QuickCast: Fast and Efficient Inter-Datacenter Transfers Using Forwarding Tree Cohorts

Several organizations have built multiple datacenters connected via dedicated wide area networks over which large inter-datacenter transfers take place. Since many such transfers move the same data from one source to multiple destinations, using multicast forwarding trees can reduce bandwidth needs and improve completion times. However, using a single forwarding tree per transfer can lead to poor performance as the slowest receiver dictates the completion time for all receivers. Using multiple forwarding trees per transfer alleviates this concern-the average receiver could finish early; however, if done naively, bandwidth usage would also increase and it is apriori unclear how best to partition receivers, how to construct the multiple trees and how to determine the rate and schedule of flows on these trees. This paper presents QuickCast, a first solution to these problems. Using simulations on real-world network topologies, we see that QuickCast can speed up the average receiver's completion time by as much as $10\times$ while only using $1.04\times$ more bandwidth; further, the completion time for all receivers also improves by as much as $1.57 >$ faster at high loads. Thereby, while some implementation challenges remain, we advocate using a cohort of forwarding trees.