Full system "end-to-end" measurements in physical testbeds are the gold standard for network systems evaluation but are often not feasible. When physical testbeds are not available we frequently turn to simulation for evaluation. Unfortunately, existing simulators are insufficient for end-to-end evaluation, as they either cannot simulate all components, or simulate them with inadequate detail.We address this through modular simulation, flexibly combining and connecting multiple existing simulators for different components, including processor and memory, devices, and network, into virtual end-to-end testbeds tuned for each use-case. Our architecture, SimBricks, combines well-defined component interfaces for extensibility and modularity, efficient communication channels for local and distributed simulation, and a co-designed efficient synchronization mechanism for accurate timing across simulators. We demonstrate SimBricks scales to 1000 simulated hosts, each running a full software stack including Linux, and that it can simulate testbeds with existing NIC and switch RTL implementations. We also reproduce key findings from prior work in congestion control, NIC architecture, and in-network computing in SimBricks.
