Bob, Rong Let me present our rationale for choosing the PI controller (hope it will be helpful to present the rationale for PIE).
When we were working on the issue in 2001-2001, RED had been deployed everywhere but also disabled everywhere. People had trouble understanding and tuning RED. Our fluid modeling and control theoretic analysis showed that RED was a classical Proportional Controller with a delay element (the queue averaging mechanism. We identified the following flaws with RED 1. The “Proportional” control meant that RED could not control the queue length to a specified value, it is an inherent limitation of Proportional control. RED always controlled the queue (and hence delay) to some value between min_thresh and max_thresh depending on the load. 2. The delay element (queue averaging) introduced an additional instability to the control loop making it more difficult to control (and as Bob has said, the instability is oscillations causing jitter and loss of some link throughput due to the oscillations). We decided to redesign the controller but since RED was already implemented, create a controller that could use the same deployment with some minor modifications. So what we did to fix the issues was: 1. In classical control theory Integral control drives the steady state error to zero, so we changed the Proportional Control to Proportional Integral, or PI control which lets you control the buffer queue length to a specific, target value independent of the load. 2. We simply removed the queue averaging mechanism. The resultant controller could work with existing RED implementations with a slight modification to the update equation. -Vishal -- http://www.cs.columbia.edu/~misra/ > On May 22, 2015, at 9:38 AM, Bob Briscoe <[email protected]> wrote: > > > I didn't mean that you need to justify better why you chose a PI controller. > I said you need to "Articulate the Rationale for a PI Controller"
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