In a non-SACK case, any non-retransmitted segment acknowledged will
set FLAG_ORIG_SACK_ACKED in tcp_clean_rtx_queue even if there is
no indication that it would have been delivered for real (the
scoreboard is not kept with TCPCB_SACKED_ACKED bits in the non-SACK
case). This causes bogus undos in ordinary RTO recoveries where
segments are lost here and there, with a few delivered segments in
between losses. A cumulative ACKs will cover retransmitted ones at
the bottom and the non-retransmitted ones following that causing
FLAG_ORIG_SACK_ACKED to be set in tcp_clean_rtx_queue and results
in a spurious FRTO undo.

We need to make the check more strict for non-SACK case and check
that none of the cumulatively ACKed segments were retransmitted,
which would be the case for the last step of FRTO algorithm as we
sent out only new segments previously. Only then, allow FRTO undo
to proceed in non-SACK case.

Signed-off-by: Ilpo Järvinen <>
 net/ipv4/tcp_input.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 0305f6d..1a33752 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -2629,8 +2629,13 @@ static void tcp_process_loss(struct sock *sk, int flag, 
bool is_dupack,
        if (tp->frto) { /* F-RTO RFC5682 sec 3.1 (sack enhanced version). */
                /* Step 3.b. A timeout is spurious if not all data are
                 * lost, i.e., never-retransmitted data are (s)acked.
+                *
+                * As the non-SACK case does not keep track of 
+                * we need to ensure that none of the cumulative ACKed segments
+                * was retransmitted to confirm the validity of 
                if ((flag & FLAG_ORIG_SACK_ACKED) &&
+                   (tcp_is_sack(tp) || !(flag & FLAG_RETRANS_DATA_ACKED)) &&
                    tcp_try_undo_loss(sk, true))

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