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Improving the Start-Up Behavior of a Congestion Control Scheme for TCP.

Improving the Start-Up Behavior of a Congestion Control Scheme for TCP. Janey C. Hoe Laboratory for Computer Science at MIT. 노상훈 , 권민혁 @ Pllab. Abstract. Introduction. Related Work. The Congestion Control Scheme. Slow-start problem. Fast retransmit problem #1 : Timeout. CWND is full

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Improving the Start-Up Behavior of a Congestion Control Scheme for TCP.

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  1. Improving the Start-Up Behavior of a Congestion Control Scheme for TCP. Janey C. Hoe Laboratory for Computer Science at MIT 노상훈, 권민혁 @ Pllab

  2. Abstract

  3. Introduction

  4. Related Work

  5. The Congestion Control Scheme

  6. Slow-start problem

  7. Fast retransmit problem #1: Timeout • CWND is full • 51 is recovered • By fast retransmit • 53 couldn’t recovered • Network is idle • Retransmission time0ut • Ignored cue • Max seq is 54 and Last ack is 53 • Obviously 53 was lost

  8. Fast retransmit problem #2: False Fast Retransmit

  9. Modification: Slow-start • Use better initial value of ssthresh • Estimated value is better than arbitrary default value • Estimation method • At first, initialize as previous default value (=64 seg) • During 3 ack (= establishing connection) • Least square estimation of bandwidth-delay product • Even if default value is high, CWND not ramp up riskily

  10. Modification: Slow-start • Prevent multiple packet losses during start-up period on severe environment

  11. Modification: Fast retransmit & Fast Recovery #1

  12. Modification: Fast retransmit & Fast Recovery #2 Yes, Still in fast retransmit phase • Fast retransmit phase • Setup • set ssthresh = ½ cwnd • cwnd = 1 segment • snd_next = snd_una • (2) set snd_high = snd_max • save_cwnd = ssthresh + 1 segment Retransmit (3) send using slow start algorithm (4) allow congestion window to increase as long as congestion window <= save_cwnd (5) do not start a new fast retransmit phase (6) upon receiving 2 duplicate ACK’s, send out 1 new packet beyond snd_high Seq number ACKed < snd_high Upon receiving 3 duplicate ACK’s begin fast restransmit Phase No cwnd=save_cwnd

  13. Modification: Fast retransmit & Fast Recovery #3 • Modified algorithm can handle multiple packet losses

  14. Simulation Environment • Bottle-neck link HOST 1 Switch1 Switch2 HOST 2 10Mbps 1600Kbps 10Mbps 50ms delay

  15. Simulation result - original • Multiple packet losses occurred during start-up period

  16. Simulation result – improved • Graph show new algorithm is better than original algorithm

  17. Conclusion • Proposes several possible changes to improve the start-up behavior of the congestion control scheme. • Estimated ssthresh value. • Modification Fast retransmit algorithm. • Tested in simulations and over real network. • These changes can significantly improve TCP’s performanceduringstart-up period • Byeliminating the wait for unnecessary timeout • These changes arenoticeable inshort TCP data transfers

  18. Reference • [1] Janey C. Hoe, Improving the Start-up Behavior of a Congestion Control Scheme for TCP, Proceedings of the ACM SIGCOMM Conference, 1996 • [2] Kevin Fall. Kannan Varadhan, ns Notes and Documentation, UC Berkeley, LBL, USC/ISI, and Xerox PARC

  19. Supplement 1:About Implementation. [2] ns Notes and Documentation

  20. Supplement 2:About Implementation

  21. Supplement 3:About graph • There is no TCL variable which is contain current sequence number. • We added new variable “just_seq_” and compiled.

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