netdev - Re: scp stalls mysteriously

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Date:	Thu, 03 Dec 2009 15:27:10 +0100
From:	Arnd Hannemann <hannemann@...s.rwth-aachen.de>
To:	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
Cc:	Frederic Leroy <fredo@...rox.org>,
	Damian Lukowski <damian@....rwth-aachen.de>,
	Netdev <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Greg KH <gregkh@...e.de>
Subject: Re: scp stalls mysteriously

Ilpo Järvinen wrote:

[snipped]

> Also, we have the another mystery to be solved, the fast retransmission is 
> not triggered for some reason (or alternatively not captured in to a 
> log), even in the working .9. case. It would be easy to see whether it 
> works at all from TCP point of view by looking into mibs once you have 
> have some transfers in a working configuration:
> 
> grep -A1 TCP /proc/net/netstat
> 
> ...luckily this fast retransmit issue is less crucial as almost all people 
> are pretty happy already if their RTO-based recovery works even if the 
> fast recovery would not. So figuring it out can be postponed (if one has 
> to prioritize) until the silent death issue is out of the way.
> 
> 

I looked at the working .9 case stream from 192.168.1.15 to 192.168.1.19.
I don't think it is a mystery that fast retransmit does not trigger.
The condition SACKED_DATA > 3* SMSS is simply not fulfilled.
Neither are there 3 non-continuous SACK sequences.
The segments sent are too small :-(
Interesting though, seems to me in this case non-SACK would be better than SACK.
Or did I miss something?

Hey we could cook up a draft for this problem ;-)

Anyway, real problem is, RTO does not trigger...

Best regards,
Arnd
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