Re: [PATCH net 2/5] tcp: prevent bogus FRTO undos with non-SACK flows

On Wed, Mar 7, 2018 at 7:59 AM, Ilpo Järvinen [off-list ref] wrote:

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.

Hi Ilpo - Do you have a packet trace or (even better) packetdrill
script illustrating this issue? It would be nice to have a test case
or at least concrete example of this.

Thanks!
neal