On 6/12/18, 8:39 AM, "Bloat on behalf of Geoff Huston"
<bloat-boun...@lists.bufferbloat.net on behalf of g...@apnic.net> wrote:
>
>I do agree that bbr treats aqm drops as "noise", not backpressure. And
>bbr scares me.
>I look forward very much to bbr one day soon doing some sort of sane,
>conservative, response to ecn marks.
I’m not sure that I understand this comment.
Part of the pressure going on here is the issue of whether the endpoints
can and should trust the signals and.or manipulation that they get from the
network infrastructure. BBR is using a different form of feedback to control
its send rate. Essentially BBR is taking a delay variance measurement 1 / 8 of
the time to adjust its internal model of the end-to-end delay bandwidth product
(every 8th RTT). ECN provides a constant information flow, and this certainly
matches the requirements of loss-based TCP, where every ACK contributes to the
TCP flow dynamic, but it does not seem to me to be a good match to BBR’s
requirements.
The idea with BBR is to drive the network path such at the internal routers
are sitting just at the initial onset of queuing. In theory ECN will not
trigger at the onset of queuing, but will trigger later in the cycle of queue
buildup.
[GW] That's "Classic" ECN. The proposed new version
https://tools.ietf.org/html/draft-ietf-tsvwg-ecn-l4s-id-02 starts marking right
at the onset of queuing, which would allow a congestion controller to much more
accurately stay right at the optimal congestion window, without all of the
cycling phenomena and inferences that BBR uses.
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