1. Routing and Transport challenges in mobility-assisted communication Konstantinos Psounis Assistant Professor EE and CS departments, USC

2. 2  Intermittent connectivity  lack of contemporaneous end-to-end paths  Disaster communication  Vehicular ad hoc networks  Sensor networks for environmental monitoring and wildlife tracking  Ad hoc networks for low cost Internet provision to remote areas  Inter-planetary networks  Ad-hoc military networks  Routing: “store-carry-and-forward” model  Transport: message-oriented approach, link-layer retransmissions  Interoperability with “traditional” network segments also a goal The need for mobility-assisted communication

3. 3 3 Example of store and forward routing S D 1 2 4 5 6 7 8 9 10 11 12 13 14 16 15

4. 4 Routing  Redundant copies reduce delay  Too much redundancy is wasteful and induces a lot of interference  Middle ground:  spray a small number of copies to distinct nodes  use carefully chosen relay-nodes to route each copy towards the destination  Challenges  How many copies to use? derive formal expressions that take into account real world limitations and compute number of copies that guarantee performance targets  How to optimally spray the copies use stochastic optimization and portfolio theory to find optimal policy  How to optimally choose relays? find a good utility function that indicates the goodness of a node as a relay

5. 5 How well spraying-based routing works? 500x500 grid, 200 nodes, medium traffic load  Spraying schemes outperform flooding schemes in terms of both transmissions and delay  As connectivity increases  delay of spraying schemes decreases  delay of other schemes increases due to severe contention Tx Range K (connectivity: % of nodes in max cluster)

6. 6 How many copies to use? Number of Copies L (M = 100) α = 2 α = 5 α = 10   = expected delay of spraying schemes over the expected delay of an oracle-based optimal scheme to be within some distance from optimal

7. 7 How to spray the copies? l th B closer to D A closer to D  Practical heuristic:  if l  l th (a few copies) the best node should keep/get all copies  else (a lot of copies) do binary spraying (split copies in half)  Optimal policy:  node A has l copies for node D  node A encounters node B 150x150 grid, 40 nodes, K=20

8. 8 Transport  Message oriented transport  rather than stream-oriented (no concept of flow)  Link layer retransmissions  hard to support end-to-end feedback mechanisms  Congestion control:  short term relief: if a node is congested give it priority over other nodes that contend for the same medium challenging to identify and coordinate these nodes in practice  medium term relief: use congestion information to dynamically adapt routing paths e.g. lower utility of congested nodes  Of course, source rate adaptation should eventually occur if network is oversubscribed

9. 9 Set of contending nodes S R  Congestion control and fairness require coordination among contending nodes  Which are those nodes?  assume, for simplicity, a single disk model for the transmission and interference range

10. 10 Interoperability  Future network:  Wired core  Wireless edge single-hop wireless sub- networks (SWN) multi-hop wireless sub- networks (MWN)  Use core-edge elements to break connections into sub- connections  mask differences Delay/disruptive tolerant MWN Mobile Ad-Hoc MWN Sensor/Mesh MWN Core-Edge Element A BcBc B AcAc WiFi SWN Base station WiMax SWN

11. 11 Core-edge element functionality examples  Transport connection management  Hide latencies and disconnections from the wired core  e.g. delay the start of successive sub connections until enough data are accumulated  Packet caching  Core-edge element acts as proxy of sender or receiver  e.g retransmit cached packets in case of loses no requirement to contact (hard to locate) source

12. 12 Experimentation and applications  Human mote experiments  students carry motes within main campus and on its vicinity  USC testbed  hundreds of static nodes arranged in disconnected clusters (tutornet platform) and a handful of radio-capable robots (robomote project) to bridge the gaps between them  Applications  offer connectivity for delay tolerant applications to USC commuters in collaboration with the university transportation office  customize protocols for VANET applications

13. 13 Selected Publications and funding sources more info available at http://ee.usc.edu/research/netpd/publications/  Publications:  Routing  Efficient Routing in Intermittently Connected Mobile Networks: The Multi-copy Case, T. Spyropoulos, K. Psounis, and C.Raghavendra, to appear in IEEE/ACM Transactions on Networking, February 2008.  Efficient Routing in Intermittently Connected Mobile Networks: The Single-copy Case, T. Spyropoulos, K. Psounis, and C. Raghavendra, to appear in IEEE/ACM Transactions on Networking, February 2008.  Performance Analysis of Mobility-Assisted Routing, T. Spyropoulos, K. Psounis, and C. Raghavendra, ACM MOBIHOC, Florence, Italy, May 2006.  Transport  Interference-aware fair rate control in wireless sensor networks S. Rangwala, R. Gummandi, R. Govindan, and K. Psounis, ACM SIGCOMM, Pisa, Italy, September 2006.  Mobility  Modeling Time-variant User Mobility in Wireless Mobile Networks, W.-j. Hsu, T. Spyropoulos, K.Psounis and A. Helmy, IEEE INFOCOM, May 2007.  Funding:  External: NSF Nets  Internal: Zumberge foundation, startup funds