HyStart testing between Princeton and NCSU

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  • We tested HyStart in the path between Princeton and NCSU. An estimated bandwidth for this path is 100Mbps and the RTT is 25ms. We tested CUBIC with two clock resolutions (HZ=100 and HZ=1000). The buffer auto-tuning was enabled. We applied three patches in addition to the recent patches (PATCH 0/6 TCP CUBIC and Hystart) from Stephen Hemminger.
  • Tested protocols
 CUBIC with SlowStart (by disabling HyStart)
 CUBIC with HyStart (without the patches) 
 CUBIC with HyStart (with the patches)
  • Installation
 Download the latest git from Linus.
 Apply six patches from Stephen Hemminger ([1])
 Apply these three patches[2][3][4]
  • Credits
 Patches enabling high-resolution ACK time (if HZ < 1000) from Stephen Hemminger
 Problem reporting from Lucas Nussbaum
 Testing CUBIC from Bill Fink
 Discussion with Injong Rhee


  • When the clock resolution is coarse (e.g., HZ < 1000), the accuracy of HyStart is affected and it may exit slow start prematurely.
  • Even though Hystart found a safe exit point, it may be still slow to reach to maximum available bandwidth in long BDP paths due to initial slow growth of CUBIC.
  • The above three patches solve this problem.
Closer look at the first 20 secs
Clock / Protocols HZ = 100 HZ = 1000
HyStart (before fix)
HyStart (fixed)
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