From: Eric Dumazet <eduma...@google.com> Date: Tue, 20 Nov 2018 05:53:59 -0800
> Jean-Louis reported a TCP regression and bisected to recent SACK > compression. > > After a loss episode (receiver not able to keep up and dropping > packets because its backlog is full), linux TCP stack is sending > a single SACK (DUPACK). > > Sender waits a full RTO timer before recovering losses. > > While RFC 6675 says in section 5, "Algorithm Details", > > (2) If DupAcks < DupThresh but IsLost (HighACK + 1) returns true -- > indicating at least three segments have arrived above the current > cumulative acknowledgment point, which is taken to indicate loss > -- go to step (4). > ... > (4) Invoke fast retransmit and enter loss recovery as follows: > > there are old TCP stacks not implementing this strategy, and > still counting the dupacks before starting fast retransmit. > > While these stacks probably perform poorly when receivers implement > LRO/GRO, we should be a little more gentle to them. > > This patch makes sure we do not enable SACK compression unless > 3 dupacks have been sent since last rcv_nxt update. > > Ideally we should even rearm the timer to send one or two > more DUPACK if no more packets are coming, but that will > be work aiming for linux-4.21. > > Many thanks to Jean-Louis for bisecting the issue, providing > packet captures and testing this patch. > > Fixes: 5d9f4262b7ea ("tcp: add SACK compression") > Reported-by: Jean-Louis Dupond <jean-lo...@dupond.be> > Tested-by: Jean-Louis Dupond <jean-lo...@dupond.be> > Signed-off-by: Eric Dumazet <eduma...@google.com> > Acked-by: Neal Cardwell <ncardw...@google.com> Applied and queued up for -stable. Thanks Eric.