The Bump in the Stack (BIS) [RFC2767] (see Figure 5-8) translation mechanism is similar to taking the NAT-PT approach with SIIT and moving it to the OS protocol stack within each host. Unlike SIIT however, it assumes an underlying IPv6 infrastructure. Whereas SIIT is a translation interface between IPv6 and IPv4 networks, BIS is a translation interface between IPv4 applications and the underlying IPv6 network (i.e. the network interface driver). The host stack design is based on that of a dual-stack host, with the addition of three modules, a translator, an extension name resolver, and an address mapper.

The translator rewrites outgoing IPv4 headers into IPv6 headers and incoming IPv6 headers into IPv4 headers (if applicable). It uses the header translation algorithm defined in SIIT. The extension name resolver acts as the DNS-ALG in the NAT-PT mechanism. It snoops IPv4 DNS queries and creates another query asking to resolve both ‘A’ (IPv4) and ‘AAAA’ (IPv6) records, sending the returned ‘A’record back to the requesting IPv4 application. If only ‘AAAA’ records are returned, the resolver requests the address mapper to assign an IPv4 address corresponding to the IPv6 address. The address mapper maintains a pool of IPv4 addresses and the associations between IPv4 and IPv6 addresses. It will also assign an address when the translator receives an IPv6 packet from the network for which there is no mapping entry for the source address. Because the IPv4 addresses are never actually transmitted on the network, they do not have to be globally unique and a private address pool can be used.

The BIS mechanism may be useful during initial stages of IPv4 transition to IPv6 when IPv4 applications remain unmodified within IPv6 domains. However, BIS is limited in its translation capabilities. It allows IPv4 to IPv6 host communication but not vice versa. It does not send or receive any IPv4 packets to/from the network. Thus, even an IPv4 application attempting communication with another IPv4 application using BIS, will fail without additional translation mechanisms somewhere in the communication path. As with NAT-PT and SIIT, BIS will not work for multicast communications and will not work for applications that embed IP addresses in their payloads. An ALG is required for any application that exhibits this behaviour.

The BIS method is not widely used; indeed we are not aware of any 6NET sites that see a need for its deployment.