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Independent Submission                                           T. Tsou
Request for Comments: 6654                     Huawei Technologies (USA)
Category: Informational                                          C. Zhou
ISSN: 2070-1721                                                T. Taylor
                                                     Huawei Technologies
                                                                 Q. Chen
                                                           China Telecom
                                                               July 2012

Gateway-Initiated IPv6 Rapid Deployment on IPv4 Infrastructures (GI 6rd)


   This document proposes an alternative IPv6 Rapid Deployment on IPv4
   Infrastructures (6rd) deployment model to that of RFC 5969.  The
   basic 6rd model allows IPv6 hosts to gain access to IPv6 networks
   across an IPv4 access network using 6-in-4 tunnels. 6rd requires
   support by a device (the 6rd customer edge, or 6rd-CE) on the
   customer site, which must also be assigned an IPv4 address.  The
   alternative model described in this document initiates the 6-in-4
   tunnels from an operator-owned Gateway collocated with the operator's
   IPv4 network edge rather than from customer equipment, and hence is
   termed "Gateway-initiated 6rd" (GI 6rd).  The advantages of this
   approach are that it requires no modification to customer equipment
   and avoids assignment of IPv4 addresses to customer equipment.  The
   latter point means less pressure on IPv4 addresses in a high-growth

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This is a contribution to the RFC Series, independently of any other
   RFC stream.  The RFC Editor has chosen to publish this document at
   its discretion and makes no statement about its value for
   implementation or deployment.  Documents approved for publication by
   the RFC Editor are not a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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RFC 6654                  Gateway-Initiated 6rd                July 2012

Copyright Notice

   Copyright (c) 2012 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
   (http://trustee.ietf.org/license-info) 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.

Table of Contents

   1. Introduction ....................................................2
   2. Problem Statement ...............................................3
   3. Proposed Solution ...............................................4
      3.1. Prefix Delegation ..........................................5
      3.2. Relevant Differences from Basic 6rd ........................6
   4. Security Considerations .........................................7
   5. Acknowledgements ................................................7
   6. References ......................................................7
      6.1. Normative References .......................................7
      6.2. Informative References .....................................7

1.  Introduction

   6rd [RFC5969] provides a transition tool for connecting IPv6 devices
   across an IPv4 network to an IPv6 network, at which point the packets
   can be routed natively.  The network topology is shown in Figure 1.

          +--------------+     +-----------------+      +---------+
          |              |     |                 |      |         |
       +-----+        +-----+  | Provider   +--------+  |         |
       |IPv6 |        | 6rd |__|   IPv4     | Border |__|  IPv6   |
       |Host |        |  CE |  |  Network   | Router |  | Network |
       +-----+        +-----+  |            +--------+  |         |
          | Customer LAN |     |                 |      |         |
          +--------------+     +-----------------+      +---------+

                     Figure 1: 6rd Deployment Topology

   In Figure 1, the CE is the customer edge router.  It is provisioned
   with a delegated IPv6 prefix, but it is also configured with an IPv4
   address so that it is reachable through the IPv4 network.  If a
   public IPv4 address is provisioned to every customer, it will
   aggravate the pressure due to the IPv4 address shortage for operators

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RFC 6654                  Gateway-Initiated 6rd                July 2012

   faced with a high rate of growth in the number of broadband
   subscribers to their network.  The use of private addresses with 6rd
   avoids this particular difficulty but brings other complications.

2.  Problem Statement

   Consider an operator facing a high subscriber growth rate.  As a
   result of this growth rate, the operator faces pressure on its stock
   of available public IPv4 addresses.  For this reason, the operator is
   motivated to offer IPv6 access as quickly as possible.  Figure 2
   shows the sort of network situation envisioned in the present

     +----+              +-------------------+    +----------------+
     |Host|\             |                   |    |                |
     +----+ \_+---+    +----+    Metro    +----+  |    Backbone    |
             _|CPE|----| GW |   Network   | BR |--|     Network    |
     +----+ / +---+    +----+    (IPv4)   +----+  |      (IPv6)    |
     |Host|/             |                   |    |                |
     +----+              +-------------------+    +----------------+

     Host = IPv6 customer host device
     CPE  = customer edge device (customer-provided)
     GW   = provider edge device (Gateway)
     BR   = border router (dual stack)

   Specialized GW and BR functions are described in the next section.

          Figure 2: Typical Network Scenario for IPv6 Transition

   The backbone network will be the first part of the operator's network
   to support IPv6.  The metro network is not so easily upgraded to
   support IPv6, since many devices need to be modified and there may be
   some impact to existing services.  Thus, any means of providing IPv6
   access has to minimize the changes required to devices in the metro

   In contrast to the situation described for basic 6rd [RFC5569],
   the operator is assumed to have no control over the capabilities
   of the IP devices on the customer premises.  As a result, the
   operator cannot assume that any of these devices are capable of
   supporting 6rd.

   If the customer equipment is in bridged mode and IPv6 is deployed to
   sites via a Service Provider's (SP's) IPv4 network, the IPv6-only
   host needs an IPv6 address to visit the IPv6 service.  In this
   scenario, 6to4 [RFC3056] or 6rd can be used.  However, each IPv6-only

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   host may need one corresponding IPv4 address when using a public IPv4
   address in 6to4 or 6rd, which puts great address pressure on the

   If the CPE in the above figure is acting in bridging mode, each host
   behind it needs to be directly assigned an IPv6 prefix so it can
   access IPv6 services.  If the CPE is acting in routing mode, only the
   CPE needs to be assigned an IPv6 prefix, and it delegates prefixes to
   the hosts behind it.

   If the Gateway supports IPv4 only, then an IPv4 address must also be
   assigned to each host (bridging mode) or to the CPE (routing mode).
   Both of these cases, but the bridging mode in particular, put
   pressure on the provider's stock of IPv4 addresses.

   If the Gateway is dual stack, an arrangement may be possible whereby
   all communication between the Gateway and the customer site uses IPv6
   and the need to assign IPv4 addresses to customer devices is avoided.
   A possible solution is presented in the next section.

3.  Proposed Solution

   For basic 6rd [RFC5969], the 6rd CE initiates the 6-in-4 tunnel to
   the dual-stack border router (i.e., the 6rd Border Relay in 6rd
   terminology) to carry its IPv6 traffic.  To avoid the requirement for
   customer premises equipment to fulfill this role, it is necessary to
   move the tunneling function to a network device.  This document
   identifies a functional element, termed the 6rd Gateway, to perform
   this task.  In what follows, the 6rd Gateway and 6rd Border Relay are
   referred to simply as the Gateway and Border Relay, respectively.

   The functions of the Gateway are as follows:

   o  to generate and allocate Gateway-initiated 6rd delegated prefixes
      for IPv6-capable customer devices, as described in Section 3.1;

   o  to forward outgoing IPv6 packets through a tunnel to a Border
      Relay, which extracts and forwards them to an IPv6 network as
      for 6rd;

   o  to extract incoming IPv6 packets tunneled from the Border Relay
      and forward them to the correct user device.

   In the proposed solution, there is only one tunnel initiated from
   each Gateway to the Border Relay, which greatly reduces the number of
   tunnels the Border Relay has to handle.  The deployment scenario
   consistent with the problem statement in Section 2 collocates the
   Gateway with the IP edge of the access network.  This is shown in

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RFC 6654                  Gateway-Initiated 6rd                July 2012

   Figure 2 and is the typical placement of the Broadband Network
   Gateway (BNG) in a fixed broadband network.  By assumption, the metro
   network beyond the BNG is IPv4.  Transport between the customer site
   and the Gateway is over Layer 2.

   The elements of the proposed solution are as follows:

   o  The IPv6 prefix assigned to the customer site contains the
      compressed IPv4 address of the network-facing side of the Gateway,
      plus a manually provisioned or Gateway-generated customer site
      identifier.  This is illustrated in Figure 3.

   o  The Border Relay is able to route incoming IPv6 packets to the
      correct Gateway by extracting the compressed Gateway address from
      the IPv6 destination address of the incoming packet, expanding it
      to a full 32-bit IPv4 address, and setting it as the destination
      address of the encapsulated packet.

   o  The Gateway can route incoming packets to the correct link after
      decapsulation using a mapping from either the full IPv6 prefix or
      the customer site identifier extracted from that prefix to the
      appropriate link.

3.1.  Prefix Delegation

   Referring back to Figure 2, prefix assignment to the customer
   equipment occurs in the normal fashion through the Gateway/IP edge,
   using either DHCPv6 or Stateless Address Autoconfiguration (SLAAC).
   Figure 3 illustrates the structure of the assigned prefix, and how
   the components are derived, within the context of a complete address.

   |  32-bit Gateway IPv4 address   |
   |<---IPv4MaskLen --->|  o bits   |   Gateway or manually
                        /           /    generated value, unique
      Configured       /           /   / for the Gateway
       |              /           /   |
       |             /           /    V
   |   V  p bits    |  o bits    | n bits  |m bits |     64 bits    |
   |                |  Gateway   |Customer |       |                |
   | Common prefix  | Identifier |  Site   |subnet | interface ID   |
   |                |            | Index   |  ID   |                |
   |<------ GI 6rd delegated prefix ------>|

    Figure 3: Gateway-Initiated 6rd Address Format for a Customer Site

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   The common prefix, i.e., the first p bits of the GI 6rd delegated
   prefix, is configured in the Gateway.  This part of the prefix is
   common across multiple customers and multiple Gateways.  Multiple
   common prefix values may be used in a network either for service
   separation or for scalability.

   The Gateway Identifier is equal to the o low-order bits of the
   Gateway IPv4 address on the virtual link to the Border Relay.  The
   number of bits o is equal to (32 - IPv4MaskLen), where the latter is
   the length of the IPv4 prefix from which the Gateway IPv4 addresses
   are derived.  The value of IPv4MaskLen is configured in both the
   Gateways and the Border Relays.

   The Customer Site Index is effectively a sequence number assigned to
   an individual customer site served by the Gateway.  The value of the
   index for a given customer site must be unique across the Gateway.
   The length n of the Customer Site Index is provisioned in the Gateway
   and must be large enough to accommodate the number of customer sites
   that the Gateway is expected to serve.

   To give a numerical example, consider a 6rd domain containing ten
   million IPv6-capable customer devices (a rather high number given
   that 6rd is meant for the early stages of IPv6 deployment).  The
   estimated number of 6rd Gateways needed to serve this domain would be
   on the order of 3,300, each serving 30,000 customer devices.
   Assuming best-case compression for the Gateway addresses, the Gateway
   Identifier field has length o = 12 bits.  If 6-in-4 tunneling is
   being used, this best case is more likely to be achievable than it
   would be if the IPv4 addresses belonged to the customer devices.  The
   customer device index, which is a more controllable parameter, has
   length n = 15 bits.

   Overall, these figures suggest that the length p of the common prefix
   can be 29 bits for a /56 delegated prefix, or 21 bits if /48
   delegated prefixes need to be allocated.

3.2.  Relevant Differences from Basic 6rd

   A number of the points in [RFC5969] apply, with the simple
   substitution of the Gateway for the 6rd CE.  When it comes to
   configuration, the definition of IPv4MaskLen changes, and there are
   other differences as indicated in the previous section.  Since
   special configuration of customer equipment is not required, the 6rd
   DHCPv6 option is inapplicable.

   Since the link for the customer site to the network now extends only
   as far as the Gateway, Neighbor Unreachability Detection on the part
   of customer devices is similarly limited in scope.

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4.  Security Considerations

   No further security considerations are raised in this document to
   those described in the Security Considerations section of [RFC5969].

5.  Acknowledgements

   Thanks to Ole Troan for his technical comments on an early version of
   this document.

6.  References

6.1.  Normative References

   [RFC5969]  Townsley, W. and O. Troan, "IPv6 Rapid Deployment on IPv4
              Infrastructures (6rd) -- Protocol Specification",
              RFC 5969, August 2010.

6.2.  Informative References

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

   [RFC5569]  Despres, R., "IPv6 Rapid Deployment on IPv4
              Infrastructures (6rd)", RFC 5569, January 2010.

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Authors' Addresses

   Tina Tsou
   Huawei Technologies (USA)
   2330 Central Expressway
   Santa Clara, CA  95050

   EMail: Tina.Tsou.Zouting@huawei.com

   Cathy Zhou
   Huawei Technologies
   Bantian, Longgang District
   Shenzhen  518129
   P.R. China

   EMail: cathy.zhou@huawei.com

   Tom Taylor
   Huawei Technologies
   Ottawa, Ontario

   EMail: tom.taylor.stds@gmail.com

   Qi Chen
   China Telecom
   109 Zhongshan Ave. West
   Tianhe District, Guangzhou  510630
   P.R. China

   EMail: chenqi.0819@gmail.com

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