From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-lf1-x12e.google.com (mail-lf1-x12e.google.com [IPv6:2a00:1450:4864:20::12e]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by lists.bufferbloat.net (Postfix) with ESMTPS id 896A43B29D; Mon, 14 Nov 2022 12:05:39 -0500 (EST) Received: by mail-lf1-x12e.google.com with SMTP id g12so20369260lfh.3; Mon, 14 Nov 2022 09:05:39 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; h=to:references:message-id:content-transfer-encoding:cc:date :in-reply-to:from:subject:mime-version:from:to:cc:subject:date :message-id:reply-to; bh=cq+Cslj/luPeGmnV4afb/vjXHbm7nD0GpNZRaKEt++Y=; b=j3dIz0lkh2Tz9ml9QafCvMnFq2PF1z423eEoMcoswDb9BTVYvptau9nUsln5eGvM0M qeVFSNI066qQkzxSDaj5NLtHemw1hISF03xqsBzwOVDpTTiJKa7xNc+CtFSpwy7EDutv K0GSDV1XUIGFjFiSyOt/Bxr4Mro3rimqDza/eYqS4EraZKct4Cpza36mJgKIR8/ngkxw h9uoTuTsWO8CfOr69yP3FiAN+znqttZLcnVcZoLLzUrZQBw4URINQV0g+6eW0UZb1t9L tTqkvYcWLkfUeGvPCaF/3JIVQaxZy9eeUqZCOzlHjzkBJ0Kai+lmtw5ooIO6mwAocG48 CnTQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=to:references:message-id:content-transfer-encoding:cc:date :in-reply-to:from:subject:mime-version:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=cq+Cslj/luPeGmnV4afb/vjXHbm7nD0GpNZRaKEt++Y=; b=lzGYUC2QkIKc0di/bBqUAxUGyFLgM5GlT1cr7x1EAHKsfNrW2TMNE91pGQwbs5K7MN +pRmjFBtf6kx4kK677G/VxUDjxxJPOooixkDxFRr89kawrYy7WeyVAQP4T0uKNxaYN+N mh0sbRZMOgsU9pAKGQJmCmA2aayBbJF+A6zyIXG7FBo9eEVcH239tlsoohjzF0ONKGzK x0lphe8OxaMzNvLtPZ6G4oqOCRVnf4HOd/Op/FfsB5v8Ar86yNcP8fklQF2F4bWu7oCl m2nunYBxWo14u84MZdJ9pKrIwOsaQ4A97E5D0h+gUL+jh9QupLtX6WlcV3xdTGlBAVsc 3E/g== X-Gm-Message-State: ANoB5pkIIRAIH/rpihzgDbbhkCrSe78aLj5IQo8a95kxhmhuY20uf7bj 6Qo+dPWkREJWxhf47M0eKW4= X-Google-Smtp-Source: AA0mqf7oaQHzpEeoDVTdRHbDSFuQS8ZFxotTMGmOh3XyH0CEvMOBuT8dYrs7pHmI/QdtwyOq0ngNNQ== X-Received: by 2002:a05:6512:2987:b0:4b2:af4:65b2 with SMTP id du7-20020a056512298700b004b20af465b2mr4098186lfb.189.1668445538069; Mon, 14 Nov 2022 09:05:38 -0800 (PST) Received: from smtpclient.apple (178-55-88-210.bb.dnainternet.fi. [178.55.88.210]) by smtp.gmail.com with ESMTPSA id e12-20020a05651c038c00b0026fc822c264sm2096052ljp.87.2022.11.14.09.05.36 (version=TLS1_2 cipher=ECDHE-ECDSA-AES128-GCM-SHA256 bits=128/128); Mon, 14 Nov 2022 09:05:37 -0800 (PST) Content-Type: text/plain; charset=us-ascii Mime-Version: 1.0 (Mac OS X Mail 14.0 \(3654.100.0.2.22\)) From: Jonathan Morton In-Reply-To: Date: Mon, 14 Nov 2022 19:05:35 +0200 Cc: libreqos , bloat Content-Transfer-Encoding: quoted-printable Message-Id: <4EE62BEA-256D-4157-A1CE-310D9253ABD3@gmail.com> References: To: Dave Taht X-Mailer: Apple Mail (2.3654.100.0.2.22) Subject: Re: [Bloat] summarizing the bitag latency report? 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: Mon, 14 Nov 2022 17:05:39 -0000 > On 12 Nov, 2022, at 1:16 am, Dave Taht via Bloat = wrote: >=20 > If you were to try to summarize this *in a paragraph*, what would you = say? >=20 > https://www.bitag.org/documents/BITAG_latency_explained.pdf I can get it down to *three* paragraphs while conveying the essentials: The quality of an Internet path is measured by three factors: = throughput, latency, and packet loss. Of these three measures, = throughput is typically the least important for application performance, = so long as a modest threshold is met - for example the US "broadband" = definition of 25Mbps. Packet loss is interpreted by computers as = indicating congestion, which causes them to slow down network transfers = unnecessarily; it also causes objectionable glitches in video and audio = streams, and should thus be minimised. Latency is the primary driver of = perceived Internet quality for most applications in most circumstances. Latency can be divided into "inherent" and "induced" components. = Inherent latency is simply the time it takes for a packet to traverse = all the links in the path, outward and return. Induced latency is the = additional time spent deciding which of several links to direct the = packet to, waiting for a shared medium, and/or stuck in a queue full of = other packets going the same way. Most applications are able to adapt = to reasonable levels of inherent latency, but induced latency is much = more difficult to manage due to its variability. There are several ways = to reduce induced latency without impairing throughput or packet loss, = chiefly AQM and Fair Queuing, which can fruitfully be combined as in = SQM. SQM is widely, but not yet universally, deployed on the Internet, = and works very well. AQM is the practice of observing how big queues get, and signalling = congestion in a deliberate way based on those observations. ECN can be = used to perform that signalling without any packet loss. On traffic = that doesn't support ECN, deliberately dropping packets in a controlled = way is necessary. These congestion signals cause applications to reduce = their load on the network to match available capacity, and thereby = reduce queuing. Fair Queuing works orthogonally to this by treating = each flow of traffic individually, so that one flow inducing heavy = delays in its queue doesn't affect another flow which is lighter. This = makes it easy for very different applications to coexist on the same = path, which often happens when there are several users in the same = household or office. SQM uses Fair Queuing, and also applies a separate = AQM to each flow, so that congestion signals are directed solely to = heavy flows. If you really need it to be only *one* paragraph, the middle one might = be the most essential. > Also QoS, vs QoE. Try to imagine explaining the need to a CFO, or > congresscritter. Feel free to take more than a paragraph. QoS is Quality of Service. QoE is Quality of Experience. The two are = very different concepts. To illustrate this, consider a railway manager tasked with modernising = his line by replacing steam trains with diesel ones. He's a modern = businessman keen to apply modern thinking to this task, so he delegates = some underlings to gather data about the expected traffic flows on the = line, as well as the types of train that are available for hire. In the answers that come back, he focuses on two key figures: the line = carries 1000 passengers per day, and each carriage can seat 50 = passengers. Simple arithmetic shows that this demand can be met by = running 20 carriages per day, but the manager rounds this up to 24 = carriages to allow some margin for error. After all, with the = tremendous efficiency of diesel traction (compared to steam traction), = he can afford to be a little generous. One of the trains on offer is a 2000hp locomotive hauling 12 carriages - = a very impressive sight, to be sure. "Splendid," he thinks, "we can run = that one twice a day, and that will meet demand with some margin to = spare." So that's what he does; once in the morning, and once in the = evening. The timetables are very easy to publish, too. In the first month of operation, all of these trains turn up on time and = with the correct number of carriages, and there are no breakdowns or = accidents. The specified capacity is therefore supplied. This is an = excellent "Quality of Service". Yet the complaints start rolling in almost immediately. Passengers who = turn up wanting to travel at any other time than the two trains serve = find themselves with an exceptionally long wait ahead of them. Local = police even report an increase in vagrancy complaints, due to passengers = missing the evening train and having to sleep in the waiting rooms = overnight. This represents a very poor "Quality of Experience". Learning from this misadventure, the manager goes back to his data and = notes that one-carriage "railcars" are also available for hire. For the = next month's timetable, instead of the two 12-carriage trains each day, = he will run one of these railcars every hour. These will provide = exactly the same seating capacity over the course of the day, but the = waiting time will now be limited to a much more palatable duration. (In = Internet terms, he's optimised squarely for latency.) Still the complaints come in - but now from different sources. No = longer are passengers waiting for hours and sleeping overnight in = stations. Instead, rush-hour commuters who had previously found the = 12-carriage trains convenient are finding the railcars too crowded. = Even with over a hundred passengers crammed in like sardines, many more = are left on the platforms and arrive at work late - or worse, come home = to a cold dinner and an annoyed wife. Simply put, demand is not evenly = distributed through the day, but concentrated on particular times; at = other times, the railcars are sufficient for the relatively small number = of passengers, or even run almost empty. So again, even though the "Quality of Service" is provided just as = specified, the "Quality of Experience" for the passengers is very poor. = Indeed the overcrowding leads to some railcars being delayed, due to the = difficulty of getting everyone in and out of the doors, and the = conductors have great difficulty in checking tickets, hence a noticeable = reduction in fare revenue. Things improve markedly when the manager brings in 6-carriage express = trains for the morning, lunchtime, and evening commuters, and continues = to run the railcars at hourly intervals in between them, except for the = small hours when some trains are removed due to minimal demand. Now = there are enough carriages in the rush-hour trains to satisfy commuters, = and there are still trains running at other times so that nobody needs = to wait particularly long for one. In fact, demand increases substantially due to the good "Quality of = Experience" that this new timetable provides, such that by the end of = the first year, many of the railcars are upgraded to 3-carriage trains, = and the commuter expresses are lengthened to 8 carriages. Fare revenue = is more than doubled. The modernisation effort is a success. The lesson here is that QoS is merely the means by which you may attempt = to achieve high QoE. Meeting QoS does not guarantee QoE. Only if the = QoS is designed around the factors that genuinely influence QoE will you = succeed. Unfortunately, many QoS schemes are inadequate for the needs = of actual Internet users; this is because their designers have not kept = up with the appropriate QoE factors. - Jonathan Morton