  Data center applications use multicast as an effective method to reduce
bandwidth cost. However, traditional multicast protocols designed for IP
networks are usually bottlenecked by the limited state capacity on switches. In
this paper, we propose a scalable multicast solution on fat tree networks based
on the observation that data center multicast traffic has strong heterogeneity.
We propose to remove the multicast management logic from switches and use the
SDN controller to manage multicast groups. The proposed Dual-structure
Multicast (DuSM) determines elephant and mice groups according to their traffic
amounts and treats them separately. For each elephant group, the controller
installs multicast state to maintain multiple shared trees and the group
traffic will be balanced evenly among the trees to avoid congestion. For mice
groups, the controller applies state-free mutlicast that trades bandwidth
capacity for state capacity, such as multicast-to-unicast translation. Our
experiments using real multicast traffic data show that the number of groups
DuSM supports can be 300% of that of IP multicast. DuSM also achieves traffic
balance among links.
