tcp: fix cwnd undo on DSACK in F-RTO

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@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 5 insertions, 6 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index d6b46eb2f94c..3a26b3b23f16 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -2684,13 +2684,12 @@ 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 */