Re: [tcpm] CUBIC rfc8312bis / WGLC Issue 3

[Neal Cardwell <[e-mail]>]

On Tue, Jun 14, 2022 at 11:52 AM Markku Kojo <kojo=
[e-mail]> wrote:

> Hi Neal,
>
> thank for your proposal. Note that I moved your proposal here under the
> thread on Issue 3 which it is related to (see below).
>
> On Tue, 14 Jun 2022, Markku Kojo wrote:
>
> > Hi all,
> >
> > this thread starts the discussion on the issue 3: the incorrect way of
> > setting alpha to 1 when in the fast convergence mode within the
> Reno-friendly
> > region (Issue 3 a) and the issue of setting Wmax for a congestion event
> > arriving when in slow start (Issue 3 b):
> >
> >
> > 3 a) The rule for changing alpha to 1 when Wmax is reached in the
> >     Reno-friendly region is the correct thing to do during the normal
> >     steady state. However, it is incorrect action to take when in the
> >     fast convergence mode within the Reno-friendly region because it
> >     would act just *opposite* to what CUBIC should do when in the fast
> >     convergence mode; instead of slowing down the increase rate during
> >     congestion avoidance it actually accelerates because alpha becomes
> >     increased to 1 earlier than when not in the fast convergence mode.
> >     This seems an obvious mistake with the quite recent modifications
> >     to the rfc8312bis.
>
> On Mon, 6 Jun 2022, Neal Cardwell wrote:
>
> > Markku, thank you for your clear and comprehensive write-up.
> >
> > Regarding this particular, somewhat easier point:
> >
> [snip]
> > I agree this seems like a clear and straightforward thing to fix.
> > Indeed, my current draft Linux TCP CUBIC patch series for this
> > "changing alpha to 1" mechanism already uses your suggested approach,
> > and it seems to behave sensibly.
> >
> > I have posted a proposed pull request to try to encode this in the
> > draft:
> >   https://github.com/NTAP/rfc8312bis/pull/146
> >
> > Please take a look and see what you think. Thanks!
> >
> > neal
>
> This seems to correct the issue, thanks. It would be useful to introduce
> prior_cwnd in section 4.1.2 as already discussed in the github pull
> request. Also needs text specifying the initialization in other
> case than "congestion event". This also seemed to attention in
> github.
>

Thanks. For those who are interested, I just posted a new patch in the pull
request, to try to address the most recent comments from the review. Please
take a look:

https://github.com/NTAP/rfc8312bis/pull/146

thanks,
neal