Re: [tcpPrague] [aqm] L4S status update

Jonathan Morton <> Tue, 29 November 2016 01:20 UTC

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From: Jonathan Morton <>
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Date: Tue, 29 Nov 2016 03:20:28 +0200
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To: Bob Briscoe <>
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Cc: tcpm IETF list <>, tsvwg IETF list <>, "Bless, Roland (TM)" <>, TCP Prague List <>, AQM IETF list <>
Subject: Re: [tcpPrague] [aqm] L4S status update
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> On 29 Nov, 2016, at 02:45, Bob Briscoe <> wrote:
> I am particularly worried about embedding fq in the Internet. That is far worse than embedding a subtly different performance improvement for certain congestion controls. With fq, the network determines the precise departure time of each packet, completely overriding the host's choice, without any understanding of what the applications is trying to achieve.

You don’t seem to be very fond of *either* component of fq_codel, then.  A shame, since it works very well in practice.

What fq_codel does is identify latency-sensitive flows by the fact that they are not taking up their fair share of the bandwidth.  Packets belonging to these flows are typically delivered *immediately* and *without loss*.  This is a far cry from “completely overriding the hosts’s choice” of timing, as you claim.

In fact, I would actually be grateful if you retracted that particular claim.

It must be emphasised, though, that flow-isolating AQMs are not designed for the Internet core - there are just too many individual flows to manage.  Their place is at the edge, on either end of last-mile links, where the bottlenecks are most apparent.  For core, backhaul and peering links, plain AQM is sufficient and much easier to implement.

> Even worse, in Jan 2017, I am told that fq_CoDel will become hard coded into the Linux WiFi drivers, without even a framework to dynamically load any alternative(s). Of course, we can add such a framework, but we are seeing Linux become the next major middlebox problem. It might be excusable if there were not sound alternatives available,... but there are.

This tight integration is because it was necessary to solve some serious, long-standing problems with Linux wifi, which couldn’t be solved satisfactorily at the qdisc layer because information about wifi-specific things was needed - and there were *no* practical alternatives which actually solved the problem, otherwise we’d have used them.

Wifi is also a last-mile technology, and it is often the bottleneck in several types of practical deployment.  Large conferences are a particular example.  I’m rather looking forward to seeing the first large conference to deploy the new Linux wifi stuff, and seeing whether it has made the typical load there easier to cope with.  It probably has.

 - Jonathan Morton