netdev - Re: A Linux TCP SACK Question

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Message-ID: <Pine.LNX.4.64.0804161203590.17123@wrl-59.cs.helsinki.fi>
Date:	Wed, 16 Apr 2008 12:21:38 +0300 (EEST)
From:	"Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi>
To:	David Miller <davem@...emloft.net>
cc:	johnwheffner@...il.com, wenji@...l.gov,
	Netdev <netdev@...r.kernel.org>
Subject: Re: A Linux TCP SACK Question

On Wed, 16 Apr 2008, David Miller wrote:

> From: "John Heffner" <johnwheffner@...il.com>
> Date: Tue, 15 Apr 2008 15:40:05 -0700
> 
> > Subject: [PATCH] Increase the max_burst threshold from 3 to tp->reordering.
> > 
> > This change is necessary to allow cwnd to grow during persistent
> > reordering.  Cwnd moderation is applied when in the disorder state
> > and an ack that fills the hole comes in.  If the hole was greater
> > than 3 packets, but less than tp->reordering, cwnd will shrink when
> > it should not have.
> > 
> > Signed-off-by: John Heffner <jheffner@...a.(none)>
> 
> I think this patch is correct, or at least more correct than what
> this code is doing right now.
> 
> Any objections to my adding this to net-2.6.26?

I don't have objections.

But I want to note that tp->reordering does not consider the situation on 
that specific ACK because its value might originate a number of segments 
and even RTTs back. I think it could be possible to find a more 
appropriate value for max_burst locally to an ACK. ...Though it might be a 
bit over-engineered solution. For SACK we calculate similar metric anyway 
in tcp_clean_rtx_queue to find if tp->reordering needs to be updated at 
cumulative ACK and for NewReno min(tp->sacked_out, tp->reordering) + 3
could perhaps be used (I'm not sure if these would be foolproof in 
recovery though).

-- 
 i.
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