问题：
当我运行mininet时，出现以下问题：
***Creating network
***Adding controller
***Adding hosts:
h1 h2 h3 h4 h5
***Adding switches:
s1
***Adding links:
(h1, s1) (h2, s1) (h3, s1) (h4, s1) (h5, s1)
***Configuring hosts
h1 h2 h3 h4 h5
***Starting controller
Cannot find required executable controller.
Please make sure that it is installed and available in your $PATH:
(/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin)
解决：
运行以下命令：
sudo ln /usr/bin/ovs-controller /usr/bin/controller
即可解决。