Section 10.1. Dual-Stack Techniques

10.1. Dual-Stack Techniques

A dual-stack node has complete support for both protocol versions. This type of node is often referred to as an IPv6/IPv4 node. In communication with an IPv6 node, such a node behaves like an IPv6-only node; in communication with an IPv4 node, it behaves like an IPv4-only node. Implementations may have a configuration switch to enable or disable one of the stacks, so this node type can have three modes of operation. When the IPv4 stack is enabled and the IPv6 stack is disabled, the node behaves like an IPv4-only node. When the IPv6 stack is enabled and the IPv4 stack disabled, it behaves like an IPv6-only node. When both the IPv4 and IPv6 stacks are enabled, the node can use both protocols. An IPv6/IPv4 node has at least one address for each protocol version. It uses IPv4 mechanisms to be configured for an IPv4 address (static configuration or DHCP) and uses IPv6 mechanisms to be configured for an IPv6 address (static configuration or autoconfiguration).

DNS is used with both protocol versions to resolve names and IP addresses. An IPv6/IPv4 node needs a DNS resolver that is capable of resolving both types of DNS address records. The DNS A record resolves IPv4 addresses, and the DNS AAAA (referred to as quad-A) record resolves IPv6 addresses.

For a detailed discussion of IPv6 DNS record types, refer to Chapter 9.

In some cases, DNS has been configured to advertise only an IPv4 or an IPv6 address. If the host that is to be resolved is a dual-stack host, DNS might return both types of addresses. Hopefully, in this case, both the DNS resolver on the client and an application using DNS will have configuration options that let us specify orders or filters of how to use the addresses (i.e., preferred protocol settings). Generally, applications that are written to run on dual-stack nodes need a mechanism to determine whether they are communicating with an IPv6 peer or an IPv4 peer. Note that the DNS resolver may run over an IPv4 or IPv6 network, but the worldwide DNS tree will only become fully reachable over IPv6 over time.

A dual-stack network is an infrastructure in which both IPv4 and IPv6 forwarding is enabled on routers. The disadvantage of this technique is that you must perform a full network software upgrade to run the two separate protocol stacks. All tables (e.g., routing tables) are kept simultaneously with routing protocols being configured for both protocols. For network management, on some operating systems you may still have separate commands depending on the protocol (e.g., ping for IPv4 and ping6 for IPv6), and it takes more memory and CPU power.