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[83.245.235.192]) by smtp.gmail.com with ESMTPSA id t8sm3030575lfe.25.2020.04.29.09.03.02 (version=TLS1_2 cipher=ECDHE-ECDSA-AES128-GCM-SHA256 bits=128/128); Wed, 29 Apr 2020 09:03:03 -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 In-Reply-To: <537a840b-4c71-4ad2-8421-32c248eb69fa@bobbriscoe.net> Date: Wed, 29 Apr 2020 19:03:01 +0300 Cc: Dave Taht , ECN-Sane , tsvwg IETF list Content-Transfer-Encoding: quoted-printable Message-Id: <4B98C666-E91C-462D-B039-F084A86040DE@gmail.com> References: <6d925e3b-2781-0fb1-6936-7a6c006b9a21@bobbriscoe.net> <00A6CD68-E37B-4505-BB96-476509937AFC@gmail.com> <537a840b-4c71-4ad2-8421-32c248eb69fa@bobbriscoe.net> To: Bob Briscoe X-Mailer: Apple Mail (2.3445.9.1) Subject: Re: [Ecn-sane] Fwd: my backlogged comments on the ECT(1) interim call 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, 29 Apr 2020 16:03:07 -0000 >> As shown in our test results, Flent (which implements the RRUL test) = is indeed now capable of tracking the latency experienced by individual = TCP flows. It does not do so at the packet level, but at the socket = level. >>=20 >> Of course it is also possible to capture the traffic and analyse the = traces offline, if you really do want packet-level detail. >=20 > 1. So, why do you continue to use this approach? It hides all the = larger delays underneath a moving average. This seems more designed to = hide inconsistent delay results than to measure them. If a real-time = application waited only for the median delay before rendering, it would = have to discard the 50% of traffic in the upper median! TCP also = delivers nothing until the more delayed packets have been delivered to = get the stream in order. So median (or mean) delay is a meaningless = metric. Because we are not laser-focused on the five-nines tail of latency as = you are, but instead take a broader view that recognises some tradeoffs. = For us, the 99th percentile is interesting, but anything beyond that is = not worth sacrificing other desirable properties for. And, as I said, = we can extract whatever per-packet statistics we like from a packet = trace. The time-series plot, however, tells us much more about the gross = behaviour. It tells us, for example, that the reason you have such good = five-nines behaviour is because you drop out of slow-start extremely = early, so you end up having to grow linearly to the BDP instead of using = some faster function. I bet when you get around to implementing Paced = Chirping to correct that, you'll find that your five-nines latency takes = a hit from the queue pacing the probe trains. That's a prediction I'd = like to see tested. > 2. I also notice you didn't address Dave's point about using short = flows as well as long. I don't believe I have ever seen a test from you = or PeteH with anything but long flows. That's mostly a tools problem. Flent doesn't currently have a way to = generate that type of traffic and incorporate it into a test run. It's = on our todo list though. - Jonathan Morton=