[e2e] Why do we need congestion control?

Jon Crowcroft Jon.Crowcroft at cl.cam.ac.uk
Wed Apr 3 03:58:52 PDT 2013

assume your goals are
a. efficiency
b. fairness

where to meet a, 
you must not waste capacity on the links up to the bottleneck 
(for type 1 flow, this means not too much packet loss)
and to meet b,
you can use the Jain fairness equation.

now assume flows of type 2 can start and stop at some point.
what level of FEC do you use when there is 0, 1, or 2 flows of type 2?

now have a mix of flows of type 1 & 2
what level of FEC do you use when there is 
0, 1 or 2 flows of type 1 andor type 2?

repeat for 1million flows.

In missive <515C06BF.1020608 at web.de>, Detlef Bosau typed:

 >>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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