netdev - Re: [PATCH net-next] tcp: tracking packets with CE marks in BW rate sample

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Date:   Sun, 26 Sep 2021 15:31:26 -0700
From:   Dave Taht <dave.taht@...il.com>
To:     Yuchung Cheng <ycheng@...gle.com>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        Luke Hsiao <luke.w.hsiao@...il.com>,
        David Miller <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Lawrence Brakmo <brakmo@...com>,
        Neal Cardwell <ncardwell@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Luke Hsiao <lukehsiao@...gle.com>
Subject: Re: [PATCH net-next] tcp: tracking packets with CE marks in BW rate sample

On Fri, Sep 24, 2021 at 4:43 PM Yuchung Cheng <ycheng@...gle.com> wrote:
>
> On Fri, Sep 24, 2021 at 1:20 PM Jakub Kicinski <kuba@...nel.org> wrote:
> >
> > On Thu, 23 Sep 2021 21:17:07 +0000 Luke Hsiao wrote:
> > > From: Yuchung Cheng <ycheng@...gle.com>
> > >
> > > In order to track CE marks per rate sample (one round trip), TCP needs a
> > > per-skb header field to record the tp->delivered_ce count when the skb
> > > was sent. To make space, we replace the "last_in_flight" field which is
> > > used exclusively for NV congestion control. The stat needed by NV can be
> > > alternatively approximated by existing stats tcp_sock delivered and
> > > mss_cache.
> > >
> > > This patch counts the number of packets delivered which have CE marks in
> > > the rate sample, using similar approach of delivery accounting.
> >
> > Is this expected to be used from BPF CC? I don't see a user..
> Great question. Yes the commit message could be more clear that this
> intends for both ebpf-CC or other third party module that use ECN. For
> example bbr2 uses it heavily (bbr2 upstream WIP). This feature is
> useful for congestion control research which many use ECN as core
> signals now.

I am glad a common API to this is emerging. RFC3168 compliant aqms
(fq_codel, pie, cake) can also emit multiple CE marks per RTT, and
the defined response to them is inadequate. Glad you found space!

There is also this ecn problem outstanding elsewhere, that I hope
bbrv2 has looked into?

https://www.bobbriscoe.net/projects/latency/sub-mss-w.pdf

Otherwise:

Reviewed-by: Dave Taht <dave.taht@...il.com>


--
Fixing Starlink's Latencies: https://www.youtube.com/watch?v=c9gLo6Xrwgw

Dave Täht CEO, TekLibre, LLC