tcp_cubic: fix spurious HYSTART_DELAY exit upon drop in min RTT
Mirja Kuehlewind reported a bug in Linux TCP CUBIC Hystart, where Hystart HYSTART_DELAY mechanism can exit Slow Start spuriously on an ACK when the minimum rtt of a connection goes down. From inspection it is clear from the existing code that this could happen in an example like the following: o The first 8 RTT samples in a round trip are 150ms, resulting in a curr_rtt of 150ms and a delay_min of 150ms. o The 9th RTT sample is 100ms. The curr_rtt does not change after the first 8 samples, so curr_rtt remains 150ms. But delay_min can be lowered at any time, so delay_min falls to 100ms. The code executes the HYSTART_DELAY comparison between curr_rtt of 150ms and delay_min of 100ms, and the curr_rtt is declared far enough above delay_min to force a (spurious) exit of Slow start. The fix here is simple: allow every RTT sample in a round trip to lower the curr_rtt. Fixes: ae27e98a5152 ("[TCP] CUBIC v2.3") Reported-by: Mirja Kuehlewind <mirja.kuehlewind@ericsson.com> Signed-off-by: Neal Cardwell <ncardwell@google.com> Signed-off-by: Eric Dumazet <edumazet@google.com> Acked-by: Soheil Hassas Yeganeh <soheil@google.com> Signed-off-by: David S. Miller <davem@davemloft.net>
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@ -432,10 +432,9 @@ static void hystart_update(struct sock *sk, u32 delay)
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if (hystart_detect & HYSTART_DELAY) {
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/* obtain the minimum delay of more than sampling packets */
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if (ca->curr_rtt > delay)
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ca->curr_rtt = delay;
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if (ca->sample_cnt < HYSTART_MIN_SAMPLES) {
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if (ca->curr_rtt > delay)
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ca->curr_rtt = delay;
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ca->sample_cnt++;
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} else {
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if (ca->curr_rtt > ca->delay_min +
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