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Implementing High Speed TCP (aka Sally Floyd’s)

Implementing High Speed TCP (aka Sally Floyd’s). Yee-Ting Li & Gareth Fairey 1 st October 2002 DataTAG Presentation @ CERN (Kinda!). What is High Speed TCP?. Changes the way TCP behaves at high speed (ie large cwnd) Standard TCP has two modes Slow start (not very slow…)

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Implementing High Speed TCP (aka Sally Floyd’s)

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  1. Implementing High Speed TCP (aka Sally Floyd’s) Yee-Ting Li & Gareth Fairey 1st October 2002 DataTAG Presentation @ CERN (Kinda!)

  2. What is High Speed TCP? • Changes the way TCP behaves at high speed (ie large cwnd) • Standard TCP has two modes • Slow start (not very slow…) • Congestion Avoidance • Focuses on Congestion Avoidance Mode • ie when TCP knows (thinks it knows…) how well the network behaves… • BUT only when we are at high speeds, else do what normal Standard TCP does… • Readily deployable 1st step towards Equation Based Congestion Control

  3. What does it do? • TCP uses two parameters during Congestion Avoidance - AIMD( a,b ) • Increase parameter, a • Decrease parameter, b • StandardTCP uses • a=1 • b=0.5 • High Speed TCP introduces a dependance of a and b depending on the current cwnd • a->a(cwnd) • b->b(cwnd) • If we increase a more with larger cwnd we can get back up to our ‘optimal’ cwnd size for the network path • If we decrease b less we don’t lose as much bandwidth due to a small congestion window

  4. What exactly does it do? • Based on the TCP response function • Relates loss and throughput • Uses the TCP response function to investigate certain parameters • High_Window, High_Loss; largest cwnd needed for x throughput and the required loss for that throughput • Low_Window, Low_Loss; smallest cwnd when we actually switch from Standard TCP and the required loss rate for that cwnd size • High_B; the smallest decrease in b when we are at a large cwnd • Equations to transform this information into a table for a(cwnd) and b(cwnd)

  5. a(cwnd) & b(cwnd)

  6. Kernel Modifications • Focus on investigating phase space of HSTCP variables • 3 Phase approach • Hard code HSTCP as compile-time option – ‘recommended’ HSTCP by Sally Floyd • Kernel hooks to enable alteration of HSTCP variables – from user space • Allows easier testing/exploration • Only a few changes necessary: • Code for calculating the a(cwnd) and b(cwnd) values • Modification of existing code for changing cwnd (during the Congestion Avoidance phase only) • Implementation on kernel 2.4.16 with web100 (currently alpha1.1)

  7. a(cwnd) & b(cwnd) • a(cwnd) and b(cwnd) implement via a static look-up table • Using access function get_hstcp_val • Changes to congestion avoidance algorithm required only slight modification in tcp_output.c

  8. UCL->CERN • 100mbit link from UCL->CERN • Saw tooth increase is greater! • Ie a is larger • Limited by 100mbit link… (bandwidth delay ~ 180kbytes)

  9. UCL->CERN • RTT: 20ms • Not much difference! • No clear advantage to using HSTCP for this link at this speed (100mbit)!

  10. UCL->SLAC • Graph shows min and max of about 8 separate flows • RTT: 135ms • Standard TCP performance poor • HighSpeedTCP gives performance boost of about 2.5!

  11. AIMD(9,0.32) AIMD(8,0.33) AIMD(7,0.34) AIMD(6,0.35) AIMD(5,0.36) AIMD(4,0.38) AIMD(3,0.41) AIMD(2,0.44) AIMD(1,0.50) AIMD(1,0.50) UCL->SLAC • Cwnd grows a lot bigger • HSTCP is not linear cwnd growth • Grows faster as result (increased bandwidth!)

  12. Performance Issues • Slow Start • High drop rates when we send too much out during Slow Start • Means that we have to linearly increment cwnd • As HSTCP has a much steeper increase, we get up to a large cwnd faster

  13. Controlling Loss Rates • Difficult to control what is happening on real life networks • Need to be able to control packet drops to quantify effects of TCP • Need facility for Sender side to ‘unacknowledged’ acknowledgements • Artificial packet drops • Easier to test as only requires Sender side (already modified with HSTCP) to be changed • Investigate into • Constant drop rates – unrealistic but easy to implement • Statistically predictable drop rates • Can analyse implementation of b parameter easier!

  14. Test Plan • Need to verify a(cwnd) and b(cwnd) are functioning correctly • Comparison with Theoretical Results with Network Simulator by Evandro de Souza @ Berkely Lab • Investigation of ‘recommended’ HSTCP • Quantify advantages over StandardTCP • Investigate into Fairness • Investigation of RTT independence of HSTCP • Proposed by Tom Kelly & Glenn Vinnicombe • Comparison against Multiple Stream TCP • If we can do it with Multiple Stream TCP, why bother with HSTCP? • Investigation into Limited Slow Start • Important if we’re using large cwnds (UCL->SLAC) to better guess the network state at start up

  15. Conclusions • Not much use for low latency, low throughput networks • Much improved for high latency, high throughput networks • Important! • Fairness (and hence internet stability) needs much investigation • Comparisons to existing methods (ie Multistream TCP)

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