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[83.245.240.147]) by smtp.gmail.com with ESMTPSA id c5sm8738023ljd.18.2019.04.11.17.37.12 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 11 Apr 2019 17:37:13 -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: Date: Fri, 12 Apr 2019 03:37:12 +0300 Cc: Mikael Abrahamsson , "bloat@lists.bufferbloat.net" Content-Transfer-Encoding: quoted-printable Message-Id: <79BAAF74-7E32-4361-AE5C-A0008EF8BE12@gmail.com> References: <8301D679-FB99-4CB8-8A58-E83B3BA0007D@gmail.com> <6889A853-52F2-4554-9450-F59F9CB6B91A@gmail.com> To: "Holland, Jake" X-Mailer: Apple Mail (2.3445.9.1) Subject: Re: [Bloat] datapoint from one vendor regarding bloat 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: Fri, 12 Apr 2019 00:37:15 -0000 > On 12 Apr, 2019, at 2:56 am, Holland, Jake = wrote: >=20 > But in practice do you expect link speed changes to be a major issue? For wireline, consider ADSL2+. Maximum downstream link speed is 24Mbps, = impaired somewhat by ATM framing but let's ignore that for now. A basic = "poverty package" might limit to 4Mbps; already a 6:1 ratio. In rural = areas the "last mile" copper may be so long and noisy for certain = individual subscribers that only 128Kbps is available; this is now a = 192:1 ratio, turning 10ms into almost 2 seconds if uncompensated from = the headline 24Mbps rate. Mind you, 10ms is too short to get even a = single packet through at 128Kbps, so you'd need to put in a failsafe. That's on wireline, where link speed changes are relatively infrequent = and usually minor, so it's easy to signal changes back to some discrete = policer box (usually called a BRAS in an ADSL context). That may be = what you have in mind. One could, and should, also consider wireless technologies. A given = handset on a given SIM card may expect 100Mbps LTE under ideal = conditions, in a major city during the small hours, but might only have = a dodgy EDGE signal on a desolate hilltop a few hours later. (Here in = Finland, cell coverage was greatly improved in rural areas by cascading = old 2G equipment from urban areas that received 3G upgrades, so this is = not at all uncommon.) In general, wireless links change rate rapidly = and unpredictably in reaction to propagation conditions as the handset = moves (or, for fixed stations, as the weather changes), and the ratio of = possible link rates is even more severe than the ADSL example above. Often a "poverty package" is implemented through a shaper rather than a = policer, backed by a dumb FIFO on which no right-sizing has been = considered (even though the link rate is obviously known in advance). = On one of these, I have personally experienced 40+ seconds of delay, = rendering the connection useless for other purposes while any sort of = sustained download was in progress. In fact, that's one of the = incidents which got me seriously interested in congestion control; at = the time, I hacked the Linux TCP stack to right-size the receive window, = and directed most of my traffic through a proxy running on that machine. = This was sufficient to restore some usability. I find it notable that ISPs mostly consider only policers for congestion = signalling, and rarely deploy even these to all the places where = congestion may reasonably occur. - Jonathan Morton