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

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Date:	Tue, 29 Mar 2016 17:35:45 -0700
From:	Stephen Hemminger <stephen@...workplumber.org>
To:	Yuchung Cheng <ycheng@...gle.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org,
	Matt Mathis <mattmathis@...gle.com>,
	Neal Cardwell <ncardwell@...gle.com>,
	Soheil Hassas Yeganeh <soheil@...gle.com>
Subject: Re: [PATCH net-next] tcp: remove cwnd moderation after recovery

On Tue, 29 Mar 2016 17:15:52 -0700
Yuchung Cheng <ycheng@...gle.com> wrote:

> 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.
> 
> 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.
> 
> 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>

I have a concern that this might break Linux builtin protection
against hostile receiver sending bogus ACK's.  Remember Linux is
different than NewReno. You are changing something that has existed for
a long long time.