Dummynet is a simple and flexible network simulator that allows experiments on network protocols to be conducted on a standalone system with minimal modifications to an existing protocol stack. It simulates the effects of finite queues, bandwidth limitations, and communication delays by intercepting communications at the protocol layer under test. This approach allows the use of real traffic generators and protocol implementations, enabling the simulation of unusual environments. Dummynet runs in a fully operational system, providing great control over operating parameters, simplicity, and the ability to use real traffic generators. Experiments can be run as easily as running the desired applications on a workstation. Dummynet is implemented as a kernel module in FreeBSD and is highly portable, compatible with other BSD-derived systems, and requires less than 300 lines of kernel code. It allows the simulation of various network settings, including routers with bounded queue sizes, communication links with given bandwidth and delay, and random packet reordering and losses. The tool is particularly useful for debugging, studying new protocols, and performance evaluation. It can also be used as a simple selective bandwidth limiter for servers. Dummynet's principle of operation involves intercepting communication between the protocol layer under analysis and the underlying one, simulating the presence of a real network with finite-size queues, bandwidth limitations, communication delays, and possibly lossy links. The system is able to simulate almost all settings used in the literature where a bottleneck link can be identified. It is particularly useful for studying the interactions between protocols and application programs in unusual or hard-to-reproduce settings. Dummynet has some limitations, including the granularity and precision of the operating system's timer, which can affect timing-related measurements. Additionally, the periodic task might be run late or miss timer ticks, depending on the system load. Events in Dummynet occur synchronously with the system's timer, which might hide or amplify some real-world phenomena. However, the presence of Dummynet does not change the essentially non-deterministic behavior of a system when analyzed with a fine-grain resolution. Dummynet can be extended to simulate more complex network topologies by defining multiple data structures, each comprising one router, one pipe, and a routing table. It can also be used to simulate complex topologies involving multiple hosts using a standalone system. The current implementation lacks an automated tool to setup an arbitrary network topology, but this is not necessary as most simulations can be run with a few command lines. Dummynet is particularly useful for developing new protocols as a suitable testbed might not exist. It can speed up the analysis and development of protocols, making the simulation environment readily available in a production environment.Dummynet is a simple and flexible network simulator that allows experiments on network protocols to be conducted on a standalone system with minimal modifications to an existing protocol stack. It simulates the effects of finite queues, bandwidth limitations, and communication delays by intercepting communications at the protocol layer under test. This approach allows the use of real traffic generators and protocol implementations, enabling the simulation of unusual environments. Dummynet runs in a fully operational system, providing great control over operating parameters, simplicity, and the ability to use real traffic generators. Experiments can be run as easily as running the desired applications on a workstation. Dummynet is implemented as a kernel module in FreeBSD and is highly portable, compatible with other BSD-derived systems, and requires less than 300 lines of kernel code. It allows the simulation of various network settings, including routers with bounded queue sizes, communication links with given bandwidth and delay, and random packet reordering and losses. The tool is particularly useful for debugging, studying new protocols, and performance evaluation. It can also be used as a simple selective bandwidth limiter for servers. Dummynet's principle of operation involves intercepting communication between the protocol layer under analysis and the underlying one, simulating the presence of a real network with finite-size queues, bandwidth limitations, communication delays, and possibly lossy links. The system is able to simulate almost all settings used in the literature where a bottleneck link can be identified. It is particularly useful for studying the interactions between protocols and application programs in unusual or hard-to-reproduce settings. Dummynet has some limitations, including the granularity and precision of the operating system's timer, which can affect timing-related measurements. Additionally, the periodic task might be run late or miss timer ticks, depending on the system load. Events in Dummynet occur synchronously with the system's timer, which might hide or amplify some real-world phenomena. However, the presence of Dummynet does not change the essentially non-deterministic behavior of a system when analyzed with a fine-grain resolution. Dummynet can be extended to simulate more complex network topologies by defining multiple data structures, each comprising one router, one pipe, and a routing table. It can also be used to simulate complex topologies involving multiple hosts using a standalone system. The current implementation lacks an automated tool to setup an arbitrary network topology, but this is not necessary as most simulations can be run with a few command lines. Dummynet is particularly useful for developing new protocols as a suitable testbed might not exist. It can speed up the analysis and development of protocols, making the simulation environment readily available in a production environment.