If SACK is not enabled and the first cumulative ACK after the RTO
retransmission covers more than the retransmitted skb, a spurious
FRTO undo will trigger (assuming FRTO is enabled for that RTO).
The reason is that 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 so the check for that bit won't help like it does with SACK).
Having FLAG_ORIG_SACK_ACKED set results in the spurious FRTO undo
in tcp_process_loss.

We need to use more strict condition for non-SACK case and check
that none of the cumulatively ACKed segments were retransmitted
to prove that progress is due to original transmissions. Only then
keep FLAG_ORIG_SACK_ACKED set, allowing FRTO undo to proceed in
non-SACK case.

Signed-off-by: Ilpo Järvinen <ilpo.jarvi...@helsinki.fi>
 net/ipv4/tcp_input.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 4a26c09..c60745c 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -3166,6 +3166,15 @@ static int tcp_clean_rtx_queue(struct sock *sk, u32 
                                pkts_acked = rexmit_acked + newdata_acked;
                        tcp_remove_reno_sacks(sk, pkts_acked);
+                       /* If any of the cumulatively ACKed segments was
+                        * retransmitted, non-SACK case cannot confirm that
+                        * progress was due to original transmission due to
+                        * lack of TCPCB_SACKED_ACKED bits even if some of
+                        * the packets may have been never retransmitted.
+                        */
+                       if (flag & FLAG_RETRANS_DATA_ACKED)
+                               flag &= ~FLAG_ORIG_SACK_ACKED;
                } else {
                        int delta;

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