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

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

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

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

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

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

7. 7 Contributions

8. 8 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. … Solution (effective loss rate ): destination relay source Assumptions: 1. Paths are independent 2. Continuous-time Gilbert model for packet losses: average loss rate average loss burst length data to send protection (FEC) Model: schedule

9. 9 2. Observation that ∆t>0 ∆t = t 2 - t 1 [ms] t1t1 t2t2 ∆t = max(t 2,t 3 ) - t 1 [ms] t1t1 t2t2 t3t3

10. 10 Time t =0 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 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 3. P roposition of SPREAD T Usually very good, but not necessarily optimal Might schedule a packet departure before it is generated!

11. 11 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 4. Evaluation: Loss rate improvement

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

13. 13 5. Trace-driven evaluation

14. 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. 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.

16. 16 SPREAD Space Packets Regularly Exploiting Asymmetry in Delays MASTErPIECE MultipAth Scheduler That Exploits PropagatIon diffErenCEs bETTER Exploit Transmission Time diffERences SPLENDiD Space Packets evenLy and Exploit Network Delay Differences Sponsored article Not so easy to find? Try AcronymCreator.net !AcronymCreator.net

17. 17 Thank you !

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