Application migration and coexistence overview

Many IPv6 stacks support both IPv4 and IPv6 interfaces and are capable of receiving and sending native IPv4 and IPv6 packets over the corresponding interfaces. This type of TCP/IP stack is generally referred to as a dual-mode stack IP node. This does not mean that there are two separate TCP/IP stacks running on this type of node. It means that the TCP/IP stack has built-in support for both IPv4 and IPv6. In this topic, the term dual-mode stack or IP node is a TCP/IP stack that supports both IPv4 and IPv6 protocols.

Figure 1. Dual-mode stack IP host

IPv4-only applications communicate with IPv4 nodes, IPv6-enabled applications communicate with both IPv4 and IPv6 nodes

For a multihomed dual-mode IP host, it is a likely configuration that the host has both IPv4 and IPv6 interfaces over which requests for host-resident applications are received or sent. Older AF_INET applications can communicate using IPv4 addresses only. IPv6-enabled applications that use AF_INET6 sockets can communicate using both IPv4 and IPv6 addresses (on a dual-mode host). AF_INET and AF_INET6 applications are able to communicate with one another, but only using IPv4 addresses.

If the socket libraries on the IPv6-enabled host are updated to support IPv6 sockets (AF_INET6), applications can be IPv6 enabled. When an application on a dual-mode stack host is IPv6 enabled, the application is able to communicate with both IPv4 and IPv6 partners. This is true for both clients and server on a dual-mode stack host.

Table 1. Application communication on a dual-mode host
	IPv4-only	IPv6-enabled
IPv4-only partner	Yes	Yes
IPv6-only partner		Yes

IPv6-enabling both sockets libraries and applications on dual-mode hosts therefore becomes a migration concern. As soon as IPv6-only hosts are being deployed in a network, applications on those IPv6-only nodes cannot communicate with the IPv4-only applications on the dual-mode hosts, unless one of multiple migration technologies are implemented either on intermediate nodes in the network or directly on the dual-mode hosts.