[PATCH 3.13 214/259] tcp: fix cwnd undo on DSACK in F-RTO

From: Kamal Mostafa
Date: Fri Aug 08 2014 - 16:52:22 EST -stable review patch. If anyone has any objections, please let me know.


From: Yuchung Cheng <ycheng@xxxxxxxxxx>

[ Upstream commit 0cfa5c07d6d1d7f8e710fc671c5ba1ce85e09fa4 ]

This bug is discovered by an recent F-RTO issue on tcpm list

The bug is that currently F-RTO does not use DSACK to undo cwnd in
certain cases: upon receiving an ACK after the RTO retransmission in
F-RTO, and the ACK has DSACK indicating the retransmission is spurious,
the sender only calls tcp_try_undo_loss() if some never retransmisted
data is sacked (FLAG_ORIG_DATA_SACKED).

The correct behavior is to unconditionally call tcp_try_undo_loss so
the DSACK information is used properly to undo the cwnd reduction.

Signed-off-by: Yuchung Cheng <ycheng@xxxxxxxxxx>
Signed-off-by: Neal Cardwell <ncardwell@xxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Kamal Mostafa <kamal@xxxxxxxxxxxxx>
net/ipv4/tcp_input.c | 11 +++++------
1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index c53b7f3..320cfe1 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -2680,13 +2680,12 @@ static void tcp_process_loss(struct sock *sk, int flag, bool is_dupack)
bool recovered = !before(tp->snd_una, tp->high_seq);

if (tp->frto) { /* F-RTO RFC5682 sec 3.1 (sack enhanced version). */
- if (flag & FLAG_ORIG_SACK_ACKED) {
- /* Step 3.b. A timeout is spurious if not all data are
- * lost, i.e., never-retransmitted data are (s)acked.
- */
- tcp_try_undo_loss(sk, true);
+ /* Step 3.b. A timeout is spurious if not all data are
+ * lost, i.e., never-retransmitted data are (s)acked.
+ */
+ if (tcp_try_undo_loss(sk, flag & FLAG_ORIG_SACK_ACKED))
- }
if (after(tp->snd_nxt, tp->high_seq) &&
(flag & FLAG_DATA_SACKED || is_dupack)) {
tp->frto = 0; /* Loss was real: 2nd part of step 3.a */

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