1. 1-1 CMPE 259 Sensor Networks Katia Obraczka Winter 2005 Transport Protocols II

2. 1-2 Announcements r Feedback on project proposals. r Project resources.

3. 1-3 Transport protocols (cont’d) r RMST r CODA r Summary

4. 1-4 RMST

5. 1-5 RMST r Reliable Multi-Segment Transport. r Where to do reliability? m MAC. m Transport. m Application.

6. 1-6 MAC reliability r 802.11. m RTS/CTS, Data, Ack. m Basic stop-and-wait ARQ. m No ARQ when in broadcast or multicast modes. Random slot selection. r Options: m No ARQ. m AEQ always. m Selective ARQ.

7. 1-7 MAC reliability (cont’d) r Without ARQ: m Use broadcast mode. m For unicast: address screening at routing layer. m +’s: no overhead. r With ARQ: m Unicast transmissions. m For broad- & multicast, use multiple unicast. m Number of retries is configurable. r Selective ARQ: m Unicast uses ARQ. m Broad- and multicast use no ARQ. E.g., route discovery.

8. 1-8 Transport reliability r Strictly e2e. m Initiated by sink. r Local recovery. m Intermediate nodes trigger repair when loss is detected. m Nodes cache packets. r NACK-based.

9. 1-9 Application-layer reliability r Directed-diffusion based. m Sink sends out request (“interest”). m When complete data received, sink removes request.

10. 1-10 Question? r Benefits of lower-layer reliability? r Additional overhead?

11. 1-11 RMST overview r Functions: m Fragmentation/reassembly. m Guaranteed delivery. r Unique identifiers: m “No fragments”. m Fragment id’s and number of fragments. r Loss detection and repair: m Sequence # holes and timers. m Loss detection at either sinks or intermediate nodes. m NACKs.

12. 1-12 Preliminary analysis r Demonstrate the benefits of hop-by-hop reliability.

13. 1-13 RMST evaluation r MAC-only reliability. r Local recovery. m With and without MAC reliability. r End-to-end reliability. m With and without MAC reliability.

14. 1-14 Observations r When there is no transport reliability: m MAC reliability critical in lossy links. r Hop-by-hop transport reliability: m Adds little to reliable MAC. m But, hop-by-hop transport reliability only more efficient than adding MAC reliability. MAC ARQ overhead incurred in every packet. r E2E transport reliability: m When no MAC reliability is used, simulation does not terminate: hop-by-hop recovery is critical. m If MAC reliability used, hop-by-hop and e2e transport reliability are equivalent.

15. 1-15 Observations (cont’d) r Experiments with high error rates: m Hop-by-hop transport reliability without MAC reliability. m Hop-by-hop transport reliability+Sel. ARQ. m E2e transport reliability+ Sel. ARQ. r Hbh transport reliability without ARQ breaks down at high error rates. m Routing has hard time establishing routes.

16. 1-16 CODA

17. 1-17 COngestion Detection and Avoidance r Importance of congestion control.

18. 1-18 What is CODA ? r Energy efficient congestion control. r Three mechanisms are involved: m Congestion detection m Open-loop hop-by-hop backpressure. m Closed-loop multi-source regulation.

19. 1-19 Congestion detection r Accurate and efficient congestion detection is important m Channel loading – sample channel at appropriate rate to detect congestion.

20. 1-20 Open-loop h-by-h backpressure 6 1 2 4 5 3 Congestion detected Upstream node decides to propagate backpressure or not.

21. 1-21 Closed loop multi-source regulation 12 1,2,3 ACK 4,5,6 Congestio n detected 7,8 Regulate bit is set ACK

22. 1-22 Congestion detection schemes r Buffer occupancy. m Not reliable in CSMA networks. r Channel loading. m Good for the immediate neighborhood. m Energy considerations. r Report rate. m Report rate goes down, congestion. m Detection based on report rate needs to react on longer time scale.

23. 1-23 CODA overview r Combination of backpressure (fast time scale) with closed-loop congestion control. r Backpressure targets “local” congestion, whereas closed-loop regulation targets persistent congestion. r Backpressure is cheaper/simpler since it’s open loop. r Congestion control requires a feedback loop. m Uses ACK from sink to self-clock.

24. 1-24 CODA performance metrics r Average Energy Tax = Total packets dropped in network / Total packets received at sink r Average Fidelity Penalty = Difference between average number of packets delivered at sink using CODA and using ideal congestion scheme.

25. 1-25 Simulation Setup r Random network topologies with network size from 30 to 120 nodes. r 2Mbps IEEE 802.11 MAC (RTS/CTS are disabled). r Directed diffusion is used as routing core. r Fixed work load, 6 sources and 3 sinks. r Source generate data at different rates. r Event packet is 64 bytes and interest packet is 36 bytes.

26. 1-26 Simulation Results (Case 1: Dense Source, High Rate)

27. 1-27 Simulation Results (Case 2: Sparse Sources, Low Rate)

28. 1-28 Simulation Results Case 2: Sparse Source, Low Rate

29. 1-29 Simulation Results (Case 3: Sparse Sources, High Rate) Network Size (#no of nodes)

30. 1-30 Conclusion r CODA’s energy efficiency. r CODA’s ability to handle persistent and transient congestion.

31. 1-31 Transport protocols: summary

32. 1-32 Pump Slow Fetch Quickly PSFQ r For sink-to- source communication (e.g. network reprogramming) r Reliability via retransmissions r Sequence-driven loss detection C.Y. Wan, A.T. Campbell, and L. Krishnamurthy. PSFQ: A Reliable Transport Protocol for Wireless Sensor Networks. WSNA'02, September 28, 2002, Atlanta, Georgia, USA.

33. 1-33 RMST r End-to-end or hop-by-hop repair (the latter is generally better) r Suggests that repair could be done at either MAC layer (ARQ retransmissions) or Transport Layer (requests based on fragment numbers etc.) r Timer-driven loss detection and local data caches r Fits with the Directed Diffusion API F. Stann and J. Heidemann. RMST: Reliable Data Transport in Sensor Networks. IEEE SNPA'03.

34. 1-34 ESRT r Aim for overall quality of service rather than node-to-node reliability Sankarasubramaniam, Y., Akan, O.B., and Akyildiz, I.F., "ESRT: Event-to-Sink Reliable Transport in Wireless Sensor Networks ", In Proc. ACM MobiHoc`03

35. 1-35 CODA Sankarasubramaniam, Y., Akan, O.B., and Akyildiz, I.F., "ESRT: Event-to-Sink Reliable Transport in Wireless Sensor Networks ", In Proc. ACM MobiHoc`03 r Receiver based congestion detection r Open loop hop-by-hop backpressure r Closed-Loop multi-source regulation

36. 1-36 Summarizing Transport Issues r Because of harsh conditions and severe constraints, it may be better to implement reliability in a hop-by-hop rather than end-to-end manner at either the MAC or transport layer r For energy efficiency, it is best to avoid congestion entirely, or have packet losses occur close to the source. Back pressure is a useful technique. r Where possible, scheduled solutions are preferable. s