IPv4 Residual Deployment
|IPv6 transition mechanisms|
4rd is similar tunneling technology to IPv6 rapid deployment (6rd), the mechanism specified for IPv6 rapid deployment across IPv4 networks (RFC 5569 and RFC 5969). While 6rd targets early phases of the transition period form IPv4 to IPv6, when some networks do not directly route IPv6 packets, 4rd targets advanced phases when some networks route IPv6 packets and no longer IPv4 packets. On these networks, 4rd maintains a residual IPv4-service for customers that still need it.
4rd belongs to the family of transition mechanisms based on IP-in-IP tunnels. This approach, unlike those based on IPv4-IPv6 translation mechanisms (RFC 6145), fully preserves network end-to-end transparency to IPv4 packets.
Within this family, and like 6rd, 4rd uses automatic and stateless mappings between IPv6 and IPv4 addresses. Also like 6rd, it uses direct tunnel paths between customer nodes, a difference with centralized approaches of the Hubs and Spokes model of RFC 4925.
In recognition of IPv4 address exhaustion, 4rd includes a mechanism to statically share IPv4 addresses among several customers. With it, each 4rd customer node is assigned an exclusive set of ports that it can use freely for TCP, UDP, etc. This assignment is such that no customer has any of the first 4,096 ports (they have more value than others), that all other ports are distributed, and that the process is algorithmic (no parameter needs to be configured by ISP's).
Before that, four Japanese ISPs had expressed their intention to adopt 4rd as a solution for IPv4 across their IPv6 networks.