ns-3 error: endless cwnd growth

[Michael Welzl < >]

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:

Iā€™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.

Iā€™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 ā€œgood caseā€ plotted below.

Now, add ONE packet to the queue, making it 29 packets long => and you get 
the other plot (ā€œbad caseā€). 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?  Iā€™d be happy!  And the ns-3 people should be 
happy too !

Cheers,
Michael

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