tcp: prevent CUBIC ACKs from reducing cwnd - #13958
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Clamp the computed CUBIC window to the current congestion window before processing ACKs. This prevents ACK processing from reducing cwnd when the computed target falls below it, as required by RFC 9438 section 4.2.
EtiennePerot
requested review from
nybidari
and removed request for
shailend-g and
xinzhong
August 3, 2026 18:19
nybidari
approved these changes
Aug 7, 2026
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CUBIC currently applies the per-ACK window update even when the computed
W_cubic(t + RTT)value is below the current congestion window. In that case, the update becomes negative and receiving an ACK can reducecwndwithout a congestion signal.Clamp the computed CUBIC window to the current
cwndbefore processing ACKs. This applies the lower-bound rule from RFC 9438, Section 4.2, and ensures that ACK processing does not reduce the congestion window.