Packet Sink perhop_times attribute is empty after running the fattree.py example

[jzzhang8]

Dear Developers,
I just ran the code of "fattree.py" and tried to print the "perhop_times" attribute of each packet sink in order to check the detailed information for each packet. I found that the "perhop_times" attribute of packet sink is always empty after running the simulation. Could I know whether this is as expected after running the simulation?

Brief Description
The the dict items of flow.pkt_sink.perhop_times are all empty after running the simulation.

To Reproduce
Steps to reproduce the behavior:

1. Add the "print(all_flows[flow_id].pkt_sink.perhop_times)" following the line "print(all_flows[flow_id].pkt_sink.arrivals)" at the end of the file "fattree.py".
2. Run the "fattree.py"
3. Check the output. Found all the items of perhop_times are empty.

Thank you for reading my request!