Internet Engineering Task Force                              W. Townsley
Internet-Draft                                                  O. Troan
Intended status: Standards Track                                   Cisco
Expires: April 29, 2010                                 October 26, 2009

                IPv6 via IPv4 Service Provider Networks

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
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   This Internet-Draft will expire on April 29, 2010.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.


   This document specifies a protocol mechanism tailored to advance
   deployment of IPv6 to end users via a Service Provider's IPv4 network
   infrastructure.  Key aspects include automatic IPv6 prefix delegation

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   to sites, stateless operation, simple provisioning, and service which
   is equivalent to native IPv6 at the sites which are served by the

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Requirements Language  . . . . . . . . . . . . . . . . . . . .  4
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  6rd Prefix Delegation  . . . . . . . . . . . . . . . . . . . .  5
   5.  Address selection  . . . . . . . . . . . . . . . . . . . . . .  7
   6.  Provisioning the 6rd CE router . . . . . . . . . . . . . . . .  7
     6.1.  6rd DHCP option  . . . . . . . . . . . . . . . . . . . . .  8
     6.2.  6rd with PPP . . . . . . . . . . . . . . . . . . . . . . .  9
   7.  Provisioning the Service Provider 6rd Border Relay . . . . . .  9
   8.  Encapsulation considerations . . . . . . . . . . . . . . . . . 10
     8.1.  Receiving Rules  . . . . . . . . . . . . . . . . . . . . . 11
   9.  Transition Considerations  . . . . . . . . . . . . . . . . . . 11
   10. Address space usage  . . . . . . . . . . . . . . . . . . . . . 11
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   12. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
   13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     14.1. Normative References . . . . . . . . . . . . . . . . . . . 13
     14.2. Informative References . . . . . . . . . . . . . . . . . . 14
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14

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1.  Introduction

   The original idea and the name of the mechanism (6rd) specified in
   this document is described in draft-despres-6rd [I-D.despres-6rd],
   which details a successful commercial "rapid deployment" of the 6rd
   mechanism by a residential Service Provider and is recommended
   background reading.

   This document describes the 6rd mechanism, extended for use in more
   general environments, and intended for advancement on the IETF
   Standards Track.  Throughout this document, the term 6rd is used to
   refer to the new mechanisms described here and 6to4 as that which is
   described in RFC 3056.

   6rd specifies a protocol mechanism to deploy IPv6 to sites via a
   Service Provider's (SP's) IPv4 network.  It builds on 6to4 [RFC3056],
   with the key differentiator that it utilizes an SP's own IPv6 address
   prefix rather than 2002::/16.  By using the SP's IPv6 prefix, the
   operational domain of 6rd is limited to the SP network and under its
   direct control.  From the perspective of customer sites and the IPv6
   Internet at large connected to the 6rd-enabled SP network, the IPv6
   service provided is equivalent to native IPv6.

   6rd encapsulates IPv6 in IPv4 with a destination IPv4 address which
   is either encoded within the IPv6 destination address itself (for
   packets destined to an RG within a 6rd domain), or is the destination
   address of a preconfigured 6rd Border Relay (BR) router that will
   decapsulate the IPv4 header and route the IPv6 packet to its
   destination.  This way, IPv6 packets encapsulated by 6rd follow the
   IPv4 routing topology within the SP network among RGs and BRs.  BRs
   are traversed only for IPv6 packets that are destined to or are
   arriving from outside the SP's 6rd domain .  The 6rd mechanism is
   fully stateless, so the Border Relay routers may be addressed via
   anycast within the SP network for added resiliency.

   The 6rd Customer Edge router (6rd CE) plays a critical role in a 6rd
   deployment. 6rd decouples deployment of IPv6 on the "LAN" side of the
   6rd CE router from that on the "WAN" side, allowing IPv6 on either
   side to be deployed and evolve independently.  On the LAN (e.g.,
   "Home") side of the 6rd CE router, 6rd expects that IPv6 is
   implemented as it would be for any native dual-stack IP service
   delivered by the SP.  On the WAN side of the 6rd CE router, the 6rd
   CE WAN interface itself, the access network between it and partnering
   6rd equipment, and the OSS system including DHCP, AAA, etc. may
   remain IPv4-only (e.g., there is no need to deploy DHCPv6 [RFC3315],
   IPv6 Neighbor Discovery, IPv6 routing, create IPv6 address plans,
   etc. within the SP network to deliver IPv6 to the customer site).

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   6rd relies on IPv4 and is designed to deliver "production-quality"
   IPv6.  IPv6 deployment within the SP network itself (rather than
   tunneled via 6rd) may continue for the SP's own purposes aside of
   delivering IPv6 service to its subscribers.  Once the SP access
   network can support native IPv6 access and transport, 6rd may be
   discontinued.  IPv4 may also eventually be turned off or tunneled
   over IPv6 as described in draft-ietf-softwire-dual-stack-lite

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  Terminology

   6rd Delegated Prefix  The IPv6 prefix determined by the 6rd CE device
                         for use by hosts within the customer site.
                         This prefix can be considered logically
                         equivalent to a DHCPv6 IPv6 Delegated Prefix
                         [RFC3633], though it is calculated by combining
                         the 6rd SP Prefix and the end user's IPv4
                         address obtained via IPv4 configuration

   6rd SP Prefix         An IPv6 prefix selected by the Service Provider
                         for use by a given 6rd deployment.  This may be
                         the entire IPv6 prefix obtained from an RIR and
                         announced to the IPv6 Internet, or a more-
                         specific assigned just to given 6rd deployment.

   6rd CE                The 6rd "Customer Edge" router that sits
                         between an IPv6-enabled site and an IPv4-enable
                         SP network.  In a residential broadband
                         deployment this is sometimes referred to as the
                         "Residential Gateway (RG)," "Customer Premises
                         Equipment," (CPE) or "Internet Gateway Device"
                         (IGD).  This router has a one internal 6rd
                         Virtual Interface acting as an endpoint for the
                         IPv6 in IPv4 encapsulation and forwarding, at
                         least one "6rd CE LAN Side" interface and "6rd
                         CE WAN Side" interface, respectively.

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   6rd CE LAN Side       The functionality of a 6rd CE router that
                         serves the "Local Area Network (LAN)" or "Home"
                         side of a broadband service provider
                         connection.  The 6rd CE LAN Side interface is
                         fully IPv6 enabled.

   6rd CE WAN Side       The functionality of a 6rd CE Router that
                         serves the "Wide Area Network" or "Service
                         Provider" side of the 6rd CE Router.  The 6rd
                         CE WAN side is IPv4-only, except that it
                         delivers IPv6 packets encapsulated in IPv4 by
                         the 6rd Virtual Interface.

   6rd BR                A 6rd-enabled "Border Relay" router located at
                         the SP premises.  The 6rd BR router has at
                         least one IPv4 interface, an internal 6rd
                         Virtual Interface for multi-point tunneling,
                         and at least one IPv6 interface that is
                         reachable via the IPv6 Internet or IPv6-enabled
                         portion of the SP network.

   6rd Virtual Interface Internal multi-point tunnel interface where 6rd
                         encapsulation and decapsulation of IPv6 packets
                         inside IPv4 occurs.  A typical 6rd CE or 6rd BR
                         implementation requires one 6rd Virtual

   Subscriber IPv4 address  The IPv4 address given to the subscriber as
                         part of normal IPv4 Internet access (i.e.,
                         configured via DHCP, PPP, or otherwise).  This
                         address may be global or private (RFC1918)
                         within the 6rd domain.  This address is used by
                         6rd to create the 6rd IPv6 prefix, and to send
                         and receive IPv6 packets encapsulated in IPv4
                         by the 6rd mechanism.

4.  6rd Prefix Delegation

   In 6rd, a customer site's IPv6 Delegated Prefix is derived from 2

   1.  An IPv6 Prefix selected by the SP to be the common 6rd SP Prefix
       for the given 6rd deployment (an SP can have multiple 6rd
       deployments called domains).

   2.  An assigned IPv4 address for the subscriber.  This IPv4 address
       may be a global IPv4 address, or a Private RFC 1918 [RFC1918]

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       IPv4 address.

   From these two elements, the 6rd Delegated Prefix is automatically
   created for the customer site when IPv4 service is obtained.  From
   the perspective of the 6rd CE LAN-Side functionality, this IPv6
   delegated prefix is used in the same manner as a prefix obtained via
   DHCPv6 Prefix Delegation [RFC3633].

   In 6to4, the location of the stored 32-bit IPv4 address is at a fixed
   location within the IPv6 address.  In 6rd it varies, so the size of
   the SP IPv6 prefix is important.  Also, in 6rd the SP chooses how
   many suffix bits of the IPv4 prefix are used in the algorithm to
   create the IPv6 prefix for its subscribers.  This allows the SP to
   adjust the balance between IPv6 addresses used by the 6rd mechanism,
   and how many are left to be delegated to end user sites.  To allow
   for stateless address auto-configuration and sub delegation a 6rd
   delegated prefix MUST be shorter than a /64.

   The 6rd Delegated Prefix is created by concatenating the 2 items
   above in order.  The sum of the number of bits used by each
   determines the size of the prefix that is delegated to the 6rd CE
   router for use by the customer site.

       /n            + (<= 32)  + (<= 16)   +      64       = 128 bits
   | 6rd SP Prefix   |v4 address| Subnet ID |      Interface ID       |
   |<---6rd Delegated Prefix--->|<---  End user's address space  ---->|

                                 Figure 1

   For example, if the 6rd SP Prefix is a /32, the v4suffix-length for
   the 6rd domain is 24, the shortest possible delegated IPv6 prefix for
   each subscriber is /56 (32 + 24 = 56).

   Embedding less than the full 32 bits of an IPv4 address is possible
   only with an aggregated block of IPv4 addresses for a given 6rd SP
   Prefix.  This may not be practical for global IPv4 addresses at a
   given SP, but is quite likely in a deployment where private addresses
   are being assigned to end users (for example  If private
   addresses overlap within a given 6rd deployment, the deployment may
   be subdivided into separate 6rd Domains, likely along the same
   topology lines the NAT-based IPv4 deployment itself would also
   require.  In this case, each domain is addressed with a different 6rd
   SP Prefix.

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   Multiple encodings are possible within a single 6rd deployment.  For
   example, if global and private IPv4 addresses are used within the
   same 6rd site, and the number of IPv4 bits encoded in the IPv6
   Delegated Prefix varies between the two (e.g., 32 bits for global,
   and 24 bits for private), then a different 6rd SP Prefix must be used
   to disambiguate the two different encoding settings.

   Since 6rd IPv6 prefixes are selected algorithmically from an IPv4
   address, changing the IPv4 address will cause a change in the IPv6
   delegated prefix which would normally ripple through the site's
   network and be disruptive.  As such, if possible the service provider
   should utilize a long-lived IPv4 address assignment for a given end

   6rd IPv6 address assignment and hence the IPv6 service itself is tied
   to the IPv4 address lease (whether set via DHCP, PPP, or otherwise),
   thus the 6rd service is also tied to this in terms of authorization,
   accounting, etc.  For example, the 6rd Delegated Prefix has the same
   DHCP lease time as its associated IPv4 address.  The prefix lifetimes
   advertised in Router Advertisements or used by DHCP on the 6rd CE LAN
   side MUST be equal or shorter than the IPv4 address lease time.

   For trouble-shooting and traceability, a 6rd IPv6 address and the
   associated IPv4 address for the same site can always be determined
   algorithmically.  This may be useful for referencing logs and other
   data at an SP that may be limited to IPv4 address assignment

5.  Address selection

   A 6rd delegated prefix is a native IPv6 prefix in the LAN-side of 6rd
   CE.  For the purpose of source and destination address selection the
   prefix should be treated as native IPv6 and no changes to the source
   address selection or destination address selection policy table
   [RFC3484] is needed.

6.  Provisioning the 6rd CE router

   The 6rd CE router must be configured with the 6rd SP prefix, the
   common IPv4 prefix length and a 6rd BR router IPv4 address.  A given
   6rd CE router is expected to exist in only one 6rd Domain (as
   indicated by the 6rd SP prefix).

   As the information is the same for every user within a 6rd domain,
   this information can be configured into the device in a variety of
   ways.  Automatic provisioning methods such as the Broadband Forum's

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   "TR-69" Residential Gateway management interface, a specific XML-
   based object retrieved after IPv4 connectivity is established, DNS,
   SNMP MIB, or manual configuration are all possible methods of
   configuration.  This document describes how to configure the
   nececssary parameters via a single DHCP object.

6.1.  6rd DHCP option

   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_6RD   |      len      |v4suffix-length|v6prefix-length|
   |              6rd Border Relay IPv4 Address (4 octets)         |
   |                                                               |
   |                        SP 6rd SP Prefix                       |
   |                   (variable, up to 16 octets)                 |
   |                                                               |
   |                                                               |
   |                                                               |

                                 Figure 2

   option code               OPTION_6RD(TBD)

   len                       Total length of option in octets.

   v4suffix-length           v4suffix-length is the number of low-order
                             bits that are not identical across all
                             subscriber IPv4 addresses within a given
                             6rd domain.  If there are no identical
                             bits, the v4suffix-length is 32 and the
                             entire subscriber IPv4 address is used to
                             create a 6rd delegated prefix.  A value of
                             0, and any value greater than 32, is
                             invalid and should cause 6rd setup to fail.

   v6prefix-length           IPv6 Prefix length of the SP IPv6 prefix in
                             number of bits.

   6rd BR IPv4 address       The IPv4 address of the 6rd Relay (may be

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   SP 6rd IPv6 prefix        Variable length field containing the
                             Service Provider's 6rd IPv6 prefix for this
                             deployment and this 6rd CE router, zero
                             padded to at least a full octet.  Length of
                             the field is determined by the reported
                             length of the entire DHCP option (len).  An
                             implementation must handle receipt of this
                             option with zero padding up to a full 16
                             octets, for deployments preferring to send
                             a fixed size option.

   The client must validate the values received in the option as

   The 6rd IPv6 prefix includes the domain ID embedded within it, sizing
   the v6prefix-length accordingly to cover both the 6rd SP prefix size
   and domain ID for this 6rd route entry.

   The 6rd CE router MUST install a default route to the relay.  It
   should also install a sink route for the delegated prefix.  As an
   example using a subscriber IPv4 address of, a 6rd IPv4
   relay address of, a v4suffix-length of 24 and 2001:ABC0::/32
   as the SP 6rd IPv6 prefix, the RIB will look like:

      ::/0 -> 2001:ABC0:0000:0100::   (default route)
      2001:ABC0:6464:0100::/56 -> Null0 (6rd prefix sink route)

6.2.  6rd with PPP

   For PPP-based CE devices, some IPv4 parameters are received via IPCP
   and others via the DHCPINFORM message which is sent after IPCP
   completes.  While the 6rd provisioning information could be sent in
   IPCP as well, use of DHCPINFORM is advised as it does not require
   changes to the access network equipment to support the new PPP
   option.  Note that a DHCP server which replies to all DHCPINFORM
   messages with nothing but the configured 6rd parameters is sufficient
   for a given 6rd domain (i.e., there is no need to deploy DHCP in a
   manner which can differentiate between users and offer different 6rd
   parameters to each).  Alternatively, the DHCP Server may be embedded
   within the router terminating PPP sessions, allowing for user-
   specific policy similar to as if the function were part of IPCP.

7.  Provisioning the Service Provider 6rd Border Relay

   As the 6rd IPv4 relay address is configurable, there is no need for a
   well known anycast address as specified in RFC3068 [RFC3068].  For

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   increased reliability and load-balancing, the relay address can be an
   anycast address shared by all of the SP BRs for a given 6rd Domain.
   As 6rd is stateless, any BR may be used at any time.  The 6rd relay
   MUST use its anycast IPv4 address as the source address in packets
   relayed via the SP network to the 6rd CE router.

   Since 6rd uses provider address space, no specific routes need to be
   advertised externally for 6rd to work, neither in IPv6 nor IPv4 BGP.
   However, the 6rd IPv4 relay anycast addresses must be advertised in
   the providers IGP.

   This example shows how the 6rd prefix is created based on a /32 IPv6
   prefix using RFC1918 address space from

       SP IPv6 prefix: 2001:0DB8::/32
       v4suffix-length: 24 (from 10/8, first octet (10) is excluded
                            from the encoding)
       6rd CE router IPv4 address:
       6rd site IPv6 prefix: 2001:0DB8:6464:0100::/56

8.  Encapsulation considerations

   IPv6 in IPv4 encapsulation is done as specified in 6to4 [RFC3056] and
   in Basic Transition Mechanisms for IPv6 Hosts and Routers [RFC4213].

   IPv6 packets from a 6rd CE router are encapsulated in IPv4 packets
   when they leave the site via its 6rd CE WAN side interface.  The
   Subscriber IPv4 address MUST be configured to send and receive
   packets on this interface.

   MTU and fragmentation issues for IPv6 in IPv4 tunnelling is discussed
   in detail in section 3.2 of RFC4213 [RFC4213]. 6rd's scope is limited
   to a service provider network.  If the MTU is well-managed such that
   the IPv4 MTU on the 6rd CE WAN interface is set so that no
   fragmentation occurs within the boundary of the SP, then the IPv6 MTU
   should be set to the IPv4 MTU minus the size of the encapsulating
   IPv4 header (20 bytes).  IPv4 Path MTU discovery MAY be used to
   adjust the MTU of the tunnel as described in section 3.2.2 of RFC4213
   [RFC4213] or the IPv6 tunnel MTU may be explicitly configured.

   The IPv6 tunnel MTU, whether determined automatically or configured
   directly, SHOULD be advertised on the LAN-side by setting the MTU
   option in Router Advertisements [RFC4861] messages to the IPv6 tunnel

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8.1.  Receiving Rules

   In order to prevent spoofing of IPv6 addresses, the BR and CE MUST
   validate the source address of the encapsulated IPv6 packet with the
   address of the IPv4 it is encapsulated by.  If the addresses do not
   match, the packet is dropped.

   The 6rd CE router should drop packets received on the 6rd virtual
   interface for destinations not covered by the 6rd Delegated prefix.

9.  Transition Considerations

   6rd is intended to deliver a production-level service to customer
   sites.  Once 6rd IPv6 access is available, the SP network can migrate
   to IPv6 at its own pace with little or no affect on the customer.
   When native IPv6 is fully available, the 6rd CE router is provisioned
   with IPv6 on its WAN side. 6rd and native IPv6 can coexist for a time
   while the customer site is adopts the new IPv6 native prefix, and
   then 6rd deprovisioned.  Alternatively, the same numbering plan for
   6rd may be used for the native service, though this might require a
   "flag day" when the 6rd service is turned off and native service is

   While 6rd bears resemblance to 6to4 and utilizes the same
   encapsulation and base mechanisms, it is not intended as a
   replacement for 6to4.  Unlike 6to4, 6rd is for use only in an
   environment where a service provider cooperates closely to deliver
   the IPv6 service. 6to4 routes with the 2002::/16 prefix may exist
   alongside 6rd in the 6rd CE router, and doing so may offer some
   efficiencies when communicating directly with 6to4 routers.

10.  Address space usage

   The 6rd prefix is an RIR delegated IPv6 prefix.  It must encapsulate
   an IPv4 address and must be short enough so that a /56 or /60 can be
   given to subscribers.  Using the full IPv4 address assigning a /56
   for subscribers would mean that each service provider using 6rd would
   require a /24 for 6rd in addition to other IPv6 address needs they
   have.  Assuming that 6rd would be stunningly successful and taken up
   by almost all AS number holders (32K) then the total address usage of
   6rd would be equivalent to a /9.  If instead delegated /60s to
   subscribers the service provider would require a /28 and the total
   global address consumption by 6rd would be equivalent to a /13.
   Again, this assumes that 6rd is used by all AS number holders in the
   IPv4 Internet today at the same time, and that none have moved to
   native IPv6 and reclaimed the 6rd space which was being used.

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   As 6rd uses service provider address space, 6rd uses the normal
   address delegation a service provider gets from its RIR and no global
   allocation of a single 6rd address block like for example the 6to4
   2002::/16 is needed.

   The 6rd address block can be reclaimed when all users of it has
   transitioned out of it into native IPv6 service.  This requires
   renumbering and usage of additional address space during the
   transition period.

   To alleviate concerns about address usage 6rd allows for leaving out
   redundant IPv4 prefix bits in the encoding of the IPv4 address inside
   the 6rd IPv6 address.  This is most useful where the IPv4 address
   space is very well aggregated.  For example to provide each customer
   with a /60, if a service provider has all its IPv4 customers under a
   /12 then only 20 bits needs to be used to encode the IPv4 address and
   the service provider would only need a /40 IPv6 allocation for 6rd.
   If private address space is used then a 10/8 would require a /36.  If
   multiple 10/8 domains are used then up to 16 could be supported
   within a /32.

11.  Security Considerations

   A 6to4 router as specified in RFC 3056 [RFC3056] can be used as an
   open relay.  It can be used to relay IPv6 traffic and as a traffic
   anonymizer.  By restricting the 6rd Domain to within a provider
   network a 6rd CE router only needs to accept packets from a single or
   small set of known 6rd relay routers.  As such many of the threats
   against 6to4 as described in RFC3964 [RFC3964] do not apply.

   When applying the receiving rules Section 8.1 IPv6 packets are as
   well protected against spoofing as IPv4 packets are within an SP

   A malicious user that is aware of a 6rd domain and the 6rd BR IPv4
   address could use this information to construct a packet that would
   cause a Border Relay Router to reflect tunneled packets outside of
   the domain that it is serving.  If the attacker constructs the packet
   accordingly, and can inject a packet with an IPv6 source address that
   looks as if it originates from within the 6rd domain of the second
   border relay, forwarding loops between 6rd domains may be created,
   allowing the malicious user to launch a packet amplification attack
   between 6rd domains.

   One possible mitigation for this is to simply not allow the 6rd BR
   IPv4 address to be reachable from outside the SP's 6rd domain.  In
   this case, carefully constructed IPv6 packets still may be reflected

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   off a single BR, but the looping condition will not occur.  Tunnelled
   packets with the 6rd BR IPv4 address as the source address may also
   be filtered to prohibit 6rd tunnels from exiting the 6rd domain.

   To avoid forwarding loops via other internal relays, the 6rd BR
   should employ outgoing and incoming IPv4 packets filters, filtering
   out all known relay addresses for internal ISATAP or 6to4 relays,
   including the well known anycast address space for 6to4.

   The 6rd BR MUST install a sink route for its 6rd delegated prefix
   created based on its 6rd BR IPv4 address.

12.  IANA Considerations

   IANA is requested to assign a new DHCP Option code point for

13.  Acknowledgements

   This draft is based on Remi Despres' original idea described in
   [I-D.despres-6rd] and the work done by Rani Assaf, Alexandre Cassen,
   and Maxime Bizon at Free Telecom.  Brian Carpenter and Keith Moore
   documented 6to4, which all of this work is based upon.  Review and
   encouragement have been provided by many others and in particular
   Chris Chase, Thomas Clausen, Wojciech Dec, Bruno Decraene, Remi
   Despres, Alain Durand, Martin Gysi, Eric Voit and David Ward.

14.  References

14.1.  Normative References

   [RFC1332]  McGregor, G., "The PPP Internet Protocol Control Protocol
              (IPCP)", RFC 1332, May 1992.

   [RFC1661]  Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51,
              RFC 1661, July 1994.

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
              E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, February 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2516]  Mamakos, L., Lidl, K., Evarts, J., Carrel, D., Simone, D.,

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              and R. Wheeler, "A Method for Transmitting PPP Over
              Ethernet (PPPoE)", RFC 2516, February 1999.

   [RFC3056]  Carpenter, B. and K. Moore, "Connection of IPv6 Domains
              via IPv4 Clouds", RFC 3056, February 2001.

   [RFC3315]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
              and M. Carney, "Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 3315, July 2003.

   [RFC3633]  Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
              Host Configuration Protocol (DHCP) version 6", RFC 3633,
              December 2003.

   [RFC3964]  Savola, P. and C. Patel, "Security Considerations for
              6to4", RFC 3964, December 2004.

   [RFC4213]  Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
              for IPv6 Hosts and Routers", RFC 4213, October 2005.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.

14.2.  Informative References

              Despres, R., "IPv6 Rapid Deployment on IPv4
              infrastructures (6rd)", draft-despres-6rd-03 (work in
              progress), April 2009.

              Durand, A., Droms, R., Haberman, B., and J. Woodyatt,
              "Dual-stack lite broadband deployments post IPv4
              exhaustion", draft-durand-softwire-dual-stack-lite-01
              (work in progress), November 2008.

   [RFC3068]  Huitema, C., "An Anycast Prefix for 6to4 Relay Routers",
              RFC 3068, June 2001.

   [RFC3484]  Draves, R., "Default Address Selection for Internet
              Protocol version 6 (IPv6)", RFC 3484, February 2003.

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Authors' Addresses

   Mark Townsley

   Phone: +33 15 804 3483

   Ole Troan

   Phone: +47 917 38519

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