Several studies have considered control theory tools for traffic control in communication networks, as for example the congestion control issue in IP (Internet protocol) routers. In this paper, we propose to design a linear observer for time-delay systems to address the traffic monitoring issue in TCP/AQM (transmission control protocol/active queue management) networks. Due to several propagation delays and the queueing delay, the set TCP/AQM is modeled as a multiple delayed system of a particular form. Hence, appropriate robust control tools as quadratic separation are adopted to construct a delay dependent observer for TCP flows estimation. Note that, the developed mechanism enables also the anomaly detection issue for a class of DoS (denial of service) attacks. At last, simulations via the network simulator NS-2 and an emulation experiment validate the proposed methodology.