Explicit congestion control (XCC) is emerging as one potential solution for overcoming limitations inherent to the current TCP algorithm, characterized by unstable throughput, high queuing delay, RTT-limited fairness, and a static dynamic range that does not scale well to high bandwidth delay product networks. In XCC, routers provide multibit feedback to sources, which, in turn, adapt throughput more accurately to the path bandwidth with potentially faster convergence times. Such systems, however, require precise knowledge of link capacity for efficient operation. In the presence of variable-capacity media, e.g., 802.11, such information is not entirely obvious or may be difficult to extract. We explore three possible algorithms for XCC which retain efficiency under such conditions by inferring available bandwidth from queue dynamics and test them through simulations with two relevant XCC protocols: XCP and RCP. Additionally, preliminary results from an experimental implementation based on XCP are presented. Finally, we compare our proposals with TCP and show how such algorithms outperform it in terms of efficiency, stability, queuing delay, and flow-rate fairness.

Real-time multimedia applications, such as VoIP, video conferencing or on-line gaming are key applications to the success of 4G. In today's Internet these applications are not subject to congestion control, therefore the growth of popularity of these applications may endanger the stability of the Internet. On the other hand, 4G networks will be much more dynamic due to mobility and multi-homing, and the network conditions may vary abruptly requiring a fast response from the control mechanism. In this article we compare the feedback-based to the reservation-based congestion control approach and focus on the first one, by evaluating some mechanisms with respect to media friendliness, scalability and dynamic behaviour. We also present a set of requirements for the ideal congestion control mechanism of real-time flows in 4G networks