netdev - Re: [PATCH 1/3] net: TCP thin-stream detection

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Date:	Fri, 30 Oct 2009 16:24:02 +0100
From:	Arnd Hannemann <hannemann@...s.rwth-aachen.de>
To:	"apetlund@...ula.no" <apetlund@...ula.no>
Cc:	William Allen Simpson <william.allen.simpson@...il.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"shemminger@...tta.com" <shemminger@...tta.com>,
	"ilpo.jarvinen@...sinki.fi" <ilpo.jarvinen@...sinki.fi>,
	"davem@...emloft.net" <davem@...emloft.net>
Subject: Re: [PATCH 1/3] net: TCP thin-stream detection

apetlund@...ula.no schrieb:
> As Ilpo writes, the mechanism we propose is simpler than the ID, and
> slightly more aggressive. The reason why we chose this is as follows: 1)
> The ID and Limited Transmit tries to prevent retransmission timeouts by
> retransmitting more aggressively, thus keeping the congestion window open
> even though congestion may be the limiting factor. If their limiting
> conditions change, they still have higher sending rates available. The
> thin-stream applications are not limited by congestion control. There is
> therefore no motivation to prevent retransmission timeouts in order to
> keep the congestion window open because in the thin-stream scenario, a
> larger window is not needed, but we retransmit early only to reduce
> application-layer latencies. 2) Our suggested implementation is simpler.
> 3) I believe that the reason why the ID has not been implemented in Linux
> is that the motivation did not justify the achieved result. We have
> analysed a wide range of time-dependent applications and found that they
> very often produce thin streams due to transmissions being triggered by
> human interaction. This changes the motivational picture since a thin
> stream is an indicator of time-dependency.

Both mechanism prevent retransmission timeouts, thereby reducing latency.
Who cares, that they were motivated by performance?

I agree, that you are more aggressive, and that your scheme may have
latency advantages, at least for the Limited Transmit case. And there are
probably good reasons for your proposal. But I really think you should
bring your proposal up in IETF TCPM WG. I have the feeling that there are
a lot of corner cases we didn't think of.

One example: Consider standard NewReno non-SACK enabled flow:
For some reasons two data packets get reordered.
The TCP sender will produce a dupACK and an ACK.
The dupACK will trigger (because of your logic) a spurious retransmit.
The spurious retransmit will trigger a dupACK.
This dupACK will again trigger a spurious retransmit.
And this game will continue, unless a packet is dropped by coincidence.

P.S.: The Early-Rexmit ID has not been implemented in Linux,
because our student who was working on that is busy with something
else...

Best regards,
Arnd Hannemann
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