Re: [Cerowrt-devel] [Make-wifi-fast] [tsvwg] Comments on draft-szigeti-tsvwg-ieee-802-11e

[Jonathan Morton <chromatix99@gmail.com>]

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> I think that is achievable, *even if there is a WiFi network in the
middle*, by thinking about the fact that the shared airwaves in a WiFi
network behaves like a single link, so all the queues on individual
stations are really *one queue*, and that the optimal behavior of that link
will be achieved if there is at most one packet queued at a time.

I agree that queues should be kept short in general. However I don't think
single packet queues are achievable in the general case.

The general case includes Wi-Fi networks, whose TXOP overhead is so
ruinously heavy that sending single MTU sized packets is inefficient.
Aggregating multiple packets into one TXOP requires those several packets
to be present in the buffer at that moment.

The general case includes links which vary in throughput frequently,
perhaps on shorter timescales than an RTT, so either packets must be
buffered or capacity is left unused. This also happens to include Wi-Fi,
but could easily include a standard wired link whose competing load varies.

The endpoints do not have and do not receive sufficient information in
sufficient time to reliably make packets arrive at nodes just in time to be
transmitted. Not even with ECN, not even with the wet dreams of the DCTCP
folks, and not even with ELR (though ELR should be able to make it happen
under steady conditions, there are still transient conditions in the
general case).

- Jonathan Morton

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