# Delay Bounded Multi-Source Multicast in Software-Defined Networking
## 1 Introduction  
As a promising centralized control architecture, Software-defined networking (SDN) enabled by OpenFlow protocol, allows the control plane to be programmable for effective optimization of the network resources and support various network dynamics [1; 2; 3]. This is made possible by separating the control plane and data plane in a network into two main components; controllers (SDN-Cs) and forwarding elements (SDN-FEs) [4; 5; 6; 7]. Forwarding elements in switches are flexible to support various forwarding rules for different traffic. One or more controllers are responsible for taking control plane from the switches and installs the corresponding forwarding policies through secured interfaces. Figure 1 illustrate the basic architecture of SDN. The separation of the application logic from the forwarding substrate greatly facilitates the deployment and operation of new services and enables SDN to react gracefully to changing network demands and modifications to the substrate topology [8]. OpenFlow is the most common protocol used in SDN networks. It enables the controller to communicate with all the Network Elements. Multicast benefits in saving the bandwidth consumption and reducing the load on the source by avoiding unnecessary duplication of transmissions among a group of independent unicast paths towards destinations, after multiplexing a shared multicast tree. Despite such bandwidth superiority, multicast has not been optimally utilized by the Internet during the past decades. Recently, it has achieved some successful network-level deployments in datacenter networks [9], enterprise networks and IPTV networks [10; 11; 12; 13]. Research proposals dealing with the construction of multicasting trees that satisfy various constraints under single source scenario are abundant in the literature. For example, the problem of constructing a single source serving a single multimedia stream wherein minimum delay is desired can be solved efficiently by constructing a single source shortest path tree and pruning subtrees that do not have a destination node. In cases where the delay bound, delay variation bound, node degree bound, and others are desired the multicasting tree construction problems have been shown to be NP-hard [14; 15]. One of the existing multicast routing standard protocols is Protocol Independent Multicast Spare Mode (PIM-SM) [16], which connects the source and destination nodes through a shortest-path tree. PIM-SM presented a solution to the problem of routing multicast packets in large, wide-area internets. This approach uses constrained, receiver-initiated, membership advertisement for sparsely distributed multicast groups. In most multicast techniques, however, the characteristics of a multicast group, like a source node is known in advance. This case is referred to as a deterministic multicast. In many cases, however, the characteristics of a multicast transfer are not known in advance. It is not always necessary for the source of a multicast transfer to be in a specific location, as long as certain constraints are met. A major reason is the widely used content replica design for improving the robustness and efficiency in various networks, such as the content distribution network [17; 18], IPTV networks and datacenter networks. In those networks, any node with a replica content can act as a potential source. When delivering a content file to multiple destinations, the source of such a multicast transfer can be any of the replica. This causes multi-source multicast due to the source flexibility. The SDN controller is able to determine which nodes have the replica as it constantly gets updated information regarding the network. In this paper, we focus on the multicast tranfer, the _delay bounded multi-source multicast_, which aims to provide and demonstrate the effectiveness of utilizing more than one source node in an SDN  
Figure 1: SDN Architecture. environment.