Tuning Skype’s Redundancy Control Algorithm for User Satisfaction
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Determining how to transport delay-sensitive voice data has long been a problem in multimedia networking. The difficulty arises because voice and best-effort data are different by nature. It would not be fair to give priority to voice traffic and starve its best-effort counterpart; however, the voice data delivered might not be perceptible if each voice call is limited to the rate of an average TCP flow. To address the problem, we approach it from a user-centric perspective by tuning the voice data rate based on user satisfaction. Our contribution in this work is threefold. First, we investigate how Skype, the largest and fastest growing VoIP service on the Internet, adapts its voice data rate (i.e., the redundancy ratio) to network conditions. Second, by exploiting implementations of public domain codecs, we discover that Skype’s mechanism is not really geared to user satisfaction. Third, based on a set of systematic experiments that quantify user satisfaction under different levels of packet loss and burstiness, we derive a concise model that allows user-centric redundancy control. The model can be easily incorporated into general VoIP services (not only Skype) to ensure consistent user satisfaction.
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Tuning Skype’s Redundancy Control Algorithm for User Satisfaction
- 1. Network and Systems Laboratory nslab.ee.ntu.edu.tw Te‐Yuan Huang, Kuan‐Ta Chen, Polly Huang Network and Systems Laboratory National Taiwan University Institute of Information Science Academia Sinica, Taiwan INFOCOM, 2009
- 2. Network and Systems Laboratory nslab.ee.ntu.edu.tw Motivation Voice traffic is sensitive to network impairment Why VoIP sending rate is important? Most important factors on user satisfaction Sending Rate and its Variation High and Stable voice quality g q y Why adapting sending rate is difficult? Aggressively? Conservatively?
- 3. Network and Systems Laboratory nslab.ee.ntu.edu.tw Motivation – Cont. Skype – one of the most popular VoIP software Q1: How Skype adapts its voice traffic? Q1: How Skype adapts its voice traffic? Q2: Is their mechanism good enough? Q2 I th i h i d h? Q3: How can Skype’s policy be improved?
- 4. Network and Systems Laboratory nslab.ee.ntu.edu.tw Related Work Skype’s voice traffic is governed by: [Bonfiglio et al.] Bit Rate Bit Rate Framing Time Redundant Data Redundant Data PC‐PSTN calls G.729 G 729 PC‐PC calls iSAC Only Redundant Only Redundant Data is controlled by Skype is controlled by Skype
- 5. Network and Systems Laboratory nslab.ee.ntu.edu.tw Outline Motivation Related Work How does Skype adapt the redundancy ratio? How does Skype adapt the redundancy ratio? Is Skype’s mechanism good enough? How can we do better? Conclusion
- 6. Network and Systems Laboratory nslab.ee.ntu.edu.tw Outline Motivation Related Work How does Skype adapt its redundant data? How does Skype adapt its redundant data? Is Skype’s mechanism good enough? How can we do better? Conclusion
- 7. Network and Systems Laboratory nslab.ee.ntu.edu.tw Experiment Setup p p PC‐PSTN(G.729) PC PSTN(G 729) PC‐PC(iSAC)
- 8. Network and Systems Laboratory nslab.ee.ntu.edu.tw Observation 10% 9% G.729 (PC‐PSTN) 8% Constant bit rate 7% 6% Constant framing Constant framing 5% time 4% 3% 2% 1% 0% 0%
- 9. Network and Systems Laboratory nslab.ee.ntu.edu.tw Redundancy Ratio y Definition The percentage of packets that carry redundant voice Th t f k t th t d d t i data 4 3 2 1 Redundancy Ratio = 2/4 = 0.5 edu da cy at o / 0.5
- 10. the latter is used in Taiwan and Hong Kong, and the former is used more often in Mainland Network and Systems Laboratory nslab.ee.ntu.edu.tw Estimate Redundancy Ratio of G.729 stimate Redundancy Ratio of G.7 9 10% 9% G.729 (PC‐PSTN) 8% Constant bit rate 7% 6% Constant framing Constant framing 5% time 4% 3% 2% 1% 0% 0%
- 11. Network and Systems Laboratory nslab.ee.ntu.edu.tw Identify Redundancy Ratio y y Redundancy Ratio of G.729 (PC‐PSTN Calls)
- 12. Network and Systems Laboratory nslab.ee.ntu.edu.tw Identify Redundancy Ratio y y Redundancy Ratio of iSAC (PC‐PC Calls)
- 13. Network and Systems Laboratory nslab.ee.ntu.edu.tw Outline Motivation Related Work How does Skype adapt its redundancy ratio? How does Skype adapt its redundancy ratio? Is Skype’s mechanism good enough? How can we do better? Conclusion
- 14. Network and Systems Laboratory nslab.ee.ntu.edu.tw Skype s Redundancy Control Algorithm Skype’s Redundancy Control Algorithm Adapt to network loss rate Adapt to network loss rate Adapt to other factors? Codec Network Loss Burstiness N t kL B ti
- 15. Network and Systems Laboratory nslab.ee.ntu.edu.tw Effect of Codec G.729 iSAC
- 16. Network and Systems Laboratory nslab.ee.ntu.edu.tw Effect of Network Loss Burstiness G.729 (PC‐PSTN) BR=2 BR 1 BR=1 BR=1.5
- 17. Network and Systems Laboratory nslab.ee.ntu.edu.tw Outline Motivation Related Work How does Skype adapt its redundancy ratio? How does Skype adapt its redundancy ratio? Is Skype’s mechanism good enough? How can we do better? Conclusion
- 18. Network and Systems Laboratory nslab.ee.ntu.edu.tw Optimal Redundancy Control Policy p y y What’s the Optimal Policy? Minimum amount of redundancy data we need to sustain the same audio quality need to sustain the same audio quality under different network conditions
- 19. Network and Systems Laboratory nslab.ee.ntu.edu.tw Emulation Flow Optimal Redundancy Ratio G.729
- 20. Network and Systems Laboratory nslab.ee.ntu.edu.tw Skype vs. Optimal – G.729 yp p Skype Burst Ratio = 1
- 21. Network and Systems Laboratory nslab.ee.ntu.edu.tw Optimal Redundancy for the urst Ratio Optimal Redundancy for the Burst Ratio BR=2 BR=1.5 BR=1 Skype G.729, MOS=3.5
- 22. Network and Systems Laboratory nslab.ee.ntu.edu.tw Modeling Optimal Redundancy Ratio Modeling Optimal Redundancy Ratio Based on the targeted voice quality Take codec and burstiness into consideration Optimal Policy for G.729, targeted MOS=3.5
- 23. Network and Systems Laboratory nslab.ee.ntu.edu.tw Conclusion Explore how Skype adapts its voice traffic Redundancy Ratio Skype s policy does not factor in the individual Skype’s policy does not factor in the individual codec and loss patterns in to consideration Propose a general model for optimal policy Consistent user satisfaction Extensible to general VoIP software
- 24. Network and Systems Laboratory nslab.ee.ntu.edu.tw