[Bloat] *under*bloated networks

[Neal Cardwell]

On Mon, Nov 22, 2021 at 10:35 PM Dave Taht <dave.taht at gmail.com> wrote:

> In the last two weeks I have found two dramatically underbuffered Gbit
> fiber networks.
>
> This one appears to have about a 400 full size packet uplink buffer
> (5ms)[1]
>
> https://imgur.com/a/Bm9hdNf
>
> It was pretty remarkable to see how well multiple tcp flows still
> achieved close to the full rate with such a small fixed size queue,
> eventually.
>
> A single bbr flow can't crack 150mbits: https://imgur.com/a/DpydL5K.
>

Thanks, Dave. The single-flow BBR upload case is interesting.

I took a look at the packet traces in the later thread:
  https://www.reddit.com/r/eero/comments/qxbkcl/66_is_out/hltlep0/

For the single-flow BBR case it seems that....

(1) the BBR(v1) flow is running into a 300 Mbps bottleneck rate (visible in
the slope of the green ACK line in the zoomed-in trace, attached).

(2) the BBR(v1) flow is achieving an average rate a bit above 150 Mbps
because it repeatedly runs into receive window limits (the yellow line in
the zoomed-out trace, attached). The frequent receive window limits mean
that the flow spends a lot of time unable to send anything, thus leading to
lower average throughput.

cheers,
neal
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