Exploiting the Path Propagation Time Differences in Multipath Transmission with FEC

1. Infocom 2009, April 19-25, Rio de Janeiro Exploiting the Path Propagation Time Differencesin Multipath Transmission with FEC Maciej Kurant EPFL, Switzerland Acknowledgements: Patrick Thiran (EPFL), Dan Jurca (DoCoMo, Munich), Pascal Frossard (EPFL)

2. T protection (FEC) data to send received reconstructed schedule Multipath transmission with FEC source destination relay • Requirements: • few losses • small delay (no retransmission possible)

3. Example Assumptions: 1. Paths are independent T=5ms source destination relay

4. Example 1%, 10ms 1% 10ms Time t=0 Effective loss rate : 0.553% 0.148% State of the art - IMMEDIATE Assumptions: 1. Paths are independent 2. Continuous-time Gilbert model for packet losses: • average loss rate • average loss burst length T=5ms source destination relay Schedule: 1.000%

5. Example Time t=0 Effective loss rate : 0.113% ∆t = 50ms source destination relay slow! ∆t >> 0 (here assume 50ms) Schedule: 1.000% 0.553% 0.148% State of the art - IMMEDIATE We use ∆t to SPREADthe packets on the green path, so that the total block delay is not changed.

6. Example Time t=0 Effective loss rate : 0.113% 0.016% The optimal rates on the paths may change. source destination relay slow! Schedule: The gain over state of the art is close to one order of magnitude! 1.000% 0.553% 0.148% State of the art - IMMEDIATE We use ∆t to SPREADthe packets on the green path, so that the total block delay is not changed.

7. Contributions

8. Model: Assumptions: 1. Paths are independent 2. Continuous-time Gilbert model for packet losses: • average loss rate • average loss burst length source destination relay protection (FEC) schedule Solution (effective loss rate ): data to send 1. Exact solution of the model • Previous attempts (using approximations): • L. Golubchik et al, “Multi-path continuous media streaming. what are the benefits?” Performance Evaluation Journal, 2002. • E. Vergetis, R. Guerin, and S. Sarkar, “Realizing the benefits of user-level channel diversity,” SIGCOMM Comput. Commun. Rev., 2005. • Y. Li, Y. Zhang, L. Qiu, and S. Lam, “SmartTunnel: Achieving reliability in the internet,” Proc. of INFOCOM’07, 2007. • …

9. ∆t = max(t2,t3) - t1 [ms] t1 t2 t3 2. Observation that ∆t>0 ∆t = t2 - t1 [ms] t1 t2

10. 3. Proposition of SPREAD T Time t=0 • IMMEDIATE rules: • Send each packet immediately after it is generated at the source • Find the best packet rates for each path • Assign packets to paths to minimize the loss rate Usually very good, but not necessarily optimal • SPREAD rules: • On each path,spread the packets evenly in time within all the available time budget • Find the best packet rates for each path • Might schedule a packet departure • before it is generated!

11. 4. Evaluation: Loss rate improvement i.e., a relative loss rate improvement over the state of the art path propagation time difference [ms] source packet generation period [ms] number of packets per FEC block

12. 4. Evaluation: Minimizing delays Reverse the problem, i.e.: “Minimize the FEC block delay of SPREAD, but keep .” FEC block delay gain [ms] path propagation time difference [ms] Useful, e.g., to minimize the effect of jitter.

13. 5. Trace-driven evaluation

14. Limitations • Simple performance metric • Not (yet) adaptive • Assumes path independence • Assumes that we control when packets are actually sent • Needs more experimental verification But the general principle remains.

15. Conclusions • Paths often have different propagation times. • This can be exploited in multipath FEC transmission to reduce: • the effective loss rate • the total FEC block delay. • To achieve this, we: • solved the multipath FEC model, • proposed and evaluated SPREAD.

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17. Thank you !