[e2e] Why do we need congestion control?

Detlef Bosau detlef.bosau at web.de
Wed Apr 3 03:38:55 PDT 2013

Am 03.04.2013 11:41, schrieb Jon Crowcroft:
> lets do a simple thought experiment lets say you have two users 
> sharing at least one router's output port/link as part of their path, 
> and both users are greedy lets say you have a choice in each user 
> whether to use either 1) feedback based rate adjustment (don't care if 
> its VJCC cwnd or TFRC based) or 2) a rateless erasure code you could 
> have both users' flows use 1 or both 2 or a mix, i.e. 3 cases 1+1, 2+2 
> or 1+2 now, how do you choose the code to get max goodput for the 3 
> cases... 

Hm. Maybe I miss something essential. However, I don't know whether 
there is "that only" and "simple" answer to this question. As Joe wrote 
some days ago, in networks with huge (R)TT, FEC dominated approaches 
would often be prefarable, so perhaps 2+2. In networks with small (R)TT, 
ARQ based approaches would be preferable, so perhaps 1+1.

However: What about two flows which only share a part of their paths, so 
one flow has a huge RTT, the other a small one?

And one point is missing: In case 2+2, we have to distribute the path's 
capacity. "Somehow". In case 1+1, it is VJCC which distributes available 
resources among the flows.  How is this achieved in 2+2?

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