tuning skype’s redundancy control algorithm for user satisfaction
DESCRIPTION
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.TRANSCRIPT
Network and Systems Laboratorynslab.ee.ntu.edu.tw
Te‐Yuan Huang, Kuan‐Ta Chen, Polly Huang
Network and Systems LaboratoryNational Taiwan University
Institute of Information ScienceAcademia Sinica, Taiwan
INFOCOM, 2009
Network and Systems Laboratorynslab.ee.ntu.edu.tw
MotivationVoice traffic is sensitive to network impairmentWhy VoIP sending rate is important?
Most important factors on user satisfactionSending Rate and its VariationHigh and Stable voice qualityg q y
Why adapting sending rate is difficult?Aggressively?Aggressively?Conservatively?
Network and Systems Laboratorynslab.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 I th i h i d h?Q2: Is their mechanism good enough?
Q3: How can Skype’s policy be improved?
Network and Systems Laboratorynslab.ee.ntu.edu.tw
Related WorkSkype’s voice traffic is governed by: [Bonfiglio et al.]
Bit RateBit RateFraming TimeRedundant DataRedundant Data
PC‐PSTN callsG 729G.729
PC‐PC calls iSACiSAC
Only Redundant Data is controlled by SkypeOnly Redundant Data is controlled by Skype
Network and Systems Laboratorynslab.ee.ntu.edu.tw
OutlineMotivation
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?
ConclusionConclusion
Network and Systems Laboratorynslab.ee.ntu.edu.tw
OutlineMotivation
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?
ConclusionConclusion
Network and Systems Laboratorynslab.ee.ntu.edu.tw
Experiment Setupp p
PC PSTN(G 729)PC‐PSTN(G.729)
PC‐PC(iSAC)
Network and Systems Laboratorynslab.ee.ntu.edu.tw
Observation 10%
G.729 (PC‐PSTN) 8%9%
10%
Constant bit rate
Constant framing 6%7%
Constant framing time
3%4%
5%
2%1%
3%
0%1%
0%
Network and Systems Laboratorynslab.ee.ntu.edu.tw
Redundancy RatioyDefinition
Th t f k t th t d d t i The percentage of packets that carry redundant voice data
1234
Redundancy Ratio = 2/4 = 0.5edu da cy at o / 0.5
Network and Systems Laboratorynslab.ee.ntu.edu.tw
the latter is used in Taiwan and Hong Kong, and the former is used more often in Mainland
Estimate Redundancy Ratio of G.729 10%stimate Redundancy Ratio of G.7 9
8%9%
10%
G.729 (PC‐PSTN)
6%7%Constant bit rate
Constant framing
3%4%
5%Constant framing time
2%1%
3%
0%1%
0%
Network and Systems Laboratorynslab.ee.ntu.edu.tw
Identify Redundancy Ratioy yRedundancy Ratio of G.729 (PC‐PSTN Calls)
Network and Systems Laboratorynslab.ee.ntu.edu.tw
Identify Redundancy Ratioy yRedundancy Ratio of iSAC (PC‐PC Calls)
Network and Systems Laboratorynslab.ee.ntu.edu.tw
OutlineMotivation
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?
ConclusionConclusion
Network and Systems Laboratorynslab.ee.ntu.edu.tw
Skype’s Redundancy Control AlgorithmSkype s Redundancy Control Algorithm
Adapt to network loss rateAdapt to network loss rate
Adapt to other factors?
Codec
N t k L B tiNetwork Loss Burstiness
Network and Systems Laboratorynslab.ee.ntu.edu.tw
Effect of Codec
G.729
iSAC
Network and Systems Laboratorynslab.ee.ntu.edu.tw
Effect of Network Loss BurstinessG.729 (PC‐PSTN)
BR=2
BR=1BR 1
BR=1.5
Network and Systems Laboratorynslab.ee.ntu.edu.tw
OutlineMotivation
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?
ConclusionConclusion
Network and Systems Laboratorynslab.ee.ntu.edu.tw
Optimal Redundancy Control Policyp y yWhat’s the Optimal Policy?
Minimum amount of redundancy data we need to sustain the same audio qualityneed to sustain the same audio quality under different network conditions
Network and Systems Laboratorynslab.ee.ntu.edu.tw
Emulation Flow
OptimalRedundancyRatio
G.729
Network and Systems Laboratorynslab.ee.ntu.edu.tw
Skype vs. Optimal – G.729yp p
Skype
Burst Ratio = 1
Network and Systems Laboratorynslab.ee.ntu.edu.tw
Optimal Redundancy for the Burst RatioOptimal Redundancy for the urst Ratio
BR=1.5BR=2BR=2
BR=1
Skype
G.729, MOS=3.5
Network and Systems Laboratorynslab.ee.ntu.edu.tw
Modeling Optimal Redundancy RatioModeling Optimal Redundancy RatioBased on the targeted voice quality
Take codec and burstiness into considerationconsideration
Optimal Policy for G.729, targeted MOS=3.5
Network and Systems Laboratorynslab.ee.ntu.edu.tw
ConclusionExplore how Skype adapts its voice trafficRedundancy Ratio
Skype’s policy does not factor in the individualSkype s policy does not factor in the individual codec and loss patterns in to consideration
Propose a general model for optimal policyConsistent user satisfaction
Extensible to general VoIP software
Network and Systems Laboratorynslab.ee.ntu.edu.tw