Network Working Group                                             Y. Cui
Internet-Draft                                                     J. Wu
Intended status: Informational                                     P. Wu
Expires: August 29, 2013                             Tsinghua University
                                                              O. Vautrin
                                                        Juniper Networks
                                                                  Y. Lee
                                                       February 25, 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 August 29, 2013.
Copyright Notice

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

Internet-Draft                Public 4over6                February 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.  Requirements Language  . . . . . . . . . . . . . . . . . . . .  4
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Scenario and Use Cases . . . . . . . . . . . . . . . . . . . .  4
   5.  Public 4over6 Address Provisioning . . . . . . . . . . . . . .  6
     5.1.  Basic Provisioning Steps . . . . . . . . . . . . . . . . .  6
     5.2.  Public IPv4 Address Allocation . . . . . . . . . . . . . .  7
   6.  4over6 CE Behavior . . . . . . . . . . . . . . . . . . . . . .  7
   7.  4over6 BR Behavior . . . . . . . . . . . . . . . . . . . . . .  8
   8.  Fragmentation and reassembly . . . . . . . . . . . . . . . . .  9
   9.  DNS  . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   10. Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   11. Change Log from the -03 Version  . . . . . . . . . . . . . . . 10
   12. Author List  . . . . . . . . . . . . . . . . . . . . . . . . . 10
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 11
     13.2. Informative References . . . . . . . . . . . . . . . . . . 12

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

Internet-Draft                Public 4over6                February 2013
1.  Introduction
   When deploying IPv6 networks, IPv4 connectivity is still a required
   functionality 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, carrier-side address translation should be avoided and
   public IPv4 addresses should be provisioned to end users.  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 keeps IPv4-IPv6 addressing independent:
   end user IPv4 address is not embedded in its IPv6 address.  The IPv6
   infrastructure in the middle is not involved with the IPv4-over-IPv6

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

Internet-Draft                Public 4over6                February 2013
   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.  Requirements Language
   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].
3.  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 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.
4.  Scenario and Use Cases
   The general scenario of Public 4over6 is shown in Figure 1.  Users in

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

Internet-Draft                Public 4over6                February 2013
   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.
   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

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

Internet-Draft                Public 4over6                February 2013
   collision will come up.
5.  Public 4over6 Address Provisioning
5.1.  Basic Provisioning Steps
   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.

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

Internet-Draft                Public 4over6                February 2013
   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.
5.2.  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 here: DHCPv4 over IPv6, and static
   configuration.  Public 4over6 MUST support both.  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 MUST 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).
6.  4over6 CE Behavior
   A CE MUST be provisioned with IPv6 before Public 4over6.  It MUST
   also learn the BR's IPv6 address.  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 SHOULD implement the AFTR-
   Name DHCPv6 option defined in [RFC6334].
   A CE MUST support DHCPv4 over IPv6[I-D.ietf-dhc-dhcpv4-over-ipv6], to
   dynamically require IPv4 address over IPv6 and assign it to the IPv4-
   in-IPv6 tunnel interface.  The CE considers 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 MUST also support static configuration of the tunnel interface.
   In the case of prefix provisioning, Well-Known IPv4 Address defined
   in section&nbsp;5.7 of [RFC6333] SHOULD be assigned to the tunnel

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

Internet-Draft                Public 4over6                February 2013
   interface, rather than using an address from the prefix.  If the CE
   has multiple IPv6 addresses on its WAN interface, it MUST use 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
   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 MAY
   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 (maybe because the DHCPv4 over IPv6
   process fails or the DHCPv4 server refuses to allocate a public IPv4
   address to it, etc.).
7.  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 MUST 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

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

Internet-Draft                Public 4over6                February 2013
      the synchronization happens within the BR.  It installs a binding
      when send 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
      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 MUST check 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 MUST support hairpinning of traffic between two CEs, by
   performing de-capsulation and re-encapsulation of packets.
8.  Fragmentation and reassembly
   The same considerations as described in section 5.3 and section 6.3
   of [RFC6333] are to be taken into account.
9.  DNS
   The procedure described in Section 5.5 and Section&nbsp;6.4 of [RFC6333]
   is to be followed.
10.  Security Considerations
   The 4over6 BR SHOULD implement methods to limit service only to
   registered customers.  The first step is to allocate IPv4 addresses
   only to registered customers.  One simple solution is to 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.

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

Internet-Draft                Public 4over6                February 2013
11.  Change Log from the -03 Version
   1.  Change the Intended Status to Informational.
   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.
12.  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 August 29, 2013               [Page 10]

Internet-Draft                Public 4over6                February 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
13.  References
13.1.  Normative References
   [RFC2119]                        Bradner, S., "Key words for use in
                                    RFCs to Indicate Requirement
                                    Levels", BCP 14, RFC 2119,
                                    March 1997.
   [RFC4925]                        Li, X., Dawkins, S., Ward, D., and
                                    A. Durand, "Softwire Problem
                                    Statement", RFC 4925, July 2007.

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

Internet-Draft                Public 4over6                February 2013
   [RFC4966]                        Aoun, C. and E. Davies, "Reasons to
                                    Move the Network Address Translator
                                    - Protocol Translator (NAT-PT) to
                                    Historic Status", RFC 4966,
                                    July 2007.
   [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.
13.2.  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), September 2012.
Authors' Addresses
   Yong Cui
   Tsinghua University
   Department of Computer Science, Tsinghua University
   Beijing  100084
   Phone: +86-10-6260-3059

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

Internet-Draft                Public 4over6                February 2013
   Jianping Wu
   Tsinghua University
   Department of Computer Science, Tsinghua University
   Beijing  100084
   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 August 29, 2013               [Page 13]