[e2e] admission control vs congestion control

[Detlef Bosau]

Dah Ming Chiu wrote:
> Hi Lisong,
> 
> This is an interesting question!  I believe your intuition is quite 
> right. In fact
> I had the same intuition. So the question is how do you show it is true.
> This question cannot be easily answered using the currently widely accepted
> models for studying congestion controls.
>

I tend to disagree here.

 From my point of view, the objective of admission control and 
congestion control is bascially different.

Perhaps, I´m somewhat disappointed by experiences I made myself during 
the last about seven years. However, _basically_, and I´m ready to see 
contradiction here, bringing multimedia to the Internet is reinventing 
the wheel.

There are dozens of PhD-theses which provide mechanisms for admission 
control and discuss how to provide for QoS and so on.

Excuse me: What´s the very difference between these mechanisms and good 
ol´ TDM as it is used in line switched networks with excellent results 
for decacdes now? (I know, this question is provoking and it´s intended 
to be.) Most of the proposed protocols and mechanism do nothing else 
them implement Telco protocols and mechanisms using IP packets,
including replacing best effort store´n forwared nodes with switches 
using schedulers. That´s nothing else than a digital Strowger Gear, 
which is known to work fine for decades. We should leave it for the 
Kansas City cemetery, perhaps at good ol´ Almons tombstone ;-)


As a concrete example consider wide area mobile networks. Why do we 
discuss "multimedia over IP" in this context for no reasons, when 
actually availabe line switching mechanisms marvelously serve this very 
purpose?

O.k., perhaps there _are_ resons to consider multimedia over IP, but 
then we should agree upon a concrete system model which is to be 
discussed here.

So, _admission_ _control_ serves the purpose to maintain QoS agreements 
and to allocate ressources to streams applying for them.

_Congestion_ _control_ prevents the path capacity from being exceeded in 
a best effort environment.

In a very concise manner we can say: Admission control is a 
_hierarchical_ concept whereas congestion control is a _heterarchical_ 
concept.

Often we follow an hybrid approach: We reserve parts of a network´s 
capacity for hierarchical purposes and QoS whereas the rest of the 
network is left to heterarchical best effort communication.

Of course, we can define arbitrary utility functions to have the 
ressource partitioning follow a set of rules or policies. One of these 
policies could be to make a real time rate controlled flow "TCP friendly".

However, all of these policies are basically a matter of definition.

And definitions are generally that difficult to prove ;-)

Detlef


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Detlef Bosau
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