netdev - Re: [PATCH 2.6.22] TCP: Make TCP_RTO

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Date:	Fri, 13 Jul 2007 09:55:10 -0700
From:	Rick Jones <rick.jones2@...com>
To:	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
Cc:	Stephen Hemminger <shemminger@...ux-foundation.org>,
	noboru.obata.ar@...achi.com, David Miller <davem@...emloft.net>,
	yoshfuji@...ux-ipv6.org, Netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH 2.6.22] TCP: Make TCP_RTO_MAX a variable (take 2)

Ilpo Järvinen wrote:
> On Thu, 12 Jul 2007, Rick Jones wrote:
> 
> 
>>>One question is why the RTO gets so large that it limits failover?
>>>
>>>If Linux TCP is working correctly,  RTO should be srtt + 2*rttvar
>>>
>>>So either there is a huge srtt or variance, or something is going
>>>wrong with RTT estimation.  Given some reasonable maximums of
>>>Srtt = 500ms and rttvar = 250ms, that would cause RTO to be 1second.
>>
>>I suspect that what is happening here is that a link goes down in a trunk
>>somewhere for some number of seconds, resulting in a given TCP segment being
>>retransmitted several times, with the doubling of the RTO each time.
> 
> 
> But that's a back-off for the retransmissions, the doubling is 
> temporary... Once you return to normal conditions, the accumulated backoff 
> multiplier will be immediately cut back to normal. So you should then be 
> back to 1 second (like in the example or whatever) again...

Fine, but so?  I suspect the point of the patch is to provide a lower cap on the 
accumulated backoff so data starts flowing over the connection within that lower 
cap once the link is restored/failed-over.

rick jones

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