netdev - [PATCH] tcp: frto should not set snd

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Date:	Sun, 03 Feb 2013 11:13:05 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Neal Cardwell <ncardwell@...gle.com>,
	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
Cc:	Pasi Kärkkäinen <pasik@....fi>,
	David Miller <davem@...emloft.net>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Stephen Hemminger <stephen@...workplumber.org>,
	Netdev <netdev@...r.kernel.org>,
	Yuchung Cheng <ycheng@...gle.com>
Subject: [PATCH] tcp: frto should not set snd_cwnd to 0

From: Eric Dumazet <edumazet@...gle.com>

Commit 9dc274151a548 (tcp: fix ABC in tcp_slow_start())
uncovered a bug in FRTO code :
tcp_process_frto() is setting snd_cwnd to 0 if the number
of in flight packets is 0.

As Neal pointed out, if no packet is in flight we lost our
chance to disambiguate whether a loss timeout was spurious.

We should assume it was a proper loss.

Reported-by: Pasi Kärkkäinen <pasik@....fi>
Signed-off-by: Neal Cardwell <ncardwell@...gle.com>
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Cc: Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
Cc: Yuchung Cheng <ycheng@...gle.com>
---
 net/ipv4/tcp_input.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 8aca4ee..680c422 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -3484,7 +3484,8 @@ static bool tcp_process_frto(struct sock *sk, int flag)
 	    ((tp->frto_counter >= 2) && (flag & FLAG_RETRANS_DATA_ACKED)))
 		tp->undo_marker = 0;
 
-	if (!before(tp->snd_una, tp->frto_highmark)) {
+	if (!before(tp->snd_una, tp->frto_highmark) ||
+	    !tcp_packets_in_flight(tp)) {
 		tcp_enter_frto_loss(sk, (tp->frto_counter == 1 ? 2 : 3), flag);
 		return true;
 	}


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