Re: [aqm] think once to mark, think twice to drop: draft-ietf-aqm-ecn-benefits-02

"Scheffenegger, Richard" <rs@netapp.com> Fri, 27 March 2015 18:16 UTC

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From: "Scheffenegger, Richard" <rs@netapp.com>
To: David Lang <david@lang.hm>, "De Schepper, Koen (Koen)" <koen.de_schepper@alcatel-lucent.com>
Thread-Topic: [aqm] think once to mark, think twice to drop: draft-ietf-aqm-ecn-benefits-02
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Date: Fri, 27 Mar 2015 18:11:33 +0000
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Cc: "aqm@ietf.org" <aqm@ietf.org>, Michael Welzl <michawe@ifi.uio.no>, "davecb@spamcop.net" <davecb@spamcop.net>
Subject: Re: [aqm] think once to mark, think twice to drop: draft-ietf-aqm-ecn-benefits-02
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Hi David,


> > - low latency AND high throughput (compared to low latency OR high
> > throughput for drop based congestion controllers)
> 
> I think that you are overstating things when you say that without ECN you
> are forced to choose between low latency OR high throughput. that doesn't
> match what people are reporting when they use simple fq_codel without ECN


I disagree. At the least when you are using TCP, a drop will cause head-of-line blocking on the receiver, for at least 1 RTT; Agreed, there are ways to mitigate this (FEC-encoded transports).

But the "OR" is exactly the correct description here: FEC has inheritent overhead thus reduced bandwidth, and loss-recovery suffers from head-of-line blocking, thus higher latency (to the application, where it actually matters).

FQ-Codel runs with high bandwidth, but the drops induce latency in the end-hosts nevertheless...
Thus FQ-Codel with ECN would still be more effective than without ECN.


Richard