# A Multicast Routing Scheme for the Internet: Simulation and Experimentation in Large-Scale Networks
## 6 Experimental Analysis
### Results  
Figure 11a depicts the average GCMR and PIM-SSM stretch of 10 separate runs. In all executions, GCMR has less stretch (0.14 better on average) and some degradation (0.095 on average) when compared to the optimum MDT (stretch-1 is the reference). This finding is consistent with the simulation results presented in Section 5, which show that GCMR achieves about 0.1 better stretch than PIM-SSM in considerably larger networks and a higher number of leafn nodes.  
Figure 10: Particularity of the AS1-AS6 connectivity.  
Figure 9: Setup of the experiments.  
The number of ASes part of the MDT is shown in Figure 11b. GCMR is considerably closer to the optimum MDT than PIM-SSM. From the cost perspective, this is a significant benefit of GCMR since providers charge customers depending on the resources used to deliver the multicast service. Hence, the cost for the costumers should be lower when compared to PIM-SSM since GCMR utilises fewer transit ASes.  
Figure 11c compares the two protocols in terms of memory space ratio. We can observe that GCMR requires, on average, around 2.9 times less memory than PIM-SSM. Remember that we only consider the memory space stored in the nodes part of the MDT using the formats defined in Section 5.1: unlike GCMR, which only requires MRIB and TIB entries, PIM-SSM additionally requires certain unicast information from BGP in order to calculate the shortest path to the multicast source.  
To analyse the control overhead of the protocols, we determine the communication cost of PIM-SSM and GCMR in Figure 11d. Results shown that GCMR needs an average of 20% more messages than PIM-SSM to set up the MDT.  
Again, both the memory space and the communication cost here are consistent with the previous simulation results. For instance, observing the memory space ratio in Table 1, we can see that the ratio for PIM-SSM increases with the size of the network from 146.81 with 16k nodes to 270.38 with 46k nodes. As we are considering a network with 207 nodes in these experimental runs, the gain of GCMR vs. PIM-SSM is reflected by this reduced scenario and drops to 2.9. Similar trends can be seen in communication cost which is around 10 times more when simulating GCMR in a larger network and drops to only 1.2 times more in emulation.  
The final results concern the recovery time. As commented on above, we count the maximum time elapsed to receive back the multicast service at the receiver in Barcelona after emulating a failure in the link between AS1 and AS6. In the case of GCMR, recovery time includes the time it takes for the failure-detecting node to start a search and receive  
Figure 11: Comparison between GCMR and PIM-SSM in the emulation platform: (**a**) stretch; (**b**) number of ASes in the MDT; (**c**) routing table size ratio; (**d**) communication cost ratio.  
responses from its neighbours pointing to the least cost branching path, as well as the time it takes to send a Join message and reconnect the MDT. The experiments show that the recovery time for GCMR in this particular scenario is approximately one second. In the case of PIM-SSM, firstly we need MP-BGP to detect the failure, withdraw the entries of the affected paths to the multicast source, and find the new routes [35], i.e., MP-BGP needs to converge to the new stable state. Secondly, we need PIM-SSM to find the new adjacencies with its Hello messages and exchange the Join message with the new next hops toward the multicast source [19]. Because of the specific nature of the AS1-AS6 interconnection, BGP tends to check all options (an issue known as path exploration [37]) before reaching the convergence. This means that the service interruption experimented with PIM-SSM is significant high, approximately 2 min and half.