The Bump in the API (BIA) [RFC3338] translation mechanism is very similar to that of BIS. However, instead of translating between IPv4 and IPv6 headers, BIA inserts an API translator between the socket API and the TCP/IP modules of the host stack (see Figure 5-9).

Thus, IPv4 socket API functions are translated into IPv6 socket API functions and vice versa. In this
way, IPv4/IPv6 translation can be achieved without the overhead of translating every packet header. Like BIS, BIA is based on the addition of three modules: the extension name resolver, the function mapper and the address mapper. Both the extension name resolver and the address mapper modules operate in exactly the same way as the corresponding modules in BIS. The function mapper is the entity that maps IPv4 socket calls to IPv6 socket calls and vice versa. The function mapper does this by intercepting IPv4 socket API function calls and invoking corresponding IPv6 socket API function calls in their place. These IPv6 socket function calls are used to communicate with the peer IPv6 host and are transparent to the IPv4 application that invoked the original IPv4 socket function calls.

The BIA mechanism is intended for systems that have an IPv6 stack but contain applications that have not been upgraded to IPv6. Thus, BIA may be useful in early stages of transition when there are many unmodified IPv4 applications within IPv6 domains. BIA allows IPv4 to IPv6 host communication, but does not specify the reverse case. However, it could be easily extended to cater to IPv6 to IPv4 host communication (this is also applicable to BIS). The advantage BIA has over BIS is that it is independent of the network interface driver and does not introduce the overhead of per-packet header translation. However, BIA exhibits similar limitations to BIS. It will not support multicast communication without some additional functionality in the function mapper module, and it will not work for applications that embed IP addresses in their payloads.

The BIA method is, like the BIS method, not widely used. No 6NET sites see a deployment requirement for it.