Re: [Ecn-sane] paper idea: praising smaller packets

[Vint Cerf <vint@google.com>]

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fully congested: zero throughput, maximum (infinite) delay....

v


On Wed, Sep 29, 2021 at 6:36 AM Jonathan Morton <chromatix99@gmail.com>
wrote:

> > On 29 Sep, 2021, at 1:15 am, David P. Reed <dpreed@deepplum.com> 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
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