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

[Yuchung Cheng <ycheng@google.com>]

On Wed, Mar 7, 2018 at 11:24 AM, Neal Cardwell <ncardwell@google.com> wrote:
>
> On Wed, Mar 7, 2018 at 7:59 AM, Ilpo Järvinen <ilpo.jarvinen@helsinki.fi> 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.
a packetdrill or even a contrived example would be good ... also why
not just avoid setting FLAG_ORIG_SACK_ACKED on non-sack? seems a much
clean fix.

>
> Thanks!
> neal