## 4 Performance Evaluation  
In this section, we evaluate CCSM in term of scalability and compare it with the traditional multicast routing protocol. Since the main difference between CCSM and traditional multicast is in the core network, we mainly analyse the amount of multicast entries that CCSM and traditional multicast would install in the core network. In the evaluation, we choose PIM-SM as a representative of traditional multicast routing. PIM-SM is a distributed multicast routing protocol which cooperates with IGMP/MLD to construct the multicast delivery tree. For a given multicast group, PIM-SM first constructs a shared tree rooted in Rendezvous Point (RP) which is called as RP Tree (RPT). In RPT, multicast source sends multicast packets to RP, and the RP forwards the packets along the RPT, and the multicast entries in router are (*,G). Once multicast receiver gets the multicast packets, it may initiate a transfer from RPT to SPT to optimize latency or bandwidth utilization. Therefore the entries installed in routers will be updated to (S,G). In some implementations, this transfer is based on the load of RP. Unlike PIM-SM, CCSM achieves load balance through dynamic root election in core network, so it is advocated to install (*,G) entries for multicast group without concerning that roots are overloaded. Notice that, the 'G' in (*,G) may be MSI or MGI. For multicast group with multiple sources, PIM-SM needs to construct multiple SPTs in line with the number of sources while CCSM only needs to construct one SPT. We adopt the Inet [68] to generate the network topology and select the root/RP based on node centrality. The network topology size is 4000 and the number of edges is 6659. All the nodes locate in a 10,000 \(\times\) 10,000 square. For the sake of simplification, we select the node with highest degree centrality as the root, and selects the multicast receiver randomly from the rest of nodes. In the analysis, we mainly focus on the multicast delivery tree reconstruction cost and multicast receiver join delay. We adopt the _hop_ to measure the reconstruction cost and join delay. The reconstruction cost is measured by the total message delivery distances, and join delay is measured by the distance between multicast delivery tree and new multicast receiver. Figure 9 shows the reconstruction cost of CCSM and PIM-SM. During this simulation, we randomly select a node as multicast receiver to join the group, and we perform the CCSM multicast reconstruction procedure and PIM-SM graft procedure, respectively. We can find that when the number of multicast receiver is small, to be more specific, less than 54, the reconstruction cost of CCSM is lower than PIM-SM. When the number of receivers is greater than 54, the reconstruction cost of PIM-SM is smaller than CCSM. This is because that with the increase of multicast receivers, the PIM-SM graft procedure will have a high probability to encounter the existing multicast delivery tree, while as for CCSM, all the reconstruction has been initiated by MMC and MMC distributes the multicast routing entries to ASR or CR to update the multicast delivery tree. Therefore, the reconstruction procedure cannot benefit from the existing multicast delivery tree. This result also shows that comparing with PIM-SM, the CCSM is more effective when the multicast receiver density is small. Figure 10 shows the average multicast receiver join delay of CCSM and PIM-SM. We simulate multicast join procedure by randomly selecting a new node as the multicast receiver. When a new multicast receiver joins the group, the multicast delivery tree will be reconstructed. Once the multicast delivery tree finished the update, the new multicast receiver will get the multicast packets. We calculate the average distance of all multicast receivers to get the multicast packets. From Figure 10, we can find that the average join delay of CCSM is lower than PIM-SM although CCSM will take more cost to rebuild the multicast tree when the multicast receiver density is large.