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Motivation

TCP SPC: Statistic Process Control for Enhanced Transport over Wireless Links Yantai Shu , Dawei Gao, and Li Yu Tanjn University M.Y. Sanadidi, Mario Gerla UCLA. TCP SPC. Motivation. New congestion control scheme Using RTT as congestion indicator

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Motivation

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  1. TCP SPC: Statistic Process Control for Enhanced Transport over Wireless Links Yantai Shu, Dawei Gao, and Li Yu Tanjn UniversityM.Y. Sanadidi, Mario Gerla UCLA

  2. TCP SPC Motivation • New congestion control scheme • Using RTT as congestion indicator • Using SPC (Statistic Process Control) as monitoring method • Overcome sporadic losses in wireless networks • Improve TCP performance • Improve TCP fairness

  3. Network model • Model on transport layer • Input: data packet • Output: packet Loss and RTT • Packet loss: • employed in TCP congestion control • problematic as congestion indicator in wireless environments • inferred from RTT • RTT: more essential for congestion • Insensitive to wireless links

  4. Throughput, RTT and load • When load is less than cliff point • delay changes slightly • When load is between knee point and cliff point • delay are proportional to load • When the load exceeds cliff point • delay increases sharply • network status changes → the pattern of delay variance changes → different congestion levels RTT is suitable as a congestion indicator

  5. RTT Throughput Load

  6. Distribution of RTT • Assumption: RTT obeys the normal distribution under any invariable-load situation  Using SPC (Statistic Process Control) • Proof: • Samples sampled continuously from a stable process obey the normal distribution. • The distribution of RTT in the Internet was shown to be proximate to a Normal distribution. • We verified that the Normal Distribution also suits the wireless network by simulating a wireless network with a linear topology that consists of 8 nodes and one flow.

  7. Monitoring RTT • Assumption: the network load is stable • Sampling 15 consecutive RTT values • Calculating parameters: • the mean m • the standard deviation σ

  8. + 3 σ + 2 σ + σ T m T R - σ - 2 σ - 3 σ t Control chart

  9. Adjusting window • congestion criteria met • Decreasing congestion window drastically • over-load criteria met • Decreasing congestion window gently • chart-invalidation criteria met • Re-estimating parameters • under-load criteria met • Increasing the congestion window Re-estimate parameters after adjustment!

  10. Window adjustment • Slow Start • Doubling the window when a RTT get • Congestion Avoidance • Increasing 1 every 15 RTTs • When under-load criteria are met • Increasing 1 immediately • When congestion criteria are met • Halving the window • When over-load criteria are met • Decreasing the window by one quarter

  11. Simulation • Line • Cross • Grid

  12. Effects by hops • Line topology, Single flow

  13. Impact of packet error • Line topology, 7 nodes, Single flow

  14. Effects by flows • Line topology

  15. Effects by flows • Cross topology • 2 flows

  16. Effects by flows • Grid topology • 4,6,8 flows

  17. Discussion • RTT is suitable to be used as the congestion indicator • The throughput of TCP SPC is higher than that of TCP Reno and TCP SACK • TCP SPC is better to overcome the sporadic loss in wireless networks • The fairness index of TCP SPC is also considerably good

  18. Future work • Testifying the distribution assumption • Evaluating TCP SPC in testbed • Applying TCP SPC in wired/wireless mixed environments and in mesh networks

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