  Traditional IP multicast routing is not suitable for cloud data center (DC)
networks due to the need for supporting large numbers of groups with large
group sizes. State-of-the-art DC multicast routing approaches aim to overcome
the scalability issues by, for instance, taking advantage of the symmetry of DC
topologies and the programmability of DC switches to compactly encode multicast
group information inside packets, thereby reducing the overhead resulting from
the need to store the states of flows at the network switches. However,
although these scale well with the number of multicast groups, they do not do
so with group sizes, and as a result, they yield substantial traffic control
overhead and network congestion. In this paper, we present Bert, a scalable,
source-initiated DC multicast routing approach that scales well with both the
number and the size of multicast groups, and does so through clustering, by
dividing the members of the multicast group into a set of clusters with each
cluster employing its own forwarding rules. Compared to the state-of-the-art
approach, Bert yields much lesser traffic control overhead by significantly
reducing the packet header sizes and the number of extra packet transmissions,
resulting from the need for compacting forwarding rules across the switches.
