netdev - Re: TCP connection stops after high load.

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Date:	Fri, 13 Apr 2007 18:41:11 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Daniel Schaffrath <danielschaffrath@....com>
Cc:	Ben Greear <greearb@...delatech.com>,
	Andi Kleen <andi@...stfloor.org>, netdev@...r.kernel.org,
	bcrl@...ck.org
Subject: Re: TCP connection stops after high load.

On Fri, 13 Apr 2007 18:10:12 +0200
Daniel Schaffrath <danielschaffrath@....com> wrote:
> 
> On 2007/04/12  , at 20:19, Eric Dumazet wrote:
> >
> > Warning : tcpdump can lie, telling you packets being transmited  
> > several time.
> Maybe you have further pointers how come that tcpdump lies about  
> duplicated packets?
> 

dev_queue_xmit_nit() is called before attempting to send packet to device.

If device could not accept the packet (hard_start_xmit() returns an error), packet is requeued and retried later.
each retry means call ev_queue_xmit_nit() again, so tcpdump/sniffers can 'see' packet transmited several times. 

This is why I asked for "tc -s -d qdisc" results : to check the requeue counter (not its absolute value, but relative to number of packets sent)

See dev_hard_start_xmit() in net/core/dev.c

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