[e2e] admission control vs congestion control
detlef.bosau at web.de
Mon Apr 17 15:18:51 PDT 2006
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
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
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
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_
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 ;-)
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