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Subject[PATCH 3.16.y-ckt 133/144] tcp: fix FRTO undo on cumulative ACK of SACKed range
3.16.7-ckt10 -stable review patch.  If anyone has any objections, please let me know.


From: Neal Cardwell <>

commit 666b805150efd62f05810ff0db08f44a2370c937 upstream.

On processing cumulative ACKs, the FRTO code was not checking the
SACKed bit, meaning that there could be a spurious FRTO undo on a
cumulative ACK of a previously SACKed skb.

The FRTO code should only consider a cumulative ACK to indicate that
an original/unretransmitted skb is newly ACKed if the skb was not yet

The effect of the spurious FRTO undo would typically be to make the
connection think that all previously-sent packets were in flight when
they really weren't, leading to a stall and an RTO.

Signed-off-by: Neal Cardwell <>
Signed-off-by: Yuchung Cheng <>
Fixes: e33099f96d99c ("tcp: implement RFC5682 F-RTO")
Signed-off-by: David S. Miller <>
Signed-off-by: Luis Henriques <>
net/ipv4/tcp_input.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index a1bbebb03490..486f70050563 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -3068,10 +3068,11 @@ static int tcp_clean_rtx_queue(struct sock *sk, int prior_fackets,
if (!first_ackt.v64)
first_ackt = last_ackt;

- if (!(sacked & TCPCB_SACKED_ACKED))
+ if (!(sacked & TCPCB_SACKED_ACKED)) {
reord = min(pkts_acked, reord);
- if (!after(scb->end_seq, tp->high_seq))
+ if (!after(scb->end_seq, tp->high_seq))
+ }

if (sacked & TCPCB_SACKED_ACKED)

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