IPv4 address exhaustion is a reality. There are already several tunnel based solutions  in the industry to ease the introduction of IPv6 while continuously offering IPv4 services during the transition period. Dual-Stack Lite (DS-Lite ), which defines a model for providing IPv4 access over an IPv6 network , aims to better align the costs and benefits of deploying IPv6 in operator’s networks. The DS-Lite model proposes a NAPT function in the operator’s network to dynamically assign public IPv4 addresses and ports to internal IPv4 packets.
Lightweight 4over6  (Lw4o6) is an optimization of DS-Lite that aims to reduce the NAPT states in the operator’s network. The underlying idea of Lw4o6 is to relocate the NAPT function from the Tunnel Concentrator (lwAFTR) to Initiators (lwB4s). The lwB4 element is provisioned with a public IP address and a port set. In this way, the lwAFTR can be transformed into a simple router.
Lightweight 4over6 overview
There are three main components in the Lightweight 4over6 architecture:
lwB4: performs the NAPT function and encapsulation/decapsulation of IPv4/IPv6. It is provisioned with a Public IPv4 address and a port set. This port set is used to restrict the external ports used by the NAPT function to source packets. The NAPT function is co-located in the lwB4.
lwAFTR: performs the encapsulation/decapsulation of IPv4/IPv6. It is also responsible for forwarding incoming packets to the appropriate lwB4 and outgoing packets to the IPv4 network.
The Provisioning System: configures the lwB4 with the Public IPv4 address and port set.
Lightweight 4over6 decouples IPv4 and IPv6 address architectures. It does not require any IPv4 address information to be embedded in the IPv6 address. Therefore, flexible and independent IPv4/IPv6 address schemes can be used. This allows operators to efficiently utilize public IPv4 addresses without affecting existing IPv6 address schema, and have full backward compatibility with other technologies, e.g. dual-stack, DS-Lite, etc.
In the same time, by offloading NAT functionality from AFTR to lwB4 (CPE), this solution can greatly increase the lwAFTR scalability. The newly introduced CPE model is quite similar with today, e.g. NAT traversal and ALG, etc. Therefore, it would be easier to support servers behind lwB4s then CGN-based solutions.
(2) Binding Table Maintenance in Lw4o6
The lwAFTR maintains an address binding table containing the binding between the lwB4's IPv6 address, the allocated IPv4 address and restricted port-set. Unlike the normal 5-tuple NAT table, the entry in the Lightweight 4over6 binding table contains 3-tuples: IPv6 Address for a single lwB4; Public IPv4 Address and the restricted port-set. The volume of per-subscriber state is well below a PPP context.
Currently, the Lightweight 4over6 technology has several related documents  in IETF (Internet Engineering Task Force). Two of them have been accepted as working group documents: Lightweight 4over6 architecture  in IETF Softwire Working Group and PCP (Port Control Protocol) supporting port set allocation  in IETF PCP Working Group.
Demo and Prototype
Two Interoperability tests have been carried out with the participation of Huawei, GreenNet, Fiberhome, Yamaha, BII, Tsinghua and China Telecom. These tests consisted of seven lwB4s and four lwAFTRs. Over 1400 test cases have been run between them.
The Lightweight 4over6 prototypes have also been demonstrated during IETF-85, which support a variety of IPv4 applications, including web browsing, video streaming, VoIP apps (e.g., Skype), peer-to-peer multimedia apps (e.g., PPLive), running on a range of devices, including smart phones, laptops and tablets. The Lightweight4over6 demo attracted many visitors from both operators and vendors, receiving many valuable comments.
Lightweight 4over6 Deployment
Lightweight4over6 can be deployed in IPv6-only access network, but continues to provide IPv4 connectivity as a service. In this way, IPv6 deployment can be accelerated by default, and legacy IPv4 services can still be reached.
Lightweight4over6 has little impact on CPE implementation. All the existing modules including NAT and IP tunneling can be reused by simple configuration, and only port-set allocation process should be added.
The technology of Lightweight4over6 is already supported by Huawei product NE40E and has been tested in the field trial of China Telecom since 2012.
Currently, operators around the world are carrying out, or planning to carry out, testing/deployment of this mechanism. China Telecom has been running a Lightweight4over6 field trial in Hunan Province, China since 2012. There will be a larger-scale deployment plan in multiple provinces this year. The lwAFTR is centralized deployed at the entrance to the MAN, and the lwB4 function is deployed in the subscriber’s CPE . The CPEs can be remotely upgraded through CPE management system. The supporting system can distinguish different types of subscribers, e.g. IPv4-only, dual-stack, and lw4over6, and the fallback mechanism among lw4over6, dual-stack, and IPv4-only users have been fully supported.
Lightweight4over6 is a simple extension of DS-Lite, but they are compatible and can be deployed together to provide different services to users based on the service agreement. Lightweight4over6 provides flexible and independent IPv4/IPv6 address schemes, so there is no extra requirement on address planning. This allows operators to have full flexibility on addressing planning and configuration. The per-subscriber state maintenance on LwAFTR could provide good scalability for overwhelming traffic.
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. Durand A., Droms R., et al, “Dual-Stack Lite Broadband Deployments Following IPv4 Exhaustion”, IETF RFC 6333, August 2011
. Cui Y., Wu P., Xu M., et al., “4over6: Network Layer Virtualization for IPv4-IPv6 Coexistence”, IEEE Network, October 2012
. Cui Y., Sun Q., Boucadair M., Tsou T., Lee Y., and Farrer I., "Lightweight 4over6: An Extension to the DS-Lite Architecture", draft-ietf-softwire-lw4over6-00, April 2013.
. Cui Y., Wu J., Wu P., Vautrin O., Lee Y., "Public IPv4 over IPv6 Access Network", draft-ietf-softwire-public-4over6-04, October 2012.
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. Sun Q., Lee Y., Sun Q., Bajko G., Boucadair M., “Dynamic Host Configuration Protocol (DHCP) Option for Port Set Assignment”, draft-sun-dhc-port-set-option-00, October 2012
. Sun Q., Boucadair M., S. Sivakumar, Zhou C., Tsou T., Perreault S., “Port Control Protocol (PCP) Extension for Port Set Allocation”, draft-ietf-pcp-port-set-00, November 2012
. Sun Q., Xie C., Lee Y., Chen M., “Deployment Considerations for Lightweight 4over6”, draft-sun-softwire-lightweight-4over6-deployment-02, July 2012
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