The Internet has enhanced the sharing and transmission of data in the world today. Transmission Control Protocol (TCP) is the most commonly used protocol which provides reliable data transfer and congestion control. There is substantial literature analyzing TCP control of Internet traffic using fluid models to capture the average behavior of Internet traffic. Much of this research has been concerned with network stability under specific cases. Much of this research has been concerned with network stability under specific cases. For example, dynamic synchronization is studied for small buffer regimes; bifurcation is examined using simplified models. In this book, we focus on the collective behavior with heterogeneous network model with TCP users and bottleneck queue (router).