netdev - RE: TCP delayed ACK heuristic

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Date:	Tue, 18 Dec 2012 16:39:59 -0000
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Cong Wang" <amwang@...hat.com>, <netdev@...r.kernel.org>
Cc:	"Ben Greear" <greearb@...delatech.com>,
	"David Miller" <davem@...emloft.net>,
	"Eric Dumazet" <eric.dumazet@...il.com>,
	"Stephen Hemminger" <shemminger@...tta.com>,
	"Rick Jones" <rick.jones2@...com>, "Thomas Graf" <tgraf@...hat.com>
Subject: RE: TCP delayed ACK heuristic

> David's point is that we can do some heuristics for TCP
> delayed ACK, so the question is that what kind of heuristics
> can we use?
> 
> RFC1122 explicitly mentions:
> 
>             A TCP SHOULD implement a delayed ACK, but an ACK should not
>             be excessively delayed; in particular, the delay MUST be
>             less than 0.5 seconds, and in a stream of full-sized
>             segments there SHOULD be an ACK for at least every second
>             segment.
> 
> so this prevents us from using any heuristic for the number
> of coalesced delayed ACK.

There are problems with only implementing the acks
specified by RFC1122.

I've seen problems when the sending side is doing (I think)
'slow start' with Nagle disabled.
The sender would only send 4 segments before waiting for an
ACK - even when it had more than a full sized segment waiting.
Sender was Linux 2.6.something (probably low 20s).
I changed the application flow to send data in the reverse
direction to avoid the problem.
That was on a ~0 delay local connection - which means that
there is almost never outstanding data, and the 'slow start'
happened almost all the time.
Nagle is completely the wrong algorithm for the data flow.

	David