netdev - Re: TCP and reordering

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Message-ID: <1354127585.14302.467.camel@edumazet-glaptop>
Date:	Wed, 28 Nov 2012 10:33:05 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Rick Jones <rick.jones2@...com>
Cc:	Saku Ytti <saku@...i.fi>, netdev@...r.kernel.org
Subject: Re: TCP and reordering

On Wed, 2012-11-28 at 10:24 -0800, Rick Jones wrote:
> On 11/28/2012 12:54 AM, Saku Ytti wrote:
> > On (2012-11-28 00:35 -0800), Vijay Subramanian wrote:
> >
> >> Also note that reordering is tracked on the sender side using the
> >> per flow variable tp->reordering . This measures the amount of
> >> reordering on the connection so that fast retransmit and other loss
> >> recovery mechanisms are not entered prematurely. Doesn't this
> >> behavior at the  sender already provide the behavior you seek?
> >
> > Sorry I don't seem to understand what you mean. Do you mind explaining how
> > the sender can help to restore performance on reordering network?
> 
> tp->reordering is initialized via the sysctl net.ipv4.tcp_reordering 
> which controls how anxious TCP will be to fast retransmit.
> 
> By increasing net.ipv4.tcp_reordering you make the sending TCP less 
> "sensitive" to duplicate ACKs and so less sensitive to reordering 
> detected by the receiver.  The receiver is generating as many 
> (duplicate) ACKs as before, it is just that the sender is ignoring them 
> a bit more.

Note that this sysctl controls the initial value of the per socket
reordering value. It _does_ increase automatically (assuming SACK is
enabled of course)



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