netdev - Re: [PATCH] tcp: ECN blackhole should not force quickack mode

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Date:	Fri, 23 Sep 2011 20:55:45 -0400
From:	Jamal Hadi Salim <jhs@...atatu.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>, Jerry Chu <hkchu@...gle.com>,
	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>,
	Jim Gettys <jg@...edesktop.org>,
	Dave Taht <dave.taht@...il.com>
Subject: Re: [PATCH] tcp: ECN blackhole should not force quickack mode

On Fri, 2011-09-23 at 08:02 +0200, Eric Dumazet wrote:
> While playing with a new ADSL box at home, I discovered that ECN
> blackhole can trigger suboptimal quickack mode on linux : We send one
> ACK for each incoming data frame, without any delay and eventual
> piggyback.
>
> This is because TCP_ECN_check_ce() considers that if no ECT is seen on a
> segment, this is because this segment was a retransmit.

If you have Linux as the sender, that assumption is legit (i.e we never
set ECT on retransmits).

> Refine this heuristic and apply it only if we seen ECT in a previous
> segment, to detect ECN blackhole at IP level.

So  small confusion for me, Eric:
Is theres some middlebox clearing the ECT but allowing the TCP header
ECN flags to be set during SYN echange?  If yes, why would not allowing
quickack help?

cheers,
jamal





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