Approximate fair bandwidth allocation: A method for simple and flexible traffic management

The Internet architecture uses congestion avoidance mechanisms implemented in the transport layer protocol like TCP to provide good service under heavy load. If network nodes distribute bandwidth fairly, the Internet would be more robust and accommodate a wide variety of applications. Various congestion and bandwidth management schemes have been proposed for this purpose and can be classified into two broad categories: packet scheduling algorithms such as fair queueing (FQ) which explicitly provide bandwidth shares by scheduling packets. They are more difficult to implement compared to FIFO queueing. The second category has active queue management schemes such as RED which use FIFO queues at the routers. They are easy to implement but don't aim to provide (and, in the presence of non-congestion-responsive sources, don't provide) fairness. An algorithm called AFD (approximate fair dropping), has been proposed to provide approximate, weighted max-min fair bandwidth allocations with relatively low complexity. AFD has since been widely adopted by the industry. This paper describes the evolution of AFD from a research project into an industry setting, focusing on the changes it has undergone in the process. AFD now serves as a traffic management module, which can be implemented either using a single FIFO or overlaid on top of extant per-flow queueing structures and which provides approximate bandwidth allocation in a simple fashion. The AFD algorithm has been implemented in several switch and router platforms at Cisco sytems, successfully transitioning from the academic world into the industry.