netdev - Re: [PATCH v2 net-next] tcp: remove cwnd moderation after recovery

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Date:	Sat, 02 Apr 2016 20:14:21 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	ycheng@...gle.com
Cc:	netdev@...r.kernel.org, mattmathis@...gle.com,
	ncardwell@...gle.com, soheil@...gle.com
Subject: Re: [PATCH v2 net-next] tcp: remove cwnd moderation after recovery

From: Yuchung Cheng <ycheng@...gle.com>
Date: Wed, 30 Mar 2016 14:54:20 -0700

> For non-SACK connections, cwnd is lowered to inflight plus 3 packets
> when the recovery ends. This is an optional feature in the NewReno
> RFC 2582 to reduce the potential burst when cwnd is "re-opened"
> after recovery and inflight is low.
> 
> This feature is questionably effective because of PRR: when
> the recovery ends (i.e., snd_una == high_seq) NewReno holds the
> CA_Recovery state for another round trip to prevent false fast
> retransmits. But if the inflight is low, PRR will overwrite the
> moderated cwnd in tcp_cwnd_reduction() later regardlessly. So if a
> receiver responds bogus ACKs (i.e., acking future data) to speed up
> transfer after recovery, it can only induce a burst up to a window
> worth of data packets by acking up to SND.NXT. A restart from (short)
> idle or receiving streched ACKs can both cause such bursts as well.
> 
> On the other hand, if the recovery ends because the sender
> detects the losses were spurious (e.g., reordering). This feature
> unconditionally lowers a reverted cwnd even though nothing
> was lost.
> 
> By principle loss recovery module should not update cwnd. Further
> pacing is much more effective to reduce burst. Hence this patch
> removes the cwnd moderation feature.
> 
> v2 changes: revised commit message on bogus ACKs and burst, and
>             missing signature
> 
> Signed-off-by: Matt Mathis <mattmathis@...gle.com>
> Signed-off-by: Neal Cardwell <ncardwell@...gle.com>
> Signed-off-by: Soheil Hassas Yeganeh <soheil@...gle.com>
> Signed-off-by: Yuchung Cheng <ycheng@...gle.com>

Applied, thanks.