netdev - Re: [PATCH] make _minimum_ TCP retransmission timeout configurable

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Date:	Wed, 29 Aug 2007 15:23:54 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	ian.mcdonald@...di.co.nz
Cc:	rick.jones2@...com, netdev@...r.kernel.org
Subject: Re: [PATCH] make _minimum_ TCP retransmission timeout configurable

From: "Ian McDonald" <ian.mcdonald@...di.co.nz>
Date: Thu, 30 Aug 2007 10:10:37 +1200

> Understand what you are saying. That is why I questioned as 200 msecs
> makes no sense on a LAN with < 1 msec RTT. So if the current is
> ridiculous and 1000 is even more so, why do we use? Just because that
> is how TCP is written I'm guessing.

We considered getting rid of the lower bound several times, but didn't
want to investigate it fully back then.

> I know that in DCCP CCID3 the RTO is 4 x RTT (from memory - it might
> be a slight variation) but we ended up putting a minimum on it as you
> also face a problem if it fires too frequently (i.e. link is in
> usecs).
> 
> I might ask around on research lists and see why this issue has never
> been revisited.

There is also the argument that on a local lan congestion control
stops to make any sense.  The problem it that you can't detect what is
a local lan, and any config knob to indicate this is an unacceptable
hack.

Any "congestion" you see on a local high speed lan will be gone before
you can react to it, so it's pretty pointless to do anything.
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