[e2e] Reacting to corruption based loss

[Detlef Bosau]

mbgreen at dsl.cis.upenn.edu wrote:
> 
>    Tue, 28 Jun 2005 21:34:29 +0200
>    Detlef Bosau <detlef.bosau at web.de>
> 
>    For the traditional AIMD scheme, it´s quite easy to see that AIMD
>    sequences starting from different initial vaules will end up in the
>    same sawtooth as long as alpha and beta are equally chosen for all
>    competing flows.
> 
> [Digression not directly relevant to your main argument:]
> 
> I don't think this is "easy to see" if the RTTs of the competing flows
> are different --- I don't even think it is necessarily true that they
> end up in the same sawtooth.  In simulations it is easy to have many
> flows with exactly the same RTT.  In the real world this is less
> common.


You´re correct. However, for competing flows, I assume identical RTT. Of
course, this is a simplification. And it depends on how you define
"competing flows".

If the senders of the competing flows reside on node A and the receivers
reside on node B and the paths taken by the packets is identical, these
"competing flows" have the same RTT.

However, if you consider the typical dummbell scenario, sources at S1,
S2, receivers at R1 and R2 respecitively, the two "backbone routers"
being B1, B2,
then e B1-B2 may be the bottleneck, the flows S1-R1 and S2-R2 my be
competing and the RTT of both may differ greatly.

There are funny studies around how competing flows behave with different
values for alpha and beta, and I realy think they be can be used for a
very nice screensaver. I´m still looking for an appropriate choice of
colours ;-)

In fact, it is essentially the old problem with "theory" and "practice". 

However, what are the alternatives? Basically two ones. We either do a
centralized congestion control or do a decentralized congestion control.

If you have a centralized congestion control, which not needs to be
controlled by a physical single node but some centralized mechanism,
reservation scheme etc., then we all can refer to Srinivasan Keshav´s
PhD dissertation and anything will be fine. 

If you ave a decentralized congestion control scheme, it´s a little bit
religious. (Isiah 53,6; NIV) "We all, like sheep, have gone astray, each
of us has turned on his own way." (Hm, which was the basis for Handel´s
Messiah? King James? Sounds nicer :-) I just put the CD in my player :-)
Handels "Fugato" as an allegory for flows, competing for the audience´s
attention :-) Even different network load situations are modeled by the
music´s dynamics :-)

The situation is similar to Alex´ criticism on the dumbbell scenario
some time ago: What is an adequate system model? Not only for the ivory
tower but for engineers as well?

I don´t know - but who am I?

So, I use what I have. Dumbbell, AIMD, strange system models. E.g.:
Competing flows have similar RTT. (Or better: _Comparable_ flows have
similar RTT? What do we really want, if flows are not somewhat
"comparable".)

And honestly: I´m much too stupid to even understand AIMD.

DB



-- 
Detlef Bosau
Galileistrasse 30
70565 Stuttgart
Mail: detlef.bosau at web.de
Web: http://www.detlef-bosau.de
Mobile: +49 172 681 9937