WARD: A deterministic fluid model

Queue management, bandwidth share and congestion control are very important to both the robustness and fairness of the Internet. A new TCP-friendly router-based active queue management scheme, called WARD, approximates the fair queueing policy. WARD is a simple packet dropping algorithm with a random mechanism and discriminates against the flows which submit more packets per second than is allowed by their fair share. By doing this, it not only protects transmission control protocol (TCP) connections from user datagram protocol (UDP) flows, but also solves the problem of competing bandwidth among different TCP versions, such as TCP Vegas and TCP Reno. Furthermore, it is stateless and easy to implement, so WARD controls unresponsive or misbehaving flows with a minimum overhead. In this article, we present a deterministic fluid model of TCP/WARD system, and explain the UDP throughput behaviour with WARD. Also, we prove that, provided the number of TCP flows is large, the UDP bandwidth share peaks at (2e)^-1 = 0.184 when UDP input rate is slightly larger than link capacity and drops to zero as UDP input rate tends to infinity.