Overview of DHCP with Prefix Delegation . . . . . . . . . . 6
6. Prefix Delegation Option . . . . . . . . . . . . . . . . . . 7
7. Prefix Information option . . . . . . . . . . . . . . . . . 7
8. Prefix Request option . . . . . . . . . . . . . . . . . . . 9
9. Appearance of these options . . . . . . . . . . . . . . . . 9
10. Delegating Router Solicitation . . . . . . . . . . . . . . . 9
10.1 Requesting router behavior . . . . . . . . . . . . . . . . . 10
10.2 Delegating router behavior . . . . . . . . . . . . . . . . . 10
11. Requesting router initiated prefix delegation . . . . . . . 10
11.1 Requesting router behavior . . . . . . . . . . . . . . . . . 11
11.2 Delegating Router Behavior . . . . . . . . . . . . . . . . . 12
12. Prefix Delegation reconfiguration . . . . . . . . . . . . . 12
12.1 Delegating Router behavior . . . . . . . . . . . . . . . . . 12
12.2 Requesting Router behavior . . . . . . . . . . . . . . . . . 13
13. Relay agent behavior . . . . . . . . . . . . . . . . . . . . 13
14. Security Considerations . . . . . . . . . . . . . . . . . . 13
15. IANA Considerations . . . . . . . . . . . . . . . . . . . . 14
16. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 14
References . . . . . . . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 14
Full Copyright Statement . . . . . . . . . . . . . . . . . . 16
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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, an authorized delegating router can delegate prefixes to
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, the Prefix Delegation and Prefix Request 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.
2. Terminology
This document uses the terminology defined in RFC2460 [2] and the
DHCP specification [5]. In addition, this document uses the
following terms:
Requesting Router: The router that acts as a DHCP client and is
requesting that prefix(es) be assigned.
Delegating Router: The router that acts as a DHCP server, and is
responding to the prefix request.
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].
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
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prefix(es). For example, the requesting router might assign a
delegated prefix to one of its interfaces, and begin sending router
advertisements for the prefix on that link.
Each prefix has an associated lease, 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 lease on a delegated prefix and is required to terminate the use
of a delegated prefix if the lease on 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, the delegating router 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
authorize the requesting router. The AAA server returns the
subscriber's prefix in an Framed-IPv6-Prefix attribute as described
in RFC 3162 [6], 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. Overview of DHCP with Prefix Delegation
Prefix delegation with DHCP is independent of address assignment and
other configuration information. A requesting router can use DHCP
for just prefix delegation or for prefix delegation along with
address assignment and configuration.
To locate a delegating router, the requesting router sends an initial
Solicit message as described in the DHCP specification [5]. The
responding delegating router includes a Prefix Delegation option that
identifies the prefix(es) to be delegated in the Advertise message it
returns to the requesting router.
When the requesting router has identified a delegating router, the
requesting router sends a Request message to the delegating router.
The delegating router returns a Prefix Delegation option with the
prefix(es) to be delegated in the Reply message it sends to the
requesting router. The requesting router records the lease duration
time for the delegated prefix(es) and uses the prefix(es) as
described in the previous section.
Under the conditions decsribed in the DHCPv6 specification [5], the
requesting router sends a Confirm message to verify that its
delegated prefix(es) are still valid. The reqeuesting router
responds with a Reply message containing a Prefix Delegation option
with the delegated prefix(es). The requesting router may choose to
extend the lease on the delegated prefix(es) in the Reply message.
Before the lease on each delegated prefix expires, the requesting
router includes the prefix in a Prefix Delegation option sent in a
Renew message to the delegating router. The delegating router
responds by returning the prefix with an updated lease to the
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requesting router.
6. Prefix Delegation Option
The Prefix Delegation option encapsulates other options that are used
for prefix delegation. The Prefix Delegation option must include at
least one Prefix Information option.
The format of the Prefix Delegation 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_PREFIXDEL | option-length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. options .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code: OPTION_PREFIXDEL (TBD)
option-length: Length of the 'options' field in octets.
options: Options associated with Prefix Delegation.
7. Prefix Information option
One or more Prefix Information options are encapsulated in a Prefix
Delegation option and are used by the delegating router to inform a
requesting router about a prefix or prefixes.
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The format of the Prefix Information 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_PREFIX_INFO | option-length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| lease-duration |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| prefix-length | |
+-+-+-+-+-+-+-+-+ IPv6 prefix |
| (16 octets) |
| |
| |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+
option-code: OPTION_PREFIX_INFO (TBD)
option-length: 21
lease duration: The duration of the lease for the IPv6 prefix in the
option
prefix-length: Length for this prefix
IPv6-prefix: An IPv6 prefix
The lease-duration is expressed in seconds, and a value of 0xFFFFFFFF
indicates infinity. The prefix-length gives the number of bits in
the prefix carried in this option. The IPv6-prefix field contains
the delegated prefix padded on the right with additional bits to fill
the field. The pad bits are set to 0 by the sender and ignored by
the receiver.
A requesting router MAY include a Prefix Delegation option in a
Solicit, Request, Renew, Rebind or Confirm message to inform the
delegating router about the prefixes the requesting router wants the
delegating router to send.
In a message sent by a requesting router to a delegating router, the
value in the lease duration field indicates the requesting router's
preference. The requesting router may send a value of zero to
indicate no preference.
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8. Prefix Request option
The Prefix Request option is only sent by the requesting router and
MAY be used to indicate properties of the prefixes it wants
delegated.
The format of the Prefix Request 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_PREFIXREQ | option-length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length |
+-+-+-+-+-+-+-+-+
option-code: OPTION_PREFIXREQ (TBD)
option-len: 1
length: The length of the requested prefix.
This option MUST not appear together with the Prefix Delegation
option.
A requesting router MUST ignore any Prefix Request options in
messages it receives.
9. Appearance of these options
The Prefix Delegation option MUST only appear in the options section
of the following DHCP messages: Solicit, Advertise, Request, Confirm,
Renew, Rebind, Release, Reply.
The Prefix Information option MUST be encapsulated by the Prefix
Delegation option.
The Prefix request option MUST only appear in the options section of
the following DHCP messages: Solicit, Request.
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 [5]. The details of the solicitation process are
described in this section.
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10.1 Requesting router behavior
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 [5].
The requesting router MUST include either an Option Request option
specifying the Prefix Delegation option or a Prefix Request option.
The requesting router processes any received Advertise messages as
described in section "Receipt of Advertise Messages" in the DHCP
specification [5]. The requesting router MAY choose to consider the
presence of advertised prefixes in its decision about which
delegating router to respond to.
10.2 Delegating router behavior
The delegating router processes Solicit messages from requesting
routers in the same way as described in section "Receipt of Solicit
messages" of the DHCP specification [5]. If the message contains
Prefix Request option or an Option Request option specifying the
Prefix Request 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.
If the requesting router includes a Prefix Request option in its
Solicit message, the delegating router MAY choose to use the
information in that option to select the prefix(es) 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 [5].
The delegating router MUST include a Prefix Delegation option,
identifying any prefix(es) that the delegating router will delegate
to the requesting router.
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 [5] to obtain or update prefix(es) from a delegating
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router. The requesting router and the delegating router use the
Prefix Delegation option to exchange information about prefix(es) in
much the same way IA Address options are used for assigned addresses.
11.1 Requesting router behavior
To obtain prefix(es) from the delegating router, the requesting
router MUST include the Prefix Information option (in a Prefix
Delegation option) in the Request message it sends to the delegating
router identifying the prefix(es) sent from the delegating router to
the requesting router in the Advertise message received by the
requesting router.
The requesting router MUST include the Prefix Information options,
identifying the prefix(es) that have been previously delegated by the
delegating router, in any Confirm, Renew, or Rebind messages sent by
the requesting router.
Each prefix has an associated lease whose duration is specified in
the Prefix Information option for that prefix. The requesting router
uses Renew and Rebind messages to request the extension of the lease
on a delegated prefix. The recommended time to begin lease extension
with a Renew message is 0.5 times the lease duration, and the
recommended time to begin lease extension with a Rebind message is
0.8 times the lease duration.
The requesting router uses a Release message to return a delegated
prefix to a delegating router.
The requesting router assigns a subnet from each of the delegated
prefixes to each of the links to which it is 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:0:0001::/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
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advertisements for the prefix on the link, the requesting router MUST
set the valid lifetime and the preferred lifetime for that prefix to
expire no later than the expiration of the lease on the prefix. The
recommended preferred lifetime is 0.8 times the lease duration and
the recommended valid lifetime is equal to the lease duration.
11.2 Delegating Router Behavior
When a delegating router receives a Request message from a requesting
router that contains a Prefix Delegation 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. If the requesting router includes a Prefix
Request option in its Solicit message, the delegating router MAY
choose to use the information in that option to select 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 to be delegated to
a requesting router.
A delegating router examines the prefix(es) identified in Prefix
Information options (in a Prefix Delegation option) in Confirm, Renew
and Rebind messages and responds according to the current status of
the prefix(es). The delegating router returns a Prefix Information
option with an updated lease duration for each valid prefix in the
message from the requesting router.
A delegating router may mark any prefix(es) in Prefix Information
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 a Prefix Information option or
options (in a Prefix Delegation 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 behavior
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 [5]. The
delegating router specifies the Prefix Delegation option in the
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Option Request option to cause the requesting router to include a
Prefix Delegation option to obtain new information about delegated
prefix(es).
12.2 Requesting Router behavior
The requesting router responds to a Reconfigure message received from
a delegating router as described in the DHCP specification [5]. The
requesting router MUST include the Prefix Information option (in a
Prefix Delegation 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 behavior
A relay agent forwards messages containing prefix delegation options
in the same way as described in section "Relay Behavior" of the DHCP
specification [5].
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 [5].
Prefix delegation can be used to mount a denial of service attack or
a man-in-the-middle attack against an organization by delegating
invalid prefixes to a requesting router, causing the requesting
router to forward outbound datagrams to an invalid destination or to
an intruder's destination host.
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 [5]. 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 is not 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
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for DHCPv6 are under development.
15. IANA Considerations
IANA is requested to assign an option code to these options from the
option-code space defined in section "DHCPv6 Options" of the DHCPv6
specification [5].
16. Acknowledgements
Thanks for the input and review by (in alphabetical order) Steve
Deering, Dave Forster, Shin Mayakawa, Pekka Savola, Yamasaki Toshi
and Bernie Volz.
References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC 2460, December 1998.
[3] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 2373, July 1998.
[4] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[5] Bound, J., Carney, M., Perkins, C., Lemon, T., Volz, B. and R.
Droms (ed.), "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", draft-ietf-dhc-dhcpv6-23 (work in progress), February
2002.
[6] Aboba, B., Zorn, G. and D. Mitton, "RADIUS and IPv6", RFC 3162,
August 2001.
Authors' Addresses
Ole Troan
Cisco Systems
4 The Square
Stockley Park
Uxbridge UB11 1BL
United Kingdom
Phone: +44 20 8824 8666
EMail: ot@cisco.com
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Ralph Droms
Cisco Systems
300 Apollo Drive
Chelmsford, MA 01824
USA
Phone: +1 978 497 4733
EMail: rdroms@cisco.com
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