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Re: ns-3 error: endless cwnd growth


Cronologico Percorso di conversazione 
  • From: Francesco Gringoli < >
  • To: Michael Welzl < >
  • Cc:
  • Subject: Re: ns-3 error: endless cwnd growth
  • Date: Sat, 20 Jan 2018 21:49:22 +0100
Hi Michael,

I’m trying to replicate the simulation but I am not sure about where to find the TEB code. So I disabled it completely and this is what I get when running a simulation with 28 packet in the queue (blue line) or 29 (red line). They match. By the way, should I expect losses in this configuration? The cwnd increases forever meaning that there are no losses (and cwnd - always in CA - is limited by the receiver window before sending I guess).

-Francesco

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On 20 Jan 2018, at 18:31, Michael Welzl < " class=""> > wrote:

Dear all, + Francesco because he has experience with ns-3, and this is 100% an ns-3 thing, not related to our code at all:

I’m seeing some problems with my evaluations because of a strange bug in ns-3. This bug prevents the congestion window from growing above a value of approximately 137k bytes, it seems.

I’m attaching an example script. Here TEB (my code) is disabled, and the setup is plain and simple:
init cwnd = 3
8 Mbit/s bottleneck capacity
100 ms RTT
queue length: 28 packets

From the parameters above, the BDP should be 100000 bytes, i.e. 66.66 packets  (packets are of size 1500).
Add to this the queue length of 28 packets, and we get a max. cwnd of 94.66 packets. Expressed in segments (of size 1448), this is 137077.33.

This is the “good case” plotted below.

Now, add ONE packet to the queue, making it 29 packets long => and you get the other plot (“bad case”). I repeat, this is not my own code or anything - just normal ns-3, and I think it happens whenever the cwnd exceeds a certain length.

This gets really disturbing in my testbed experiments, I have to tune them to limit the max cwnd  :(

Does ANYBODY have a solution?  I’d be happy!  And the ns-3 people should be happy too !

Cheers,
Michael


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<good_case.png><bad_case.png>

— —

Francesco Gringoli, PhD
Assistant Professor
Dept. of Information Engineering
University of Brescia
via Branze, 38
25123 Brescia
ITALY

Ph:  ++39.030.3715843
FAX: ++39.030.380014
WWW: http://netweb.ing.unibs.it/~gringoli

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