• With a low number of copies, HYMAD also out- performs Spray-and-Wait in terms of delivery ratio for reasons explained hereafter. • Predictably, performance increases with the num- ber of copies. The maximum number of groups (in- cluding singletons) obtained at a given time is 29. Therefore, HYMAD with 20 copies will spray prac- tically the entire network and therefore quickly and reliably reach the destination if in the same connected component as the source. • HYMAD clearly outperforms Spray-and-Wait in terms of delay and quickly achieves comparable performance with Epidemic. 6 In this paper we identified a new class of dense and highly mobile networks not well addressed by conven- tional DTN or MANET approaches. We proposed a new hybrid approach, HYMAD, that uses nodes’ knowledge of their local group topology to improve the performance of a simple DTN protocol. In our case we used diameter-constrained groups along with distance vector for intra-group routing and Spray-and-Wait for inter-group routing. Simulations of our implementa- tion in a dense and highly mobile network show signif- icant performance improvements over regular Spray- Spray-and-Wait’s simple forwarding scheme per- forms very well under the assumption of independent and identically distributed node mobility [13]. How- ever this is absolutely not the case in Rollernet where groups of friends tend to stick together. It is also usu- ally not the case in many real-world situations where underlying social dynamics are often at work. This can have a impact on performance. For exam- ple, when using just 5 copies, Spray-and-Wait simply fails to deliver about 5% of messages even after waiting for more than 15 minutes. Using 10 copies, the aver- age delay with Spray-and-Wait (133 seconds) is nearly three times that of HYMAD (48 seconds). To further illustrate this point, Fig 6 compares the propagation of 10 copies after 15 seconds for HYMAD (Fig. 6a) and Spray-and-wait (Fig. 6b). The rightmost node is the head of the rollerblading tour. The bold lines represent intra-group links while the dashed gray lines represent inter-group links. The nodes holding at least one copy are represented by a diamond. In HYMAD’s case, the destination is a diamond meaning that our hybrid ap- proach has delivered its message within 15 seconds. On the other hand, the regular Spray-and-Wait protocol distributed copies mainly within its own local group. These nodes remain close to each other thus increase the delay. In this particular case (Fig. 6b) it will take 525 seconds for a node with a copy to meet the desti- nation number of excluded members) and how many links be- tween members of its group changed (either by appear- ing or disappearing). The averages for all the nodes are shown in Fig. 4. The composition of a given group ap- pears much more stable than the links among its mem- bers. This supports the idea that small communities like groups of friends tend to stick together during the tour and that link failures do not necessarily mean that two nodes have clearly moved away from each other. Furthermore, the rate of change of group composition, unlike most other metrics, seems to smooth the accor- dion effect. This suggests that these groups are indeed a good support for our hybrid approach. To evaluate the performance of HYMAD we re- played the 3000 seconds of the trace. Every 15 sec- onds, during the first 2000 seconds, we randomly se- lected 60 pairs of nodes which were instructed to send a message to each other using Epidemic, HYMAD and Spray-and-Wait. This averages results over both the connected and disconnected phases of Rollernet. Fig- ures 5a and 5b were obtained using the aggregate data from 10 runs of this scenario with respectively 5 and 20 maximum number of copies for HYMAD and Spray- and-Wait. They compare the cumulative distribution function of the delivery probability for the three pro- tocols. A few observations can be made: Figure 5: Comparison of delivery probabilities 0 0.2 0.4 0.6 0.8 1 0 200 400 600 800 1000 Delivery probability (cdf) time (s) Epidemic HYMAD Spray and W ait (a) 5 copies 0 0.2 0.4 0.6 0.8 1 0 200 400 600 800 1000 Delivery probability (cdf) time (s) Epidemic HYMAD Spray and W ait (b) 20 copies 5 Conclusion and further work