This paper introduces dummynet, a simple and flexible network simulator that can be built with minimal modifications to an existing protocol stack. dummynet intercepts communications between the protocol layer under test and the underlying layer, simulating finite queues, bandwidth limitations, and communication delays. This allows experiments to be run on a standalone system, using real traffic generators and protocol implementations. The tool provides great control over operating parameters, simplicity, and the ability to use real traffic generators, making it suitable for various applications such as debugging, studying new protocols, and performance evaluation. The implementation of dummynet is highly portable and compatible with other BSD-derived systems, requiring less than 300 lines of kernel code. The paper also discusses the limitations of dummynet, including the granularity of the operating system's timer and the synchronicity of events. Examples of use and possible extensions are provided, demonstrating the tool's versatility in simulating complex network topologies. Overall, dummynet offers a convenient and effective approach to evaluating network protocols in a controlled and realistic environment.This paper introduces dummynet, a simple and flexible network simulator that can be built with minimal modifications to an existing protocol stack. dummynet intercepts communications between the protocol layer under test and the underlying layer, simulating finite queues, bandwidth limitations, and communication delays. This allows experiments to be run on a standalone system, using real traffic generators and protocol implementations. The tool provides great control over operating parameters, simplicity, and the ability to use real traffic generators, making it suitable for various applications such as debugging, studying new protocols, and performance evaluation. The implementation of dummynet is highly portable and compatible with other BSD-derived systems, requiring less than 300 lines of kernel code. The paper also discusses the limitations of dummynet, including the granularity of the operating system's timer and the synchronicity of events. Examples of use and possible extensions are provided, demonstrating the tool's versatility in simulating complex network topologies. Overall, dummynet offers a convenient and effective approach to evaluating network protocols in a controlled and realistic environment.