Other Transition Mechanisms

The mechanisms specified previously handle interoperability between dual nodes and IPv4 nodes, where the dual nodes have an IPv4 address. They do not handle interoperability between IPv6-only nodes (or dual nodes that have no IPv4 address) and IPv4-only nodes. While most implementations could be made dual (duality is only an issue of memory footprint for the code), the real issue is whether there will be enough IPv4 address space to assign one address for every node that needs to interoperate with IPv4-only nodes.

Several possibilities enable you to accomplish this interoperability without requiring any new transition mechanisms.

• Use application layer gateways (ALG) that sit at the boundary between the IPv6-only nodes and the rest of the Internet. Examples of ALGs in use today are HTTP proxies and mail relays.

• Companies are already selling NAT boxes (network address translators) for IPv4 that translate between the private IP addresses (for example, network 10--see RFC 1918) on the inside and other IP addresses on the outside. It is likely that these companies will upgrade their NAT boxes to also support IPv6 to IPv4 address translation.

Unfortunately, both ALG and NAT solutions create single points of failure. Using these result in a much less robust Internet. The IETF is working on a better solution for IPv6-only interoperability with IPv4-only nodes. One proposal is to use header translators with a way to allocate IPv4 compatible addresses on demand. Another proposal is to allocate IPv4 compatible addresses on demand and use IPv4 in IPv6 tunneling to bridge the IPv6-only routers.

The stateless header translator would translate between IPv4 and IPv6 header formats as long as the IPv6 addresses in use can be represented as IPv4 addresses (that is, they have to be IPv4-compatible or IPv4-mapped addresses). The support for these translators has been built into the IPv6 protocol so that, barring any encrypted packets or rarely used features like source routing, the translation can occur without any information loss.