[e2e] Congestion control as a hot topic in IETF

Detlef Bosau detlef.bosau at web.de
Sat Mar 9 12:17:52 PST 2013

Am 09.03.2013 11:28, schrieb Jon Crowcroft:
> no, its a basis for sound system design.

Up to now, and I'm looking for this for a decade now and asked many, 
many researchers, I don't know a formula which derives the possible 
throughput over a wireless channel depending on the SNR.

And no, Shannon-Hartley doesn't.
>   >>> eventually the signal (to background noise) is too low level to carry any info in any way distinguishable
>   >>>
>   >>>   >>Data corruption is a phenomenon which occurs at the receiver. The
>   >>>   >>problem is that the receiver cannot successfully rebuild a packet from
>   >>>   >>what he received. The air interface has no idea of which waves are
>   >>>   >>travelling along and whether they make any sense at all.
>   >>>
>   >>> you're confusing interference with other sources and misreading the honorably Dave Reed
>   >>
>   >>I'm quite sure that I'm not misreading Dave Reed.
>   >>
> you are conflating two (or three) completely different facets of
> wireless nets...

Just the opposite is true.

The problem is that we often observe _one_ phenomenon, e.g. packets are 
not ACKed in time, which can be the consequence of
- collision, i.e. a MAC problem,
- corruption, caused by noise, shading, interference etc.,
- congestion,
and believe that there is _the one single reason_ for the observed 
phenomenon and afterwards identify this by an educated guess or divine 

This is sometimes called "ratio ex post" and is one of the two most 
often made mistakes in science.
(The other one is to mistake coincidence for correlation and even more 
causal relation. Take this and "ratio ex post" - and I'm convinced you 
can falsify the vast majority of medical studies currently being 

When I understand Dave correctly, this is what Dave sometimes calls 
"confirmation bias".

>   >>> secondly, you are ignoring absorption (e.g. by water vapour which gets a little bit hotter)
>   >>> and also _self_ interferance (aka Ricean fading) and scattering (rayleigh fading)
>   >>>
>   >>
>   >>So, a model which correctly describes wireless channels is that flexible
>   >>that it fits anything - and has no use at all.
> incorrect - it is a sound basis for design.

No. A model with dozens of variables, hardly any of which can be 
estimated in a sound way doesn't prove anything.

A typical example is the loss differentiation debate.

There are literally hundreds of papers around which try to determine 
whether a packet loss is due to corruption or congestion.

Take any of them - and look for "ratio ex post" - I don't know at least 
one single paper which holds.

Detlef Bosau
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