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Versions: 00 01

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

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   This Internet-Draft will expire on October 30, 2002.

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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Internet-Draft       IPv6 Prefix Options for DHCPv6             May 2002


Full Copyright Statement

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Troan & Droms           Expires October 30, 2002               [Page 16]


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