The Virtual Inter-Network Testbed (VINT) project has enhanced its network simulator, ns, to provide practical innovations that allow researchers to evaluate network protocols under varied conditions. As the Internet grows, new protocols and algorithms are needed to meet changing operational requirements, such as security, multicast transport, mobile networking, policy management, and quality-of-service support. However, custom simulators, wide-area testbeds, and small-scale lab evaluations have limitations, including high costs, difficulty in reconfiguration, and limited flexibility. Additionally, some networking phenomena, such as wireless radio interference, are difficult to reproduce in simulations. Protocol design using simulation often starts with isolated protocol elements and simplified assumptions. High start-up costs prevent researchers from creating comprehensive simulation environments. This leads to a lack of standardization and reproducibility in simulations conducted by different groups. To address this, the VINT project provides a common simulator that allows researchers to compare results across studies. This common simulator supports multiple levels of abstraction, enabling researchers to study protocols at different levels of detail. Ns provides several levels of abstraction, including detailed packet forwarding, centralized routing, session-level packet forwarding, and algorithmic routing. These abstractions allow researchers to balance simulation accuracy and performance. The simulator also includes an emulation interface that permits network traffic to pass between real-world network nodes and the simulator, allowing researchers to evaluate the dynamic behavior of protocols and their implementations in end systems. Ns also supports scenario generation, which is critical for testing protocols under appropriate network conditions. The simulator includes a rich library of predefined topologies and traffic models, and it can generate random topologies using packages such as the Georgia Tech models (GT-ITM). Automatic scenario generation plays an important role in the STRESS approach to systematic protocol testing, which automatically generates test scenarios to explore protocol correctness. Ns is publicly available and has a large protocol library, making it an ideal virtual testbed for comparing protocols. It supports a wide range of protocols, including TCP, reliable multicast, and wireless networking. The simulator also includes a split-programming model that allows researchers to use C++ for high-performance simulation tasks and Tcl for dynamic simulation tasks. This model provides flexibility and allows researchers to focus on their research experiments. Ns has been widely used by the networking research community for protocol comparison and exploration of complex behavior. It has been used to study reliable multicast protocols, TCP behavior, and wireless networking. The simulator also supports multiprotocol interactions, including the impact of protocol operation at one layer upon another layer and the interaction of unrelated protocols. Despite its benefits, ns has a relatively steep learning curve and is a large system. However, the VINT project has contributed to the development of ns and has helped reduce duplication of effort within the networking research and development community. The project has also addressed challenges such as developing mechanisms for the successful integration of code from the user community, reducing the ns user’s learning curve, and developing tools for large-scale simulations with a diverse traffic mix.The Virtual Inter-Network Testbed (VINT) project has enhanced its network simulator, ns, to provide practical innovations that allow researchers to evaluate network protocols under varied conditions. As the Internet grows, new protocols and algorithms are needed to meet changing operational requirements, such as security, multicast transport, mobile networking, policy management, and quality-of-service support. However, custom simulators, wide-area testbeds, and small-scale lab evaluations have limitations, including high costs, difficulty in reconfiguration, and limited flexibility. Additionally, some networking phenomena, such as wireless radio interference, are difficult to reproduce in simulations. Protocol design using simulation often starts with isolated protocol elements and simplified assumptions. High start-up costs prevent researchers from creating comprehensive simulation environments. This leads to a lack of standardization and reproducibility in simulations conducted by different groups. To address this, the VINT project provides a common simulator that allows researchers to compare results across studies. This common simulator supports multiple levels of abstraction, enabling researchers to study protocols at different levels of detail. Ns provides several levels of abstraction, including detailed packet forwarding, centralized routing, session-level packet forwarding, and algorithmic routing. These abstractions allow researchers to balance simulation accuracy and performance. The simulator also includes an emulation interface that permits network traffic to pass between real-world network nodes and the simulator, allowing researchers to evaluate the dynamic behavior of protocols and their implementations in end systems. Ns also supports scenario generation, which is critical for testing protocols under appropriate network conditions. The simulator includes a rich library of predefined topologies and traffic models, and it can generate random topologies using packages such as the Georgia Tech models (GT-ITM). Automatic scenario generation plays an important role in the STRESS approach to systematic protocol testing, which automatically generates test scenarios to explore protocol correctness. Ns is publicly available and has a large protocol library, making it an ideal virtual testbed for comparing protocols. It supports a wide range of protocols, including TCP, reliable multicast, and wireless networking. The simulator also includes a split-programming model that allows researchers to use C++ for high-performance simulation tasks and Tcl for dynamic simulation tasks. This model provides flexibility and allows researchers to focus on their research experiments. Ns has been widely used by the networking research community for protocol comparison and exploration of complex behavior. It has been used to study reliable multicast protocols, TCP behavior, and wireless networking. The simulator also supports multiprotocol interactions, including the impact of protocol operation at one layer upon another layer and the interaction of unrelated protocols. Despite its benefits, ns has a relatively steep learning curve and is a large system. However, the VINT project has contributed to the development of ns and has helped reduce duplication of effort within the networking research and development community. The project has also addressed challenges such as developing mechanisms for the successful integration of code from the user community, reducing the ns user’s learning curve, and developing tools for large-scale simulations with a diverse traffic mix.