[e2e] Why do we need congestion control?
Jon.Crowcroft at cl.cam.ac.uk
Wed Apr 3 03:58:52 PDT 2013
assume your goals are
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
>>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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