ally the topology evolves extremely quickly. The average lifetime of a given link is 26 seconds. The average lifetime of a shortest path between two nodes is 15.5 seconds. Considering that the sam- pling period is 15 seconds, it follows that links are highly unstable and valid routes transient. We compare HYMAD to both Epidemic and reg- ular Spray-and-Wait. Epidemic provides an upper bound on achievable performance in terms of both delay and delivery ratio while Spray-and-Wait pro- vides a DTN state-of-the-art comparison. We slightly adapted Spray-and-Wait to the more connected con- text of Rollernet. A node no longer splits half of its copies with the other nodes it meets, but instead splits its copies equally among itself and its neighbors. ior profiles the 62 bluetooth loggers were distributed among groups of friends, members of rollerblading as- sociations and staff operators. In particular, one mem- ber of the staff was instructed to remain behind the tour times while another stayed in front for the entire duration of the experiment. This allows us to get a rough sense of the relative geographic position of the participants by looking at the connectivity graph. A snapshot of the connectivity graph can be seen in Fig. 6 and an animation is available online [15]. We chose to use D max = 2 for all our results because it ensures a very fast convergence rate, keeps the over- head reasonable and seemed to accurately reflect the size of separate connected components (small groups of friends for example), particularly during the accel- erating phases. We also tested greater values of D max , which yield , at the cost of greater overhead, a small but noticeable improvement of the delivery ratio. The sampling period of the Rollernet traces is 15 seconds. We did not try to extrapolate the events (link failures, new contact opportunities, etc..) in the time between multiples of 15 seconds. We also assume that 15 seconds is enough for a message to traverse any con- nected component in Rollernet. Therefore, all our re- sults on delays when simulating protocols on top over the Rollernet traces will be in multiples of 15 seconds. The Rollernet trace is ideal for evaluating HYMAD. Indeed it exhibits the following characteristics: Extremely high link instability could mean one of two things. Either nodes only briefly stay in the vicin- ity of one another or that nodes may remain geograph- ically close but that the link fails for other reasons such as briefly moving out of transmission range or exces- sive contention. We measured between each time step and from each node’s point of view, how many of its group members changed (number of new members + 5 • High density: Contrary to many DTN traces, Rollernet is not sparse. A look at Fig. 3a, shows that the average node degree of the connectivity graph oscillates between 2.9 and 7.8. The average for the whole tour is 4.8. • High mobility: Everyone eventually meets every- one else. On average, each of the 62 nodes meets 56 others during the course of the tour. Addition- • Accordion Effect: This is an interesting conse- quence of the rollerblading context. The tour alternates between acceleration and deceleration phases in which the network topology respectively expands, leading to several separate connected component, and contracts, leading to a single con- nected component. Fig. 3b shows that the number of connected components varies between 1 and 7 (17 if counting isolated nodes). In fact, Figures 3a and 3b have roughly alternating phases. 4.2 Performance Figure 4: Topology changes: average number of links within a given that appear or fail vs. number of nodes that leave or join that group 2 3 4 5 6 7 8 0 500 1000 1500 2000 2500 3000 average node degree time (s) (a) Average node degree 1 2 3 4 5 6 7 0 500 1000 1500 2000 2500 3000 number of ccs time (s) (b) Number of connected components (ccs) Figure 3: The accordion effect in Rollernet 0 0.5 1 1.5 2 2.5 0 500 1000 1500 2000 2500 3000 rate of change time (s) links within groups no des