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

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Date:	Wed, 29 Aug 2007 16:06:27 -0700
From:	Rick Jones <rick.jones2@...com>
To:	David Miller <davem@...emloft.net>
Cc:	ian.mcdonald@...di.co.nz, netdev@...r.kernel.org
Subject: Re: [PATCH] make _minimum_ TCP retransmission timeout configurable

> All of this seems to suggest that the RTO calculation is wrong.

That is a possiblity.  Or at least could be enhanced.

> It seems that packets in this network can be delayed several orders of
> magnitude longer than the usual round trip as measured by TCP.
> 
> What exactly causes such a huge delay?  What is the TCP measured RTO
> in these circumstances where spurious RTOs happen and a 3 second
> minimum RTO makes things better?

I belive the biggest component comes from link-layer retransmissions. 
There can also be some short outtages thanks to signal blocking, 
tunnels, people with big hats and whatnot that the link-layer 
retransmissions are trying to address.  The three seconds seems to be a 
value that gives the certainty that 99 times out of 10 the segment was 
indeed lost.

The trace I've been sent shows clean RTTs ranging from ~200 milliseconds 
to ~7000 milliseconds.

rick

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