[e2e] Why do we need congestion control?

Detlef Bosau detlef.bosau at web.de
Thu Mar 7 04:22:02 PST 2013

Am 07.03.2013 12:00, schrieb Emmanuel Lochin:
> Both are complementary for me : you are faster with ARQ over small RTT 
> and faster with erasure coding over long-delay link.

So, the decision whether to do ARQ or to use erasure codes depends on 
what you want to achieve.

A typical trade off.

This is, however, different from congestion control.

Congestion control has basically two - contradictory - goals:
1. Fully utilize a channel
2. Not over utilize a channel.

The problem is that we, in general, don't know a channel's capacity.

And in my view, the problem with VJCC is, that it doen't scale with
- networks becoming larger (in terms of e.g. RTT) and
- link throughputs becoming larger (actually, we talk about terabit links)
and that VJCC cannot really cope with RTT and path capacity becoming non 

Another problem is, spoken in the analogy of control theory, that VJCC 
attemps to control the filling levels in a multi tank system, while we 
only observe whether or not "liquid is dropped somewhere" and the only 
possible influence is to control a single tap.

And as we notice that this will not work, we invent sophisticated 
algorithms for controlling the tap or - which is quite modern for quite 
some time now - additional taps where liquid my drop. So, liquid drops 
not because of an overloaded system but because of an "active dropping 
management" - this sounds more scientific.

Detlef Bosau
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