[e2e] VJCC vs. Keshav

Detlef Bosau detlef.bosau at web.de
Thu Apr 18 02:20:02 PDT 2013

Am 17.04.2013 22:54, schrieb Jon Crowcroft:
> more history
> 1. keshav wrote his own simulator (a variant maybe of the mit x sim
> system)
so did I, although the thing is not published. And of course, I did not 
reinvent the wheel but used concepts known from the ns 2.

> 2. other people verified fq results in other simulators
> 3. some people have implemented lots of variants of fq and packet
> probe pair/trains as well
> maybe you could read the source materials properly,
> rather than just snippets of this maillist...

Jon, I did. So, there is no need to blame me here.

However: When you look at the source material, particularly the program 
sources of simulators, you become aware of quite some simplifications 
frequently made. That's why I made a critical remark on "packet pair" 
techniques and the ns2.

Simply and frankly spoken: Proving packet pair techniques with a 
simulator proves absolutely nothing. Because the program code in the 
simulator is nothing else than the system model used in the papers.

E.g. in the ns2 you have some kind of "bandwidth" and a serialization 
delay which packet size / bandwidth. Fine.

Now, you send a number of packets, this needs a (cumulative) 
serialization delay = Sum over all packets (packet length/Bandwidth)
and this is divided by the cumulative packet length - and renders what?


And because each and any simulator uses the same approach, they prove 
each other.

Unfortunately, these nasty CSMA/CD Ethernets or these disgusting CSMA/CA 
WLANs or these horrible mobile networks dare not always to behave 
simulator compliant....

Simulators are nice. However they are, to a certain degree, GIGO 
systems. (GIGO: Garbage In, Garbage Out.)

This is not to criticize simulators, but to be aware of their limitations.

How do we simulate CSMA/CD Ethernets with all gory details? I really 
don't know if this is possible at all.

Keeping this in mind, I have no problems with simulators. It is 
necessary, however, to keep in mind possible limitations. And the 
simplifications, whe made.


Detlef Bosau
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