Commit 814d488c authored Jan 28, 2015 by Neal Cardwell Committed by David S. Miller Jan 28, 2015

tcp: fix the timid additive increase on stretch ACKs



tcp_cong_avoid_ai() was too timid (snd_cwnd increased too slowly) on
"stretch ACKs" -- cases where the receiver ACKed more than 1 packet in
a single ACK. For example, suppose w is 10 and we get a stretch ACK
for 20 packets, so acked is 20. We ought to increase snd_cwnd by 2
(since acked/w = 20/10 = 2), but instead we were only increasing cwnd
by 1. This patch fixes that behavior.

Reported-by: Eyal Perry <eyalpe@mellanox.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent e73ebb08

net/ipv4/tcp_cong.c

+9 −6

Original line number	Diff line number	Diff line
		@@ -304,16 +304,19 @@ u32 tcp_slow_start(struct tcp_sock *tp, u32 acked)
		}
		EXPORT_SYMBOL_GPL(tcp_slow_start);

		/* In theory this is tp->snd_cwnd += 1 / tp->snd_cwnd (or alternative w) */
		/* In theory this is tp->snd_cwnd += 1 / tp->snd_cwnd (or alternative w),
		* for every packet that was ACKed.
		*/
		void tcp_cong_avoid_ai(struct tcp_sock *tp, u32 w, u32 acked)
		{
		if (tp->snd_cwnd_cnt >= w) {
		if (tp->snd_cwnd < tp->snd_cwnd_clamp)
		tp->snd_cwnd++;
		tp->snd_cwnd_cnt = 0;
		} else {
		tp->snd_cwnd_cnt += acked;
		if (tp->snd_cwnd_cnt >= w) {
		u32 delta = tp->snd_cwnd_cnt / w;

		tp->snd_cwnd_cnt -= delta * w;
		tp->snd_cwnd += delta;
		}
		tp->snd_cwnd = min(tp->snd_cwnd, tp->snd_cwnd_clamp);
		}
		EXPORT_SYMBOL_GPL(tcp_cong_avoid_ai);

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