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[83.245.226.9]) by smtp.gmail.com with ESMTPSA id y10sm3132401lfg.44.2019.04.03.13.38.48 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 03 Apr 2019 13:38:49 -0700 (PDT) Content-Type: text/plain; charset=us-ascii Mime-Version: 1.0 (Mac OS X Mail 11.5 \(3445.9.1\)) From: Jonathan Morton X-Priority: 3 (Normal) In-Reply-To: <1554322684.79873224@apps.rackspace.com> Date: Wed, 3 Apr 2019 23:38:47 +0300 Cc: Dave Taht , ECN-Sane Content-Transfer-Encoding: quoted-printable Message-Id: <8FAF4273-26F1-406D-919B-843E250C334B@gmail.com> References: <1554322684.79873224@apps.rackspace.com> To: "David P. Reed" X-Mailer: Apple Mail (2.3445.9.1) Subject: Re: [Ecn-sane] where the l4s ect1 takeover is documented X-BeenThere: ecn-sane@lists.bufferbloat.net X-Mailman-Version: 2.1.20 Precedence: list List-Id: Discussion of explicit congestion notification's impact on the Internet List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 03 Apr 2019 20:38:51 -0000 > On 3 Apr, 2019, at 11:18 pm, David P. Reed = wrote: >=20 > One would need to prove that they are compatible and cause performance = to converge when used at the same time. And that's the problem; the precise opposite has been proved, unless = either: - the traffic runs through an FQ system at the bottleneck (where the FQ = system enforces fair convergence), or - the DualQ system is at the bottleneck (where the AQMs handling each = class are coupled in a compatible manner), or - a dumb tail-drop FIFO is at the bottleneck (which we're trying to get = the heck away from, but L4S' response to drops is TCP friendly). If there's a simple single-queue ECN-enabled AQM at the bottleneck, L4S = ends up squashing any Classic ECN or Not-ECT traffic out of existence, = simply because it responds less to each CE mark and requires a = continuous stream of them (two per RTT) to hold the cwnd stable. That = same CE marking (or packet dropping) rate will cause normal TCPs to = collapse to minimum cwnd within a handful of RTTs, whether they = implement the 50% reduction of Reno, the 70% of CUBIC, or the 85% = permitted by the ABE spec - so L4S cannot safely coexist with normal = traffic. Much of the present debate revolves around how prevalent these AQMs = actually are now, or might become in the foreseeable future. It's = obvious to me that DualQ is designed as a way to make L4S work in places = where a single-queue AQM would normally be deployed, as much as been = made of the relative simplicity of implementing it in high-speed = hardware, and the initial rollout appears to be in the same class of = devices for which PIE was developed. By adopting ECT(1) as SCE and retaining the existing meaning of CE, the = two forms of AQM marking are much better integrated and have a much = better chance of coexisting in a heterogeneous environment. I and some = others are now actively engaged in proving that by example. - Jonathan Morton