[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?

Detlef Bosau detlef.bosau at web.de
Fri Feb 21 11:05:40 PST 2014

Am 21.02.2014 18:10, schrieb Andrew Mcgregor:
> 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.

Shame on me. However, I did not expect that much news from a
"thermodramatic" model ;-)

However, I'm currently reading it.

On page 2 I hang on:
> For queues driven by point arrival processes the  fluid-fl ow
> approximation is valid when the
> mean buffer length and the buffer size are large

Hm. How do we model arrival processes? (When I have a glance at the
paper, I read about Markov- and Poisson-Processes.)
In addition, large buffer lengths and sizes sometimes are not our
primary intention.

Particularly, my comments to Saverio Masolo apply for fluid flow model
as well. Particularly they do not cover some (unwanted but often not
fully avoidable) effects like unused time slots, in other words: Fluid
flow models have an inherent work conservative view. While links in
practical networks may stay idle.
> On 21 February 2014 23:17, Detlef Bosau <detlef.bosau at web.de
> <mailto: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
>     Galileistraße 30
>     70565 Stuttgart                            Tel.:   +49 711 5208031
>     <tel:%2B49%20711%205208031>
>                                                mobile: +49 172 6819937
>     <tel:%2B49%20172%206819937>
>                                                skype:     detlef.bosau
>                                                ICQ:          566129673
>     detlef.bosau at web.de <mailto:detlef.bosau at web.de>                  
>       http://www.detlef-bosau.de
> -- 
> Andrew McGregor | SRE | andrewmcgr at google.com
> <mailto:andrewmcgr at google.com> | +61 4 1071 2221

Detlef Bosau
Galileistraße 30   
70565 Stuttgart                            Tel.:   +49 711 5208031
                                           mobile: +49 172 6819937
                                           skype:     detlef.bosau
                                           ICQ:          566129673
detlef.bosau at web.de                     http://www.detlef-bosau.de

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