Re: [tsvwg] Another tunnel/VPN scenario (was RE: Reasons for WGLC/RFC asap)

Jonathan Morton <> Fri, 20 November 2020 06:31 UTC

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From: Jonathan Morton <>
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Date: Fri, 20 Nov 2020 08:31:17 +0200
Cc: "Black, David" <>, Pete Heist <>, tsvwg IETF list <>
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To: Ingemar Johansson S <>
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Subject: Re: [tsvwg] Another tunnel/VPN scenario (was RE: Reasons for WGLC/RFC asap)
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> On 20 Nov, 2020, at 8:04 am, Ingemar Johansson S <> wrote:
> I try to make it clear to me what this scenario show is about and somehow I see it more as a flow isolation problem that makes FQ non-functional rather than an L4S problem?

The original problem is the result of L4S redefining CE.  This means it interacts poorly with AIMD traffic sharing the same queue and AQM.  The solution proposed by L4S is to ensure at all times that L4S goes to a different queue than conventional AIMD traffic.  The L4S-aware method of doing so is DualPI2, but that is not present on existing networks. Instead, on existing networks, L4S proposes relying on the assumption that FQ is sufficient.

The likely existence of a mixture of L4S and AIMD traffic in a tunnel, which appears to FQ as a single flow, indicates that FQ is not necessarily sufficient to overcome the coexistence problem.  The latter is still the basic problem; relying on FQ was only ever a workaround.

 - Jonathan Morton