netdev - Re: [PATCH 1/2] tcp: fix undo after RTO for BIC

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Date:	Fri, 20 Jan 2012 14:18:26 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	ncardwell@...gle.com
Cc:	netdev@...r.kernel.org, ilpo.jarvinen@...sinki.fi,
	nanditad@...gle.com, ycheng@...gle.com, hkchu@...gle.com,
	therbert@...gle.com
Subject: Re: [PATCH 1/2] tcp: fix undo after RTO for BIC

From: Neal Cardwell <ncardwell@...gle.com>
Date: Wed, 18 Jan 2012 22:47:58 -0500

> This patch fixes BIC 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>

Applied.
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