netdev - Re: TCP delayed ACK heuristic

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Date:	Thu, 20 Dec 2012 11:23:49 +0800
From:	Cong Wang <amwang@...hat.com>
To:	David Miller <davem@...emloft.net>
Cc:	rick.jones2@...com, David.Laight@...LAB.COM,
	netdev@...r.kernel.org, greearb@...delatech.com,
	eric.dumazet@...il.com, shemminger@...tta.com, tgraf@...hat.com
Subject: Re: TCP delayed ACK heuristic

On Wed, 2012-12-19 at 12:59 -0800, David Miller wrote:
> 
> Yes, but RFC2525 makes it very clear why we should not even
> consider doing crap like this.
> 
> ACKs are the only information we have to detect loss.
> 
> And, for the same reasons that TCP VEGAS is fundamentally broken, we
> cannot measure the pipe or some other receiver-side-visible piece of
> information to determine when it's "safe" to stretch ACK.
> 
> And even if it's "safe", we should not do it so that losses are
> accurately detected and we don't spuriously retransmit.
> 
> The only way to know when the bandwidth increases is to "test" it, by
> sending more and more packets until drops happen.  That's why all
> successful congestion control algorithms must operate on explicited
> tested pieces of information.
> 
> Similarly, it's not really possible to universally know if it's safe
> to stretch ACK or not.

Sounds reasonable. Thanks for your explanation.

> 
> Can we please drop this idea?  It has zero value and all downside as
> far as I'm concerned.
> 

Yeah, I am just trying to see if there is any way to get a reasonable
heuristic.

So, can we at least have a sysctl to control the timeout of the delayed
ACK? I mean the minimum 40ms. TCP_QUICKACK can help too, but it requires
the receiver to modify the application and has to be set every time when
calling recv().

Thanks!


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