Technical Report Number
The ability of TCP (Transmission Control Protocol) or alternative congestion control algorithms to operate successfully in networks with small link buffers has recently become a subject of intensive research. In this paper, we investigate fundamental limitations on minimum buffer requirements for any congestion control. We present an idealized protocol where all flows always transmit at their fair rates. The ideally smooth congestion control causes link queuing only due to asynchrony of flow arrivals, which is intrinsic to computer networks. Our analysis and simulations for different distributions of flow interarrival times agree that the buffer size needed for a fixed loss rate at a fully utilized link is O(sqrt(N)), where N is the number of flows sharing the link. This result undermines a standpoint that small constant buffers are sufficient for links serving large numbers of flows. On the other hand, we show that fixed loss rates are achievable with constant buffers at a price of reducing average utilization of the bottleneck link. In particular, we derive analytical expressions for the maximum bottleneck link utilization that supports a fixed loss rate with arbitrarily many flows and constant buffer. We also experiment with a simple modification of RCP (Rate Control Protocol). The experiments indicate practical feasibility of compensating for larger numbers of flows by decreasing the targeted utilization of the bottleneck link.
Podlesny, Maxim and Gorinsky, Sergey, "Price of Asynchrony: Queuing under Ideally Smooth Congestion Control" Report Number: WUCSE-2007-24 (2007). All Computer Science and Engineering Research.
Permanent URL: http://dx.doi.org/10.7936/K7H41PPW