Reference:
https://www.cisco.com/c/en/us/products/ios-nx-os-software/quality-of-service-qos/index.html
https://www.wisegeek.com/what-is-qos.htm
https://www.cnblogs.com/snailrun/p/5583738.html
https://blog.csdn.net/armlinuxww/article/details/70212840
https://blog.51cto.com/kalng/888356