netdev - [PATCH net] tcp: fix RTO calculated from cached RTT

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Message-Id: <1379382260-29303-1-git-send-email-ncardwell@google.com>
Date:	Mon, 16 Sep 2013 21:44:20 -0400
From:	Neal Cardwell <ncardwell@...gle.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org, Neal Cardwell <ncardwell@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Yuchung Cheng <ycheng@...gle.com>
Subject: [PATCH net] tcp: fix RTO calculated from cached RTT

Commit 1b7fdd2ab5852 ("tcp: do not use cached RTT for RTT estimation")
did not correctly account for the fact that crtt is the RTT shifted
left 3 bits. Fix the calculation to consistently reflect this fact.

Signed-off-by: Neal Cardwell <ncardwell@...gle.com>
Cc: Eric Dumazet <edumazet@...gle.com>
Cc: Yuchung Cheng <ycheng@...gle.com>
---
 net/ipv4/tcp_metrics.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/tcp_metrics.c b/net/ipv4/tcp_metrics.c
index 4a22f3e..52f3c6b 100644
--- a/net/ipv4/tcp_metrics.c
+++ b/net/ipv4/tcp_metrics.c
@@ -502,7 +502,9 @@ reset:
 	 * ACKs, wait for troubles.
 	 */
 	if (crtt > tp->srtt) {
-		inet_csk(sk)->icsk_rto = crtt + max(crtt >> 2, tcp_rto_min(sk));
+		/* Set RTO like tcp_rtt_estimator(), but from cached RTT. */
+		crtt >>= 3;
+		inet_csk(sk)->icsk_rto = crtt + max(2 * crtt, tcp_rto_min(sk));
 	} else if (tp->srtt == 0) {
 		/* RFC6298: 5.7 We've failed to get a valid RTT sample from
 		 * 3WHS. This is most likely due to retransmission,
-- 
1.8.4

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