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Subject[PATCH 4.14 27/45] tcp_bbr: fix to zero idle_restart only upon S/ACKed data
4.14-stable review patch.  If anyone has any objections, please let me know.


From: Neal Cardwell <>

[ Upstream commit e6e6a278b1eaffa19d42186bfacd1ffc15a50b3f ]

Previously the bbr->idle_restart tracking was zeroing out the
bbr->idle_restart bit upon ACKs that did not SACK or ACK anything,
e.g. receiving incoming data or receiver window updates. In such
situations BBR would forget that this was a restart-from-idle
situation, and if the min_rtt had expired it would unnecessarily enter
PROBE_RTT (even though we were actually restarting from idle but had
merely forgotten that fact).

The fix is simple: we need to remember we are restarting from idle
until we receive a S/ACK for some data (a S/ACK for the first flight
of data we send as we are restarting).

This commit is a stable candidate for kernels back as far as 4.9.

Fixes: 0f8782ea1497 ("tcp_bbr: add BBR congestion control")
Signed-off-by: Neal Cardwell <>
Signed-off-by: Yuchung Cheng <>
Signed-off-by: Soheil Hassas Yeganeh <>
Signed-off-by: Priyaranjan Jha <>
Signed-off-by: Yousuk Seung <>
Signed-off-by: David S. Miller <>
Signed-off-by: Greg Kroah-Hartman <>
net/ipv4/tcp_bbr.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

--- a/net/ipv4/tcp_bbr.c
+++ b/net/ipv4/tcp_bbr.c
@@ -802,7 +802,9 @@ static void bbr_update_min_rtt(struct so
- bbr->idle_restart = 0;
+ /* Restart after idle ends only once we process a new S/ACK for data */
+ if (rs->delivered > 0)
+ bbr->idle_restart = 0;

static void bbr_update_model(struct sock *sk, const struct rate_sample *rs)

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