IPv6 Rapid Deployment (6RD) tunnels are an extension of 6to4 and allow a service provider to provide unicast IPv6 service to customers over its IPv4 network by using encapsulation of IPv6 in IPv4. It is defined in RFC 5969.
With 6RD, service providers do not have to use a 2002::/16 prefix; instead, they use a prefix from their own SP address block. The IPv6 operational domain is within the SP’s networks. Furthermore, with 6RD, the 32 bits of the IPv4 destination do not need to be carried within the IPv6 payload. The IPv4 destination is obtained from a combination of bits in the payload header and information on the router.
A 6RD Border Relay (BR) is a 6RD-enabled router managed by the SP. A BR has an interface in the IPv6 network, another interface in the IPv4 network, and a virtual interface that is the endpoint for the 6RD IPv6-in-IPv4 tunnel.
IPv6 ISATAP Tunnels
Another method to tunnel IPv6 over IPv4 is to use Intra-Site Automatic Tunnel Addressing Protocol (ISATAP). With ISATAP, a tunnel is automatically created between dual-stack hosts or routers to transmit IPv6 packets over an IPv4 network within a site. ISATAP does not require IPv4 to be multicast enabled.
ISATAP uses a well-defined IPv6 address format composed of any unicast prefix of 64 bits, which can be a link-local or global IPv6 unicast prefix. It then uses the 32 bits 0000:5EFE that define the ISATAP address ending with the 32-bit IPv4 address of the ISATAP link.
As an example, the link-local address is generated by concatenating FE80:0000:0000:0000: to 0000:5EFE: with the IPv4 address expressed in hexadecimal. For example, with IPv4 192.168.10.10, the link-local address is FE80:0000:0000:0000:0000:5EFE:C0A8:0A0A. ISATAP also requires the use of a routable address (for example, a global unicast IPv6 address that uses the same 0000:5EFE IANA reserved value for the interface ID along with the 32-bit IPv4 embedded address).
IPv6/IPv4 Translation Strategy
One of the mechanisms for an IPv6-only host to communicate with an IPv4-only host without using dual stacks is protocol translation. Translation is basically an extension to IPv4 NAT techniques. RFC 6144 provides a framework for IPv4/IPv6 translation. It replaces the deprecated Network Address Translation–Protocol Translation (NAT-PT), which was not considered a viable strategy for transitioning to IPv6. NAT-PT was deprecated because of its tight coupling with DNS.