netdev - Re: [net-next PATCH v4 1/3] net: TCP thin-stream detection

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Message-ID: <alpine.DEB.2.00.1002220033360.23201@melkinpaasi.cs.helsinki.fi>
Date:	Mon, 22 Feb 2010 00:36:54 +0200 (EET)
From:	"Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi>
To:	Alexander Zimmermann <zimmermann@...s.rwth-aachen.de>
cc:	Pavel Machek <pavel@....cz>, Andreas Petlund <apetlund@...ula.no>,
	lars.eggert@...ia.com, Netdev <netdev@...r.kernel.org>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Arnd Hannemann <Arnd.Hannemann@...s.rwth-aachen.de>,
	LKML <linux-kernel@...r.kernel.org>, shemminger@...tta.com,
	David Miller <davem@...emloft.net>,
	william.allen.simpson@...il.com,
	Lukowski Damian <damian@....rwth-aachen.de>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [net-next PATCH v4 1/3] net: TCP thin-stream detection

On Sun, 21 Feb 2010, Alexander Zimmermann wrote:

> 
> Am 21.02.2010 um 11:21 schrieb Pavel Machek:
> 
> > Hi!
> > 
> >> +After analysing a large number of time-dependent interactive
> >> +applications, we have seen that they often produce thin streams
> >> +and also stay with this traffic pattern throughout its entire
> >> +lifespan. The combination of time-dependency and the fact that the
> >> +streams provoke high latencies when using TCP is unfortunate.
> >> +
> >> +In order to reduce application-layer latency when packets are lost,
> >> +a set of mechanisms has been made, which address these latency issues
> >> +for thin streams. In short, if the kernel detects a thin stream,
> >> +the retransmission mechanisms are modified in the following manner:
> >> +
> >> +1) If the stream is thin, fast retransmit on the first dupACK.
> >> +2) If the stream is thin, do not apply exponential backoff.
> > 
> > 2) seems very dangerous/unfair. If network  congestion is caused just
> > by thin streams, will the network just fall apart?
> 
> and 1) can also be dangerous if we have reordering on the path.
> 
> I strongly suggest that we discuss Andreas' idea on IETF TCPM *before*
> we integrate it in the kernel and enable it for everyone

What difference you see with 1) and early rexmit when cwnd = 2, the latter 
being afaict "discussed already" on TCPM?

-- 
 i.
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