Delay and loss-based transport protocols: Buffer-sizing and stability

It is generally accepted that buffers, in Internet routers, should be much smaller than the currently deployed bandwidth-delay product rule. However, as yet, there is no consensus on the optimal buffer-sizing strategy. Our focus will be on the performance, with respect to sizing buffers, of transport protocols that use queuing delay and packet loss as their feedback signals for flow and congestion control. The protocols we choose for our study are Compound, Illinois, and AFRICA. We study these protocols in two different buffer-sizing regimes: the bandwidth-delay product rule and small buffers. In a small buffer environment, we develop fluid models for each of these protocols and analyze the local stability and local bifurcation theoretic properties. We observe that all these protocols exhibit stable limit cycles when the buffer, in a small buffer regime, crosses a certain threshold value. We also offer guidelines for protocol parameters and for the choice of buffer size to ensure stability. The analysis is broadly corroborated with packet-level simulations. As large buffers are currently implemented we also study these protocols, via simulations, in a large buffer regime. In addition to the extra queuing delay, large buffers also exhibit oscillatory dynamics which serves to degrade end-to-end performance. As Compound TCP is widely implemented in the windows operating system, our work demonstrates how variations in buffer size may affect today's network performance. © 2012 IEEE.