Strategies of Transition to IPv6

The consumption of IPv4 address resources is delayed with the support of Classless Inter-Domain Routing (CIDR) and Network Address Translation (NAT) technologies. However, a high-speed development of the Internet, tremendous demands for broadband access and the predictable construction of the 3G network and Next Generation Network (NGN) all need huge address space. Therefore, it is inevitable to introduce IPv6 and deploy IPv6-based networks on a large scale.

    IPv6 has been developing for more than 10 years, but the IPv6-based networks cannot yet compare with IPv4 networks in network scale, number of subscribers and network application. Although application and deployment of the IPv6 network have a tendency to accelerate, it can be anticipated that the transition from IPv4 to IPv6 will have a long course.

    In respect of network scale, the relationship between the IPv6 network and the IPv4 network is like that between the island and the ocean. At the beginning of network evolution, there are a large number of IPv4 networks, just like the ocean, whereas relatively isolated IPv6 networks or nodes are islands. And then, due to the greatly large scale of the Internet, the IPv4 and IPv6 networks will develop themselves respectively. Therefore, the ocean of IPv4 networks and that of IPv6 networks are expected to coexist at this time. Eventually by further development of network application the pattern will change as the IPv4 networks will be the islands in the ocean of IPv6 networks.

    Therefore, the strategies related to long-time coexistence and development of IPv4 and IPv6 networks attract much attention besides the IPv6 standardization work of IETF. There are a number of technologies that support the transition from IPv4 to IPv6 networks and their coexistence.

1 Technologies for Transition to  IPv6
For different possibilities of coexistence of the IPv4 and IPv6, multiple technical solutions have been developed, including Dual Stack, Tunneling and Translation technologies[1].

1.1 Dual Stack
Dual Stack, also called Dual IP Layer, is 2 IP protocol versions to simultaneously support IPv4 and IPv6. That is to support the simultaneous implementation of IPv4 and IPv6 at the router and host. Thus both, IPv4 and IPv6, can work in a mixed environment: application of IPv4 follows the IPv4 system, while the IPv6 applications follow the IPv6 system. The dual stack technology involves the Domain Name System (DNS), assignment of IPv6 addresses, the routing protocols, access license of users and network application.

    The IPv6 applications are available directly through a connection to the IPv6 network, and so do IPv4 applications. However, if IPv4 applications are expected to be used through the IPv6 network or vice versa, it is necessary to obtain corresponding v6/v4 addresses through DNS for communication, or implemented by the translation mechanism. Due to different design considerations on the DNS server, implementations of the dual stack technology may differ from one another.

    In an IPv4/IPv6 mixed environment, relevant routing protocols are possible to conflict with each other. But the Intermediate System-to-Intermediate System (IS-IS) with multiple topologies helps resolve the problems occurring during the usage of IS-IS protocols.

1.2 Tunneling
Just as its name implies, the tunneling technology enables IPv6 network access via a passage in the IPv4 network. It works by encapsulating the IPv6 messages within packets carried by the IPv4 network. Several tunneling technologies have been developed to support the transition of IPv4 to IPv6 and their coexistence, such as the manual tunneling, auto tunneling and MPLS tunneling. Main tunneling technologies are listed in Table 1.

    The interconnection of IPv6 sites may also be implemented by some Layer 2 network technologies, such as by Asynchronous Transfer Mode (ATM). Such technologies are not listed in Table 1, and there is no further description either.

1.3 Translation
The communication between pure IPv4 and pure IPv6 networks can be implemented through the translation mechanism. When an IPv6 node is communicating with an IPv4 node, the edge equipment of the IPv4 network will translate the IPv6 message into an equal IPv4 message once it reaches the edge of the IPv4 network. It is the IPv4 message, rather than its IPv6 version, that arrives at the destination in the IPv4 network. The returned IPv4 message will be translated into IPv6 at the edge node, and then be sent to the source node in the IPv6 network. Main translation technologies are listed in Table 2.

2 Strategic Analysis of Introducing IPv6 into Operator´s Network

2.1 Strategies for Transition to IPv6

(1) Updating the existing IPv4 network to IPv6
    The current high-end router products can well support IPv6 and some relevant characteristics. Therefore, the equipment of the backbone network and the equipment of the core and convergence layers of Metropolitan Area Network (MAN) have had the capability to support IPv6, but relevant functions have not yet started.

    By upgrading the current IPv4 network, the functions and applications of IPv6 can directly start up. (There have been successful cases in the existing networks of foreign operators.) In this way, the IPv4/IPv6 dual stack function works to implement the migration to the IPv6 network. The main reason of using the dual stack technology to implement the transition is that now most services available are based on IPv4, whereas the number of users of the IPv6 networks is small. The smooth transition is a good solution when the application of IPv6 has not grown up. With the development of IPv6 application, especially in Next Generation Network (NGN) and 3G networks, the IPv6 network can be introduced by implementing the conversion from IPv4 into IPv6 at the edge of the access network.

    However, using the dual stack technology to upgrade the current network to the IPv6 network may make the network perform worse. After all, the current network is built for IPv4 services. When the traffic of an IPv4 network approximates to its equipment capacity, such an upgrade will make a bad impact on the current IPv4 services, as well as limit the development of IPv6.

(2) Building a new IPv6 backbone network
    This strategy may also implement the introduction of IPv6. This IPv6 backbone network can be either a pure IPv6 network or an IPv6 network supported by the dual stack. As the IPv4 services can use the existing IPv4 network, a pure IPv6 network is recommended.

    The interconnection of the new network with existing networks can be implemented by the Network Address Translation-Protocol Translation (NAT-PT) function of the gateway equipment. In the MAN, the demands of IPv6 access can be met by either introducing new IPv6 equipment or upgrading the existing equipment with qualified function capability to support IPv6. At this time, the IPv6 network coexists with the existing network. Moreover, with tunneling technologies, the interconnection can be implemented in the existing network.

2.2 Analysis of IPv6 User Networks
The network migration of operators should be able to meet with the diversified access requirements of different IPv6 users. According to practical situations of users and their demands for IPv6 access, the IPv6 user networks can be categorized by the following: 
(1) Pure IPv6 Network 
    It can be a single PC, the Small Office Home Office (SOHO) network, or a large network. It may be either a pure IPv6 network or a dual stack IPv6 network implemented by network transition.

(2) Mixed IPv6 Network 
    Most connections are IPv4 connection and only a few nodes support IPv6.

(3) Customer Premises Network (CPN) 
    By NAT, the IPv6 nodes in it implement the interconnection with the IPv6 network.

    For the first category, the direct user connection may be implemented through the IPv6 link. However, if such link is difficult to obtain, it is necessary to use the tunneling technologies to implement the access of users. According to the characteristics of the tunnel and access demands of users, the tunnel can be the configured tunnel, 6to4 tunnel and MPLS tunnel. Other factors that should be considered include the technologies used by and the scalability of the networks of operators. For the second category, the network access may be implemented through the Inter-Site Automatic Tunnel Addressing Protocol (ISATAP) tunnel. The connecting router between the user network and the network of the operator supports ISATAP, and IPv6 nodes in the user network will connect with the external IPv6 network through ISATAP.

    For the third category, the network access may be implemented through the Teredo tunnel, which supports the IPv6 node behind NAT to connect with the IPv6 network through NAT.

    In order to satisfy access demands of various users, the network equipment of operators would better support the above-mentioned tunneling technologies. Moreover, the operators should take the tunneling function support of their equipment and the structure of user networks into consideration to select a suitable mode for the access of IPv6 users.

3 Conclusions
The IPv6 technologies have been able to supply abundant network characteristics and strategy of coexistence with and transition from IPv4. Therefore, they satisfy the demands for building the IPv6 network.

    However, with the development of the IPv6 network, new functions are being required, such as the IPv6 function support with Broadband Access Server (BAS), IPv6-based MPLS and the support and implementation of its relevant functions, the improvement of OoS, the IPv6 Multicast support, network security support and mobility support. All of them will become important aspects of transition to and deployment of the IPv6 network and will be important guideline for development of network equipment.

Reference
[1] Waddington G D, Chang Fangzhe. Realizing the Transition to IPv6[J]. IEEE Communications Magazine, 2002, 40(6): 138-148.

Manuscript received: 2005-02-21