netdev - Re: [PATCH net] tcp: fix ssthresh and undo for consecutive short FRTO episodes

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Date:	Thu, 14 Aug 2014 14:41:06 -0700 (PDT)
From:	David Miller <davem@...hat.com>
To:	ncardwell@...gle.com
Cc:	netdev@...r.kernel.org, ycheng@...gle.com
Subject: Re: [PATCH net] tcp: fix ssthresh and undo for consecutive short
 FRTO episodes

From: Neal Cardwell <ncardwell@...gle.com>
Date: Thu, 14 Aug 2014 16:13:07 -0400

> Fix TCP FRTO logic so that it always notices when snd_una advances,
> indicating that any RTO after that point will be a new and distinct
> loss episode.
> 
> Previously there was a very specific sequence that could cause FRTO to
> fail to notice a new loss episode had started:
> 
> (1) RTO timer fires, enter FRTO and retransmit packet 1 in write queue
> (2) receiver ACKs packet 1
> (3) FRTO sends 2 more packets
> (4) RTO timer fires again (should start a new loss episode)
> 
> The problem was in step (3) above, where tcp_process_loss() returned
> early (in the spot marked "Step 2.b"), so that it never got to the
> logic to clear icsk_retransmits. Thus icsk_retransmits stayed
> non-zero. Thus in step (4) tcp_enter_loss() would see the non-zero
> icsk_retransmits, decide that this RTO is not a new episode, and
> decide not to cut ssthresh and remember the current cwnd and ssthresh
> for undo.
> 
> There were two main consequences to the bug that we have
> observed. First, ssthresh was not decreased in step (4). Second, when
> there was a series of such FRTO (1-4) sequences that happened to be
> followed by an FRTO undo, we would restore the cwnd and ssthresh from
> before the entire series started (instead of the cwnd and ssthresh
> from before the most recent RTO). This could result in cwnd and
> ssthresh being restored to values much bigger than the proper values.
> 
> Signed-off-by: Neal Cardwell <ncardwell@...gle.com>
> Signed-off-by: Yuchung Cheng <ycheng@...gle.com>
> Fixes: e33099f96d99c ("tcp: implement RFC5682 F-RTO")

Applied and queued up for -stable, thanks.
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