Routing and Transport challenges in mobility-assisted communication

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

2. The need for mobility-assisted communication • 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

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

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. Tx Range K (connectivity: % of nodes in max cluster) 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

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

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

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. Set of contending nodes • 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 R S

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 Sensor/Mesh MWN WiMax SWN A Ac Base station WiFi SWN Bc Core-Edge Element B Mobile Ad-Hoc MWN

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. 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. Selected Publications and funding sourcesmore infoavailable 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