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[37.136.219.147]) by smtp.gmail.com with ESMTPSA id a25sm728939ljp.71.2021.07.09.16.56.46 (version=TLS1_2 cipher=ECDHE-ECDSA-AES128-GCM-SHA256 bits=128/128); Fri, 09 Jul 2021 16:56:47 -0700 (PDT) Content-Type: text/plain; charset=us-ascii Mime-Version: 1.0 (Mac OS X Mail 11.5 \(3445.9.7\)) From: Jonathan Morton X-Priority: 3 (Normal) In-Reply-To: <8C38E940-8B97-4767-A39B-25F043AE0856@cs.ucla.edu> Date: Sat, 10 Jul 2021 02:56:46 +0300 Cc: "David P. Reed" , Cake List , Make-Wifi-fast , Bob McMahon , starlink@lists.bufferbloat.net, codel@lists.bufferbloat.net, cerowrt-devel , bloat , Ben Greear Content-Transfer-Encoding: quoted-printable Message-Id: References: <1625188609.32718319@apps.rackspace.com> <989de0c1-e06c-cda9-ebe6-1f33df8a4c24@candelatech.com> <1625773080.94974089@apps.rackspace.com> <1625859083.09751240@apps.rackspace.com> <8C38E940-8B97-4767-A39B-25F043AE0856@cs.ucla.edu> To: Leonard Kleinrock X-Mailer: Apple Mail (2.3445.9.7) Subject: Re: [Cerowrt-devel] [Bloat] Little's Law mea culpa, but not invalidating my main point X-BeenThere: cerowrt-devel@lists.bufferbloat.net X-Mailman-Version: 2.1.20 Precedence: list List-Id: Development issues regarding the cerowrt test router project List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Fri, 09 Jul 2021 23:56:51 -0000 > On 10 Jul, 2021, at 2:01 am, Leonard Kleinrock wrote: >=20 > No question that non-stationarity and instability are what we often = see in networks. And, non-stationarity and instability are both topics = that lead to very complex analytical problems in queueing theory. You = can find some results on the transient analysis in the queueing theory = literature (including the second volume of my Queueing Systems book), = but they are limited and hard. Nevertheless, the literature does contain = some works on transient analysis of queueing systems as applied to = network congestion control - again limited. On the other hand, as you = said, control theory addresses stability head on and does offer some = tools as well, but again, it is hairy.=20 I was just about to mention control theory. One basic characteristic of Poisson traffic is that it is inelastic, and = assumes there is no control feedback whatsoever. This means it can only = be a valid model when the following are both true: 1: The offered load is *below* the link capacity, for all links, = averaged over time. 2: A high degree of statistical multiplexing exists. If 1: is not true and the traffic is truly inelastic, then the queues = will inevitably fill up and congestion collapse will result, as shown = from ARPANET experience in the 1980s; the solution was to introduce = control feedback to the traffic, initially in the form of TCP Reno. If = 2: is not true then the traffic cannot be approximated as Poisson = arrivals, regardless of load relative to capacity, because the degree of = correlation is too high. Taking the iPhone introduction anecdote as an illustrative example, = measuring utilisation as very close to 100% is a clear warning sign that = the Poisson model was inappropriate, and a control-theory approach was = needed instead, to capture the feedback effects of congestion control. = The high degree of statistical multiplexing inherent to a major ISP = backhaul is irrelevant to that determination. Such a model would have found that the primary source of control = feedback was human users giving up in disgust. However, different = humans have different levels of tolerance and persistence, so this = feedback was not sufficient to reduce the load sufficiently to give the = majority of users a good service; instead, *all* users received a poor = service and many users received no usable service. Introducing a = technological control feedback, in the form of packet loss upon overflow = of correctly-sized queues, improved service for everyone. (BTW, DNS becomes significantly unreliable around 1-2 seconds RTT, due = to protocol timeouts, which is inherited by all applications that rely = on DNS lookups. Merely reducing the delays consistently below that = threshold would have improved perceived reliability markedly.) Conversely, when talking about the traffic on a single ISP subscriber's = last-mile link, the Poisson model has to be discarded due to criterion 2 = being false. The number of flows going to even a family household is = probably in the low dozens at best. A control-theory approach can also = work here. - Jonathan Morton=