When you upgrade your hosts and routers to IPv6, the hosts and routers retain their IPv4 capability. Consequently, IPv6 provides compatibility for all IPv4 protocols and applications. These hosts and routers are known as dual-stack.
These hosts and routers use the name service (for example, DNS) to carry information about which nodes are IPv6 capable.
IPv6 address formats can contain IPv4 addresses.
The term dual-stack normally refers to a complete duplication of all levels in the protocol stack from applications to the network layer. An example of complete duplication is the OSI and TCP/IP protocols that run on the same system. However, in the context of IPv6 transition, dual-stack means a protocol stack that contains both IPv4 and IPv6. The remainder of the stack is identical. Consequently, the same transport protocols (TCP, UDP, and so on) can run over both IPv4 and IPv6. Also, the same applications can run over both IPv4 and IPv6.
The following figure illustrates dual-stack protocols through the OSI layers.
In the dual-stack method, subsets of both hosts and routers are upgraded to support IPv6, in addition to IPv4. The dual-stack approach ensures that the upgraded nodes can always interoperate with IPv4-only nodes by using IPv4.
A dual node must determine if the peer can support IPv6 or IPv4 in order to check which IP version to use when transmitting. The control of the information that goes in the name service enables a dual node to determine which IP version to use. You define an IPv4 node's IP address and the IPv6 node's IP address in the name service. Thus, a dual node has both addresses in the name service.
The presence of an IPv6 address in the name service also signifies that the node is reachable by using IPv6. However, the node is only reachable by nodes that obtain information from that name service. For example, placing an IPv6 address in NIS implies that the IPv6 host is reachable by using IPv6. However, the IPv6 host is only reachable by IPv6 and dual nodes that belong to that NIS domain. The placement of an IPv6 address in global DNS requires that the node is reachable from the Internet IPv6 backbone. This situation is no different than in IPv4. For example, the mail delivery operation requires that IPv4 addresses exist for nodes that can be reached by using IPv4. The same situation is true for the HTTP proxy operation. When no reachability exists in IPv4, for instance, because of firewalls, the name service must be partitioned into an inside firewall and outside firewall database. Consequently, the IPv4 addresses are visible only where the IPv4 addresses are reachable.
The protocol that is used to access the name service is independent of the type of address that can be retrieved from the name service. This name service support, coupled with dual-stacks, allows a dual node to use IPv4 when communicating with IPv4-only nodes. Also, this name service support allows a dual node to use IPv6 when communicating with IPv6 nodes. However, the destination must be reachable through an IPv6 route.
000 ... 000
000 ... 000
The compatible format is used to represent an IPv6 node. This format enables you to configure an IPv6 node to use IPv6 without having a real IPv6 address. This address format enables you to experiment with different IPv6 deployments because you can use automatic tunneling to cross IPv4–only routers. However, you cannot configure these addresses by using the IPv6 stateless address autoconfiguration mechanism. This mechanism requires existing IPv4 mechanisms such as DHCPv4 or static configuration files.
The mapped address format is used to represent an IPv4 node. The only currently defined use of this address format is part of the socket API. An application can have a common address format for both IPv6 addresses and IPv4 addresses. The common address format can represent an IPv4 address as a 128-bit mapped address. However, IPv4–to-IPv6 protocol translators also allow these addresses to be used.
To minimize any dependencies during the transition, all the routers in the path between two IPv6 nodes do not need to support IPv6. This mechanism is called tunneling. Basically, IPv6 packets are placed inside IPv4 packets, which are routed through the IPv4 routers. The following figure illustrates the tunneling mechanism through routers (R) that use IPv4.
The different uses of tunneling in the transition follow:
Configured tunnels between two routers (as in the previous figure)
Automatic tunnels that terminate at the dual hosts
A configured tunnel is currently used in the Internet for other purposes, for example, the MBONE (the IPv4 multicast backbone). Operationally, the tunnel consists of two routers that are configured to have a virtual point-to-point link between the two routers over the IPv4 network. This kind of tunnel is likely to be used on some parts of the Internet for the foreseeable future.
The automatic tunnels have a more limited use during early experimental deployment. Automatic tunnels require IPv4–compatible addresses and can be used to connect IPv6 nodes when IPv6 routers are not available. These tunnels can originate either on a dual host or on a dual router by configuring an automatic tunneling network interface. The tunnels always terminate on the dual host. These tunnels work by dynamically determining the destination IPv4 address (the endpoint of the tunnel) by extracting the address from the IPv4–compatible destination address.
Even on a node that has been upgraded to IPv6, the use of IPv6 is dependent on the applications. An application might not use a networking API that asks the name service for IPv6 addresses. The application might use an API (such as sockets) that requires changes in the application. Also, the provider of the API, such as an implementation of the java.net class might not support IPv6 addresses. In either situation, the node only sends and receives IPv4 packets like an IPv4 node would.
The following names have become standard terminology within the Internet community:
IPv6–unaware—This application cannot handle IPv6 addresses. This application cannot communicate with nodes that do not have an IPv4 address.
IPv6–aware—This application can communicate with nodes that do not have an IPv4 address, that is, the application can handle the larger IPv6 addresses. In some situations, the address might be transparent to the application, for example, when the API hides the content and format of the actual address.
IPv6–enabled—This application can, in addition to being IPv6–aware, can use some IPv6–specific feature such as flow labels. The enabled applications can still operate over IPv4, though in a degraded mode.
IPv6–required—This application requires some IPv6–specific feature and cannot operate over IPv4.