netdev - Re: [PATCH 2/2] tcp: fix undo after RTO for CUBIC

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Message-ID: <20120119184921.314d3d88@nehalam.linuxnetplumber.net>
Date:	Thu, 19 Jan 2012 18:49:21 -0800
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Neal Cardwell <ncardwell@...gle.com>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	ilpo.jarvinen@...sinki.fi, Nandita Dukkipati <nanditad@...gle.com>,
	Yuchung Cheng <ycheng@...gle.com>,
	Jerry Chu <hkchu@...gle.com>, Tom Herbert <therbert@...gle.com>
Subject: Re: [PATCH 2/2] tcp: fix undo after RTO for CUBIC

On Wed, 18 Jan 2012 22:47:59 -0500
Neal Cardwell <ncardwell@...gle.com> wrote:

> This patch fixes CUBIC so that cwnd reductions made during RTOs can be
> undone (just as they already can be undone when using the default/Reno
> behavior).
> 
> When undoing cwnd reductions, BIC-derived congestion control modules
> were restoring the cwnd from last_max_cwnd. There were two problems
> with using last_max_cwnd to restore a cwnd during undo:
> 
> (a) last_max_cwnd was set to 0 on state transitions into TCP_CA_Loss
> (by calling the module's reset() functions), so cwnd reductions from
> RTOs could not be undone.
> 
> (b) when fast_covergence is enabled (which it is by default)
> last_max_cwnd does not actually hold the value of snd_cwnd before the
> loss; instead, it holds a scaled-down version of snd_cwnd.
> 
> This patch makes the following changes:
> 
> (1) upon undo, revert snd_cwnd to ca->loss_cwnd, which is already, as
> the existing comment notes, the "congestion window at last loss"
> 
> (2) stop forgetting ca->loss_cwnd on TCP_CA_Loss events
> 
> (3) use ca->last_max_cwnd to check if we're in slow start
> 
> Signed-off-by: Neal Cardwell <ncardwell@...gle.com>

I was unsure of this, so forwarded it to the author 
and received his confirmation.

Acked-by: Stephen Hemminger <shemminger@...tta.com>
Acked-by: Sangtae Ha <sangtae.ha@...il.com>
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