[e2e] Why do we need congestion control?

Detlef Bosau detlef.bosau at web.de
Thu Mar 7 01:10:29 PST 2013

Am 06.03.2013 13:29, schrieb shun cai:
> As discussed in chapter 1 of your PhD thesis,  when network is 
> congested, retransmission dominate the traffic and effective 
> throughput diminshes rapidly, leading to a deteriorating situation. 
> This can be illustrated in the well known figure with two turning 
> points Knee and Cliff.
>      I wonder, however, does the situation the same if rateless 
> erasure code (say fountain codes) is used?  As with erasure code, no 
> ACK and retransmission is needed except when the whole file is 
> completed. So even heavy loaded, the network is still busy with 
> effective data packet, right?  Although queueing delay will increase, 
> I believe that  the network throughput  will not  suffer the plunge as 
> un-coded network.

Very short answer, because I'm not quite familiar with erasure codes.

However, we're moving around overhead here. In the end, it doesn't 
matter whether your network suffers from retransmission overload and the 
goodput as seen by the user runs to zero or of the network suffers from 
"unsatisfactory code rates" where 1 packet is encoded into 
packets so that it can be recovered.

To the point: Flow control and congestion control are supposed to 
/solve/ a problem, not to /worsen/ it.

And from what I read in some off list discussions which developed from 
this thread here, there are quite some people who claim that VJCC is 
quite a kludge. At the moment, I have no clear position to that claim, 
though in some of the PMs I got, I missed the necessary respect towards 
Van's work. And quite some mails argued more historically than 
scientifically which I personally find annoying.

Sliding window & Co. were invented to fully utilize network capacities. 
You cannot over utilize them. Or, wrt the 12 basic network truths: No 
matter how you push or pull, no matter what priority your project may 
have: You cannot increase the speed of light.

Detlef Bosau
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