> On 12 Apr, 2019, at 2:56 am, Holland, Jake <[email protected]> wrote: > > 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 _______________________________________________ Bloat mailing list [email protected] https://lists.bufferbloat.net/listinfo/bloat
