Huawei Technologies Telecom Italia Huawei Technologies Huawei Technologies Like other internet activities, multicast is affected by the fact that the transition from IPv4 to IPv6 is occurring over a period of years, via multiple transition paths. As a result, mechanisms need to be added to the basic multicast architecture to assist in specific transition scenarios. This document describes detailed use cases so that the requirements for the multicast transition mechanisms can be better understood. The considered opinion in discussions to date on multicast transition requirements has been that the two most important transition scenarios in the near future will be the "4-6-4" and "6-4" network scenarios. These scenarios are described in detail below, in their several possible variants, showing the new issues that IPv6 transition raises for multicast operation. There is further general agreement that Any-Source Multicast (ASM) (the service, not necessarily the technology) is no longer an important use case. As a result, this document restricts itself to scenarios where the set of multicast sources is separate from the set of multicast receivers. As a final restriction, it assumes a single administrative provider domain.