fully congested: zero throughput, maximum (infinite) delay.... v On Wed, Sep 29, 2021 at 6:36 AM Jonathan Morton wrote: > > On 29 Sep, 2021, at 1:15 am, David P. Reed wrote: > > > > Now, it is important as hell to avoid bullshit research programs that > try to "optimize" ustilization of link capacity at 100%. Those research > programs focus on the absolute wrong measure - a proxy for "network capital > cost" that is in fact the wrong measure of any real network operator's cost > structure. The cost of media (wires, airtime, ...) is a tiny fraction of > most network operations' cost in any real business or institution. We don't > optimize highways by maximizing the number of cars on every stretch of > highway, for obvious reasons, but also for non-obvious reasons. > > I think it is important to distinguish between core/access networks and > last-mile links. The technical distinction is in the level of statistical > multiplexing - high in the former, low in the latter. The cost structure > to the relevant user is also significantly different. > > I agree with your analysis when it comes to core/access networks with a > high degree of statistical multiplexing. These networks should be built > with enough capacity to service their expected load. When the actual load > exceeds installed capacity for whatever reason, keeping latency low > maintains network stability and, with a reasonable AQM, should not result > in appreciably reduced goodput in practice. > > The relevant user's costs are primarily in the hardware installed at each > end of the link (hence minimising complexity in this high-speed hardware is > often seen as an important goal), and possibly in the actual volume of > traffic transferred, not in the raw capacity of the medium. All the same, > if the medium were cheap, why not just install more of it, rather than > spending big on the hardware at each end? There's probably a good > explanation for this that I'm not quite aware of. Perhaps it has to do > with capital versus operational costs. > > On a last-mile link, the relevant user is a member of the household that > the link leads to. He is rather likely to be *very* interested in getting > the most goodput out of the capacity available to him, on those occasions > when he happens to have a heavy workload for it. He's just bought a game > on Steam, for example, and wants to minimise the time spent waiting for > multiple gigabytes to download before he can enjoy his purchase. Assuming > his ISP and the Steam CDN have built their networks wisely, his last-mile > link will be the bottleneck for this task - and optimising goodput over it > becomes *more* important the lower the link capacity is. > > A lot of people, for one reason or another, still have links below 50Mbps, > and sometimes *much* less than that. It's worth reminding the gigabit > fibre crowd of that, once in a while. > > But he may not the only member of the household interested in this > particular link. My landlord, for example, may commonly have his wife, > sister, mother, and four children at home at any given time, depending on > the time of year. Some of the things they wish to do may be > latency-sensitive, and they are also likely to be annoyed if > throughput-sensitive tasks are unreasonably impaired. So the goodput of > the Steam download is not the only metric of relevance, taken > holistically. And it is certainly not correct to maximise utilisation of > the link, as you can "utilise" the link with a whole lot of useless junk, > yet make no progress whatsoever. > > Maximising an overall measure of network power, however, probably *does* > make sense - in both contexts. The method of doing so is naturally > different in each context: > > 1: In core/access networks, ensuring that demand is always met by capacity > maximises useful throughput and minimises latency. This is the natural > optimum for network power. > > 2: It is reasonable to assume that installing more capacity has an > associated cost, which may exert downward pressure on capacity. In > core/access networks where demand exceeds capacity, throughput is fixed at > capacity, and network power is maximised by minimising delays. This > assumes that no individual traffic's throughput is unreasonably impaired, > compared to others, in the process; the "linear product-based fairness > index" can be used to detect this: > > > https://en.wikipedia.org/wiki/Fairness_measure#:~:text=Product-based%20Fairness%20Indices > > 3: In a last-mile link, network power is maximised by maximising the > goodput of useful applications, ensuring that all applications have a > "fair" share of available capacity (for some reasonable definition of > "fair"), and keeping latency as low as reasonably practical while doing > so. This is likely to be associated with high link utilisation when demand > is heavy. > > > Operating at fully congested state - or designing TCP to essentially > come close to DDoS behaviour on a bottleneck to get a publishable paper - > is missing the point. > > When writing a statement like that, it's probably important to indicate > what a "fully congested state" actually means. Some might take it to mean > merely 100% link utilisation, which could actually be part of an optimal > network power solution. From context, I assume you actually mean that the > queues are driven to maximum depth and to the point of overflow - or beyond. > > - Jonathan Morton > _______________________________________________ > Ecn-sane mailing list > Ecn-sane@lists.bufferbloat.net > https://lists.bufferbloat.net/listinfo/ecn-sane > -- Please send any postal/overnight deliveries to: Vint Cerf 1435 Woodhurst Blvd McLean, VA 22102 703-448-0965 until further notice