netdev - Re: [PATCH] tcp: bound RTO to minimum

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Date:	Thu, 25 Aug 2011 13:14:38 +0300 (EEST)
From:	"Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi>
To:	Eric Dumazet <eric.dumazet@...il.com>
cc:	Arnd Hannemann <arnd@...dnet.de>,
	Alexander Zimmermann <alexander.zimmermann@...sys.rwth-aachen.de>,
	Yuchung Cheng <ycheng@...gle.com>,
	Hagen Paul Pfeifer <hagen@...u.net>,
	netdev <netdev@...r.kernel.org>,
	Lukowski Damian <damian@....rwth-aachen.de>
Subject: Re: [PATCH] tcp: bound RTO to minimum

On Thu, 25 Aug 2011, Eric Dumazet wrote:

> Le jeudi 25 août 2011 à 11:46 +0200, Arnd Hannemann a écrit :
> > Hi Eric,
> > 
> > Am 25.08.2011 11:09, schrieb Eric Dumazet:
> 
> > > Maybe we should refine the thing a bit, to not reverse backoff unless
> > > rto is > some_threshold.
> > > 
> > > Say 10s being the value, that would give at most 92 tries.
> > 
> > I personally think that 10s would be too large and eliminate the benefit of the
> > algorithm, so I would prefer a different solution.
> > 
> > In case of one bulk data TCP session, which was transmitting hundreds of packets/s
> > before the connectivity disruption those worst case rate of 5 packet/s really
> > seems conservative enough.
> > 
> > However in case of a lot of idle connections, which were transmitting only
> > a number of packets per minute. We might increase the rate drastically for
> > a certain period until it throttles down. You say that we have a problem here
> > correct?
> > 
> > Do you think it would be possible without much hassle to use a kind of 
> > "global" rate limiting only for these probe packets of a TCP connection?
> >
> > > I mean, what is the gain to be able to restart a frozen TCP session with
> > > a 1sec latency instead of 10s if it was blocked more than 60 seconds ?
> > 
> > I'm afraid it does a lot, especially in highly dynamic environments. You
> > don't have just the additional latency, you may actually miss the full
> > period where connectivity was there, and then just retransmit into the next
> > connectivity disrupted period.
> 
> Problem with this is that with short and synchronized timers, all
> sessions will flood at the same time and you'll get congestion this
> time.
>
> The reason for exponential backoff is also to smooth the restarts of
> sessions, because timers are randomized.

But if you get a real congestion the system will self-regulate using 
exponential backoffs due to lack of ICMPs for some of the connections?


-- 
 i.