functions:
1.get topology of the whole network
2.find the shortest path to forward the packet
2.avoid repeated forwarding in the loop
using method:
1.first use the ryu package under utils to replace your default ryu in python2.7 or just replace your python2.7 with our pyhton 2.7
2../run.sh to run c2 and c3, run test.py to run c1