KarpelesLab / slirp / 23946466579

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Implement RFC 3649 HighSpeed TCP congestion control

HighSpeed TCP (HSTCP) replaces NewReno as the default congestion control
algorithm. It uses more aggressive increase/decrease functions for large
congestion windows, scaling much better on high-BDP networks.

RFC 3649 implementation:
- a(w) increase function: grows faster than standard TCP when w > 38 segs
- b(w) decrease function: less severe backoff for large windows (0.1 at
  High_Window vs 0.5 for standard TCP)
- Below Low_Window (38 segments): identical to standard NewReno
- Parameters: Low_Window=38, High_Window=83000, High_Decrease=0.1

Configurable via ConnConfig.CongestionControl: "highspeed" (default) or
"newreno" for standard RFC 5681 behavior.

Throughput improvements (100MB transfers, 5-second cap):
  Clean:    570 MB/s  (was 291)
  5ms RTT:   80 MB/s  (was 31)
  25ms RTT:  17.6 MB/s (was 7.1)
  50ms RTT:   8.2 MB/s (was 4.0)
  10% reorder: 14.3 MB/s (was 0.3)

Stress tests updated: transferUntil() streams data up to 100MB or 5
seconds, giving steady-state throughput measurements instead of
small-transfer ramp-up artifacts.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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