Network Working Group                                             Y. Cui
Internet-Draft                                                     J. Wu
Intended status: Informational                                     P. Wu
Expires: October 29, 2013                            Tsinghua University
                                                              O. Vautrin
                                                        Juniper Networks
                                                                  Y. Lee
                                                          April 27, 2013

                  Public IPv4 over IPv6 Access Network


   When the service provider networks are upgraded to IPv6, end users
   will continue to demand IPv4 connectivity.  This document proposes a
   mechanism for hosts or customer networks in IPv6 access network to
   build bidirectional IPv4 communication with the IPv4 Internet.  The
   mechanism follows the hub and spokes softwire model, and uses IPv4-
   over-IPv6 tunnel as basic method to traverse IPv6 network.  The bi-
   directionality of this IPv4 communication is achieved by explicitly
   allocating public IPv4 addresses to end users, as well as maintaining
   IPv4-IPv6 address binding on the border relay.  This mechanism
   features the allocation of full IPv4 address over IPv6 network, and
   has been used in production for high-end IPv4 users, IPv6 transition
   of ICPs, etc.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on October 29, 2013.

Copyright Notice

Cui, et al.             Expires October 29, 2013                [Page 1]

Internet-Draft                Public 4over6                   April 2013

   Copyright (c) 2013 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.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Scenario and Use Cases . . . . . . . . . . . . . . . . . . . .  4
   4.  Public 4over6 Address Provisioning . . . . . . . . . . . . . .  5
     4.1.  Basic Provisioning Steps . . . . . . . . . . . . . . . . .  6
     4.2.  Public IPv4 Address Allocation . . . . . . . . . . . . . .  7
   5.  4over6 CE Behavior . . . . . . . . . . . . . . . . . . . . . .  7
   6.  4over6 BR Behavior . . . . . . . . . . . . . . . . . . . . . .  8
   7.  Fragmentation and reassembly . . . . . . . . . . . . . . . . .  9
   8.  DNS  . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   10. Change Log from the -03 Version (RFC Editors please remove
       this part) . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   11. Author List  . . . . . . . . . . . . . . . . . . . . . . . . . 10
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 11
     12.2. Informative References . . . . . . . . . . . . . . . . . . 12

Cui, et al.             Expires October 29, 2013                [Page 2]

Internet-Draft                Public 4over6                   April 2013

1.  Introduction

   When deploying IPv6 networks, IPv4 connectivity is still a
   functionality required by end users.  It is used for IPv4
   communication with IPv4-only part of the Internet during the IPv4-
   IPv6 transition period.  IPv4-over-IPv6 tunnel mechanisms are the
   general solutions to provide this type of IPv4 services.

   This document describes a mechanism for providing IPv4 connectivity
   in this situation.  The mechanism is similar to the Binding approach
   of the Unified IPv4-in-IPv6 Softwire CPE effort that is documented in
   [I-D.bfmk-softwire-unified-cpe] Section 2.  Although the
   functionality documented in the standard is similar, this document
   describes existing practice that differs from the standard, but that
   has been deployed in China Next Generation Internet (CNGI) - China
   Education and Research Network 2 (CERNET2).

   The purpose of this draft is to document the protocol that was
   deployed, both for historical purposes and for the benefit of users
   of that protocol in the field at the time of publication.  Future
   deployments with similar requirements should simply use the related
   mechanism in [I-D.bfmk-softwire-unified-cpe].

   The advantage of IPv4-over-IPv6 tunnel mechanisms is the transparency
   to the IPv6 infrastructure: since IPv4 is actually only needed on the
   end user side as well as beyond the tunnel concentrator, most parts
   and functionalities of the ISP network can remain IPv6 only.
   Therefore, operators can run an IPv6-only infrastructure instead of a
   fully dual-stack network, as well as save the IPv4 address resource
   from being assigned all over the network.

   While different IPv4-over-IPv6 mechanisms are developed for different
   application scenarios, the mechanism proposed in this document
   focuses on providing full end-to-end transparency to the user-side.
   Therefore, public IPv4 addresses are expected to be provisioned to
   end users and carrier-side address translation can be avoided.
   Further more, full address is preferred to port-restricted address.
   With full address provisioned, user-side address translation is not
   necessarily needed either.  This means minimal changes to the user
   side: operating system could support the mechanism smoothly, while
   transparency on upper-layer applications is guaranteed.  For many
   ISPs which are actually capable of provisioning full IPv4 addresses,
   the mechanism provide a pure, suitable solution.

   Another focus of this mechanism is deployment and operation
   flexibility.  This mechanism allows IPv4 addressing and IPv6
   addressing schemes to be independent of each other: end user IPv4
   address is not embedded in its IPv6 address.  The IPv6 infrastructure

Cui, et al.             Expires October 29, 2013                [Page 3]

Internet-Draft                Public 4over6                   April 2013

   in the middle is not involved with the IPv4-over-IPv6 mechanism, so
   no special network planning is required; the service can be provided
   in on-demand style; the IPv4 address resources can be managed in a
   flat, centralized manner rather than distributed to customer sites
   with IPv6.  The tradeoff is per-subscriber binding state maintenance
   on the border relay.

   The mechanism follows hub and spokes softwire model, and uses IPv4-
   over-IPv6 tunnel between end host or CPE and border relay as basic
   data plane method.  Full IPv4 addresses are allocated from the ISP to
   the end host or CPE over IPv6 network.  Simultaneously, the binding
   between the allocated IPv4 address and the end user's IPv6 address
   are maintained on the border relay for encapsulation usage.

2.  Terminology

   Public 4over6: Public 4over6 is a per-subscriber stateful, IPv4-over-
   IPv6 tunnel mechanism proposed by this document.  Public 4over6
   supports bidirectional communication between IPv4 Internet and IPv4
   hosts or customer networks in IPv6 access network, by leveraging
   IPv4-in-IPv6 tunnel and public IPv4 address allocation over IPv6.

   4over6 Customer Edge (CE): A device functioning as a Customer Edge
   equipment in Public 4over6 environment.  The 4over6 CE can be either
   a dual-stack capable host, or a dual-stack CPE device, both of which
   have a tunnel interface to support IPv4-in-IPv6 encapsulation.  In
   the former case, the host supports both IPv4 and IPv6 stack but is
   provisioned with IPv6 only.  In the latter case, the CPE has an IPv6
   interface connecting to ISP network, and an IPv4 or dual-stack
   interface connecting to customer network; hosts in the customer
   network can be IPv4-only or dual-stack.

   4over6 Border Relay (BR): A router functioning as the border relay in
   Public 4over6 environment. 4over6 BR is the IPv4-in-IPv6 tunnel
   concentrator located in IPv6 ISP network.  It is a dual-stack router
   which connects to both the IPv6 ISP network and IPv4 Internet.  The
   4over6 BR also works as a DHCPv4 over IPv6
   [I-D.ietf-dhc-dhcpv4-over-ipv6] server/relay for assigning public
   IPv4 address to 4over6 CEs.

3.  Scenario and Use Cases

   The general scenario of Public 4over6 is shown in Figure 1.  Users in
   an IPv6 network take IPv6 as their native service.  Some users are
   end hosts which face the ISP network directly, while the others are
   customer LAN networks behind CPEs, such as a home LAN, an enterprise
   network, etc.  The ISP network is IPv6-only rather than dual-stack,
   which means the ISP cannot provide native IPv4 service to users.

Cui, et al.             Expires October 29, 2013                [Page 4]

Internet-Draft                Public 4over6                   April 2013

   However, it is acceptable that some router(s) on the carrier side
   becomes dual-stack and connects to IPv4 Internet.  So if network
   users require IPv4 connectivity, the dual-stack router(s) will work
   as their "entrance".

                   |    IPv6 ISP Network     |
                +------+                     |
                |4over6|Host                 |
                |  CE  |=================+-------+   +-----------+
                +------+                 |4over6 |   |   IPv4    |
   +---------+     |      IPv4-in-IPv6   |  BR   |---| Internet  |
   |Customer |  +------+                 |       |   |           |
   |IPv4 LAN |--|4over6|=================+-------+   +-----------+
   | Network |  |  CE  |CPE                  |
   +---------+  +------+                     |
                   |                         |

   Figure 1 Public 4over6 scenario

   Public 4over6 can be applicable in several use cases.  If an ISP
   which switches to IPv6 still has plenty of IPv4 address resource, it
   can deploy Public 4over6 to provide transparent IPv4 service for all
   its customers.  If the ISP does not have so much IPv4 addresses, it
   can deploy Dual-Stack Lite [RFC6333] as the basic IPv4-over-IPv6
   service.  Along with DS-Lite, Public 4over6 can be deployed as a
   value-added service, overcoming the service degradation caused by the
   CGN.  The two mechanisms can be integrated, because the IPv4-in-IPv6
   tunnel functions are the same; the difference is that DS-Lite employs
   a CGN while Public 4over6 employs an IPv4 provisioning process.  A
   typical case of the high-end users that could use Public 4over6 is
   IPv4 application server.  Full, public IPv4 address brings
   significant convenience in this case, which is important to IPv6
   transition for ICPs.  The DNS registration can be direct using
   dedicated address; the access of the application service can be
   straightforward, with no translation involved; there will be no need
   to hold the "pinhole" for incoming traffic, and no well-known port
   collision will come up.

4.  Public 4over6 Address Provisioning

4.1.  Basic Provisioning Steps

Cui, et al.             Expires October 29, 2013                [Page 5]

Internet-Draft                Public 4over6                   April 2013

   The following figure shows the basic provisioning steps for Public

       4over6                  DHCPv6         4over6         DHCPv4
         CE                    Server           BR           Server
         |Assign IPv6 Addr/Pref +|              |              |
         |  BR's IPv6 Addr Info  |              |              |
         |<----------------------|              |              |
         |     DHCPv6/Other      |              |              |
        WAN                                     |              |
     IPv6 Configure                             |              |
         |                                      |              |
         | Assign Public IPv4 Addr(DHCPv4-over-v6/Static Conf) |
         |                                      | IPv4-IPv6    |
         |                                      | Binding SYN  |
        Tunnel                                  |
     IPv4 Configure                       Binding Update
         |                                      |
         |          IPv4-in-IPv6 Tunnel         |
         |                                      |

   Figure 2 Public 4over6 Address Provisioning

   The main steps are:

   o  Provision IPv6 address/prefix to 4over6 CE, along with the
      information of 4over6 BR's IPv6 address, by DHCPv6 or other means.

   o  4over6 CE configures its WAN interface with globally unique IPv6
      address which is a result of IPv6 provisioning, including DHCPv6,
      SLAAC or manual configuration.

   o  Provision IPv4 address to 4over6 CE, by DHCPv4 over IPv6 or static

   o  Synchronize the IPv4-IPv6 address binding between DHCPv4 server
      and 4over6 BR, simultaneously with DHCPv4 provisioning.

   o  4over6 CE configures its tunnel interface, as a result of IPv4

   o  4over6 BR updates the IPv4-IPv6 address binding table, as a result
      of address binding synchronization.

Cui, et al.             Expires October 29, 2013                [Page 6]

Internet-Draft                Public 4over6                   April 2013

4.2.  Public IPv4 Address Allocation

   Usually each CE is provisioned with one public IPv4 address.  However
   it is possible that a CE would require an IPv4 prefix.  The key
   problem here is the mechanism for IPv4 address provisioning over IPv6

   There are two possibilities: DHCPv4 over IPv6, and static
   configuration.  Public 4over6 supports both these methods.  DHCPv4
   over IPv6 enables DHCPv4 message to be transported in IPv6 rather
   than IPv4; therefore, the DHCPv4 process can be performed over an
   IPv6 network, between BR and CE.  [I-D.ietf-dhc-dhcpv4-over-ipv6]
   describes the DHCP protocol extensions to support that.  As to static
   configuration, 4over6 users and the ISP operators negotiate
   beforehand to authorize the IPv4 address(es).  Then the tunnel
   interface and the address binding are configured by the user and the
   ISP respectively.

   While regular users would probably take DHCPv4 over IPv6, the manual
   configuration is usually seen in two cases: application server, which
   requires a stable IPv4 address, and enterprise network, which usually
   requires an IPv4 prefix rather than one single address (Note that
   DHCPv4 does not support prefix allocation).

5.  4over6 CE Behavior

   A CE is provisioned with IPv6 before Public 4over6.  It also learns
   the BR's IPv6 address beforehand.  This IPv6 address can be
   configured using a variety of methods, ranging from an out-of-band
   mechanism, manual configuration, or DHCPv6 option.  In order to
   guarantee interoperability, the CE element implements the AFTR-Name
   DHCPv6 option defined in [RFC6334].

   A CE supports DHCPv4 over IPv6[I-D.ietf-dhc-dhcpv4-over-ipv6], to
   dynamically acquire IPv4 address over IPv6 and assign it to the IPv4-
   in-IPv6 tunnel interface.  The CE regards the BR as DHCPv4-over-IPv6
   server/relay for public IPv4 address allocation, whose IPv6 address
   is learned by the CE as described above.

   A CE also supports static configuration of the tunnel interface.  In
   the case of prefix provisioning, the tunnel interface is assigned
   with the Well-Known IPv4 Address defined in section&nbsp;5.7 of [RFC6333],
   rather than using an address from the prefix.  If the CE has multiple
   IPv6 addresses on its WAN interface, it uses the same IPv6 address
   for DHCPv4 over IPv6/negotiation of manual configuration, and for
   data plane encapsulation.

   A CE performs IPv4-in-IPv6 encapsulation and decapsulation on the

Cui, et al.             Expires October 29, 2013                [Page 7]

Internet-Draft                Public 4over6                   April 2013

   tunnel interface.  When sending out an IPv4 packet, it performs the
   encapsulation, using the IPv6 address of the 4over6 BR as the IPv6
   destination address, and its own IPv6 address as the IPv6 source
   address.  The decapsulation on 4over6 CE is simple.  When receiving
   an IPv4-in-IPv6 packet, the CE just removes the IPv6 header, and
   either hands it to upper layer or forward it to customer network
   according to the IPv4 destination address.

   A CE runs a regular IPv4 NAPT for its customer network when it is
   provisioned with one single IPv4 address.  In that case, the assigned
   IPv4 address of the tunnel interface would be the external IPv4
   address of the NAPT.  Then the CE performs IPv4 private-to-public
   translation before encapsulation of IPv4 packets from the customer
   network, and IPv4 public-to-private translation after decapsulation
   of IPv4-in-IPv6 packets.

   IPv4 NAPT is not necessarily when the CE is provisioned with an IPv4
   prefix.  In this case, the detailed customer network planning is out
   of scope.

   4over6 CE supports backward compatibility with DS-Lite.  A CE can
   employ Well-Known IPv4 Address for B4 [RFC6333] and switch to Dual-
   Stack Lite for IPv4 communications, if it can't get a public IPv4
   address from the DHCPv4 server (for instance, the DHCPv4 over IPv6
   process fails or the DHCPv4 server refuses to allocate a public IPv4
   address to it, etc.).

6.  4over6 BR Behavior

   4over6 BR maintains the bindings between the CE IPv6 address and CE
   IPv4 address (prefixes).  The bindings are used to provide correct
   encapsulation destination address for inbound IPv4 packets, as well
   as validate the IPv6-IPv4 source of the outbound IPv4-in-IPv6

   The BR is bound to synchronize the binding information with the IPv4
   address provisioning process.  For static configuration, the BR
   configures the binding right after negotiation with the customer.  As
   for DHCPv4-over-IPv6, there are multiple possibilities which are

   o  The BR can be collocated with the DHCPv4-over-IPv6 server.  Then
      the synchronization happens within the BR.  It installs a binding
      when sending out an ACK for a DHCP lease, and delete it when the
      lease expires or a DHCP RELEASE is received.

   o  The BR can play the role of TRA as described in
      [I-D.ietf-dhc-dhcpv4-over-ipv6], and snoop for the DHCPv4 ACK and

Cui, et al.             Expires October 29, 2013                [Page 8]

Internet-Draft                Public 4over6                   April 2013

      Release messages, as well as keep a timer for each binding
      according to the DHCP lease time.

   On the IPv6 side, the BR decapsulates IPv4-in-IPv6 packets coming
   from 4over6 CEs.  It removes the IPv6 header of every IPv4-in-IPv6
   packet and forwards it to the IPv4 Internet.  Before the
   decapsulation, the BR checks the inner IPv4 source address against
   the outer IPv6 source address, by matching such a binding entry in
   the binding table.  If no binding is found, the BR silently drops the
   packet.  On the IPv4 side, the BR encapsulates the IPv4 packets
   destined to 4over6 CEs.  When performing the IPv4-in-IPv6
   encapsulation, the BR uses its own IPv6 address as the IPv6 source
   address, uses the IPv4 destination address in the packet to look up
   IPv6 destination address in the address binding table.  After the
   encapsulation, the BR sends the IPv6 packet on its IPv6 interface to
   reach a CE.

   The BR supports hairpinning of traffic between two CEs, by performing
   de-capsulation and re-encapsulation of packets.

7.  Fragmentation and reassembly

   The same considerations as described in section 5.3 and section&nbsp;6.3
   of [RFC6333] are taken into account.

8.  DNS

   The procedure described in Section 5.5 and Section&nbsp;6.4 of [RFC6333]
   is followed.

9.  Security Considerations

   In some cases, the 4over6 BR is expected to implement methods to
   limit service only to registered customers.  One simple solution is
   to make sure the BR allocates IPv4 addresses only to registered
   customers, i.e. the BR can filter on the IPv6 source addresses of
   incoming DHCP packets and only respond to the ones which have
   registered IPv6 source address.  The BR can also perform
   authentication during DHCP, for example, based on the MAC address of
   the CEs.  As to data packets, the BR can implement an IPv6 ingress
   filter on the tunnel interface to accept only the IPv6 address range
   defined in the filter, as well as check the IPv4-IPv6 source address
   binding by looking up the binding table.

10.  Change Log from the -03 Version (RFC Editors please remove this

   1.  Change the Intended Status to Informational, and reword some text

Cui, et al.             Expires October 29, 2013                [Page 9]

Internet-Draft                Public 4over6                   April 2013

   to not use RFC2119 language.

   2.  Specify the feature of Public 4over6 and circumstances requiring
   the mechanism in Abstract.

   3.  Explain the motivation of IPv4-over-IPv6 for Public 4over6 in
   section 1.

   4.  Explain the relationship between Public 4over6 and Unified CPE,
   as well as the purpose of this doc.

   5.  Clarify that customer network behind the 4over6 CE could be IPv4-
   only or dual-stack in section 3.

   6.  Explain how to integrate Public 4over6 and DS-lite as a typical
   use case in section 4 and section 5.

   7.  Clarify that IPv6 address/prefix can both be supported by 4over6
   CEs in section 5.

   8.  Improve the preciseness of the texts.

   9.  Remove the text that describes the BR not participating the
   DHCPv4-over-IPv6 process.

   10.  Updating the references.

11.  Author List

   The following are extended authors who contribute to the effort:

      Huiling Zhao
      China Telecom
      Room 502, No.118, Xizhimennei Street
      Beijing 100035

      Phone: +86-10-58552002

      Chongfeng Xie
      China Telecom
      Room 708, No.118, Xizhimennei Street
      Beijing 100035

Cui, et al.             Expires October 29, 2013               [Page 10]

Internet-Draft                Public 4over6                   April 2013

      Phone: +86-10-58552116

      Qiong Sun
      China Telecom
      Room 708, No.118, Xizhimennei Street
      Beijing 100035

      Phone: +86-10-58552936

      Qi Sun
      Tsinghua University
      Beijing 100084

      Phone: +86-10-62785822

      Chris Metz
      Cisco Systems
      3700 Cisco Way
      San Jose, CA 95134


12.  References

12.1.  Normative References

   [RFC4925]                        Li, X., Dawkins, S., Ward, D., and
                                    A. Durand, "Softwire Problem
                                    Statement", RFC 4925, July 2007.

   [RFC4966]                        Aoun, C. and E. Davies, "Reasons to
                                    Move the Network Address Translator
                                    - Protocol Translator (NAT-PT) to
                                    Historic Status", RFC 4966,
                                    July 2007.

Cui, et al.             Expires October 29, 2013               [Page 11]

Internet-Draft                Public 4over6                   April 2013

   [RFC5549]                        Le Faucheur, F. and E. Rosen,
                                    "Advertising IPv4 Network Layer
                                    Reachability Information with an
                                    IPv6 Next Hop", RFC 5549, May 2009.

   [RFC5565]                        Wu, J., Cui, Y., Metz, C., and E.
                                    Rosen, "Softwire Mesh Framework",
                                    RFC 5565, June 2009.

   [RFC6333]                        Durand, A., Droms, R., Woodyatt, J.,
                                    and Y. Lee, "Dual-Stack Lite
                                    Broadband Deployments Following IPv4
                                    Exhaustion", RFC 6333, August 2011.

   [RFC6334]                        Hankins, D. and T. Mrugalski,
                                    "Dynamic Host Configuration Protocol
                                    for IPv6 (DHCPv6) Option for Dual-
                                    Stack Lite", RFC 6334, August 2011.

12.2.  Informative References

   [I-D.bfmk-softwire-unified-cpe]  Boucadair, M. and I. Farrer,
                                    "Unified IPv4-in-IPv6 Softwire CPE",
                                    (work in progress), January 2013.

   [I-D.ietf-dhc-dhcpv4-over-ipv6]  Cui, Y., Wu, P., Wu, J., and T.
                                    Lemon, "DHCPv4 over IPv6 Transport",
                                    (work in progress), March 2013.

Authors' Addresses

   Yong Cui
   Tsinghua University
   Department of Computer Science, Tsinghua University
   Beijing  100084

   Phone: +86-10-6260-3059

   Jianping Wu
   Tsinghua University
   Department of Computer Science, Tsinghua University
   Beijing  100084

Cui, et al.             Expires October 29, 2013               [Page 12]

Internet-Draft                Public 4over6                   April 2013

   Phone: +86-10-6278-5983

   Peng Wu
   Tsinghua University
   Department of Computer Science, Tsinghua University
   Beijing  100084

   Phone: +86-10-6278-5822

   Olivier Vautrin
   Juniper Networks
   1194 N Mathilda Avenue
   Sunnyvale, CA  94089


   Yiu L. Lee
   One Comcast Center
   Philadelphia, PA  19103


Cui, et al.             Expires October 29, 2013               [Page 13]