[e2e] Why do we need congestion control?

Detlef Bosau detlef.bosau at web.de
Wed Apr 10 06:14:15 PDT 2013

Am 06.03.2013 19:19, schrieb Richard G. Clegg:
> On 06/03/13 15:02, Jon Crowcroft wrote:
>> ok - i see your point - this is true if your sources have a peak rate 
>> they can send at
>> this could be the line rate of their uplink  -
>> that would be embarrasingly bad
>> (see keshav's followup on escalating costs of coding)
>> or the rate they can get data off disk (which could be as bad, but 
>> might be lower)
>> or an application specific rate (e.g. streamed video) for which 
>> you're suggestion is
>> quite reasonable...
> Apologies if this has been mentioned already -- the "blast it out at 
> full whack and code against loss" strategy is explored in
> Is the ''Law of the Jungle'' Sustainable for the Internet? from 
> Infocom 2009.
> Nice maths in that paper actually -- I was lucky enough to see them 
> present it.  The conclusions are interesting.
> http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5061903

I'm generally reluctant to "nice maths" in this discussion - quite a lot 
of the "mathwork" is an impressive envelope with hardly a letter inside.

I think we should reconsider our goals here in order not to give another 
confirmation to the well known quote: "Having lost sight of our goals, 
we endoubled our efforts".

In a PM, Matt Mathis mentioned that we should be particularly careful to 
use maths from "statistics" and "stochastics" - when actually there is 
hardly any stochastic behaviour is present. Matt pointed out that in 
many TCP scenarios, the behaviour of the net is mainly deterministic - 
and hence some of our statistical apparatus simply does not apply here, 
e.g. Little's Law.  And  I fear, the same holds true for erasure codes 
and statistic rationales for "fair goodput reduction".

Let me sketch a very simple scenario here.

Think of four nodes, e.g. PC, attached to a simple coax Ethernet.

                     PC 1                  PC 2             PC 3                 PC 4
                       |                     |                |                    |


Think of two bidirectional TCP flows here.

One betwenn PC 1 and PC 3, the other between PC 2 and PC 4.

And now allow me to ask some questions.

Q1: What do we want to achieve at all in this scenario? With particular 
respect to the categories
       - goodput
       - throughput
       - congestion avoidance
       - fairness?

Q2: Do we need VJCC in this scenario?

Q3: Which of the goals stated in response to Q 1 are achieved?

Q4: If goals are achieved, refer to Q3, what is the particular 
contribution of VJCC here?

This scenario is quite easy - and when I answered this questions for 
myself, it became clear that I simply asked the wrong question when I 
asked how many flows would fit into the Internet.

To some degree, VJCC is a stroke of genius - while being a kludge at the 
same time.

So, I would like to ask: What are our goals? What do we achieve? Did we 
achieve our goals? And is that what we achieve identical to our 
intentions in the first?

Detlef Bosau
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