On Nonblocking Multicast Fat-Tree Data Center Networks with Server Redundancy

Fat-tree networks have been widely adopted as network topologies in data center networks (DCNs). However, it is costly for fat-tree DCNs to support nonblocking multicast communication, due to the large number of core switches required. Since multicast is an essential communication pattern in many cloud services and nonblocking multicast communication can ensure the high performance of such services, reducing the cost of nonblocking multicast fat-tree DCNs is very important. On the other hand, server redundancy is ubiquitous in today's data centers to provide high availability of services. In this paper, we explore server redundancy in data centers to reduce the cost of nonblocking multicast fat-tree data center networks (DCNs). First, we present a multirate network model that accurately describes the communication environment of the fat-tree DCNs. We then show that the sufficient condition on the number of core switches required for nonblocking multicast communication under the multirate model can be significantly reduced when the fat-tree DCNs are 2-redundant, i.e., each server in the data center has exactly one redundant backup. We also study the general redundant fat-tree DCNs where servers may have different numbers of redundant backups depending on the availability requirements of services they provide, and show that a higher redundancy level further reduces the cost of nonblocking multicast fat-tree DCNs. Then, to complete our analysis, we consider a practical faulty data center, where one or more active servers may fail at any time. We give a strategy to re-balance the active servers among edge switches after server failures so that the same nonblocking condition still holds. Finally, we give a multicast routing algorithm with linear time complexity to configure multicast connections in fat-tree DCNs.