[3.13.y.z extended stable] Patch "tcp: fix cwnd undo on DSACK in F-RTO" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    tcp: fix cwnd undo on DSACK in F-RTO

to the linux-3.13.y-queue branch of the 3.13.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.13.y-queue

This patch is scheduled to be released in version 3.13.11.6.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.13.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From ce2d4ab437e58c8c570980894a20bb352718c441 Mon Sep 17 00:00:00 2001
From: Yuchung Cheng <ycheng at google.com>
Date: Fri, 30 May 2014 15:25:59 -0700
Subject: tcp: fix cwnd undo on DSACK in F-RTO

[ Upstream commit 0cfa5c07d6d1d7f8e710fc671c5ba1ce85e09fa4 ]

This bug is discovered by an recent F-RTO issue on tcpm list
https://www.ietf.org/mail-archive/web/tcpm/current/msg08794.html

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 at google.com>
Signed-off-by: Neal Cardwell <ncardwell at google.com>
Signed-off-by: David S. Miller <davem at davemloft.net>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 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))
 			return;
-		}
+
 		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 */
--
1.9.1