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Improving TCP Performance in High Bandwidth High RTT Links Using Layered Congestion Control

Improving TCP Performance in High Bandwidth High RTT Links Using Layered Congestion Control. Sumitha Bhandarkar Saurabh Jain A. L. Narasimha Reddy Texas A & M University. Layering Concepts. Design Constraints Fairness among flows of similar RTT RTT unfairness no worse than TCP

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Improving TCP Performance in High Bandwidth High RTT Links Using Layered Congestion Control

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  1. Improving TCP Performance in High Bandwidth High RTT Links Using Layered Congestion Control Sumitha Bhandarkar Saurabh JainA. L. Narasimha ReddyTexas A & M University

  2. Layering Concepts • Design Constraints • Fairness among flows of similar RTT • RTT unfairness no worse than TCP • Fair to TCP in slow networks • Two dimensional congestion control • Increase layers, if no losses for extended period • Per-RTT window increase more aggressive at higher layers

  3. Layering Concepts (Cont.) • Layering • Start layering when window > WT • Associate each layer with a step size K • When window increases from previous addition of layer by K, increment number of layers • For each layer K, increase window by K per RTT Number of layers determined dynamically based on current network conditions.

  4. Layering Concepts (Cont.) Minimum Window Corresponding to the layer Layer Number K + 1 WK+1 dK K K WK dK-1 K - 1 WK-1 Number of layers = K when WK W  WK+1

  5. Framework • Constraint 1 : • rate of increase for flow at higher layer should be lower than flow at lower layer • Constraint 2 : • After a loss, recovery time for a larger flow should be more than the smaller flow (K1 > K2, for all K1, K2  2)

  6. A Design Choice • Decrease behavior : • Multiplicative decrease • Increase behavior : • Additive increase with additive factor = layer number W = W + K/W

  7. A Design Choice (Cont.) • After loss, drop at most one layer • Constraint for choice of K: • We choose 

  8. A Design Choice (Cont.) • Choice of  : Since after loss, at most one layer is dropped,  (We choose  = 0.15 corresponding to K = 19)

  9. Analysis Time to claim bandwidth Speedup inPacket recovery time

  10. Analysis (Cont.) • Steady state throughput where K' is the layer corresponding to steady state window size,  is the window decrease factor and p is the steady state loss probability

  11. Analysis (Cont.) • RTT Unfairness • With random losses, RTT unfairness similar to TCP • With synchronized losses, RTT unfairness is • Can be easily compensated • Modify increase behavior W = W + (KR * K) / W • When KR RTT (1/3), RTT unfairness similar to TCP • When KR RTT, linear RTT unfairness (window size independent of RTT) • Loss model depends on type of queue management, level of multiplexing etc.

  12. Experimental Evaluation Window Comparison

  13. Experimental Evaluation Link Utilization

  14. Experimental Evaluation Fairness among multiple flows

  15. Experimental Evaluation Dynamic Link Sharing

  16. Experimental Evaluation Interaction with TCP

  17. Experimental Evaluation RTT Unfairness

  18. Conclusions • Why LTCP ? • Current design remains AIMD • Dynamically changes increase factor • Retains convergence and fairness properties • Simple to understand/implement • RTT unfairness similar to TCP

  19. Future Work • Characterize losses on actual high speed links • Study alternate designs for LTCP framework • Compare with other TCP based high speed solution. Preliminary results show • observed loss probability with LTCP is lower than other schemes • improved RTT unfairness • better TCP tolerance in high speed networks

  20. Comparison with BIC(Preliminary Results) RTT Unfairness

  21. Thank You... Questions ? Additional questions/feedback welcome at {sumitha,saurabhj,reddy}@ee.tamu.edu

  22. Simulation Topology

  23. Related Work • HS-TCP Sally Floyd, “HighSpeed TCP for Large Congestion Windows”, RFC 3649 Dec 2003. • Scalable TCP Tom Kelly, “Scalable TCP: Improving Performance in HighSpeed Wide Area Networks”, ACM Computer Communications Review, April 2003. • FAST Cheng Jin, David X. Wei and Steven H. Low, “FAST TCP: motivation, architecture, algorithms, performance”, IEEE Infocom, March 2004. • BIC Lisong Xu, Khaled Harfoush, and Injong Rhee, “Binary Increase Congestion Control for Fast Long-Distance Networks”, IEEE Infocom, March 2004. • HTCP R. N. Shorten, D. J. Leith, J. Foy, and R. Kilduff, “H-TCP Protocol for High-Speed Long Distance Networks”, PFLDnet 2004, February 2003.

  24. RTT Fairness(Random Loss Model) • Probability of loss for LTCP • Probability of loss for TCP

  25. Comparison with BIC(Preliminary Results) Observed Loss Rates Single Flow, 1Gbps bottleneck link

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