I already notified the authors.
Hesham
On 8/12/2022 7:21 AM, Livingood, Jason via Starlink wrote:
IMO this is all good feedback for the authors to have. I don’t know
them but will send them a note that this list may have feedback on the
paper as well as for future research*.
JL
* I will also note that this research is the kind of thing a grant
program I lead at work might be interested in for 2023.
*From: *Starlink <[email protected]> on behalf of
"David P. Reed via Starlink" <[email protected]>
*Reply-To: *"David P. Reed" <[email protected]>
*Date: *Thursday, August 11, 2022 at 15:34
*To: *"[email protected]" <[email protected]>
*Subject: *Re: [Starlink] SIGCOMM MIT paper: Starvation in e2e
congestion control
On Thursday, August 11, 2022 10:29am,
[email protected] said:
> From: Hesham ElBakoury <[email protected]>
> To: "David P. Reed" <[email protected]>,
> [email protected]
> Subject: Re: [Starlink] SIGCOMM MIT paper: Starvation in e2e
> congestion control
>
> Hi David,
>
> I think someone such as Professor Hari who got many awards including the
> sigcomm 2021 life-achievement award or his student Venkat need to be
> educated on Fair Queuing. There are many publications and text books
> which describe FQ. The results of this paper is for network paths that
> do not use FQ or ECN. Venkat/Hari can provide more details.
I would think that he knows about FQ in AQM, too. He should.
My point is that this paper, which talks about *starvation*, doesn't
mention FQ at all, even though it is well known to mitigate
"starvation effects" - it was invented to solve exactly that problem!
I'd suggest at minimum that the paper should point out that it
*excludes* FQ from consideration. And if possible, explain why it was
excluded...
I can think of reasons for excluding FQ in the specific paper, but
shouldn't the title and abstract say it applies only narrowly:
Proposed revised title: "Starvation in e2e congestion control if FQ is
excluded within the network"
Particularly since the paper makes *broad* generalizations - the only
2-out-of-3 argument is stated as if it applies to ALL congestion control.
>
> For the CAP theorem, do you think I can get C,A,P, if this is what I
> need ? if this is the case, then this theorem is wrong or has limited
> applicability, correct ?
It has limited applicability for sure. Yet, it has become fashionable
to act as if it is a completely general truth.
The CAP theorem, in the limited space of its assumptions, appears to
be correct. But because it is so easily trivialized, as encouraged by
the "you can have any two of C A an P, but not 3" without any
qualification, problems with the definitions of the words C A and P -
serious problems indeed that matter to a first order in real
distributed systems - it is often used to derive "impossibility".
I'll give you another example of a serious misuse of a theorem outside
its range of applicability:
Shannon proved a channel capacity theorem: C = W log(S / N). The proof
is mathematical, and correct.
But hiding in the assumptions are some very strong and rarely
applicable conditions. It was a very useful result in founding
information theory.
But... it is now called "Shannon's Law" and asserted to be true and
applicable to ALL communications systems.
This turns out not to be correct. And it is hardly ever correct in
practice. An example of non-correct application turns out to be when
multiple transmissions of electromagnetic waves occur at the same
time. EE practice is to treat "all other signals" as Gaussian Noise.
They are not - they never are.
So, later information theorists discovered that where there are
multiple signals received by a single receiving antenna, and only a
little noise (usually from the RF Front End of the receiver, not the
environment) the Slepian-Wolf capacity theorem applies C = W
log(\sum(S[i]. i=1,N) /W). That's a LOT more capacity than Shannon's
Law predicts, especially in narrowband signalling.
And noise itself is actually "measurement error" at the receiver,
which is rarely Gaussian, in fact it really is quite predictable
and/or removable.
So a theorem can be correct (based on its assumptions) and
inapplicable in most cases, because of its narrowness.
And this is why a limited (not very general) theorem of the 2-out-of-3
form is dangerous.
As for the CAP theorem, my Ph.D. thesis was in this very area -
multi-copy consistency in distributed data systems. That was in 1978,
45 years ago. I've followed that work since the time - both the
pragmatics and the theory. I think I fully understand both the context
and how the axioms chosen by Brewer simplify reality in radical ways.
C A and P are not booleans or binary quantities. So in a real sense
the CAP theorem is always inapplicable. But worse, the proof structure
falls apart as a mathematical proof if you assume any metric for C A
or P that isn't homomorphic to boolean algebraic quantities.
And worse, there is no standard measure of C A and P that captures
what matters on any dimension.
So, aside from an intuition that maybe C, A, and P trade off in some
way in some model of reality, the theorem is meaningless, and not very
useful.
I hope this helps understand what's behind my comments.
At core, a referee ought to have asked - how is this conclusion
justified as a general conclusion about ALL e2e congestion control in
all networks, when it is only shown in a narrow, unrealistic case?
In my nearly 50 years of publishing in the computing and
communications world, I've done a LOT of refereeing, and served on
program committees as well. The obligation of a referee is to look at
the conclusions of the paper, in the context of the state of the
science, and figure out if the conclusion is supported by the paper's
contents.
I'm not sure why this didn't happen here.
David
PS: compared to the post-publication comments to my first CS
publication, in a letter to my mentor from Edsgar Dijkstra, I think
I'm being gentle. It's motivated by getting the science right.
>
> Thanks
>
> Hesham
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