Not able to ping hosts in POX controller in Mininet.

I am a beginner to SDNs and I wanted to learn how to congest a network in mininet and alter buffer size of switches to monitor the congestion. I came across this PDF online with some programs in it. I'm on Ubuntu and I started a mininet VM and SSH'ed into it. I saved the lab4_controller.py in ~/pox/ext. 

However, when I run the program and try h1 ping h2, I get the error message "Destination host unreachable". The full log is shown here.

Please help. Also some pointers on how to design an SDN where congestion ( and consequently buffering) takes place would be appreciated.