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Examination of the issue of IPv6 address allocation for multicast

The support of multicast using IPv6 was recognised early in the project as being of great importance, as there are many applications using multicast technology. These applications require globally unique group addresses, which may be permanent or transient, to exchange data amongst their peers. Whilst systems such as SDR or web based mechanisms provide mechanisms for domain specific allocation they cannot offer a globally reliable Internet wide service. A generic, simple and scalable mechanism was required, which can provide globally unique multicast addresses for user groups within specified time periods. Moreover the notion of Ĝscopeĝ in the IPv6 multicast address introduces an additional requirement: to be able to allocate a globally unique multicast address within a certain scope. Such mechanisms did not even exist for IPv4, let alone for IPv6. The Westfaelische Wilhelms University worked on this topic and documented a complete taxonomy of the IPv6 multicast addresses allocation problem. The main need today for an allocation mechanism is collision avoidance. In the SSM model, as the channel is defined by a group address and a source, a collision could only occur only if a host would like to be a sender for 2 different groups having the same address. Therefore, it is sufficient for a sender to choose different addresses for different channels, this is a purely local decision. In the ASM model, we can consider two kinds of collision. There can be an address collision, and addresses and ports collision. In the former case, the receivers would get both flows but would discard the bad one. The risk is a loss of bandwidth in the receiver sites. Note that with IGMPv3/MLDv2, a host can exclude unwanted sources. In this case the loss of bandwidth will be beyond the first hop router. In the case of address and port collision, there would be a loss of resources in receivers, applications receiving both flows. We can also differentiate collisions due from errors and collisions due to malicious users, wanting to disturb multicast sessions. The session announcement problem is closely related to the address allocation issue. Some implementations today combine IPv6 multicast address allocation and session announcement.

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