Re: [tsvwg] [Ecn-sane] Comments on L4S drafts

"De Schepper, Koen (Nokia - BE/Antwerp)" <> Thu, 04 July 2019 13:43 UTC

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From: "De Schepper, Koen (Nokia - BE/Antwerp)" <>
To: Jonathan Morton <>, Bob Briscoe <>
CC: "" <>, "" <>
Thread-Topic: [tsvwg] [Ecn-sane] Comments on L4S drafts
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Date: Thu, 4 Jul 2019 13:43:08 +0000
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Subject: Re: [tsvwg] [Ecn-sane] Comments on L4S drafts
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>> 2: DualQ can be defeated by an adversary, destroying its ability to isolate L4S traffic.

Not correct. DualQ works not different than any (single q) FIFO, which can be defeated by non-responsive traffic.
It even does not matter what type of traffic the adversary is (L4S or Classic drop/mark), as the adversary will push away the responsive traffic only by the congestion signal it invokes in the AQM (drop or classic or L4S marking). The switch to drop for all traffic from 25% onwards avoids that ECN flows get a benefit under overload caused by non-responsive flows. This mechanism protects also Classic ECN single Q AQMs, as defined in the ECN RFCs.

So conclusion:   a DualQ works exactly the same as any other single Q AQM supporting ECN !!
Try it, and you'll see...


-----Original Message-----
From: tsvwg <> On Behalf Of Jonathan Morton
Sent: Thursday, July 4, 2019 2:24 PM
To: Bob Briscoe <>
Subject: Re: [tsvwg] [Ecn-sane] Comments on L4S drafts

> On 4 Jul, 2019, at 2:54 pm, Bob Briscoe <> wrote:
> The phrase "relative to a FIFO" is important. In a FIFO, it is of course possible for flows to take more throughput than others. We see that as a feature of the Internet not a bug. But we accept that some might disagree...

Chalk me up as among those who consider "no worse than a FIFO" to not be very reassuring.  As is well documented and even admitted in L4S drafts, L4S flows tend to squash "classic" flows in a FIFO.

So the difficulty here is twofold:

1: DualQ or FQ is needed to make L4S coexist with existing traffic, and

2: DualQ can be defeated by an adversary, destroying its ability to isolate L4S traffic.

I'll read your reply to Jake when it arrives.

 - Jonathan Morton