Public IPv4 over IPv6 Access Network
draft-ietf-softwire-public-4over6-08

Abstract

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.

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This Internet-Draft will expire on November 03, 2013.

Copyright Notice

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Table of Contents

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

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 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. 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

The following figure shows the basic provisioning steps for Public 4over6.

    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:

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 network.

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 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 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 packets.

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 deployment-specific:

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 6.3 of [RFC6333] are taken into account.

8. DNS

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

9. IANA Considerations

This document has no IANA actions.

10. 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.

11. Change Log from the -03 Version (RFC Editors please remove this part)

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

12. Author List

The following are extended authors who contribute to the effort:

13. References

13.1. Normative References

13.2. Informative References

Authors' Addresses