Communication Networks Fluid flow analysis of TCP flows in a DiffServ environment

The differentiated services (DiffServ) architecture allows IP networks to offer different QoS levels to different users and applications. In this architecture, routers in the core network offer the same per-hop behaviour (PHB) to all packets classified as belonging to the same class at the edge of the network. One of the most important types of PHB is assured forwarding (AF) PHB. Within each AF class, IP packets can be marked with different drop precedence (DP) values, and treated differently in congested DS nodes. To this end, DiffServ nodes in the core network implement active queue management (AQM) mechanisms. The target of this paper is to provide network designers with an accurate fluid-flow analytical model of a DiffServ network, where the RED with in/out and coupled average queues (RIO-C), RED with in/out and decoupled average queues (RIO-DC) and Weighted RED (WRED) AQM techniques are implemented. We address a network simultaneously loaded with both greedy and data-limited TCP flows, and we consider one AF class in which two DPs are defined, one for packets complying with the negotiated profile (IN packets), and another for packets that do not respect it (OUT packets). A token bucket marking mechanism is modelled for this purpose. The proposed model is applied to a complex network topology. Comparison between model and simulation results demonstrates that the model is able to capture both transient and steady-state network behaviour with a high degree of accuracy, even when not all network routers implement the same AQM technique. These characteristics make our modelling approach suitable to address the issue of network parameter optimisation. As an example, the link capacity dimensioning problem in a DiffServ domain by means of an iterative optimisation algorithm is presented. Copyright# 2006 AEIT. 1