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Versions: 00 01

Network Working Group                                           O. Troan
Internet-Draft                                                  R. Droms
Expires: August 14, 2002                                   Cisco Systems
                                                       February 13, 2002


                     IPv6 Prefix Options for DHCPv6
            draft-troan-dhcpv6-opt-prefix-delegation-00.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
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   This Internet-Draft will expire on August 14, 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.  Conceptually,
   IPv6 prefixes are assigned with these options in the same manner as
   IPv6 addresses.  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.  Prefix delegation options  . . . . . . . . . . . . . . . . . .  6
   5.1 IA Prefix option . . . . . . . . . . . . . . . . . . . . . . .  7
   5.2 IA Prefix Request option . . . . . . . . . . . . . . . . . . .  8
   6.  Message Validation . . . . . . . . . . . . . . . . . . . . . .  8
   7.  Delegating Router Solicitation . . . . . . . . . . . . . . . .  8
   7.1 Requesting router behavior . . . . . . . . . . . . . . . . . .  9
   7.2 Delegating router behavior . . . . . . . . . . . . . . . . . .  9
   8.  Requesting-router-initiated prefix delegation  . . . . . . . . 10
   8.1 Requesting router behavior . . . . . . . . . . . . . . . . . . 10
   8.2 Delegating Router Behavior . . . . . . . . . . . . . . . . . . 11
   9.  Delegating Router-initiated prefix delegation reconfiguration  11
   9.1 Delegating Router behavior . . . . . . . . . . . . . . . . . . 11
   9.2 Requesting Router behvaior . . . . . . . . . . . . . . . . . . 12
   10. Relay agent behavior . . . . . . . . . . . . . . . . . . . . . 12
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 12
       References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 13
       Full Copyright Statement . . . . . . . . . . . . . . . . . . . 14



























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1. Introduction

   This document describes two new options for DHCP, which provide a
   mechanism for delegation of IPv6 prefixes.  Through these options, an
   authorized delegating router can delegate prefixes to a requesting
   router.

   The prefix delegation mechanism described in this document is
   intended for simple delegation of prefixes from a delegating router
   to a requesting router.  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 or prefixes 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 DHCP
   [5].  In addition, this document uses the following terms:

   Requesting Router: The router that acts as a DHCP client and is
      requesting that a prefix 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 8.2.  A
   requesting router requests a prefix or prefixes from the delegating
   router, as described in Section 8.1.  The delegating router chooses a
   prefix or prefixes for delegation, and returns those prefixes to the
   requesting router.  The requesting router is then responsible for the
   delegated prefix or prefixes.  For example, the requesting router



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   might assign a delegated prefix to a link to which the router has an
   interface, and begin sending router advertisements for the prefix on
   that link.

   Delegated prefixes are managed in the same way as assigned IPv6
   addresses in DHCP.  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.

   For example, 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 the
   identity of the requesting router.  The AAA server returns an
   acknowledgment of the requesting router's identity to the delegating
   router.  The delegating router locates the prefix that has been
   assigned to the subscriber and returns it to the requesting router.



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   In the case where the subscriber's network consists of a single
   internal link, the requesting router assigns the delegated prefix to
   the internal link.  If there are multiple internal links, as shown in
   figure 1, the requesting router can subnet a single delegated prefix
   into longer prefixes and assign them to the internal links.  Or, if
   the delegating router has delegated multiple prefixes, the requesting
   router can assign those prefixes to the internal links.

   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 delegating router in
   the requesting router.

5. Prefix delegation options

   Prefix delegation is accomlished with two options:

   IA Prefix option:           Used to inform a requesting router of a
      delegated prefix

   Prefix Request option:      Used by a requesting router to explicitly
      request a prefix or prefixes

























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5.1 IA Prefix option

   The format of the IA 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         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        lease-duration                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | prefix-length |          IPv6-prefix                          |
      +-+-+-+-+-+-+-+-+       (variable length)                       |
      .                                                               .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   option-code:     OPTION_IAPREFIX (TBD)

   option-length:   See section 23 of the DHCP specification

   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.  The prefix-length gives
   the number of bits in the prefix carried in this option.  To reduce
   the number of octets used for this option, the IPv6 prefix is
   represented in ceiling(prefix-length/8) octets.

   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 for those parameters.  The requesting router may send 0 if
   it has no preference for the lease duration.

   An IA Prefix option MUST only appear in an IA option.  One or more IA
   Prefix Options can appear anywhere in an IA option.










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5.2 IA Prefix Request option

   The format of the IA 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         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | prefix-length |  num-global   |    num-site   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   option-code:   OPTION_PREFIXREQ (TBD)

   option-len:    See section 23 of the DHCP specification

   prefix-length: Prefix length for the requested global-scope prefixes.
      A value of zero (0) indicates that the requesting router will
      accept any prefix length provided by the delegating router.

   num-global:    The number of global-scope prefixes requested.  A
      value of 0 indicates that the requesting router is not requesting
      any prefixes.  A value of -1 indicates that the requesting router
      does not indicate a preference.

   num-site:      The number of site-scope prefixes requested.  A value
      of 0 indicates that the requesting router is not requesting any
      prefixes.  A value of -1 indicates that the requesting router does
      not indicate a preference.

   A Prefix Request option MUST only appear in an IA in a message from a
   requesting router.

6. Message Validation

   A requesting router or a delegating router MUST ignore any IA Prefix
   option or Prefix Request option that does not appear in an IA option
   in messages it receives.

   A requesting router MUST ignore any Prefix Request options in
   messages it receives.

7. 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.  The details of the solicitation process are



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   described in this section.

7.1 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.  The
   requesting router MUST include at least one IA in which the
   delegating router will list any prefixes it advertises that it will
   delegate to the requesting router.  The requesting router MAY include
   a Prefix Request option to indicate the requesting router's
   preferences about prefixes it is requesting.

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

7.2 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.  If the message contains one or
   more IA options and the delegating router is configured to delegate a
   prefix or prefixes to the requesting router, the delegating router
   selects the prefix or prefixes to be delegated to the requesting
   router.  The mechanism through which the delegating router selects
   prefixes for delegation is not specified in this document.  Examples
   of ways in which the delegating router might select prefixes for a
   requesting router include: static assignment based on subscription to
   an ISP; dyanmic assignment from a pool of available prefixes;
   selection based on an external authority such as a RADIUS server.

   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 or prefixes 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.  The
   delegating router MUST include an IA Prefix option or options
   identifying any prefix or prefixes that the delegating router will
   delegate to the requesting router in an IA option or options in the
   Advertise message.





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8. 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 to obtain or update delegated prefixes from a
   delegating router.  The requesting router and the delegating router
   use the IA Prefix option to exchange information about prefixes in
   much the same way IA Address options are used for assigned addresses.

8.1 Requesting router behavior

   To obtain prefixes from the delegating router, the requesting router
   MUST include IA Prefix options (in IA options) identifying the prefix
   or prefixes sent from the delegating router to the requesting router
   in the Advertise message received by the requesting router.

   The requesting router MUST include IA Prefix options (in IA options)
   identifying the prefix or prefixes that have previously delegated
   from the delegating router in any Confirm, Renew, or Rebind messages
   send by the requesting router.

   Each prefix has an associated lease whose duration is specified in
   the IA Prefix option for that prefix.  The requesting router uses
   Renew and Rebind messages to request the extension of the lease on a
   delegated prefix.

   The requesting router uses a Release message to return a delegated
   prefix to a delegating router.

   The requesting router extracts any delegated prefixes as identified
   in IA Prefix options in Reply messages it receives.

   The way in which the requesting router uses delegated prefixes is not
   specified in this document.  As an example, the requesting router
   might subnet a delegated prefix and assign the longer prefixes to the
   internal links in the subscriber network shown in Figure 1.

   If the 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 were delegated
   DEAD:BEEF:CAFE:0::/48, it might generate DEAD:BEEF:CAFE:0001::/64 and
   DEAD:BEEF:CAFE:0002::/64 for assignment to the two links in the
   subscriber network.

   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 and the preferred lifetime for that prefix to



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   expire no later than the expiration of the lease on the prefix.

8.2 Delegating Router Behavior

   When a delegating router receives a Request message from a requesting
   router that contains one or more IA options and the delegating router
   is authorized to delegate a prefix or prefixes to the requesting
   router, the delegating router selects the prefix or prefixes 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 or prefixes to be delegated to the requesting
   router.  The mechanism through which the delegating router selects
   prefixes for delegation is not specified in this document.  Section
   7.2 gives examples of ways in which a delegating router might select
   prefixes to be delegated to a requesting router.

   A delegating router examines the prefixes identified in IA Prefix
   options in Confirm, Renew and Rebind messages and responds according
   to the current status of the prefix.  The delegating router returns
   an IA Prefix option with an updated lease duration for each valid
   prefix in the message from the requesting router.

   Upon the receipt of a valid Decline message, the delegating router
   examines the IA options and the IA Prefix options for validity.  If
   the IAs in the message are in a binding for the requesting router and
   the prefixes in the IAs have been assigned by the delegating router
   to those IA, the delegating router deletes the prefix(es) from the
   IAs.  The delegating router MAY choose to make a notification that
   prefixes were declined.

   A delegating router marks any prefixes in IA Prefix options in a
   Release message as "available".

   The delegating router MUST include an IA Prefix option or options in
   an IA option or options identifying any delegated prefixes in Reply
   messages sent to a requesting router.

9. Delegating Router-initiated prefix delegation reconfiguration

   This section describes prefix delegation in Reconfigure message
   exchanges.

9.1 Delegating Router behavior

   The delegating router initiates a configuration message exchange with
   a requesting router in the same way as a DHCP server, as described in
   the section "DHCP Server-Initiated Configuration Exchange" of the



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   DHCP specification.  The delegating router specifies the IA option in
   the Option Request option to cause the requesting router to include
   an IA option to obtain new information about delegated prefixes.

9.2 Requesting Router behvaior

   The requesting router responds to a Reconfigure message received from
   a delegating router in the same way as a DHCP client, as described in
   the DHCP specification.  The requesting router MUST include IA Prefix
   options for any prefixes that have been delegated to the requesting
   router by the delegating router from which the Reconfigure message
   was received.

10. 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.

11. Security Considerations

   Security considerations in DHCP are described in the section
   "Security Considerations" of the DHCP specification.

   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.

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



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        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.


Authors' Addresses

   Ole Troan
   Cisco Systems
   4 The Square
   Stockley Park
   Uxbridge  UB11 1BN
   United Kingdom

   Phone: +44 20 8756 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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Full Copyright Statement

   Copyright (C) The Internet Society (2002).  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
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   The limited permissions granted above are perpetual and will not be
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   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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   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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