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[83.245.229.102]) by smtp.gmail.com with ESMTPSA id w16sm7196471lfc.1.2019.12.01.08.54.36 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sun, 01 Dec 2019 08:54:37 -0800 (PST) Content-Type: text/plain; charset=us-ascii Mime-Version: 1.0 (Mac OS X Mail 11.5 \(3445.9.1\)) From: Jonathan Morton In-Reply-To: <02703449-D6CE-497D-BDBD-D79542D0EACF@gmx.de> Date: Sun, 1 Dec 2019 18:54:35 +0200 Cc: Carsten Bormann , ECN-Sane , bloat Content-Transfer-Encoding: quoted-printable Message-Id: <349F14BC-683C-431E-BE8F-8574880F04B9@gmail.com> References: <63E9C0E4-C913-4B2F-8AFC-64E12489BC65@gmail.com> <297503679.4519449.1575069001960@mail.yahoo.com> <54C976BC-DEC7-4710-9CFF-0243559D9002@gmail.com> <156EA284-C01D-4FAA-89F4-DB448795F7FC@gmx.de> <385CF47C-17AD-4A62-9924-068E1485FFD5@gmail.com> <8C5FD2CE-D24F-4998-A636-8F85279C67BA@gmail.com> <02703449-D6CE-497D-BDBD-D79542D0EACF@gmx.de> To: Sebastian Moeller X-Mailer: Apple Mail (2.3445.9.1) Subject: Re: [Bloat] [Ecn-sane] sce materials from ietf X-BeenThere: bloat@lists.bufferbloat.net X-Mailman-Version: 2.1.20 Precedence: list List-Id: General list for discussing Bufferbloat List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sun, 01 Dec 2019 16:54:39 -0000 > On 1 Dec, 2019, at 6:35 pm, Sebastian Moeller wrote: >=20 > Belt and suspenders, eh? But realistically, the idea of using an = accumulating SCE counter to allow for a lossy reverse ACK path seems = sort of okay (after all TCP relies on the same, so there would be a nice = symmetry ). Sure, we did think of several schemes that used a counter. But when it = came down to actually implementing it, we decided to try the simplest = possible solution first and see how well it worked in practice. It = turned out to work very well, and can recover cleanly from as much as = 100% relative feedback error caused by ack loss: If less feedback is observed by the sender than intended by the AQM, = growth will continue and the AQM will increase its marking to = compensate, ultimately resorting to a CE mark. This is, incidentally, = exactly what happens if the receiver *or* sender are completely = SCE-ignorant, and looks very much like RFC-3168 behaviour, which is = entirely intentional. If feedback is systematically doubled by the time it reaches the sender, = perhaps through faulty ack filtering on the return path, it will back = off more than intended, the bottleneck queue will empty, and AQM = feedback will consequently reduce or cease entirely. Only a very = serious fault would re-inject ESCE feedback once SCE marking has = completely ceased, so the sender will then grow back towards the correct = cwnd after a relatively small negative excursion. The above represents both extremes of 100% relative error in the = feedback, which is shown to be safe and reasonably tolerable. Smaller = errors due to random ack loss are more likely, and consequently easier = to tolerate in a closed negative-feedback control loop. - Jonathan Morton=