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From: Eric Dumazet <eric.dumazet@gmail.com>
To: Jonathan Morton <chromatix99@gmail.com>
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Date: Wed, 16 May 2012 12:10:30 +0200
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Cc: "codel@lists.bufferbloat.net" <codel@lists.bufferbloat.net>,
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Subject: Re: [Codel] [Bloat] Exploring the potential of codel, fq_codel,
	and qfq
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On Wed, 2012-05-16 at 12:59 +0300, Jonathan Morton wrote:
> That's a pretty specific type of blackhole - one that can pass enough
> ECN information to permit negotiation of an ECN flow, but which then
> squashes Congestion Experienced codepoints. How common are those?
> 

Very common since TCP ECN negociation doesnt use ECN on IP header.

(SYN, SYNACK, and ACK messages dont use ECT)

So a network playing games with TOS field can be a ECN blackhole, still
allowing TCP to negociate ECN enabled sessions.

> If fq_codel drops from the head of the longest flow when the queue is
> physically full, rather than the next one to be serviced, all should
> be well even in that case. 

Not really. Unless you afford 3 second delay in queue.