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Date:	Mon, 09 Nov 2009 16:24:52 +0100
From:	Andreas Petlund <apetlund@...ula.no>
To:	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
CC:	Arnd Hannemann <hannemann@...s.rwth-aachen.de>,
	William Allen Simpson <william.allen.simpson@...il.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"shemminger@...tta.com" <shemminger@...tta.com>,
	"davem@...emloft.net" <davem@...emloft.net>
Subject: Re: [PATCH 1/3] net: TCP thin-stream detection

Ilpo Järvinen wrote:
> On Thu, 5 Nov 2009, Andreas Petlund wrote:
> 
>> Arnd Hannemann wrote:
>>> One example: Consider standard NewReno non-SACK enabled flow:
>>> For some reasons two data packets get reordered.
>>> The TCP sender will produce a dupACK and an ACK.
>>> The dupACK will trigger (because of your logic) a spurious retransmit.
>>> The spurious retransmit will trigger a dupACK.
>>> This dupACK will again trigger a spurious retransmit.
>>> And this game will continue, unless a packet is dropped by coincidence.
>> Such an effect will be extremely rare. It will depend on the application 
>> producing an extremely even flow of packets with just the right 
>> interarrival time, and also on reordering of data (which also will 
>> happen very seldom when the number of packets in flight are so low). 
>> Even though it can happen, the data flow will progress (with spurious 
>> retransmissions). The effect will stop as soon as the application sends 
>> more than 4 segments in an RTT (which will disable the thin-stream 
>> modifications) or less than 1 (which will cause all segments to be 
>> successfully ACKed), or if, as you say, a packet is dropped.
> 
> I'd simply workaround this problem by requiring SACK to be enabled for 
> such a connection. This is reinforced by the fact that small windowed 
> transfers want it certainly to be on anyway to get the best out of ACK 
> flow even if there were some ACK losses.
> 

Thanks. I will revise the patches based on all the feedback I have gotten
and get back to the list with a new version when I have done some more
testing.

Best regards,
Andreas



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