Internet congestion control: Complete stability region for PI AQM and bandwidth allocation in networked control

The Internet represents a shared resource, wherein users contend for the finite network bandwidth. Contention among independent user demands can result in congestion, which, in turn, leads to long queueing delays, packet losses or both. Congestion control regulates the rate at which traffic sources inject packets into a network to ensure high bandwidth utilization while avoiding network congestion. In this thesis, we present contributions pertaining to two specific areas in the Internet congestion control: PI AQM and bandwidth allocation in Cyber-Physical Systems (CPSs). In the area of PI AQM, we present an analytic derivation of the complete stability region. The stability region represents the entire set of the feasible design parameters that stabilize the closed-loop TCP-AQM system. Utilizing the complete stability region, we show that the PI parameters used in the literature can be excessively conservative. We also show that provably stable controller parameters can exhibit widely different levels of performance. Furthermore, we present examples of PI controllers that are stable and have significantly better performance than previously proposed ones. These facts explain the previous observation about PI sluggish responsiveness and stress the importance of obtaining the complete stability region for the PI AQM. As for CPSs bandwidth allocation, we devise a bandwidth allocation scheme for Cyber-Physical Systems that have their control loops closed over a distributed network. We formulate the bandwidth allocation as a convex optimization problem. We then present an allocation scheme that solves this optimization problem in a fully distributed manner. In addition to being fully distributed, the proposed scheme is asynchronous, scalable, dynamic and flexible. Furthermore, we design robust and resilient queue controllers to enhance the performance of the bandwidth allocation scheme to better fulfill the requirements of the CPSs control loops. Throughout the thesis, we present analytical results and we validate them with packet-level simulations via ns-2.