As I mentioned in one of my previous blogs (/blogs/2012/02/03/ipv6-migration-%e2%80%93-what-why-and-how/ ) there are various possibilities for transitioning to IPv6. IPv6 Rapid Deployment (6rd) is one of the transition technology popularized by successful deployment by Free Telecom in France. 6rd is a technique that enables Service Providers to provision IPv6 addresses to end customers without upgrading their core infrastructure to IPv6. 6rd enables IPv6 hosts separated by IPv4 networks to communicate with each other by establishing an IPv4 tunnel. The tunnel origination point on the sender’s side of the tunnel encapsulates the IPv6 traffic within IPv4 packets, and sends them over IPv4 to the device at the remote end of the tunnel. The device on the other end of the tunnel decapsulates the packets and sends them over the IPv6 network to their destination.

For 6rd to work, Service Providers need to deploy 6rd Customer Edge (CE) at the customer network and 6rd Border Relay (BR) at the ISP IPv4 network edge. 6rd gateways (CE and BR) act as encapsulator and decapsulator for tunneled IPv6 traffic across IPv4 network.  Any traffic destined to cross ISP network crosses the 6rd border relay and gets routed natively to IPv6 internet. On the other hand, traffic destined for IPv6 network in the same ISP’s 6rd domain gets routed internally. 6rd tunneled traffic follows IPv4 routing as 6rd devices communicate to each other on their IPv4 addresses.

Central to 6rd are the concepts of 6rd Prefix, 6rd Delegated Prefix and 6rd domain. 6rd Prefix is and IPv6 prefix selected by the ISP for use in a given 6rd domain – there is only one 6rd prefix for a 6rd domain. A 6rd delegated prefix is obtained by combining SP’s 6rd prefix and CE’s IPv4 address and is used within the customer site for assigning IPv6 addresses to end hosts. And a 6rd domain is collection of CEs and BRs connected to same virtual 6rd link sharing the common 6rd Prefix and is managed by a single ISP.  An ISP can manage multiple 6rd domains – but each 6rd CE can only be in one 6rd domain and 6rd BR be aware of multiple 6rd domains under the control of same ISP.

6rd is built over the automatic 6to4 tunnel concepts, which is a point to multipoint tunnel and treats IPv4 network as NBMA link. 6to4 requires using special IPv6 address also known as 6to4 address that starts with prefix 2002::/16 followed by 32 bit IPv4 address. It derives the tunnel destination IPv4 address from the IPv6 address. 6RD improves over automatic 6to4 tunneling with key difference that it utilizes service provider’s own prefix instead of reserved prefix 2002::/16.

Though 6rd helps ISPs to provision IPv6 connectivity to end users but it does not allow IPv6 clients to talk with IPv4 servers. For that to work solutions like NAT64 / SLB64 are required.