[e2e] Are our models correct? Re: Just a very quick remark on system theory Re: Why don't we talk about segments/objects instaead of layers? Re: Lost Layer?
andrewmcgr at google.com
Fri Feb 21 09:10:56 PST 2014
You didn't read the paper I was pointing you at then. It models the
window, and all the timers, and includes provision for cwnd clamping and
various other optional features of TCP.
The limitations you point out are indeed things that happen in naiive
models. Not all models are that naiive.
On 21 February 2014 23:17, Detlef Bosau <detlef.bosau at web.de> wrote:
> In addition: The Laplace transform is not the problem. The problem is
> that the models are inadequate, no matter whether they are stated in
> differential equations (where particularly state variables are
> unbounded, hence packets may travel with tenfold the speed of the light
> or buffers must keep 1000 terabytes of data) or whether the same system
> is written in "frequency space".
> I know dozens of paper which pursue a control theoretic approach to
> Internet congestion control - I did not yet see even a single one which
> correctly modelled congestion drop. Most of the work even did not model
> the sliding window mechanism but was rate based.
> This is always funny as a game with numbers.
> But it does not describe the real world!
> E.g,, I'm actually sitting here in front of my PC and in a few seconds,
> I'm going to have finished this post and will click the "send" button in
> my MUA. As we all can imagine, my MUA will establish a TCP connection to
> my ISP's Remailer then and submit my mail for delivery.
> This TCP connection is most likely (I did not yet verify this in the
> Ubuntu kernel code) to be credit driven and not rate driven.
> And the relationship between credits/windows on the one hand and rates
> on the other is anything but trivial.
> I can't help it but I cannot split my brain and think about reality with
> the one half and about models with the other one.
> I don't want to bother anyone. But I think it is allowed to point out
> differences between the real world and models. Particularly as those
> differences are not rare in computer networking.
> Detlef Bosau
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Andrew McGregor | SRE | andrewmcgr at google.com | +61 4 1071 2221
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