Re: ns-3 error: endless cwnd growth

[Michael Welzl < >]

… and in fact it’s not so easy  :(

I tried halving the bandwidth but doubling the RTT, and the effect was the 
same (28 packet queue is fine, but 29 isn’t).
However, when I varied parameters more, it seemed that the problem can also 
occur at lower capacities as long as the RTT gets large enough.
Generally, the queue seems to be the main factor creating this problem.

Maybe it’s something with the RTT estimation?  I don’t know…

I suspect that this can ruin the results we get for our simulation tests!   
:-(

Cheers,
Michael


> On Jan 20, 2018, at 6:31 PM, Michael Welzl 
> <
 >
>  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
> 
> 
> <michaelssim.cc>
> <good_case.png><bad_case.png>