Problems on IPv4-IPv6 network transition

IPv6 (Internet protocol version 6) is now available in various OSs and applications. It is also deployed in major ISP backbone networks. Although most of all IPv6 features work as they were designed, we may have potential problems with them and they might exist in IPv4-IPv6 dual stack environment. On the other hand, we don\´t have much opportunity to exchange enough information about dual stack network operation. We build a special research group named "IPv6-Fix" in WIDE Project in Japan, to investigate the potential issues related to IPv6 deployment. We concern about the problems on IPv4-IPv6 dual stack environment from various perspectives, such as specification, implementation and network operation. From our survey and analysis, we found some problems to be fixed. In this paper, we show some error cases taken by harmful specification, poor implementations and wrong operation. For the healthy coexistence of IPv4-IPv6, we share and consider how we should design and operate dual stack network