netdev - Re: [PATCH] tcp: detect loss above high

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Date:	Thu, 29 Dec 2011 10:21:18 -0800
From:	Yuchung Cheng <ycheng@...gle.com>
To:	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
Cc:	David Miller <davem@...emloft.net>, ncardwell@...gle.com,
	nanditad@...gle.com, Netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH] tcp: detect loss above high_seq in recovery

Ilpo,
On Wed, Dec 28, 2011 at 7:42 AM, Ilpo Järvinen
<ilpo.jarvinen@...sinki.fi> wrote:
>
> On Tue, 27 Dec 2011, Yuchung Cheng wrote:
> > The new sequences sent during fast-recovery maybe marked as lost
> > and/or retransmitted. It is possible these (new) losses are recovered
> > within the current fast recovery and the state moves back to CA_Open
> > without entering another fast recovery / cwnd redunction. This is
> > especially possible for light losses.  Note RFC 3517 (implicitly)
> > allows this as well.
>
> No it doesn't! It does not allow you to avoid the second cwnd reduction.
> They're from different RTT. What you now claim is that we never need to
> do cwnd reduction until we visit CA_Open in between. That's very very
> dangerous if the congestion suddently spikes because nobody reduces twice
> causing everyone to get losses and continues in the recovery forever.

Should TCP perform another CWR for sequences lost after recovery point?
my colleagues and I all agree with you it should. But my commit message did a
poor job explain some subtle deviations in standard and current Linux.

RFC3517 supports this by principle; however, it may repair such new data losses
in the current recovery: Rue 1.b and 1.c in NextSeg() allow retransmitting any
segment below highest sacked sequence. Thus it may repair every loss in the
current recovery and won't never go into another.

As a matter of fact, Linux already does that today, by implementing rule 3 in
NextSeg(), aka forward-retransmit. But Linux won't mark these packets lost even
if there are >= tp->reordering packets sacked above. This change fixes this.

How often is second CWR avoided? it's probably rare b/c it requires new data
during the recovery RTT and a complete repair any losses of them. It's arguably
 OK to fix small new data losses during current recovery, and RFC/Linux seem
to do so.

To summarize:
1) this change does NOT forbid TCP performing further CWR on new losses
2) this change (merely) marks some forward-retransmitted packets LOST.
3) the commit message is to be explicit that CWR may not always be performed,
    in both my change and current kernel.

HTH. I will update commit message if people can make sense of it :)
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