netdev - Re: [PATCH net] tcp: defer SACK compression after DupThresh

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Date:   Wed, 21 Nov 2018 15:50:51 -0800 (PST)
From:   David Miller <davem@...emloft.net>
To:     edumazet@...gle.com
Cc:     netdev@...r.kernel.org, jean-louis@...ond.be, ncardwell@...gle.com,
        ycheng@...gle.com, eric.dumazet@...il.com
Subject: Re: [PATCH net] tcp: defer SACK compression after DupThresh

From: Eric Dumazet <edumazet@...gle.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-louis@...ond.be>
> Tested-by: Jean-Louis Dupond <jean-louis@...ond.be>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Acked-by: Neal Cardwell <ncardwell@...gle.com>

Applied and queued up for -stable.

Thanks Eric.